IMTS Mechanical Eng. (Manufacturing Technology) by IMTS INSTITUTE

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MANUFACTURING TECHNOLOGY

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MANUFACTURING TECHNOLOGY CONTENTS:

UNIT 1

01-48 FOUNDRY

UNIT II

49-78 FORGING AND WELDING

UNIT III

79-112 LATHE & THEORY OF METAL CUTTING

UNIT IV

113-158 METROLOGY PRESS WORKING & NON-CONVENTIONAL MACHINING PROCESSES

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UNIT-I FOUNDRY PATTERNS Definition A pattern is one of the important tools used for making cavities in the mould into which molten metal is poured to produce a casting. We have already seen that it is the model of the part to be produced however there are certain essential differences. It may also have several projections or bosses called core point. It also has extra projections to produce runners, risers and gates during the moulding process. Pattern materials The usual pattern materials are wood, metal and plastics. The most commonly used pattern material is wood, the main reason being the easy availability and the low weight. Also it can b easily shaped and is relatively cheap. But the main disadvantage of wood is its absorption of moisture as a result of which distortion and dimensional changes occur. A good construction may be able to reduce the warpage to some extent. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for large scale use of wood as a pattern material. The usual varieties of wood commonly used for making patterns are pine, mahogany, teak, walnut and deodar. Besides the wood, the plywood boards of the veneer type as well as the particle boards are also used for making patterns. Because of their availability in various thicknesses, their higher strength and no need for seasoning are the reasons for their usage. However, they can be used only in patterns which are of flat type (pattern plates) and no dimensional contours. Choice of the pattern material depends essentially on the size of the casting, the number of castings to be made from pattern, and the dimensional accuracy required. For every large casting, wood may be the only practical pattern material. Moulding sand being highly abrasive for large scale production, wood may not be suitable as a pattern material and one may have to opt for metal patterns. Because of their durability and smooth surface finish metal patterns are extensively used for large scale casting production and for closer dimensional tolerances. Though many materials such as cast iron, brass, etc. can be used as pattern material, aluminium and white metal are most commonly used. These are light, can be easily worked, and are corrosion resistant. Since white metal has very small shrinkage, the white metal pattern can be made use of making additional patterns without worrying about the double shrinkage allowances. Most metal patterns are cast in sand moulds from a master wood pattern provided with the double shrinkage allowances. Comparative advantages and disadvantages of various pattern materials are shown below.

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Comparative characteristics of metallic pattern materials Sl.No. 1

Pattern metal Aluminium alloys

Advantages

Disadvantages

Good machinability

Low strength

High corrosion resistance

High cost

Low density Good surface finish 2

Grey cast iron

Good machinability

Corrosion prone

High strength

High density

Low cost 3

Steel

Brass and bronze

Good surface finish

Corrosion prone

High strength

High density

Good surface finish

High cost

High strength

High density

High corrosion resistance 5

Lead alloys

Good machinability

High cost High density Low strength

Pattern materials based on expected life Sl.No.

Number of castings produced before pattern equipment repair Pattern

Small castings (under 600 mm)

Medium castings (6001,800 mm)

Pattern material

Core

2,000

Hard wood

6,000

Aluminium, plastic

100,000

1,000

750

3,000

Cast iron Hard wood Aluminium plastic

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Large castings (above 1,800 mm)

3 200

150

Soft wood

500

Hard wood metal reinforced

Plastics are also used as pattern materials because of their low weight, easier formability, smooth surfaces and durability. They do not absorb moisture and are therefore, dimensionally stable and can be cleaned easily. The making of a plastic pattern can be done in sand clay moulds or moulds made of plaster of paris. The most generally used plastics are cold setting epoxy resins with suitable fillers. With a proper combination it is possible to obtain a no shrink plastic materials and as double shrinkage allowances may not be required. Polyurethane foam is also used as pattern material. It is very light and can be easily formed into any shape required. It can be used for light duty work for small number of castings for the conventional casting and for single casting in the case of full mould process where the pattern is bounded inside the mould without withdrawing. This plastic has very low ash content and hence can be burned inside the mould. The pattern material is to be chosen based on the expected life of the pattern. The above table gives the comparative values of pattern material choices.

Factors for selecting pattern materials

The following factors to be considered for selecting pattern materials.

 Design of casting.  Number of castings to be produced.  Degree of accuracy and surface finish required,  Shape, complexity and size of the castings.  Castings or moulding method adopted.

Single piece solid pattern These are inexpensive and the simplest type of patterns. As the name indicates, they are made of single piece as shown below. This type of pattern is used only in case where the job is very simple and does not create any withdrawal problems. It is also used for applications in very small scale production or in

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prototype development. This pattern is expected to be entirely in the drag. One of the surfaces is expected to be flat which is used as the plane. If no such flat surface exists, the moulding may become complicated with the necessity of a follow board as explained later.

Single piece solid pattern

Split pattern This is the most widely used type of pattern for intricate castings. When the contour of the casting makes its withdrawal from the mould difficult, or when the depth of the casting is too high, then the pattern is split into two part so that one part is in the drag and the other in the cope. The split surface of the pattern is same as the parting plane of the mould. The two halves of the pattern should be aligned properly by making use of the dowel pins which are fitted to the cope half. These dowel pins match with the precisely made holes in the drag half of the pattern and thus align the two halves properly as seen below figure.

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Upper Part

Dowel Pin Middle Part

Lower Part

Three piece pattern

Upper die

Dowel pin

Lower die

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Split pattern Pattern allowances Patterns are not made into the exact size of the castings to be produced. Patterns are made slightly larger than the required castings. This extra given on the pattern is called pattern allowances. Pattern allowances are given for the purpose of compensating the metal shrinkage to provide extra metal which is to be removed in machining, to avoid metal distortion, for easy withdrawal of pattern from mould and for rapping. If allowances are not given on the pattern, the casting will become smaller than the required size. The various types of allowances normally provided on the pattern are

1. 2. 3. 4. 5.

Shrinkage allowance Machining or finish allowance Draft or taper allowance Distortion or chamber allowance Rapping or shake allowance

Shrinkage allowance All the metal shrinks when cooling except perhaps bismuth. This is because of the inter-atomic vibrations which are amplified by an increase in temperature. However, there is a distinction to be made between liquid shrinkage and solid shrinkage. Liquid shrinkage refers to the reduction in volume when the metal changes from liquid to solid state at the solidus temperature. Solid shrinkage is the reduction in volume caused, when the metal losses temperature in solid state. The shrinkage allowance is provided to take care of his reduction. The rate of contraction with temperature is dependent on the material. For example, steel contracts to a higher degree compared to aluminums. The contractions also depend upon the metallurgical transformation taking place during the solidification. For example, white cast iron shrinks by about 21.0 mm/m during casting. However, when annealed it grows by about 10.5 mm/m, resulting in a net shrinkage of 10.5 mm/m. similarly in grey cast iron and spheroidal graphite iron, the amount of graphitization controls the actual shrinkage. When graphitization is more, the shrinkage would be less and vice versa, the various rates of contraction for the materials are given below table. As a rule all the dimensions are going to be altered uniformly unless they are restrained in some way. For example, a dry sand core at the centre of the casting may restrain the casting from contracting but the edges are not restrained. Thus, it may be desirable to provide a higher shrinkage allowance fro outer dimensions compared to those which may be restrained. The actual value of shrinkage depends on various factors specific to a particular casting, namely the actual composition of the alloy cast, mould materials used, mould design, complexity of the pattern and component size. The pattern makerâ&#x20AC;&#x2122;s experience and a little bit of trial are to be used in arriving at the final shrinkages provided on the pattern. The values shown in the below are average values and higher values are to be used for smaller dimensions and vice versa.

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The shrinkage allowance is always to be added to the linear dimensions. Even in case of internal dimensions (e.g. internal diameters of cylinders), the material has a tendency to contract towards the centre and thus are to be increased. It is possible to obtain shrink rulers for specific materials such as steels which are nothing but special scales where dimensions shown are actually longer by a measure equal to the shrinkage allowance. Dimensions provided b such a rule can be used at the time of making the pattern. Different shrink rulers are used for different casting materials.

Shrinkage allowances for various metals Sl.No.

Material

Grey cast iron

Pattern dimension, mm

Section thickness, mm

Shrinkage allowances, mm

Up to 600

10.5

600 to 1200

8.5

Over 1200

7.0

White cast iron

16.0 to 23.0

Ductile iron

8.3 to 10.4

Malleable iron

11.8

10.5

9.2

7.9

6.6

4.0

2.6

Up to 600

21.0

600 to 1800

16.0

Over 1800

13.0

Plain carbon steel

Chromium steel

20.0

Manganese steel

25.0 to 38.0

Aluminium

13.0

Aluminium bronze

20.0 to 23.0

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Copper

16.0

Brass

15.5

Bronze

15.5 to 22.0

Gunmetal

10.0 to 16.0

Manganese bronze

15.6

Silicon bronze

10.4

Tin bronze

10.4

Chromium copper

20.8

Lead

26.0

Monel

20.0

Magnesium

13.0

Magnesium alloys

16.0

White metal

6.0

Zinc

10.0 to 15.0

Machining or finish allowance

The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally require to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Also, ferrous materials would have scales on the skin which are to be removed by cleaning. Hence extra material is to be provided which is to be subsequently removed by machining or cleaning process. This depends on dimensions, the type of casting material and the finish required. This may range from 2 to 20mm. general guidelines for machining allowances are provided in the table below. The machining allowances provided would ultimately have to be removed by machining. Hence the cost of providing additional machining allowance should be carefully examined before finalizing. The type of machining allowance provided would depend on the metal cast, the type of moulding used, the class of accuracy required on the surface and the complexity of surface details. One way of reducing the machining allowance is to keep entire casting in the drag flask such that dimensional variation and other defects due to the parting plane are reduced to a minimum.

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Machining allowances on patterns for sand castings Material

Cast iron

Cast steel

Non ferrous

Dimensions in mm

Allowance, mm Bore

Surface

Cope side

Up to 300

3.0

5.5

301 to 500

5.0

4.0

6.0

501 to 900

6.0

5.0

6.0

Up to 150

3.0

6.0

151 to 500

6.0

5.5

7.0

501 to 900

7.0

6.0

9.0

Up to 200

2.0

1.5

2.0

201 to 300

2.5

1.5

3.0

301 to 900

3.0

2.5

3.0

Draft or taper allowance At the time of withdrawing the pattern from the sand mould, the vertical faces of the pattern are in continual contact with the sand, which may damage the mould cavity, as below figure. To reduce the chances of this happening, the vertical faces of the pattern are always tapered from the parting line. This provision is called draft allowance. Draft allowance varies with the pattern complexity of the job. But in general, inner details of the pattern require higher draft than outer surfaces. The below table is a general guide to the provision of drafts. The draft allowance given varies for hand moulding and machine moulding. More draft needed to be provided for had moulding compared to machine moulding. In machine moulding the actual draft given varies with the condition of the machine (new, rigid, properly aligned, etc., require less draft).

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Suggested draft values for patterns Pattern Material

Wood

Height of the given surface, mm

Draft angle of surfaces, degrees External surface

Internal surface

Up to 20

3.00

21 to 50

1.50

2.50

51 to 100

1.00

1.50

101 to 200

0.75

1.00

201 to 300

0.50

1.00

301 to 800

0.50

0.75

801 to 2000

0.35

0.50

0.25

1.50

3.00

21 to 50

1.00

2.00

51 to 100

0.75

1.00

101 to 200

0.50

0.75

201 to 300

0.50

0.75

301 to 800

0.35

0.50

Over 2000 Metal and plastic

Pattern

Draft allowance

Distortion or chamber allowance

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A metal when it has just solidified is very weak and therefore is likely to be distortion prone. This is particularly so for weaker sections, such as long flat portions, V, U sections or in a complicated casting which may have thin and long sections which are connected to thick sections. The foundry practice should be to make extra material provision for reducing the distortion. Alternatively, the shape of pattern itself should be given a distortion of equal amount is the opposite direction of the likely distortion direction. This can be done by trial and error basis to get the distortion amount.

Distortion allowance Rapping or shake allowance Before withdrawal from the cans mould, the pattern is rapped all around the vertical faces to enlarge the mould cavity slightly which facilitates its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel and practices involved. It is negative allowance and is to be applied only to those dimensions which are parallel to the parting plane. One way of reducing these allowances is to increase the draft which can be removed during the subsequent machining. Core prints For all those casting where coring is required, provision should be made to support the core inside the mould cavity. One of the methods that are universally followed is to provide core prints where possible. In below figure shown an example of the provision core prints. The size of the core prints to be provided is to be estimated based on the specific casting.

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Job

Pattern

The core prints are provided so that the cores are securely a correctly positioned in the mould cavity. The design of core prints is such as to take care of the weight if the core before pouring and the upward metallostatic pressure of the molten metal after pouring. The core prints should also ensure that the core is not shifted during the entry of the metal into the mould cavity. The main force acting on the core when metal is poured into the mould cavity is due to buoyancy. The buoyant force can be calculated as the difference in the weight of the liquid metal to that of the core material of the same volume at that of the exposed core. It can be written as

P = V (ρ - d) Where, P = buoyant force, N V = volume of the core in the mould cavity, cm ρ = weight density of the liquid metal, N/cm

-2

d = weight density of the core material = 1.65 x 10 N/cm

The below table list the weight densities of some of the important foundry materials.

The above equation would be valid for cases that are similar to the one illustrated below, which 2 are more common, where V is given by 0.25 π D H.

P = 0.25 π (D1 - D ) H ρ – Vd

Where, V = total volume of the core in the mould

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Densities of foundry materials 3

Material

Density, N/cm

Aluminium

0.0265

Copper

0.0878

Magnesium

0.0171

Zinc

0.0700

Lead

0.1113

Carbon steel

0.0771

Grey cast iron

0.0686 to 0.0735

White cast iron

0.0755

Moulding sand

0.0157

Sl.No.

Core position In order to keep the core in position, it is empirically suggested that core print will be able to 2 support a load of 3.5 N/cm of surface area. Hence to fully support the buoyant force, it is necessary that the following condition is satisfied. P< 350 A 2

Where, A = core print area, mm .

If above is not satisfied, then it would be necessary to provide additional support by way of chaplets as described later. From the core print area, necessary core print sizes can be calculated.

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The Russian practice of dimensioning the core prints is to make the pressure acting on the core bearing area (i.e., the core print surface area) to be less than 50 to 75% of the moulding sand compression strength. Hence P> (Vd/σ) Where, V = total volume of the core including prints, cm

σ = compression strength of the moulding sand, N/cm

And

Core print dimensions Diameter, D, mm

< 50

51 - 150

151 - 300

301 - 500

501 - 750

Up to 25

26 – 50

51 – 100

100

101 – 200

100

201 – 300

110

301 – 400

100

120

401 – 500

100

120

130

The draft angle to be used for these core prints are given in the below table

Draft angles for core prints Core print hl (mm)

Vertical, deg

Horizontal, (deg)

α1

< 20

3.0

21 to 50

1.5

51 to 100

1.0

101 to 200

0.75

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Uses of core boxes Core boxes are used to produce cores. It is made of wood or metal. They give the desired shape. MOULDING Definition Moulding is the process of making a mould cavity by packing prepared moulding sand around the pattern and removing the pattern from the mould to form the mould cavity. In shortly, we can define the process of making a cavity to the product required in sad is called moulding. Moulding sand The special type of sand is used for making mould. Moulding sad essentially contains the following three constituents

1. Refractory Sand 2. Binder 3. Additive

These types of sand are used in moulding for the following reasons.

1. 2. 3. 4.

It maintains shape at very high temperature. It makes a mould porous. It can be used again and again. It is inexpensive.

Constituents of moulding sand

Moulding sand has three constituents. They are 1. Sand 2. Binder 3. Additive

1. SAND Silica sand is widely used as moulding sand. Silica has 80 to 90% silicon dioxide. Silica gives refractoriness to the sand. Advantages a. It is cheap and easily available. b. It can be easily moulded and reusable c. It has high thermal stability.

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Silica sand is the main constituent of the moulding sand. According to the clay content, the moulding sand is classified into: 1. 2. 3. 4. 5.

silica sand – up to 2% clay Lean or weak sand – 2 to 10% clay Moderately strong sand – 10 to 20% clay Strong sand – up to 30% clay Loam sand – up to 50% clay

There are three main types of moulding sands available. 1.1. Natural sand As the name implies, it is available from natural deposits. It needs only 5 to 8% water. These sands are available at riverbeds and it contains 80 to 90% of silica, 5 to 10% alumina or clay, and small percentage of lime and magnetic. Natural sand is also prepared by crushing and milling the soft yellow sand stone. This sand has less refractoriness as compared to synthetic sand. This sand is generally used for making light castings in ferrous and non-ferrous metals. Advantages 1. It is cheap and easily available. 2. It is easy to repair. 3. Wide rage of grain sizes and shapes are available. Limitations 1. It has less refractoriness. 2. It has high expansion ration. 3. It may be fused with metals.

1.2. Synthetic sand The moulding sand prepared artificially by mixing clay free sand having specified grain type with a specified type of clay binder as well as water and additives. This sand is prepared with desired properties we like. Synthetic sand is used in mechanized production machine moulding and high-pressure moulding. Advantages 1. 2. 3. 4.

It has more uniform grain size. Required properties can be obtained. It has higher refractoriness. It can be easily mould able.

Limitation It is more expensive.

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1.3. Special sands Special sands are prepared for obtaining specific properties such as refractoriness, high heat conductivity and low expansion ratio. It is also prepared for applying particular place of mould. Using special sand, good quality casting with fine surface finish can be produced. The following are most often used special sands. (i) Zircon sand (Zr Si O4): It is mainly used for making cores of brass and bronze castings. It is also used as facing land. It does not react with the moulding sand. It is fine having good refractoriness and high density. (ii) Chromites sand: It is used for making chilled castings. It may be used as facing sand in steel casting. It has good refractoriness, high heat conductivity, and less expansion ratio. 2. BINDERS Binders are used to bring the property of cohesiveness to the sand. They bind the sand grains together and give strength to the moulding sand. There are basically two types of binders used. 1. Organic binders 2. Inorganic binders

Organic Binders Organic binders are mainly used for core making. They are cereal, resins, pitch, drying oils, molasses etc. Inorganic Binders Clay binders are the most common type of inorganic binder. It is natural earthy material. Clay is formed by weathering and decomposition of rocks. The common type of clay used in moulding sand is (i) Fine clay, (ii) Kaolinite, and (iii) Bentonite. Kaolinite and Bentonite clays are most popular because they have high thermo chemical stability. Portland cement and sodium silicate are also used as binders. 3. ADDITIVES Additives are added to the molding sand to improve the properties like strength, refractoriness and permeability. These additives may be necessary to give surface finish to the casting or to eliminate casting defects that arise from either the expansion of the moulding sand or contraction of the casting. Additives are not used for binding purpose. Some common additives that are used to improve the properties of moulding sand are given below. 3.1 Sea coal: Sea coal is finely powdered bituminous coal. It is used to obtain smoother and cleaner surfaces of castings and also reduces the adherence of sand particle to the casting. It is mainly used to make ferrous castings. It is added up to 8%. When the molten metal is poured into the mould, coal dust burns and gives

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off volatile substances like CO2 and CO which form gas spacing between the mould walls and metals. This improves the permeability of moulding sand. 3.2 Saw dust: It improves the permeability and deformability of the moulds. It should be dry. Otherwise, we can use peat that contains about 70 â&#x20AC;&#x201C; 73% volatile matter. 3.3 Pitch: It is distilled from soft coal. It improves hot strength. It gives fine surface finish ferrous castings. 3.4 Cereals: It is finely ground corn flour or cornstarch. It increases green and dry strength of the moulding sand. Generally, it is used about 1%. 3.5 Silica flour: It is very fine powered silica. It is generally mixed twice that of moulding sand to make facing sand to make facing sand and used around the pattern. Therefore, it improves the surface finish of the casting. It also increases hot strength and reduces sand expansion defects. 3.6 Special additives: Fuel oil, dextrin, molasses and iron oxide are also added to moulding sand. Fuel oil improves the mould ability of sand. Dextrin increases collapsibility and setting strength. Molasses improves the dry strength and collapsibility of the mould. Iron oxide improves the hot strength of the moulding sand.

Properties of Moulding sand A good casting can be produced only with the use of good quality moulding sand. For this, the moulding properties of the sand have to be controlled. These properties are 1. 2. 3. 4. 5. 6.

Porosity or permeability Plasticity or flow ability Adhesiveness Strength or cohesiveness Refractoriness Collapsibility

1. Porosity or permeability Permeability is a measure of moulding sand by which sand allows the steam and gases to pass through it. When molten metal is poured into the mould, steam and gases are formed due to moisture, binders and additives present in the sand. If the gases are not removed, casting defects such as blowholes will occur. Even though we provide vent holes sand riser, all of these gases will not escape through it. To escape these gases, the moulding sand should have good gas permeability.

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Permeability of the moulding sand depends on the following factors: i. ii. iii.

Quality and quantity of clays and quartz Moisture content Degree of compactness

The following parameters which affect the permeability of moulding sand:  If the clay content is less, the permeability will be more and vice versa.  If the grain size is larger, the permeability will be more and vice versa.  Soft ramming (i.e. less density) improves the permeability.  Higher the silica content on sand, lower will be the permeability.

2. Plasticity or flow ability This is the property of moulding sand by which the moulding sand flows around and over the pattern, and uniformly fills the flask. Thus, it gives the shape of the pattern and retains the shape after removing the pattern. This property may be improved by adding clay and water to silica sand.

3. Adhesiveness This is the property of moulding sand by which it sticks or adheres to another body. The moulding sand should cling or stick to the sides of the moulding boxes. So, it does not fall out when the flask are lifted and turned over. This property depends on the type and amount of binder used in the sand mix. Addition of clay and moisture increases and adhesiveness.

4. Strength or cohesiveness It is the property of moulding sand by which it sticks together. A moulding sand should have sufficient strength so the mould does not collapse or get partially damaged during shifting, turning or pouring the molten metal. Because of poured into the mould cavity, its bottom and walls are subjected to a high metallostatic pressure. Strength of the moulding sand depends on i. ii. iii.

Grain size and shape Moisture content Density of sand after ramming

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Strength is increased with increasing density, clay content and decreased size of sand grains. Therefore, it is obvious that as the strength of the moulding sand increases its porosity decreases. 5. Refractoriness This is the property of moulding sand to resist high temperature of molten metal. This property mainly depends on the purity of the sand particle and their size. Rough and larger grain and quartz content in moulding sand increase the refractoriness. Poor refractoriness will result in rough surface in casting.

6. Collapsibility This is the property of the moulding sand to decrease in volume to some extend under the compressive forces developed by the shrinkage of metal during freezing and subsequent cooling. This property permits the moulding sand to collapse easily after the casting solidifies. If the mould or core does not collapse, it may restrict the free contraction of the solidifying material and causes crack on the casting. This property depends on the amount of quartz and binders. Moulding sand preparation One of the important requirements for the preparation of sand is through mixing of its various ingredients. This is essential to ensure uniform distribution of the various components in the entire bulk of the sand. During the mixing process any lump present in sand is broken up and clay is uniformly enveloped around the sand grains and moisture is uniformly distributed. Besides manual mixing, equipment called Muller is normally used in foundries to mix the sands. These are essentially of two types, batch type and continuous. A batch Muller consists of one or two Muller wheel and equal number of plough blades all of them connected to a single driving source. The Muller wheels are large and heavy, and continuously roll inside the Muller bowl. The plough blades ensure that the sand is continuously agitated. The combined action of the both is a sort of kneading action which makes the clay and the moisture uniformly distributed throughout the sand. A continuous Muller consists of two bowls with Muller wheel and ploughs, such that sand, clay and moisture are fed through a hopper into one of the bowls which after getting mulled moves into the second one and then finally out. Thus, well prepared moulding sand is continuously available for use. It is generally used for large scale production.

Moulding sand is prepared by the following steps: 1. Mixing of sand 2. Tempering of sand 3. Conditioning of sand

1. Mixing of sand In this step, we are mixing the sand, binder, moisture and other additives. When a small quantity of moulding sand is required, mixing is done by manually using shovels. When large quantity is require,

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mixing is done by machine (Muller). Initially, all foreign materials like nails, fins, hard sans lumps, and other iron pieces are removed with the help of magnetic separator and screens. Then sand, clay and additives are mixing in the muller in required condition. Mixing is continued till uniform distribution of ingredients takes place.

2. Tempering of sand The process of spraying and mixing adequate amount of water with the sand in the muller is called tempering. It should be ensured that water is evenly distributed throughout the sand. 3. Conditioning of sand The conditioning process includes the following activities:

i. ii. iii. iv. v.

Removing foreign materials Distributing the binder uniformly Controlling the moisture Aerating the sand and Delivering at proper temperature

Sand taken out from the muller is aerated using air blaster to separate sand grains and to increase the flow ability of sand. Types of moulding Moulding process may be classified according to the type of material with which the mould is made. Similarly, the selection of a mould is governed by the type of metal to be cast, size of casting, accuracy and the surface finish of the casting. There are different types of moulds given below. 1. Green sand mould 2. Dry sand mould

Green sand mould We have already seen that the green sand contains silica sand, clay, water and additives. It contains 10 to 15% of clay, 4 to 6% of water and remaining percentage of silica sand. It is porous. The green sand mould is that mould on which the molten metal is poured immediately after the mould is prepared. These moulds are preferred for making small and medium size castings. It is specially used for nonferrous metals and alloy castings. The procedure for making green sand mould (Flour or Bench moulding method) is explained below.

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1. Here, we are using two-piece split pattern. One half of the split pattern is placed at the center of the moulding board as shown in figure. 2. The drag box is placed around the pattern. Dowel pins are connected on the drag box. 3. 20mm layer facing sand is first placed around the pattern and then the drag is filled up with green sand. 4. Sufficient ramming is done by the peen end of the hand rammer. During ramming, if necessary, additional sand can be put into the drag. Later, the sand around pattern and the edges of the molding box are rammed with butt-end of the rammer. 5. Excess sand is removed by strike off bar. 6. Vent holes are made by vent wire which is used to escape the steam and gases produced during pouring of the molten metal. 7. The top surface is made smooth b trowel. 8. Then the drag is tilted down as shown in figure. 9. The parting sand is sprinkled over the pattern to avoid sticking of pattern with the moulding sand. Similarly, the surface of green sand to avoid sticking of sand on copes with the sand on drag. 10. Top half of the pattern is placed correctly in position. 11. Cope box is placed correctly in position on the drag using dowel pins as shown in figure. 12. Riser pin and sprue pin are placed in correct position. 13. The operations of filling, ramming and venting of the sand on cope and done similar to that of drag. 14. Sprue pin and riser pin are removed. Then pouring basin is formed on the top of the cope box to facilitate easy pouring of the metal. 15. Cope and drag are separated. 16. Draw spike is driven into the pattern pieces and shacked lightly. Then pattern pieces are withdrawn slowly. 17. A gate is cut on the top surface of the drag. It should be exactly below the sprue on the cope. 18. The mould surfaces are coated with coating material like graphite to get smooth surface to the casting. 19. Then core is set in position, if necessary. 20. Finally, the cope and drag are assembled. Weight is placed on the cope to prevent the cope from floating up when the molten metal is poured. Now, the mould is ready for pouring.

Pattern

Drag

Board

Sand

Aligning pin

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Riser pin

Cope

Leg

Pattern Advantages

    

It needs lesser time for making mould since drying is not required. The process is less expensive. It can be used for all metals. Mould distortion is less. It does not restrict the free contraction of metal.

Limitations

 Surface finish is less.  Strength of the mould is low.  Defects like blow holes may occur.

Dry sand mould If the green sand mould is dried after making the mould, it is called dry sand mould. The dry sand is a mixture of silica sand, coal dust and binders like clay, Bentonite and molasses etc. the step-by-step procedure of making dry sand mould is the same as that of green sand moulding. Only difference is that the mould is dried after it is made by green sand mould. The drying may be done by oxyacetylene flame for large moulds whereas ovens are used for small moulds.

Dry sand moulds are used for large castings such as engine cylinders, engine blocks, and machine block and mill rolls.

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Advantages

1. 2. 3. 4.

It is stronger than green sand mould It has better dimensional accuracy Permeability is more. It can be stored for long time.

Limitations

1. It is a more time consuming process. Since, it required heating. 2. Cost is high. 3. It is subjected to hot tear.

Machine moulding Moulding machine is used for mass production. Since the hand moulding is a slow process, it can be only for making few castings. For production more castings, moulding is done by using moulding machines. It reduces the labours as well as increases the quality of the mould.

Moulding machine will do the following operations:

1. Ramming the moulding sand 2. Rapping the pattern fro easy removal 3. Removing the patter from the sand

The following types of the moulding machines are generally used

1. Jolting machine 2. Squeezing machine 3. Sand slinger.

Jolting machine In jolting machine, the pattern is placed in the flask on the table. The flank is filled up with moulding sand. The table with flask is raised to about 80 mm and suddenly dropped. The table will be operated pneumatically or hydraulically. The sudden dropping of table from a height makes the sand pack evenly

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around the pattern. This type of machine is mainly used for ramming horizontal surfaces on the mould. Operation is noisy because of jolting.

Sand Mould box

Pattern

Jolting mechanism Jolt Machine

Squeezing machine

In squeezing machine the moulding sand in the flask is squeezed between the machine table and a squeezer head.

Top Squeezer machine

A top squeezer machine is shown in figure. The mould board is clamped on the table. The flask is placed on the mould board. The pattern is placed inside the flask. The sand is filled up and leveled. The table is raised by table lift mechanism against the squeezer head. The platen enters the sand frame and packs the sand tightly. After the squeezing the flask, the table comes down the starting position. The main limitation of this machine is that the sand is rammed more densely on the top of the mould than the sand nearer the pattern.

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Squeeze head

Flask

Pattern

Top squeezing machine Bottom Squeezer machine Here, the pattern is placed on the mould table. The mould table is clamped on the ram. The flask is placed on a frame and is filled with stand. The table with the pattern is raised up against the squeezer head. Thus, the flask with the pattern is squeezed between the squeezer head and the table. Then the table returns to its original position.

Squeeze head

Flask

Mould table

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27 Ram

Bottom squeezing machine Sand Slinger Moulding process is performed by using sand slinger as shown in figure. In this, the pattern is placed on a board. The flask is placed over it. Now, the slinger is operated. The slinger with different speeds. When the impeller rotates, it will throw a stream of sand at greater velocity into flask. Hence, the sand is packed in the flask. The slinger can be moved to pack the sand uniformly around the pattern. The density of sand is controlled by the speed of the impeller. In this method, the ramming will be uniform, with good strength. It is used for large and medium size moulds.

Impeller head

Flask

Pattern Sand Slinger Core A core is a body made of sand which is used to make a cavity or a hole in a casting. The required cavity in the casting to be made. It is also used to make recessed, projections, undercuts and internal cavities. A core print is the projection on a pattern. Core print forms a seat in the mould. The core is supported in the seat formed by the core print. Cold curing CO2 process This process is used to make good quality castings in large numbers. Pure dry silica sand is mixed with sodium silicate liquid to use as a binder for making these cores. The mixing is done in the Muller. The moisture content should not exceed 3%. Additives like sawdust about 1.5%, asbestos powder up to 5% and graphite powder may be added with this core sand to get the core more deformable and collapsible.

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CO2 Gas Seal

Core

Core box

Seal

The core sand mix is filled up in the core box and rammed. Then CO 2 gas is passed through the core for 2 30 seconds at a pressure of 140 KN/m . CO2 reacts with the sodium silicate in the core sand. It forms sodium carbonate and silica jells. This silica jell binds the sand grains together to provide strength and hardness to the core. This core can be used immediately.

Advantages 1. 2. 3. 4. 5.

Baking is not necessary It has good strength and hardness The process is easy and quick process Cores can be handled and stored for long time It saves time and cost of heating.

Types of cores The cores can be classified in many ways (a) According to the state of core i. Green sand core ii. Dry sand core

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(b) According to the position of the core in the mould i. Horizontal core ii. Vertical core iii. Balanced core iv. Hanging core v. Drop core

1. Green sand core Green sand core is formed by pattern itself. When the pattern leaves a core as a part of the mould, then the body of sand is called green sand core. Therefore the green sand core is made out of the same sand from which the rest of the mould has been made. It is suitable only for vertical opening. 2. Dry sand core Dry sand core are those which are made by means of special core sands in a separate core box, baked and then placed in the mould before pouring. Some of the types of cores used I various situations. Then it is positioned in the mould. These cores are most commonly used.

3. Horizontal core The core is placed horizontally in the mould and it is very commonly in foundries. These cores are usually cylindrical in shape; it may also have any other shape depending upon the shape of the cavity required. This core is supported in core seats as both the ends. The seats are made by the core prints as shown in the figure,

Mould Cavity Core

Horizontal Core 4. Vertical core These cores are positioned vertically in the mould as shown in figure. The two ends of the core rest on core seats in cope and drag. The major portion of the core rests in the drag box.

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Core

Mould cavity

Vertical Core 5. Balancing core

This type of core is supported and balanced from its end only as shown in figure. This requires a long core seat so that the core does not sag or fall into the mould. These cores are used when the blind holes along a horizontal axis are to be produced.

Mould Cavity

Core

Balanced Core 6. Hanging core

These cores are supported at the top and hung into mould. It has no support from bottom as shown in figure. It is supported by the seat made in top portion of drag. These cores are used when a cored casting is to be completely moulded in the drag with the help of single piece solid pattern.

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Core

Mould Cavity

Hanging Core

7. Drop core This core is used when a hole is not in line with the parting surface is to be produced at a lower level as shown in figure. The hole may be above or below the parting line of the mould. Depending upon the use, the core may also be known as tail core, chair core or saddle core.

Mould Cavity

Core

Drop Core CASTING Definition Casting is one of the earliest metal shaping methods known to human being. It generally means pouring molten metal into a refractory mould with a cavity of the shape to be made, an allowing it to solidify. When solidified, the desired metal object is taken out from the refractory mould either by breaking the mould or taking the mould apart. The solidified object is called casting. This is also called founding.

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Gravity die casting This is also called Permanent Mould Casting. The mould is generally made of two halves. They are hinged at one end. These are provision for clamping them together at the other en. A permanent mould is necessary for producing large number of casting of similar shape. A permanent mould is made of heat resisting cast iron, alloy steel, graphite or other suitable material. Pouring cup, sprue, gates and riser are made in this mould itself. First, the mould is preheated. Then refractory coating is done by spraying or brushing. This coating protects mould surface from erosion and sticking. Lubricating casting may also be given for easy removal of castings. The molten metal is fed into the mould with the help of gravitational force. Hence this process is called gravity die-casting. After the solidification, the casting is removed by opening the top die. Almost all metals can be cast in this mould. Non-ferrous materials like zinc, copper, aluminium, lead, magnesium and tin alloys are most often cast in this method. This method is suitable only for making components of simple shapes and design and uniform wall thickness.

Applications It is used for carburetor bodies, oil pumps bodies, pistons, connecting rods, gear covers etc.

Advantages 1. 2. 3. 4. 5.

Accurate casting can be made It gives good surface finish to castings It is more suitable for mass production Wastage and rejection of metal is less Less floor space is enough

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6. Production rate is high 7. Production cost is less 8. Castings are free from defects.

Limitations 1. 2. 3. 4. 5.

Only small castings can be made It is only suitable for mass production Initial cast is more. Complicated shaped castings cannot be produced easily. Removal of casting from the mould is difficult.

Pressure die casting

In this previous casting processes expendable moulds are used where it must be broken in order to obtain the castings. In the die casting process, the mould used for making a casting is permanent, called a die. In this process, the molten metal is forced into the mould cavity under high pressure. The process is used for casting a low melting temperature material, e.g. Aluminium and Zinc alloys, brass etc.

The die-casting is carried out as follows: The molten metal is forced under pressure into the assembled die. The die is water-cooled. So, the molten metal cools down and becomes solid immediately. The die is opened. Then, the finished casting is ejected by pins.

The mould normally called die is made in two halves in which one is fixed and the other one is movable. Medium carbon and low alloy tool steel are the most common die material.

There are two types of die casting processes. They are,

1. Hot chamber die casting 2. Cold chamber die casting

Hot chamber die-casting

In hot chamber die-casting, the melting furnace is an integral part of the mould. There is a gooseneck vessel which is submerged in molten metal. There is a plunger at the top of the gooseneck vessel. When the plunger is in the upward position, the molten metal flows into the vessel through a port

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provided on the sidewall. When the plunger comes down, the molten metal is forced into the disc. Since, the die is cooled by water immediately and sufficient cooling is provided for solidification. The movable die is moved some distance and finished casting is moved by ejectors. The plunger and movable die are operated by hydraulic systems. The operating pressure of hydraulic plunger is 15 2 MN/m .

Cold chamber die-casting

In cold chamber die-casting, the metal unit is not an integral part of the machine. The metal is melted in a separate furnace and brought to the machine for pouring. The process is shown in below figure. The machine has a cold chamber of cylindrical shape with a hydraulic plunger. A measured quantity of molten metal is poured into the injection cylinder. Then the plunger moves to the right and forces the molten metal into die cavity. As the die is water-cooled, immediate solidification of molten metal takes place. Then the dies are separated. The finished casting is removed by ejector pin. Applications 1. Household equipments like washing machine parts, vacuum cleaner body, fan case, store parts etc. 2. Automobile parts like fuel pump carburetor body, horn, wiper and crank case. 3. Components for telephones, television sets, speakers, microphones, record players and so on. 4. Toys like pistols, electric trains, model aircraftâ&#x20AC;&#x2122;s etc.

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Advantages 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

We can get very accurate castings with the dimensional tolerance range of + 0.03 to 0.25mm. Castings with very good surface finish can be made. Rate of production (700 castings per hour) is high. Castings with varying thickness wall can be made. There is no possibility of sand inclusions. Cored holes down to 0.75mm diameter at accurate locations are possible. Casting defects are less. It can be stored and used for long time. Die has long life. Approximately 75000 castings are produced in single throughout its life period. The process depends on the metal to be cast. The spire, runners and gates can be remelted, hence scrap loss is less.

Limitations 1. 2. 3. 4.

Only small parts can be made. Only non-ferrous metals can be cast. Equipment cost is high. It is more suitable for mass production only.

Centrifugal casting Centrifugal Castings is primarily used for making hollow casting like pipe without using core. In this process, a metal mould is made to rotate. The rotating mould is mounted on a trolley as shown in below figure. The trolley moves over rails. The end of the mould is closed by end cores to prevent the flow of metal. The metal is poured into the mould through a long spout. The mould rotated by electric motor or mechanical means as well as moves axially on the rails. Due to centrifugal force, the molten metal is thrown to the walls of the mould. The outside of the mould is water cooled. So, the molten metal solidifies immediately. Centrifugal casting method is used for producing cylindrical and symmetrical objects.

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Applications Components like water pipes, gears, bush bearings, fly wheels, piston rings, brake drums, gun barrels etc. Advantages 1. 2. 3. 4. 5. 6.

Core is not requiring producing hollow components. Rate of production is high. Pattern, runner, and riser are not required. Impurities in the metal are driven out. Therefore, defects in castings are very less. This casting can be made. Castings have uniform physical properties.

Limitations 1. It is suitable only for cylindrical and symmetrical shaped castings. 2. Cost of equipment is high.

Continuous casting In this process, molten metal is poured from a ladle continuously into a log vertical mould. The mould is made of copper brass or graphite. The mould is water-cooled. Hence, the molten metal is immediately solidified. This solidified casting comes down continuously. Saw or oxy-acetylene flame is used to cut the casting of required length. X-ray unit controls the pouring rate of molten metal is flowing from the bottom surface, there will not be any impurities in castings. Lubricating oil is applied between the casting and the mould wall to reduce friction. The guide rolls at the bottom keep on pulling the casting to match with the cooling rate. Argon gas is supplied at the top of the mould to prevent atmospheric reaction with molten metal. By controlling the cooling rate, the grain size and structure of metal can be regulated.

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Applications It is used to produce rods, pipes, slabs, ingots, bars etc. Advantages 1. 2. 3. 4. 5.

Rate of production is high. Good surface finish can be obtained. There are no impurities on the castings. Segregation on castings is reduced. Grains size and structure can be regulated by controlling cooling rate. The process can be automated. Hence, the labours cost can be reduced.

Limitations 1. Cost of mould and other equipment is high. 2. Operation and maintenance of the equipment are costlier.

Cupola furnace (Melting of cast iron) This type of furnace is used for melting of cast iron.

Construction It is vertical, cylindrical shell made of 10mm thick steel plate. It is line with refractory bricks inside. Two bottom doors close the bottom of the cupola. A sand bed if laid over the bottom doors sloping towards the tap hole. Molten metal stays over this bed. The legs are set at the bottom of the furnace using concrete. There is a tap hole for taking molten metal. A plug made of clay closes the tap hole. The slag hole is provided in the shell above the tap hole. The slag floating over the molten metal is removed through this slag hole. The opening called tuyeres are provided one metal above the bottom. Fuel is supplied through those tuyeres for making complete combustion of fuel. There is a wind box and blower for the supply of air into the furnace. For charging metal and fuel into the furnace, a separate charging door is made.

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Preparation: The slag and waste from previous melting and cleaned. Broken bricks are repaired or replaced if necessary. Then bottom doors are closed. A sand bed with sloping towards tap hole is prepared up to a height of 200mm. A tap hole is formed and lined with clay. Then a slag is prepared. Finally, the cupola is dried thoroughly. Firing Oil waste and wooden pieces are placed at the bottom and the fire is started. Now, sufficient of air is supplied. When the wood starts burning. The coke is charge at several portions. Now, the coke burns. Again and again more coke is added up to the tuyeres level. The blast is turned off. Again coke is added up to the level of bed charge. Then the coke is allowed to burn for half an hour. Finally, the charging is done through the changing door.

Charging and Melting Pig iron, and iron scrap are charged into the furnace through the charging door. Then coke is charged alternatively. Limestone is added to the charge to remove impurities and also ensure through mixing of molten metal. The ration of pig iron to limestone and pig iron to coke are 25:1 and 10:1 respectively. The cupola is fully charged. Then the iron is socked for one hour. After that, the blast is turned on. Molten metal will begin to collect at the sand bed. After melting enough quantity of molten metal, clay plug is removed and collected in ladles.

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Then the molten metal can be directly poured into moulds. The floating slag on the top layer of the molten metal is tapped out through the slag hole. Again the furnace should be charged to the full level for repeating the same procedure. At the end of the cupola is shut off by stopping the air blast. Then the remaining molten metal is removed, the bottom doors are opened, the wastes are dropped down and they are quenched by water.

Application Cupola is used to melt cast iron. Advantages 1. 2. 3. 4. 5.

Initial cost is comparatively lower than other type of furnaces. It is simple in design. It requires less floor area. Operation and maintenance are simple It can be operated continuously for many hours.

Crucible furnace (Melting of non-ferrous metals)

The metal is melted in the crucible. It is made up of silicon carbide, graphite or other refractory materials. Generally, it is used for melting non-ferrous metals and meting point alloys. The fuel used may be oil, gas or coke. The capacities range from 30 to 150 kg. Types of crucible furnace

1. Pit furnace 2. Coke fired stationary furnace 3. Oil fired tilting furnace

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1. Pit furnace

The crucible is placed in a pit below the ground level. It is usually fired with coke. The furnace is made of steel shell with a grate and pit at its bottom. The steel shell above the grate if fired with firebricks. A chimney provides natural draught. The metal charge is pig iron, foundry returns and broken castings. The coke is placed around and above the crucible. The fuel is ignited and allowed to burn. After reaching maximum combustion, the coke above the crucible is shifted to sides. Then its top is covered with w lid. A blower may be used to provide necessary air. After melting the metal, the lid is removed. The crucible is lifted using tongs. Then it is taken to the place of pouring.

Application It is used for melting cast iron and non-ferrous metals and alloys in small quantity.

2. Coke and fired stationary furnace The furnace is erected above the ground level. The furnace is made of steel shell lined wire firebricks. A blower is used to create draught coke is packed around crucible. Its operation is similar to coke fired pit furnace.

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3. Oil fired tilting furnace

The tilting furnace may be coke, oil or gas fired. Mostly oil and gas fired because it has advantages. The furnace consists of steel. Firebrick lining is provided inside the shell. Crucible is placed centrally in such a way to form a hollow chamber. The crucible is placed on a refractory base and fixed well. A burner is mounted tangentially at the bottom of the furnace. The furnace is mounted on two pedestals. It can be tilted a geared stand wheel. While firing, oil and air are directed through the nozzle. The flame circulates in the hollow chamber. It heats the metal charge lying in the crucible.

Advantages

1. 2. 3. 4. 5. 6.

It is easy to a start and stops the operation. It is easier the control temperature. It occupies less floor area. It requires less labour. It provides a fast melting rate. It has less contamination of work place.

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Electric furnaces

Electric furnaces are used for melting steel, alloy steel, brasses etc. it is used for producing high quality castings. Because of the following reasons,

1. 2. 3. 4.

Oxidation losses are eliminated Furnace atmosphere can be more easily controlled Alloying elements can be added without loss Composition of the melt and its temperature can be accurately controlled

The following are the different types of electric furnaces:

1. Direct arc furnace 2. Indirect arc furnace 3. Induction furnace

1. Direct arc furnace It consists of a heavy steel shell. The shell is lined with refractory brick. It has a bowl shape bottom with a detachable roof. The roof is lined with silica brick. Three graphiteâ&#x20AC;&#x2122;s or carbon electrode pairs into shell through the roof. The electrode can be raised up or down. The furnace has twp spouts in which one is for molten metal and another one is for slag. The furnace is mounted in turn ions with the help of bearings. So, it can be tilted backward or forward for charging, running of the slag pouring the molten metal into the ladle.

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Working At first the furnace is preheated. Then the furnace is charged with steel scraps by opening the roof or through the charging door, the electrodes are lowered down and a gap between the electrodes and metal charge surface is maintained. Then the electric supply is given to produce electric arc. The heat produced by the arc melts the metal. The electrode may be consumed and become shorter. The arc gap between the electrodes and charge are maintained by automatic control. The slag formed on the top of the molten metal reduces the oxidation, refine the metal, and protects the roof and sidewalls from heat radiation.

Applications It is used to melt high quality carbon steels and alloy steels.

Advantages 1. 2. 3. 4. 5.

Thermal efficiency is high. Very pure metal can be obtained. It can make steel directly from pig iron and steel scrap. The furnace atmosphere above the molten metal can be easily controlled. There will be less losses of alloying elements

Disadvantages Heating cost is higher. 2. Indirect electric arc furnace It consists of steel with refractory lining. Two graphite electrodes are mounted at opposite ends. An opening is provided at the center of shell for charging metal. A pouring spout is built up with the charging door. The furnace is mounted on the rollers. The rollers are driven to rock the furnace by a rocking drive unit.

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Working At first, the pig iron is charged. The scrap is charged over the pig iron. With electric power on, two electrodes are brought nearer to produce arc. The heat generated by arc melts the metal. Some metal is melted as soon as possible. The furnace is set to rock to and for. The metal melts because of 1. The heat radiations from the arc 2. The hot refractory walls of the furnace and 3. Conduction from the hot linings when the furnace rocks.

When the malign is completed, the furnace is tilted to pour molten metal is the ladle. Applications To melt cast iron, steel, copper and its alloys.

3. Induction furnace It consists of a refractory crucible which is placed in an induction furnace and centrally located on the refractory lining at the bottom of shell. The crucible is surrounded by a water 窶田ooled coil of copper tubing. Some insulation is provided between the coil and the crucible. The furnace is mounted on two pedestals. It is used t tilt the furnace for pouring the molten metal, Working Steel scraps are charged into the furnace. The coil of copper tubing acts as a primary coil. The charge acts as a secondary coil. A high frequency current is passed through the coil. This induces a heavy secondary current on the metal charge. The metal charges give very high resistance to the secondary current. Due to this, the heat is released and melts the charge. When the melting is completed, the furnace is tilted to pout molten metal in the ladle.

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Applications It is used to melt the alloy tool steels, low carbons alloys in small quantity. Advantages 1. 2. 3. 4.

It gives excellent uniformly of the melt decomposition. There is no need of electrodes and simple construction. The process is quick and no noise. Furnace atmosphere can be easily controlled.

Defects in castings Any irregularities in the moulding process causes defects in castings which may sometimes be tolerated, sometimes eliminated with proper moulding practice or repaired using methods such as welding and metallization. The following are the major defects which are likely to occur in sand castings. 1. 2. 3. 4. 5.

Gas defects Shrinkage defects Moulding material defects Pouring metal defects Metallurgical defects

1. Gas defects The defects in this category can be classified into blow holes and open blows, air inclusion and pin hole, porosity. All these defects are caused to a great extent by the lower gas passing tendency of the mould which may be due to lower venting, lower permeability of the mould and/or improper design of the casting. The lower permeability of the mould is, in turn, caused by finger grain size of the sand, higher clay, higher moisture, or by excessive ramming of the moulds. 1.1. Blow holes and open blows These are the spherical, flattened or elongated cavities present inside the casting or on the surface as shown in figure. On the surface they are caller open blows and inside, they are called blow holes. These are caused by the moisture in converted into steam, part of which when entrapped in the casting ends up as blow hole or ends up as open blow when it reaches the surface. Apart from the presence moisture, they occur due to the lower permeability of the mould. Thus in green sand moulds it is very difficult to get rid of the blow holes, unless proper venting in provided. 1.2. Air inclusions The atmospheric and other gases absorbed by the molten metal in the furnace, in the ladle, and during the flow in the mould, when not allowed escaping, would be trapped inside the casting and weaken it. The main reasons fro these defects are the higher pouring temperatures which increase the amount of gas absorbed; poor gating design such as straight sprues in unpressurised getting, abrupt bends and other turbulence causing practices in the gating, which increase the air aspiration and finally the low permeability of the mould itself. The remedies would be to choose the appropriate pouring temperature and improve gating practices by reducing the turbulence.

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1.3. Pin hole porosity

This is caused by hydrogen in the molten metal. This could have been picked up in the furnace or by the dissociation of water inside the mould cavity. As the molten metal gets solidified it loses the temperature which decreases the solubility of gases and thereby expelling the dissolved gases. The hydrogen while leaving the solidifying metal cause very small diameter and long pin holes showing the path of escape. These series of pin holes cause of leakage of fluids under high operating pressures. The main reason for this is the high pouring temperature which increases the gas pick-up. 2. Shrinkage Cavities These are caused by the liquid shrinkage occurring during the solidification of the casting. To compensate this, proper feeding of liquid metal is required as also proper casting design.

3. Moulding metal defects Under this category are those defects which are caused because of the characteristic of the moulding materials. The defects that can be put in this category are: cuts and washes, metal penetration, fusion, run out, rat tails and buckles, swell and drop. These defects occur essentially because the moulding materials are not requisite properties or due to improper ramming.

3.1. Cuts and washes These appear as rough spots and areas of excess metal, and are caused by the erosion of moulding sand by the flowing molten metal. This may be caused by the moulding sand not having enough strength of the molten metal flowing at high velocity. The former can be remedied by the proper choice of moulding sand and using appropriate moulding method. The latter can be taken care of by altering the gating design to reduce the turbulence in the metal, by increasing the size of gates or by using multiple ingates.

3.2. Metal Penetration When the molten metal enters the gaps between the sand grains, the result would b ea rough casting surface. The main reason for this is that, either the grain size of the sand is too coarse, or no mould wash has been applied to the mould cavity. This can also be caused by higher pouring temperatures. Choosing appropriate grain size, together with a proper mould wash should be able to eliminate these defects.

3.3. Fusion This is caused by the fusion of sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this defect is that the clay in the moulding sand is of lower refractoriness or that the pouring temperature is too high. The choice of an appropriate type and amount of Bentonite would cure this defect.

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3.4. Run out A run out is caused when the molten metal leaks out of the mould. This may be caused either due to faculty mould making or because of the faculty moulding flask.

3.5. Rat tails and buckles Rat tail is caused by the compression failure of the skin of the mould cavity because of the excessive heat in the molten metal. Under the influence of the heat, the sand expands, thereby moving the mould wall backwards and in the process when the wall gives away, the casting surface may have this marked as a small line. With a number of such failures, the casting surface may have a number of criss-crossing small lines. Buckles are the rat tails which are severe. The main causes for defects are: the moulding sand has got poor expansion properties and hot strength of the heat in the pouring metal is too high. Also, the facing sand applied does not have enough carbonaceous material to provide the necessary cushioning effect. Proper choice of facing sand ingredients and the pouring temperature are the measures to reduce the incidence of these defects.

3.6. Swell Under the influence of the metallostatic forces, the mould wall may move back causing a swell in the dimensions of the casting. As a result of the swell, the feeding requirements of castings increase which should be taken care of by the proper choice of risering. The main cause of this is the faculty mould making procedure adopted. A proper ramming of the mould should correct these defects.

3.7. Drop The dropping of loose moulding sand or lumps normally from the cope surface into the mould cavity is responsible for these defects. This is essentially, due to improper ramming of the cope flask. 4. Pouring metal defects The likely defects in this category are mis-runs and cold shuts slag inclusions.

4.1. Mis-runs and cold shuts Mis-run is caused when the metal is unable to fill the mould cavity completely and thus leaves unfilled cavities. A cold shut is caused when the two metal streams while meeting in the mould cavity, do not fuse together properly, thus causing a discontinuity or weal spot in the casting. Sometimes a condition leading to cold shuts can be observed when no sharp corners exits in a casting. These defects are caused essentially, by the lower fluidity of the molten metal or when the section thickness of the casting is too small. The latter can be rectified by proper casting design. The

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remedy available is to increase the fluidity of the metal by changing the composition or raising the pouring temperature. These defects can also be caused when the heat removal capacity is increased such as in the case of green sand moulds. Further cause of this defect in the pressure due to gases in the mould which is not properly vented. The remedies are basically improving the mould design.

4.2. Slag inclusions During the melting process, flux is added to remove the undesirable oxides ad impurities present in the metal. At the time of tapping, the slag should be properly removed from the ladle, before the metal is poured into the mould. Otherwise any slag entering the mould cavity will be weakening the casting and also spoil the surface of the casting.

5. Metallurgical defects The defects that can be grouped under this category are hot tears and hot spots.

5.1. Hot tears Since the metal has low strength at higher temperature, any unwanted cooling stress may cause the rupture of the casting.

5.2. Hot spots These are caused by the chilling of the casting. For example, with grey cast iron having small amounts of silicon, very hard white cast iron may result at the chilled surface. This hot spot will interface with the subsequent machining of this region. Proper metallurgical control and chilling practices are essential for eliminating the hot spots.

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UNIT-II FORGING AND WELDING HOT WORKING AND COLD WORKING The metal working processes are traditionally divided into hot working and cold working processes. The division is on the basic of the amount of heating applied to the metal before applying the mechanical force.

Those processes, working above the re crystallization temperature, are termed as hot working processes whereas those below are termed as cold working processes. Under the action of heat and the force, when the atoms reach a certain higher energy level, the new crystals start forming which is termed as re crystallization. Recrystallization destroys the old grain structure deformed by the mechanical working, and entirely new crystals which are stain free are formed. The grains in fact start nucleating at the points of severest deformation. Recrystallization temperature as defined by American Society of Metals is â&#x20AC;&#x153;the approximate minimum temperature at which complete Recrystallization of a cold worked metal occurs within the specified time. In hot working process, the process may be carried above the Recrystallization temperature with or without actual heating. Foe example for lead and tin Recrystallization temperature is below the room temperature and hence working of these metals at the room temperature is always hot working. Similarly for steels, the recrystallisation temperature is of the order of 1000Â°C, and therefore working below that temperature is still cold working only. Advantages of hot working 1. As the material is above the recrystallisation temperature, any amount of working can be imparted since there is no strain hardening taking place. 2. As a high temperature, the material would have higher amount of ductility and therefore there is no limit on the amount of hot working that can be done on a material. Even brittle material can be hot worked. 3. Since the shear stress gets reduced at higher temperature, the hot working requires much less force to achieve the necessary deformation.

ROLLING Rolling is a process where the metal is compressed between two rotating rolls for reducing its cross sectional area. This is one of the most widely used of all the metal working processes, because of its higher productivity and low cost. Rolling would be able to produce components having constant cross sectional its length. Many shapes such as I, T, L and channel sections are possible, but not very complex shape. It is also possible to product special sections such as railway wagon wheel by rolling individual pieces. Rolling is normally a hot working process unless specifically mentioned as cold rolling. The metal is taken into rolls by friction and subsequently to obtain the final shape. The thickness of the metal that can be drawn into rolls depends on the roughness of the roll surface. Rougher rolls would be able to achieve

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greater than smoother rolls. But, the roll surface gets embedded into the rolled metal thus producing rough surface.

The reduction that could be achieved with a given set rolls is designated as the ‘angle of bite’. This depends on the type of rolling and the conditions of the rolls. The volume of the metal that enters the rolling stand should be the same as that leaving it except in initial passes when there might be some loss due to filling of voids and cavities in the ingots. Since the area of the cross section gets decreased, the metal enters the rolls; the surface speed of the rolls is higher than that of the incoming metal, whereas the metal velocity at the exit is higher than that of the surface speed of rolls. Between the entrance and exit, the velocity of the metal s continuously is changing, whereas the roll velocity remains constant. Somewhere in the contact length, the velocities of the metal and rolls are same which is designated as ‘neutral plane’. The pressure of the rolls gradually builds up from the entry to the neutral point where it is the highest and then decrease till it reaches the exit. The roll separating force which separates the two rolls a part can be obtained by multiplying the average roll pressure which us a function of the contact length. Though higher friction between roll and the metal is required for increasing the reduction achieved, it also increases the roll separating force.

FORGING Producing a component s to the required shape by applying a compressive or impact force is called forging. When forging is done on the metal below the recrystallisation temperatures, it is called cold forging. Example: bolt heads, rivets, nails etc. are produced by cold forging. When the forging of the metal is done above the recrystallisation temperature, it is called hot forging. Example: Connecting rod, crank shaft, etc., are produced by the forging. Smith forging The process involves heating the stock in the blacksmith’s hearth and then beating it over the anvil. To get the desired shape, the operator has to manipulate the component in between the blows. The types of operations available are fullering, flattening, bending, upsetting and swaging.

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In fullering, the material cross-section is decreased and length increased. To do this, the bottom fuller is kept in the anvil hole with the heated stock over the filler. The top fuller is then kept above the stock and then with the sledge hammer, the force is applied on the top fuller. The fullers concentrate the force over that point. Metal flows outward and away from the center of the fullering die After fullering, the stock would have the fullering marks left which are then cleaned by means of flattening. To obtain specific shapes such as round, square, hexagon, etc. open general purpose dies called swages are used. The force for shaping is applied by manual hammering or by means of the forging hammers, the latter being the industrial practice. Smith forging involves a lot of skill on the part of the operator and also is more time consuming. But since no special dies are3 used, smith forging is more beneficial in the manufacture of small lots or in trial production; because of the heavy cost of the closed impression dies cannot be justified in these cases.

Drop forging

Drop forging utilizes a closed impression die to obtain the desired shape of the component. The shapping is done by the repeated hammeing given to the material in the die cavity. The equipment used for delivering the blows is called drop hammers.

The drop forging die consists of two halves. The lower half of the die is fixed to the anvil of the machine, while the upper half is fixed to the ram. The heated stock is kept in the lower die while the ram delivers our to five blows on the metal, in quick succession so that the metal spreads and completely fills the die cavity. When in the die halves close, the complete cavity is formed.

The die impressions are machined in the die cavity, because of which more complex shapes can be obtained in drop forging, compared to smith forging, however, too complex shapes with internal cavities, deep pockets, reentrant shapes, ect. Cannot be obtained in drop forging due to the limitation of the withdrawal of the finished forging from the die. The typical products obtained in drop forging are crank, crank shaft, connecting rod, wrench, crane hook etc.

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The final shape desired in drop forging cannot be obtained directly from the stock in a single pass. Depending on the shape of the component, and the desired grain flow direction, the material should be manipulated in a number of passes used are: Fullering impression Since drop forging involves only a reduction in cross-section with no upsetting; the very first step is to reduce the stock to the desired size. Te impression machined in the die to achieve this is called fullering impression. Sometimes it may be possible to obtain this without a special die impression, in open dies.

Edging impression Also called as ‘preform’, this stage is required to gather the exact amount of material required at each cross-section of the finished component. This is the most imp[ortant stage in drop forging. Properly designrd perform ensures a defect free flow of metal, complete die fill and minimum flash loss. Bending impression This is required for those parts which have a bent shape. As shown in fig.19.1, the bent shapes can also be obtained without the bending impression, but then, the grain flow direction will not follow the bent shape and thus the point of bent may become weak. To improve the grain flow, therefore, a bending impression is incorporated after edging impression. Blocking impression Also called as ‘semi-finishing’ impression, blocking is a step before finishing. In forging, it is very difficult for the material to flow to deep pockets, sharp corners, etc. hence, before the actual shape is obtained, the matrerial is allowed to have one or more blocking impressions where is acquires the shape very near to the final one. The blocking impression is characterized by large corner radii and fillets but no flash. For complex shapes, more than one blocking inpression may be used. Finishing impression This is the final impression where the actual shape required is obtained. In order to ensure that the metal completely fills the die cavity, a little extra metal is added to the stock. This extra metal will form the flash and surrounds the forging in the parting plane.

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Trimming In this stage, the extra flash present around the forging is trimmed to get the forging in the usable form. Press forging Press forging dies are similar to drop forging dies as also the process. In press forging, the metal is shapes not by means of a series of blowa as in drop forging, but by means of a single continuous squeezing action. This aqueezing is obtained by means of hydraulic presses. Because of the continuous action of the hydraulic presses, the material gets uniformly deformed throughtout its entire depth. More hammer force is likely to be transmitted to the machine frame in drop forging, whereas in press forging it is absorbed fully by the stock. The impressions obtained in press forging are clean compared to that of the likely jarred impressions which are likely in the drop forged components. The draft angles used in press forging are less than in drop forging. But the press capacity required for deforming is higher and as a result somewhat smaller size components only are press forged in closed impression dies. But there is no such limitatyion for press forging in open dies. The presses may have capacities ranging from 5 mn to 50 mn for normal applications. For special heavy duty applications, higher capacity presses of the order of 150 mn are required. To get the equivalent weight of the falling parts in kg of drop hammer required, the capacity of the press expressed in tones is to be multiplied by a forged component. To provide the necessary alignment of the two die halves, die posts are attached to the bottom die so that the top die would slide only on the posts and thus register the correct alignment. This ensures better tolerances for them press forged components. The tong holds which are normally required for mainpulting dies in drop forging are not required in press forging, because metal is pressed only once is each of the impression and generally mechanical ejection is used for removing the pieces from the die. But for this, the press forging die is similar to drop forging die with the various impressions such as fuller, bender, blocker and finisher impressions properly arranged.

Machine forging (or) upset

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As it involves the upsetting operation, sometimes it is simply called as upset forging.though both drop and press forgings ae also done by machines, historically, only upset forging is referred to as machine forging. Originally this was developed for making the bolt heads in a continuous fashion, but now there is fairly large number of diverse uses of this process. Because of the beneficial grain flow obtaind from upsetting, it is used for making gear blanks, shafts, axles and similar parts.

Upsetting machines called upsetters are generally horizontal acting. The die set consists of a die and a corresponding punch or a heading tool. The die consists of two parts, one called the stationary gripper die which is fixed to the machine frame and the other, movable gripper die, which moves along with the die slide of the upsetter. The stock is held between these two gripper dies by fraction. The upset forging cycle stars with the movable die sliding against the stationary die to grip the stock. The two dies when in closed position, from the necessary die cavity. Having completed the upsetting, the heading tool moves back to its back position. Then the movable die releases the stock by sliding backwards. Similar to drop forging, it is not possible to get the final shape in a single pass in machine forging also. Therefore, the operation is carried out in a number of stages. The die cavities required for the various operations are all arranged vertically on the gripper dies. The stock is then moved from one stage to the other in a proper sequence till the final forging is ready. A heading tool each, for every upsetting stage is arranged on the heading slide of the upsetting machine.

WELDING Welding is the process of joining similar metals by the application of heat. Welding can be done with or without the application of pressure. While welding the edges of metal pieces are either method or bought to plastic condition. Welding can be done with the addition of filler material or without it. Welding is used for making permanent joints. It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furnitureâ&#x20AC;&#x2122;s, boilers, general repair work and ship building. Almost in all metal working industries welding is used.

Types of welding There are two main types of welding process. They are (i).

Plastic welding

(ii).

Fusion welding

In plastic welding the metal pieces are heated to a plastic state. They are pressed together to make the joint. The plastic welding is also known as pressure welding. Electric resistance welding is an example of plastic welding. In fusion welding the metal at the joint is heated to molten state. Then it is allowed to solidity. Pressure is not applied in this process. So it is known as non pressure welding. Gas welding and electric arc welding are examples for fusion welding.

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Arc welding equipment

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

A welding generator (D.C) or transformer (A.C) Two cables-one for work and one for electrode. Electrode holder Electrode Protective shield Gloves Apron Wire brush Chipping hammer Goggles

The transformer does not have any parts and as a result operates with less maintenance cost and also has higher efficiency. The power used is also expensive and there is practically no noise in the operation of the welding transformer. In AC welding, normally only transformer are used.

In DC arc welding a rectifier or a generator can be used to supply the required DC power. In the rectifier type, the power supply is first stepped down by means of a transformer to the required voltage and then silicon controlled rectifiers (SCR) are used to convert AC to DC. These rectifiers are very compact, highly reliable and have high efficiency. The type is a DC generator which is driven by either an induction motor running on AC or an oil engine. This combination is less efficient, more expensive and noisy in operation. The welding machines can also be divided into two types, based on the characteristics. The first one is the constant welding machines or droop curve machines or simply droppers, which vary the welding voltage to account for the change in the arc gap thus machining the arc current. As a result of this, the characteristics of the machine (a plot between volts and output ampere) are a dropping one. It can be seen that for a large change in output voltage, the corresponding change in current is so small that the quality of the weld can be maintained. This is very essential for manual arc welding processes since the maintenance of constant arc is nearly impossible by a human welder. Arc welding (i). Metal arc welding This is simply called arc welding. Arc welding is the process of joining two metals by melting their edges by an electric arc. The electric is produced between two conductors. The electrode is one conductor and the work piece is another conductor. The electrode and work piece are brought nearer with a small air gap (3mm approximate). When current is passed, an electric arc is produced between the electrode and the workpiece. The work piece is melted by the arc. The electrode is also melted. Both molten pieces of metal become one. The electrode also supplies additional filler metal into the joint. Temperature of arc is about 4000째C. a transformer or generator is used for supplying the current.

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Electrode used is arc welding are generally coated with a flux. This flux produces a gaseous shield around the molten metal. This prevents the reaction of the molten metal with oxygen and nitrogen in the atmosphere. The flux removes the impurities from the molten metal and form a slag. This slag gets deposited over the weld metal. This protects the weld seam from rapid cooling.

(ii). Carbon arc welding

In carbon arc welding, the intense heat of an electric arc between a carbon electrode and work piece metal is used for welding. DC power supply is used. The carbon electrode is connected to negative terminal and workpiece is connected to positive terminal. This is because the positive terminal is hotter (4000째C) than the negative terminal (3000째C) when an arc is produced. So carbons from the electrode willnot fuse and mix up with metal weld. If carbon mixes with the weld, the weld will become weak and brittle. To protect the molten metal from the atmosphere the welding is done with a long arc. In this case, a carbon monoxide gas is produced which surrounds the molten metal and protects it. Generally filler rods are not required. If needed, a flux coated filler rod may be used. Carbon arc welding is used to weld both ferrous and non ferrous metals. Sheets of steel. Copper alloys, brass and aluminium can be welded in this method. These methods can be easily done by automatic devices. Temperature control is easily done. Production of arc is very easy. But blow holes occur in the weld.

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Metal inert gas welding (MIG)

In this arc welding, the electric arc is produced between a consumable metal electrode and the work metal. During welding the arc and welding zone are surrounded by an inert gas. This inert gas cover protects the weld from atmospheric effects. Argon or helium is used as the inert gas. The electrode wire from the continuously fed from a wire reel. The electrode wire from the reel passes through the electrode holder. It is melted by the arc and deposited over the joint. The arc is surrounded by the inert gas supplied from the gas cylinder. This welding can be done manually or automatically. MIG welding is used for welding, aluminium, stainless steel and magnesium without weld defects.

Advantages 1. 2. 3. 4.

No flux is required. High welding speed. Possible to weld non-ferrous metals. Cheaper process.

Tungsten inert gas welding (TIG)

In this welding an electric arc is produce between a non consumable tungsten electrode and the work piece. There is an electrode holder in which the non-consumable

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tungsten electrode is fixed. When the arc is produce between the electrode and work, the inert gas from the cylinder passes through the welding head around the electrode.

The inert gas surrounds the arc and protects the weld from atmosphere effects. So welds are made without defects. The process is used for welding steel, aluminium, cast iron, magnesium, stainless steel etc. The inert gas used is argon or helium.

Advantages: 1. No flux is required, 2. High welding speed. 3. Can be used both for ferrous and no ferrous metals.

Atomic hydrogen welding

In this process, an electrode arc is produce between two non consumable electrodes. A stream of hydrogen passes over the arc and covers the welding zone. When the hydrogen enters the arc, the high temperature of the arc breaks the hydrogen (consisting of two atoms) into single atoms. In doing so, a large amount of heat is required and absorbed by hydrogen. These single atoms will recombine again. Very high heat will liberated during recombining. The temperature will be around 3000째C. This extra heat and the heat of the arc will melt the metal immediately. These hydrogen gases also protect the weld form atmospheric effects. Hydrogen is passed through gas nozzle in the electrode holder from the gas cylinder. Alternating current is used for striking the arc. The electrodes used are made of tungsten. The weld will be free from defects. This process is used for welding aluminium and stainless steel. It is used for the repair of steel moulds and dies. Advantages

1. Weld is free form defects. 2. Hydrogen protects the weld from atmospheric effects. 3. Hydrogen removes oxides from the surface of the work.

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4. Smooth, uniform, strong and ductile weld is obtained.

Plasma arc welding

In this process, the heat generated by an ionized gas jet called plasma is used for joining pieces together. Here an inert gas is joined by an electric arc.

The welding equipment has a gas chamber with a copper nozzle at the bottom. The copper nozzle is water cooled. A tungsten electrode is held vertically in the centre of the gas chamber. The electrode is connected to the negative terminal of a 120V, 100 to 400 amps DC supply. The copper nozzle is connected to the positive terminal.

When electric supply is given, an arc is produced between the tungsten electrode and the copper nozzle. Argon gas is supplied through the arc. The gas ionized due to the arc. This ionized gas is called plasma. A temperature of about 1400째C is developed. This plasma arc is directed on the workpiece to be welded. By the high heat, the metal at the joints gets melted and welds together.

Workpiece can be welded easily without a filler rod. If necessary a filler metal can be used.

Another steam of inert gas say helium is supplied around the nozzle on the weld zone through the outer shell of the nozzle. This acts as a shielding gas for the weld. Application This welding can be applied to almost all metals. This process can be used to weld stainless steel, maraging steel, carbon steel, monal metal, inconel, titanium, copper, brass and other alloys. This processed is used to weld sheet parts at very high speed. Advantages 1. All metals can be welded in this process.

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Welding can be done even without a filler rod. Little effect on metallurgical properties of parent metal. Deep penetration. Faster process.

Limitations 1. 2. 3. 4.

Automatic control is necessary. Welding of short lengths is difficult. Affects health of the operator-as ultra violet and infra red radiation is produces by plasma. High initial cost.

Electro-slag Welding

In this process, two thick metal plates are jointed together by the heat generated when electric current is passed through molten slag. This process is an improvement of submerged arc welding method. In electro slag welding, the metal plates are kept in a vertical position and welded.

The thick plates to be welded are kept in a vertical position maintaining a uniform gap between them. At the bottom of the gap, a bottom plate is tack welded. Copper sliding plates are arranged on both sides of the plates covering the gap. This is for holding the flux in the gap. Granular flux in the gap between copper slides and the plates. The electrode is positioned in the gap. The electrode along with the copper sliding plates can be moved upwards at the required rate.

Before starting welding, the plate at the bottom is preheated. The plate and electrode are connected to the terminals of a high current power source (1000 amph). When the supply is switched on, an electric arc is produce between the flux and the electrode. The granular flux melts and slag of 2 to 3 mm thick is formed. Now the arc stops. The current is conducted directly from the electrode through the slag. The slag offers high resistance to the heat melts the edges of the plates. The consumable filler rod from a role supplied at the slag zone also melts. A driving mechanism moves the electrode upwards at a specified speed along with the copper slides. The molten metal at the bottom gets solidified. The slag remains always at the top of the molten metal and prevents atmospheric reaction.

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Applications

Electro slag welding process is used to weld thick plates of 25mm or more. Boiler plates are often butt welded in this process. This process is used to weld hjot rolled carbon stels and stainless steel. Advantages 1. 2. 3. 4. 5. 6.

Thick plates can be welded in one pass. No edge preparation is needed for the plates. Process is very fast. Uniform property throughout the length. Weld is free from pores. The weld will be strong without impurities as it is always protected by the flux.

Limitations 1. 2. 3. 4.

Not suitable for thin plates. Hot cracks may develop. Only butt welding can be done Initial coat is high.

Gas Welding

In gas welding, a gas flame is used to melt the edges of metals to be joined. The flame is produced at the tip of welding torch. Oxygen and acetylene are the gases used to produce the welding flame. The flame will only melt the metal. So additional metal in the weld is supplied by the filler rod. A flux is used during welding to prevent oxidations and to remove impurities. Metals 2 mm to 50 mm thick are welded by as welding. The temperature of oxy-acetylene flame is about 3000째C.

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RESISTENCE WELDING

Resistance welding is “ a group of welding processes wherein coalescence is produced by the heat obtained from resistance of the work to electric current in a circuit of which the work is a part, and by the application of pressure and without the use of a filler metal”.

The amount of heat generated at the contacting area of the element to be welded, is determined from joule’s law, 2

Q = I Rt, joules Where, I = current in amperes R = resistance of the circuit at the contacting area of the elements in ohms t = time during which the current flows, in seconds

Butt welding There are two types of electric resistance butt welding process: the ‘upset’ and ‘flash’ (i). Upset welding, (UW) In this process, the ends of the two parts to be joined together clamped in position in the electrodes. The movable head is moved towards the fixed head the abutting surfaces of the work pieces are in light contact. Then the proper current is made flow across the interfaces for preset time, while the light pressure between the two parts is maintained. When the interface has been heated to the welding temperature (plastic state), the current switch off and the welding pressure is increased to form an “upset”. This results in the lateral flow of the surface oxide layers, bringing clean metal surfaces in contact. Metal is not melted the upset butt welded and there is no spatter.

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The welding voltage may vary from 5V to 15V while the current densities range from 300 to 800 2 A/cm . Because the contact resistance is inversely proportional to the pressure, therefore, the pressured is less at the start and is then increased to whatever is necessary to affect the weld. Final pressures range from 17 to 55 MPa. Upset butt welding is extensively used in the fabrication of tubular sections, 2 pipe and heavy steel rings. It is used for joining small ferrous and non ferrous strips. Areas up to 0.05m 2 have been successfully welded, but generally the process is limited to smaller areas (upto 1000 mm ) because of current limitations.

(ii) Flash butt welding (FBW) In this process, the parts to be welded approach towards each other and come into contact with the current switched on. The procedure of butt welding is follows: after the parts are properly positional and correct current, head speed and time are selected a cycle start button is actuate. This makes the movable head approach the fixed head. As the abutting surfaces come very near to each other, extremely rapid heating takes place when a surface asperity first makes contact. Molten metal is violently expelled and burns in air with considerable force and “Sparking” of “Arcing” thus giving the process its name “flash butt”. As this process continues for a few seconds, a very this layer at the interface is melted. The current is then shut off and the two parts are rapidly pressed together causing a small upset. This squeezes out liquid metal and oxides formed on the joint surfaces and the solid clean metal faces of the abutted parts are welded together.

The current densities range from 300 – 800 A/cm and the welding voltage is from 5 to 15V. The welding pressure may range from 350 to 1400 MPa. Spot welding Spot welding is one kind of resistance welding. Spot welding is used to join over lapping sheets or plates of metal at small areas. The metal plates are assembled and placed between two copper electrodes. The work pieces are clamped between the electrodes and current is passed. The pieces are heated at their area of contact by electric resistance. Then the electrodes are passed against the metal pieces by mechanical pressure. Welding takes place because of heat and pressure. Spot welding can be done on metal strips up to 12mm thick. Spot welding is used in welding of boxes, cams and frames of automobiles. Seam welding Seam welding is one kind of resistance welding. A continues joint between two overlapping plates is made by seam welding. The work pieces are placed between two rotating wheel, electrodes. When electric current is passed through the electrodes, high heat is produced on the work pieces between the wheels. At the same time pressure is applied to complete the rotating wheels. So a continuous weld is made because of the heat and pressure. Seam welding is used to make a continuous joint on leak proof tanks, drums and radiators.

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Projection welding Projection welding is one kind of resistance welding. The metal pieces to be welded are placed between two metal arms (electrodes). One of the work pieces has projections on its surface. The work pieces are clamped between the arms. Current is passed when heat is produced because of electric resistance; mechanical pressure is applied by pressuring the top arm. The projections are melted, fastened and the work pieces join together. The surface at the projection must be clean. There should not be any scale on the surface. A un cleaned surface will reduce the resistance to the current flow. So the joint will be weaker. Application Projection welding is used for joining thin sheets metals. A wire or rod may be easily welded on its length over a flat surface. This welding process is used in mass production. Welding related processes (i). Oxy-acetylene cutting

Iron and steel can be cut by using a special oxy-acetylene torch. Oxygen and acetylene mix in the mixing chamber to produce a pre-heating flame. First the job will be pre-heated by producing a preheating flame on the small holes in the tip of the cutting torch. The metal will be heated to its kindling (combustion) temperature. At this high temperature, the metal reacts with oxygen and iron oxides are formed. The metal will be bright red. The iron oxides will be very weak. Now a jet of pure oxygen will be supplied through the central hole of the torch by pressing the oxygen level in the torch. This oxygen will cu through the pre-heated hot metal and the work pieces will be cut. In flame cutting, only those metals whose combustion (oxidizing) temperature is below their melting point can be cut. So iron and steel can be cut by flame cutting easily. Metals which can not be cut by other processes can be cut by gas cutting.

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Arc cutting Arc cutting is the process of cutting a metal by the help of an electric arc and blowing the molten metal by a jet of air. The jet of air is supplied along with the electrode. Carbon electrode or metal electrode is used here. Nowadays oxy-arc cutting method is used. Carbon electrode is used. But instead of air, a jet of oxygen is used. Oxygen oxidizes the metal and removes the metal at faster rate. Plasma is also used for accurate cutting metal. Quality of the arc cutting is inferior to flame cutting. So are cutting is used only for rough work.

Hard facing Hard facing is a process of depending a hard material layer on the surface of a soft and tough material. A specially designed gun is used here. A fine powder of the material to be deposited is loaded into the gun; oxy-acetylene gas is used to develop a flame the metal power in the gun is melted by the heat of the flame. The molten metal is split (atomized) in to small particles by an aspirating gas (inert gas0 supplied separately under pressure. The hot atomized metal is sprayed on the required surface of the workpiece. This process is called thermo spraying. Application 1. 2. 3. 4.

Coating of tungsten carbide, chromium carbide or aluminium oxide on the surface of cutting tools. Depositing high temperature refractory materials on the surface of vessels. Hard facing of die surface. Reconditioning of worn out part like gears, bearings, shafts and cams.

Bronze welding This process is also known as braze welding. This process is intermediate between true welding and true brazing. In brazing the parts to be joined are not melted. A filler metal called spelter is melted and introduced between the parts to be joined. In welding, the edges to be joined are heated to their melting temperature and joined. But in bronze welding, the edges are heated to the melting temperature of the filler rod. The filler rod is a copper zinc alloy of 60% copper and 40% zinc. Silicon and Tin are also added. Bronze welding is carried out in the following manner. 1. 2. 3. 4. 5. 6.

The edges to be joined are shaped to the required form. The edges are thoroughly cleaned free from oil, grease and dust. The parts to be joined are correctly aligned and clamped by a fixture. The edges are preheated to the melting temperature of the filler rod material. Flux is applied to the edges. Filler rod is melted and deposited between the edges using oxy-acetylene flame.

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Application Steel, cast iron, copper, brass and bronze can be bronze welded. Advantages 1. Dissimilar metals can be joined together. 2. It is stronger than the brazed joint. 3. Lets heat is applied and so less distortion of metal.

Soldering In soldering, two parts are joined by the use of a molten filler metal whose melting points is below the solids (melting point of the base metals) and in all cases below 427°C. Soldered joints are weaker than brazed joint. Because of lower working temperatures, good wetting is more critical than in brazing. The soldering process comprises the same steps as the brazing process, that is, Surface preparation which involves fitting the surfaces to each other cleaning them mechanically and chemically and covering the cleaned the surfaces with a flux. The clearance in a joint is about 0.05 to 0.20mm (for steel). After this, soldering proper is done.

Commonly used soldering joints are: Lap, butt, seam and pipe joints. The fluxes used in soldering are: corrosive type and non corrosive type. The common corrosive fluxes are: Zinc chloride, mixtures of zinc chloride and ammonium chloride. The flux must be washed off after soldering to prevent corrosion. Common non corrosive fluxes are rosin and rosin plus alcohol. These are essential for electrical connections where corrosion can create local high resistance and even loss of conduction. The most widely used solders are alloys of tin and lead in various proportions. Small quantities of some other metals may be added. These filler metals used in soldering are called “Soft solders”. i. ii.

iii. iv. v. vi. vii. viii. ix. x.

63/37 Tin lead solder has the lowest melting point and solidifies at a constant temperature. It is most suitable for electrical connections. 70/30 Tin lead: Good alloy pertaining.

60/40 Tin lead: Good electric grade solder. 50/50 Tin lead: General purpose solders. 32/68 Tin lead: plumber’s solder. Tin –lead –silver solder: High temperature electrical solder for instruments. 96.5/3.5Tin – silver: High temperature electrical instrument solder. A low (<5%) tin content gives higher strength and is suitable for automotive radiators and lock seam cans, and tubes made of tin plate. 35/65 Tin lead: its wide freezing range makes it ideal as a wiping solder for the joining of copper tubes. Lead silver solders are used for higher temperature service. Ag: 1.5 to 3.5%. Certain other metals such as cadmium, bismuth and indium may be added for some specific purposes.

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Lead being toxic has adverse effects on the environment. Due to this, lead free solders are being developed and are now in wider use. They are being used in connection with supplying drinking water and other applications. Typical examples are:

Tin –silver (given above): for electronics. Tin –Bismuth (42%, 58 %): for electronics. Tin-zinc alloys with 9 to 100%. Zinc for soldering of aluminium in ion with special fluxes. Zinc – aluminium: for corrosion resistance, soldering of aluminium. Cadmium-silver: for strength at high temperature.

1. 2. 3. 4. 5.

Brazing The brazing process can be defined as the process to join two metal pies to suitable temperatures by 0 using a filler metal having a liquid above 427 c and below the solids of the base metals. The filler metal is distributed between the closely fitted surfaces of the joint by “capillary attraction”.

During brazing, the base metal of the two pieces to be joined is not melted. Some diffusion or alloying of the filler metal with the base metal takes place even though the base metal does not reach its solids temperature.

The greater the degree of adhesion and inter-diffusion between the molten filler metal and the base metals, the higher the mechanical strength of the join will be. To achieve this and to obtain a strong joint, the basic requirement is that the filler metal must thoroughly wet the base metal surfaces. Therefore, the surfaces must be cleaned and free of contaminants that would prevent adhesion. Thus the scale is removed by mechanical (with a steel wire brush or emery cloth) or chemical (pickling in acids) means and heavy oily residues (oil, grease, paint etc.) are removed by degreasing with hot alkaline solutions or organic solvents. Again, when the assembly is heated to melt the filler metal, oxides may form which will prevent wetting of the surfaces by the molten filler metal. This can be overcome by:

1. 2.

Performing brazing is vacuum or an appropriate (neutral or reducing ) atmosphere. Wetting of the surfaces is also ensured by the application of fluxes

Step in Brazing.

i. ii. iii.

The surfaces to be joined are cleaned (and subsequently rinsed and dried) and fitted closely together. A flux is applied to all surfaces where the filler metal is to flow. After that, the joint is heated to the proper brazing temperature. Solid filler metal may be replaced on the metal pieces and thus melted as the metal pieces are heated, or it may be applied to the metal pieces after the brazing temperature is reached. Only a small the various ways of placing the filler metal at the joint.

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Special welding process Welding of cast iron Some precautions are to be taken before welding cast iron. When one part of the casting is heated during welding, it expands. It will cause strain on the other portions of the casting. As cast iron has low ductility, the strain may cause cracks in the casting. To avoid this, the portion near the place of weld should be preheated to about 600°C. Other portions of the casting should heat to about 200°C. These temperatures should be maintained throughout the process of welding. The preheating can be done in a forge or by using oxy-acetylene flame. After welding, the job should be allowed to cool slowly by burying it in lime. In arc welding of cast iron, coated electrodes are used. In gas welding a neutral flame is used. A cast iron filler rod with a powder flux (Sodium carbonate or borax) is used.

Thermit welding This is a fusion welding process in which melding is done by pouring superheated liquid steel around the parts to be welded. Thermit steel is a mixture of fine aluminium powder and iron oxide. (1:3 by weight) The mixture is placed in a crucible. Then a powder of barium peroxide is added to the crucible. It is ignited using a match stick. Now a thermit reaction takes places within 30 seconds and superheated liquid steel is produced. The temperature is around 3000°C. The ends of the parts to be joined are kept parallel with a uniform gab between them. This gap is filled with max which becomes the pattern. Moulding sand is rammed around the wax pattern. Pouring gate, heating gate and risers are cut. A flame is used at the heating gate. The wax melts and goes out. The ends of the work are preheated. Then heating is stopped and heating gate is closed. The liquid thermit steel from the crucible is poured into mould between the ends of work pieces. The molten metal solidifies and the weld is completed. Thermit welding is used to weld very large works like joining of rails, pipes, broken teeth on larges gears and large frame work. This process is generally used for welding ferrous metals. LIQUID –SOLID- STATE BONDIG Here, the joint is by distributing the molten filler metal between the closely fitted surfaces of the parts, without melting the base metals. The two processes under this category, which will be discussed here, are: Brazing and soldering. The difference between the two depends upon the melting point of the filler metal. When the filler metal melts below 427 c, the process is called as “soldering “and if the melting point of the filler metal is above 427c but below the solids of the base metals, the process is called as “Brazing.” The major advantages of these processes are: similar as dissimilar materials can be joined. Parts with greatly unequal wall thicknesses can be joined. In these processes, the molten filler material is drawn to the various points of the joint by surface tension (capillary attraction). So the access to all parts of the joint is not required. Hence, complicated assemblies and assemblies consisting of many parts can be simultaneously jointed. Since the molten metal is distributed in the closely spaced joint by capillary action, the strength of the brazed or soldered joint is markedly affected by the with of the gap between the parts being joined together.

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Closing the gap will improve the strength of the joint. However, too narrow a gap may spoil the capillary flow of the molten filler metal. The optimum gap width depends on the type of filler and base metals. In brazing steel parts with copper or silver fillers, the optimum gap width ranges between 0.03 and 0.15mm. In soldering with a binary tin- lead solder, it is from 0.05 to 0.2 mm. It should be understood that the strength of the brazed joint increases upon a gap between the two joining surfaces beyond which it decreases.

In these processes, no post join heat treatment is normally necessary and joints are virtually free of internal stresses. The processes lend themselves to automation. However, these processes have the following limitations: 1. Relatively lower strength, particularly in the case of the lower melting point alloys. 2. Temperature resistance is limited by the melting point of the filler material. Inspection of the joint could be difficult.

3. Cost of joint could be high. 4. The process calls for narrow and uniformly spaced joints, due it which large parts/ assemblies, although apparently bras able or solder able, are seldom made in this way. 5. Compared to welding, these processes require more accurate surface preparation (for better spreading and wetting action of filler metal)and assembly before the job can be done. The two processes find extensive use in : the manufacturing of automobile radiators, plate and tube heat exchangers, impellers, fans, fuel and oil pumps, appliance parts and for the joining of wires.

Brazing makes stronger joints as compared to soldering. The strength of brazed joint ranges from 2 3 to 4.5N/mm2 while that of the soldered joint ranges from 0.3to 1.0 N/ mm . The term “brazing” implies the use of the brass as the filler material , but nowadays a number of other alloys are also in use. The filler metal is called “spelter”.

Ultrasonic welding

In this process, a light pressure and high frequently vibrations are combined to produce the weld. A frequency converter converts 50 cps electric power into high frequency (20000 to 60000 cps) power. A transducer converts the high frequency in to vibratory energy. These high frequency vibrations are known, as ultrasonic vibrations. This vibration is transmitted to coupling system which has welding tip. The components to be joined are clamped between

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the welding tip and the supporting anvil. A tight pressure is sufficient anvil. A light pressure is sufficient to keep the components in close contact.

Because of the high frequency vibrations, the contact surfaces get heated, reach plastic stage and get welded together. The welding takes place due to inter atomic bonding.

Applications 1. 2. 3. 4. 5.

Joining of fine wire and their metal assemblies. Joining thin sheets to thick sheets. Joining electrical and electronic components. Joining nuclear fuel elements. Joining of plastics.

Advantages 1. 2. 3. 4. 5.

Dissimilar metals can be joined easily. Welding of non metals possible. Fast process 10 meter/minutes. No filler metal, no flux and no oxidation. No distortion due to heat.

Limitations: 1. Equipment is costly. 2. Thick metals can not be welded.

Diffusion welding In this process, both heat and pressure are used to get the weld by interatomic bonding. Diffusion welding process is controlled by the following factors (i). Temperature, (ii). Pressure & (iii). Time. The high temperature makes the metal surface soft. The pressure brings the metal surfaces to have intimate contacts. The parts to be welded are kept at specified temperature and pressure till such time the interatomic is complete. Diffusion welding can be done in three basic techniques: (i) (ii) (iii)

Hot press techniques. Gas pressure techniques. Vacuum furnace techniques.

The parts to be welded are butted tightly together and enclosed will a removable mould which provides a space between the inner surfaces of a mould and the parts. The products; of the reaction are then poured into the mould in such a manner that slag enters the mould first. On contact with the metal parts. The slag cools rapidly and provides a layer of brittle. Glasslike material so that the Thermit iron which flows does

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not fuse with or adhere to the surface of the parts. The heavier Thermit iron sinks to the lower half of the mould. This and the aluminium slag on top will both give up their heat to the pieces beings welded. When the ends of the pieces to be wedded reach the welding heat, they are forced together by means of clamps to make a pressure butt weld. Non-pressure Welding process. Here, the sections to be joined are lined up and a parallel gap or vee joint is cut at the fractured interface. A wax pattern is then formed between and around the ends which are to be joined. Then a sand mould complete with sprue, gate and riser is built around the joint area. The mould is then heated to melt the wax, causing it to flow into the sand mould and leaving a void space at the joint. The Thermit powder is ignited in crucible placed directly into the mould. This shilsghlyl superheated iron results in a deep penetration; weld between the sections to be joined. After solidification, the moulds are destroyed and the still hot excess iron is chiseled off.

Explosive welding

In this process, two metal surfaces are brought together with high relative velocity under a heavy pressure. The pressure and velocity are developed by ignited an explosive. On plate is kept on the anvil. The other plate is placed at an angle to the first plate maintaining a gap. This angle is called contact angle. This angle varies according to material. For example, for aluminium it is 15째C and for steel it is 10 to 15째C. The explosive charge is placed on the top plate. It is ignited. The explosion cause very high impact and forces the top plate to join with the bottom plate at a high velocity. Metallurgical bond takes place between the surfaces. Microscope swirls are produced on the interface of the plates. This increase the strength of the weld because of interlocking action.

Application 1. Aluminium and mild steel are welded using explosive welding. 2. Tantalum can be welded to steel only by this method, as the melting point of tantalum is higher than the vaporization temperature of steel. Advantages 1. 2. 3. 4.

Process is simple. Larger surfaces can be welded Fast process. Excellent strength

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5. Heat treated sheets can be easily welded without spoiling.

Limitations

1. Metals with less impact strength can not be welded.

Electron beam welding

This welding process is recent development and it is commercially used. Here the welding is done by the intense heat produced when a fast moving electron beam is focused on an object.

Here tungsten is heated in a high vacuum chamber to thermionic-emission temperature 2000째C by a high voltage current. Now free electrons are released. By means of a control grid, accelerating anode and focusing coils, the electrons are converted into a beam (0.25 to 1mm dia).

This beam is focused on to the workpiece to be welded. The kinetic energy of the beam is completely converted into heat. A temperature of about 2500째C is produced. This melts the workpiece material and joints them together.

The electron beam is very narrow. It penetrates into the metal like a metal. The metal melts in a needle like form.

In most cases, the work is also enclosed and moved inside the vacuum chamber. This prevents contamination of the molten metal and ensures high quality weld.

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Nowadays non vacuum electron beam welding has been developed. Here the work piece is kept outside the vacuum chamber and welded.

Applications

1. 2. 3. 4. 5.

Thin foil up to 50 mm thick material can be welded. Used for welding high temperature metals like columbium, molybdenum, tungsten, and tantalum. Used widely for joining dissimilar metals. Several sheets can be welded together in one pass. Used to weld automobile and aero plane parts.

Advantages

1. 2. 3. 4. 5. 6.

As welding is done in vacuum, even highly reactive materials are welded easily. Welds are very clean. No thermal distortion to work piece. Narrow welds with deep penetration can be obtained. Temperature can be easily controlled. Vary fast process.

Limitations

1. Welding is done in vacuum chamber-hence the size of the work piece is limited. 2. Initial cost of equipment is high. 3. Skilled operators are needed.

Laser Beam welding In this process, welding is done using the heat developed by a laser beam. The work, LASER means Light Amplification by Stimulated Emission of Radiation. Laser is an electromagnetic radiation. It is a beam of light having a single wave length (Monochromatic light). This beam can be focused by a less on a very small spot (equal to the diameter of a human hair) on the work piece. Laser beam gives out very high heat which can weld the metal to make the weld joint.

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The equipment has a ruby crystal. The crystal is placed inside a 1000 Watts flash lamp coil. The flash lamp is filled with xenon gas. When the lamp is switched on, it gives out high intensity light. The ruby crystal is stimulated and it emits the laser beam. By using a lens, the beam is focused on the work pieces to be welded. The work piece is fed past the beam by the high heat of the laser beam; the welding of work piece takes place. The welding speed is about 12500 mm/mt. The movement of the workpiece and the operation of the equipment are automatically controlled by a control system. The diameter of the laser beam is about 0.005mm and so this welding is known as micro welding. Applications

1. Dissimilar metals can be joined. 2. Difficult to weld-metals like copper, nickel, tungsten, aluminium, stainless steel, titanium and columbium can be welded. 3. Objects which are sealed in a glass container can be welded without breaking the seal. 4. This process is used for making high quality welding in aero space and electrical industries.

Advantages

1. Excellent heat control 2. Heating is localized and so heat treated components are easily welded without affecting the property. 3. Welding is accurate. 4. Welding can be done in any position of the work piece. 5. As laser is ray of light, electrode is not necessary. 6. No thermal distortion. 7. No atmospheric contamination of weld.

Limitations

1. Materials up to 0.5mm thick alone can be welded. 2. High energy consumption.

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3. High cost of equipment.

Types of welded joints Different types of joint are classified as butt, lap, corner, tee and edge joints. The choice of the types of joint depends on the weldment being made and the sheet thickness. More details of these will be dealt in the respective welding processes. Each of the above joints can have any of four different welding positions; flat, horizontal, vertical and overhead. This type of edge preparation is used when the thickness of the two pieces to be joined is small, so that heat of welding penetrates the full depth of the joint. How ever, when the thickness increase it becomes necessary to prepare the edge in such a way that heat would be able to penetrate the entire depth. To facilitate this, the joint is widened. For very thick plates, the welding needs top be done from both sides. To provide the necessary access into the joint, it could be made as a V or U. the V joint is easier to make but the amount of extra metal to be filled in the joint increases greatly with increase in the thickness. From this account a U-joint is preferable, since amount of extra metal to be added to fill the joint is generally less beyond a certain plate thickness. However, machining a U-joint is more difficult compared to a Vjoint. The double V and double U edge preparations are used when welding is to be carried from both sides. Welding defects Generally the following defects may occur in welding 1. 2. 3. 4. 5. 6.

Blow holes Cracks Cavities Inclusions Porous weld Imperfect fusion

Testing of welding joints Welded joints are tested to determine the strength and to find out the defects if any. Welded joints are tested by using. 1. Non destructive tests 2. Destructive tests Non destructive tests are tests during which the welded joints are not destroyed. The welded joints will be destroyed during destructive tests. The various types on non destructive tests and destructive are explained below. Non destructive tests (i). Magnetic Particle test Cracks and slag inclusions can be formed by magnetic tests. The pieces are magnetized. Fine iron powder is sprayed on the surface of job. If there is a crack, magnetic poles will be formed at the crack.

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The iron powder will be attached to the crack and the crack will appear as a line. This method is called magnetic crack detection. This method is applicable only to ferrous metals.

(ii). X-ray test From an X-ray test tube X-rays are passed through the welded metal. The X=ray films will show weld defects like blow holes, cracks, inclusions, cavities and imperfect fusion. The cost of the test is high. This is a quicker method. It gives a permanent record of the result. We have to take special precautions to avoid radio active effects on human beings.

(iii). Ultrasonic test

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In ultrasonic test, a beam of ultrasonic waves (high frequency vibrations) produced by a transducer (inputoutput generator) is passed into the part being tested. If there are defects, the echoes will be returned and they will be picked up by a receiver. The receiver converts the ultrasonic waves into a electric signal. The signals are amplified and are projected on the screen (CRT) as shown in figure. The top surfaces A and bottom surface C of the specimen are shown in screen by signals A and C. The internal flaw at B in the specimen is indicated in the screen by the signal B. All the surface defects and internal defects can be located by this method of testing.

Destructive tests

In this test the grain size, blow holes, flash eye and foreign material inclusions are inspected. Two plates are welded together. A test specimen of 200mm long is selected. 6mm deep slots are cut with a saw at the opposite sides of the welded portion. This is called nicking. The specimen is supported on two rollers. At the nicked portion, a hammer blow is applied to break the weldment. The fractured surface of the welded portion is inspected for defects.

Safety in welding In welding, we work with electrical equipments, and inflammable gases like oxygen and acetylene. So, even a little carelessness will lead to accidents. So we must take adequate precautions while welding.

Safety precautions in gas welding 1. 2. 3. 4. 5. 6. 7.

The gas cylinders should be kept away from heat sources. The cylinders should be stored in well protected, well ventilated dry locations. Valves must be opened slowly, closed when not working. Faculty gauges and valves should not be used in the cylinder. Pressure gauges and valves should be properly maintained. Persons without proper training should not be allowed to work with gas. Quality pipes should be used.

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8. Specified pressure for oxygen and acetylene should be maintained throughout the welding process.

Safety precautions in arc welding

1. All precautions should be taken to prevent electrical shock. Specified voltage and current should be maintained. 2. Electrode holder should be in good condition with good insulation. 3. The operator should wear personal protective devices such as goggles, hand gloves, protective clothing and rubber shoes. 4. Weld place should be enclosed in all sides to protect others. 5. Fire extinguishing equipment should be readily available. 6. First aid boxes must be available for ready use.

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UNIT-III LATHE & THEORY OF METAL CUTTING

INTRODUCTION

LATHE is the father of machine tools. The main function of a lathe is to remove metal from a piece of work to give it the required shape and size. The lathe is used to machine cylindrical shapes. In a lathe, workpiece is held and rotated about its axis. Generally single point cutting tool is used as the cutting tool. The tool is moved parallel to the axis of rotation of workpiece to produce a cylindrical surface. The tool is moved perpendicular to the workpiece axis to produce a flat surface. The tool is moved at an angle to the axis of workpiece to produce a tapered surface. Straight turning, taper turning, eccentric turning, chamfering, facing, parting off, drilling, boring, reaming, tapping, knurling, forming, grooving, polishing, spinning and thread cutting are the main operations done on lathe. When the various operations are done automatically, then the lathe is called automatic lathe. Types of Lathe Various types of lathe have been developed to suit different machining requirements. The different types of lathe are:

1. Speed Lathe

2. Centre Lathe

3. Bench Lathe

4. Tool-room Lathe

5. Turret and Capstan Lathe

6. Automatic Lathe

7. Special purpose Lathe

8. Copying Lathe.

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1. Speed Lathe It has a bed, headstock, tailstock, and tool post. There is no leadscrew, feed box and apron mechanism. Tool is fed by hand only. Very high spindle speeds are used (1200 to 3600 rpm.) This type of the lathe is used for wood turning, spinning and polishing.

2. Centre Lathe This is the widely used lathe. It has the headstock, tailstock, carriage and bed. Automatic feeds are available. Different speeds and feeds are available, Lead Screw, feed rod and change gears are provided. The headstock may be back geared or all geared. It us used for many operations like turning, taper turning, facing, threading drilling, reaming, knurling and forming on different sizes of work on different metals.

3. Bench Lathe It is a small lathe mounted on a bench. It has all the parts like a centre lathe and is used for many operations. The difference is that the bench lathe is small in size. It is used for small works.

4. Tool room Lathe It is a very accurate lathe. It is designed for precision turning work on tools, gauges and dies. It has an accurate collet chuck, relieving and taper turning attachments. It has more range of speeds and feeds. This lathe is used in tool rooms.

4. Turret and capstan Lathe 5.

This is a heavy lathe used for mass production. The headstock is heavier than centre lathe and has more speeds and feeds. Speed is changed by lever. There are two tool posts, a front tool post and a rear tool post. The front tool post caries four tools. The rear tool post carries a parting tool there is no tailstock. But there is a hexagonal turret fixed in a carriage. The turret carries six tools. Any one tool can be brought into working position by indexing. Many operations can be done simultaneously by the front tool post, rear tool post and turret. The work and tools need not be changed for a complete cycle operations. The production time is very less. The accuracy will be more.

6. Automatic Lathe All the operations are automatic in this lathe. All the tool movements for machining are automatic. The loading and unloading of work is done automatically in this lathe. High speeds and many feeds are available. Speed and feed changes are also automatic. There is a cam shaft carrying a number of cams. The cams will make the speed changes, feed changes and tool changes. Machining time is very less.

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Accuracy is very high. These lathes have one or more spindles. A single operator can look after a number of automatic lathes at the same time. 7. Special purpose Lathe These are used for a single or special purpose only. Jobs which cannot be done easily in a standard lathe can be done in these lathes. The lathes are designed to suit the jobs. For example, a shell lathe is used for turning shells only. A wheel lathe is used for turning wheels. A cam shaft lathe is used for turning g crank shafts only.

8. Copying Lathe The tool in this lathe follows a template or master through a stylus or tracer. The shape of the template is copied on the job. Air, hydraulic or mechanical devices are used to copying the shape of the template over the work.

Parts of Lathe

A simple sketch of a centre lathe is shown in figure. The various parts and their functions are given below:

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1. Bed The bed is the base of the machine. The headstock is mounted on the left end, the carriage in the middle and the tailstock at the right end of bed. The carriage and the tailstock move over the bed. The bed has guide ways. The bed is very strong to resist the cutting forces and vibrations. The bed has ribbed construction. The guide ways have to be very accurate for getting accuracy in jobs. The bed is made of cast iron alloyed with nickel and chromium. The guide ways of the bed may be flat or inverted â&#x20AC;&#x2DC;Vâ&#x20AC;&#x2122;.

2. Headstock Head stock is mounted on the bed at the left end. It carries a hollow spindle. A long bar can pass through the hole of the spindle. The front end of the hole is tapered for holding tapered shanks (Morse taper). A live centre can be attached into the spindle. The spindle nose is threaded. Chucks and face plates can be attached to the nose of the spindle. The headstock has the driving and speed changing mechanisms. The headstock may be of back geared type or all geared type. There are speed changing and feed changing levers attached to the headstock.

3. Tailstock It is mounted on the bed on the right end. It is used for supporting the right end of work. It is also used for holding drill, reamer or tap for drilling, reaming or tapping operations. To support different lengths of work, the body of the tailstock can be moved along the bed and clamped at any position. The upper body of the tailstock can be moved towards or away from the operator for taper turning. The tailstock body is bored and the tailstock spindle moves through it. The spindle can be moved axially by means of a hand wheel. A dead centre can be fixed into the taper hole of the spindle for supporting the right end of work.

4. Carriage The carriage is used for giving various movements to the tool by hand or by power. The carriage gas the following parts.

i) Saddle It is a H shaped casting fitted over the bed. It moves along the guideway. It carries the cross slide and tool post. It can be moved to the required position and locked to the bed. ii) Cross slide It is attached to the saddle. It carries the compound rest and tool post. The cross slide can be moved by power of by hand. There is a micrometer dial on the cross slide hand wheel, with an accuracy of 0.05mm.

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iii) Compound rest The compound rest is marked in degrees. It is used during taper turning to set the tool for angular cuts. There is no power feed to the compound rest. It can be operated only by hand to feed the tool longitudinally or at an angle to the lathe axis. There is a micrometer dial for showing the depth of cut. The compound slide should be locked strongly with its base after any setting. iv) Tool post The tool is clamped in the tool post. The tool post is fitted over the compound rest. There are four types of tool post. a) Single screw tool post

b) Open side tool post.

c) Four bolt tool post

d) Four way tool post.

a) Single screw tool post The single screw tool post can hold only one tool. The tool is clamped by a clamping screw. The tools rest on the top flat surface of the convex rocker. The convex rocker has got a convex surface at its bottom. The convex rocker rests over a concave ring. This arrangement is used to adjust the height of the tool. Refer figure 4.03 b) Open-side tool post The open-side tool post is shown in figure 4.04. The tool is held in position by tow set screws. Parallel packing strips are used to adjust the height of the cutting point. The tool post can be tilted to any required position. This is done by loosening the clamping bolt. The clamping bolt is fitted in a T-slot. So the tool can be changed quickly. 5. Apron The apron is attached to the saddle and hangs in front of the bed. It has gears, levers and clutches for moving the carriage with the lead screw for thread cutting. The apron handwheel is used to move the carriage parallel to the lathe axis. 4.1.3. Specification of a lathe. The method of specifying a lathe is shown in figures 4.05 and 4.06.

Specification of a typical lathe is given below:

1. The length of bed

2. The length between centres

1830 mm

1065 mm

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3. The height of centres from the bed

250 mm

4. The swing diameter of work over bed

400 mm

5. The swing diameter of work over the carriage

300 mm

6. The maximum bar diameter which will pass through the hole of headstock spindle.

53 mm

To specify the lathe fully the following details are also given

1. Power input

2HP

2. floor space required

2500x800

3. lead screw details-pitch diameter etc.

4 TPI

4. number of spindle speeds

5. feeds

6. spindle nose diameter 7. width of bed

120 mm 330 mm

Headstock driving arrangements.

The two types of headstock driving arrangements are explained below:

1. Back geared head stock.

Back gear arrangement is cued for reducing the spindle speed. The back geared headstock is shown in figures 4.07 and 4.08. The cone pulley can rotate freely on the spindle. Gear A is connected to cone pulley. Gear A will rotate when the cone pulley rotates. Gear D is connected to the spindle. The spindle will rotate when gear D rotates. There are two back gears B and C on a back shaft. By shifting the handle F, gears B and C can be engaged or disengaged with A and D. There is a bull bear lock pin G. If

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the bull gear lock pin G is engaged, the cone pulley gear A, bull gear A, bull gear and spindle will rotate as one body. If pin G is taken out, the cone pulley will be free on the spindle. Four fast speeds or direct speeds can be obtained by engaging the pin G with the cone pulley. For getting four slow speeds or indirect speeds, back gear is engaged by moving the lever F. Lock pin G is disengaged. Now power will flow from A to B, B to C (same shaft), C to D and D to the spindle. As gear B is larger than A, the speed will be reduced at B. B and C will have same speeds. The sped will be further reduced at D, because gear D is larger than gear C. So by engaging back gear, the speed to spindle is reduced.

2. All geared headstock

The all geared headstock is shown in figure 4.09. A constant speed motor at the bas e of the lathe drives the lathe spindle through gears. Speed changes are made by levers. When the levers are shifted into different positions, different gear combinations are made and the spindle rotates at different speeds.

Gears Z4, Z5, Z6 are mounted on a splined shaft and receive power from the fast and loose pulley. Gear Z7. Z8, and Z9 are mounted on an intermediate shaft and cannot move axially. Gears Z11, Z12, Z13 are mounted on the headstock spindle and can be moved axially by levers.

The gear combinations for nine different speeds are:

(1)

Z4/Z7 x Z7/Z11

(2) Z5/Z8 x Z7/Z11 (3) Z6/Z9 x Z7/Z11

(4)

Z4/Z7 x Z8/Z12

(5) Z5/Z8 x Z8/Z12 (6) Z6/Z9 x Z8/Z12

(7)

Z4/Z7 x Z9/Z13

(8) Z5/Z8 x Z9/Z13 (9) Z6/Z9 x Z9/Z13

Advantages: 1. 2. 3. 4. 5.

Large number of spindle speeds. No belt shifting needed. Speed change is very quick by shifting a lever. No accident due to belt shifting. No overhead shaft; independent motor.

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Disadvantages:

1. Costlier. 2. Friction of gears will lead to power loss. 3. In case of overloading there is no belt slipping and so there will be damage to parts.

Feed mechanism The movement of the tool relative to the work is termed as feed. Three types of feed can be given to a lathe tool. They are longitudinal, cross and angular. When the tool moves parallel to the lathe axis, the movement is called longitudinal feed. This is achieved by moving the carriage. When the tool moves at right angles to the lathe axis, the movement is called cross feed. This is achieved by moving the cross slide. When the tool moves at an angle to the lathe axis, the movement is called cross feed. This is achieved by moving the cross slide. When the tool moves at an angle to the lathe axis, the movement is called angular feed. This is achieved by moving the compound slide after swiveling it at an angle to the lathe axis. The above three types of feed can be given by hand. The longitudinal and cross feed can also be given automatically. For this, the motion is to be transmitted from the headstock spindle to the carriage. This is done by means of the following units.

1. Tumbler gear mechanism

2. Quick change feed gear box

3. Apron mechanism

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1. Tumbler gear mechanism

Tumble gear mechanism is used to change the direction of lead screw and feed rod. By engaging tumbler gear, the change can be moved automatically from tailstock end to headstock end or moved from headstock end to tailstock end. During thread cutting and automatic feed, tumbler gear is used. E is the gear attached to the spindle. It is called spindle gear. It always rotates in clockwise direction. A and B are the tumbler gears. These gears are fitted in a bracket. The lever M of the bracket can be shifted upwards or downwards. D is the stud gear. The stud gear D is connected to the lead screw gear through a set of intermediate gears. Two positions of tumbler gears are shown. In the position 1, the lever M is in the upward position. Now gear A connects spindle gear E and stud gear D. So the lead screw rotates in the same direction. That is, lead screw (or feed rod) rotates in the same direction of the spindle rotations. The carriage will move towards headstock. This arrangement is used for cutting right hand threads. In position 2, the shift lever is in the horizontal position. Now E is connected to gear B. B to A and A to D. So the lead screw rotates in anti-clockwise direction. That is the lead screw (or feed rod0 rotates in opposite direction to the spindle rotation. The carriage will move towards tail stock. This arrangement is used for cutting left hand threads. When the lever M is in the middle position (i.e) neutral position, tumbler gears are not engaged. So automatic feed is not possible. Only hand feed is given to the carriage.

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2. Quick change gear box

Quick change gear box is shown in figure. The motion is transmitted from the spindle gear to the shaft A through the tumbler gear and change gears. Shaft A has got 12 cone gears keyed to it. So shaft b can get 12 different speeds from A by the use of sliding gear. Shaft B is connected to shaft C through 4 cone gears.

With the four additional gears, shaft C can have 12*4=48 speeds. The driver shaft C is connected to lead screw by a clutch. In some lathes, the lead screw will be used for thread cutting and also for automatic feeds. In some lathes, both lead screw and feed rod are available. In these lathes, the lead screw will be used for thread cutting only. Feed rod is used for automatic feeds. The figure shows a lathe having lead screw and feed rod to receive 48 different speeds and feeds from the quick change gear box.

3. Apron mechanism

The inner details of apron mechanism are shown below. When the spindle gear rotates, the lead screw and feed rod will rotate through tumbler gears. Splined shaft K will be rotating from lead screw and gears F and G will be always rotating. There is a feed check knob E, which has three positions, neutral, push-in and pull-out. For hand feeds, the knob E is put in neutral position. Now, gears F and G have no connection with H and R. Now handwheel C is rotated. Pinion I rotates through J and H. Pinion I will move on the rack and so, longitudinal hand feed takes place. For hand cross feed, handwheel D is rotated. Now cross slide screws S rotates and hand cross feed takes place. For automatic longitudinal feed, the knob E is pushed in. Now, rotating gear G will be engaged to H and H to I. I will rotate on rack and so automatic longitudinal feed takes place.

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For automatic cross feed, the knob E is pulled out. Now, the rotating gear F is connected to R and R to Q. Cross slide screw S rotates and automatic cross feed takes place. For thread cutting, half nut is engaged by lever B after putting E in neutral position. Now, lead screw will move the carriage for thread cutting. Both longitudinal and cross feed can be reversed by operating the tumbler gear handle. Carriage The carriage has saddle, cross slide, compound rest, tool post and apron.Saddle is an H shaped casting, fitted over the bed. The saddle moves on bed ways. It carries the cross slide, compound rest and tool post. The cross slide is attached to the saddle. The cross slide carries the compound rest and the tool post. The cross slide moves perpendicular to the lathe axis for cross feed. The cross slide can be moved by handwheel or automatically. The cross slide handwheel has a micrometer dial marked in 0.05 mm divisions. The compound rest is mounted on the cross slide. The compound rest base is graduated in degrees. The compound rest can be rotated on its base and clamped in an angular position for taper turning. There is no power feed to the compound rest. It can be moved only be a handwheel. There is a micrometer dial on the handwheel of the compound rest to measure depth of cut. The tool post is fitted on the top of compound rest. It carries the tool for machining. The tool post may be of a single tool type or four way tool post. The tool can be moved in three direction 1) parallel to the lathe axis for longitudinal feed 2) perpendicular to the lathe axis for cross feed. 3) at an angle to the lathe axis for angular feed.

The apron is the front portion of the carriage. It has gears, clutches and levers for operating the carriage by hand and by power. There is a split nut for engaging the carriage with the lead screw during thread cutting.

Tailstock The tailstock is mounted on the bedways at the right end of the lathe. The uses of tailstock are: 1. It supports the right end of the workpiece.

2. Tailstock spindle can carry a drill, reamer or tap for drilling reaming and tapping operations.

3. Tailstock body can be moved perpendicular to the lathe axis for taper turning operations.

The body of the tailstock can be moved on the bedways and fixed at any position for machining different lengths of work. The tailstock spindle has a standard taper into which the dead centre or cutting tools like drill and reamer can be fixed. When the tailstock handwheel is rotated, the spindle will be advanced by the screw thread. The spindle can be clamped in position by the spindle clamp.

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Work holding devices

The work can be held on a lathe by the following ways:

1. Work held between centres and driven by catch plate and carriers.

2. Work held in chuck.

3. Work held on face plate.

4. Work held on angle plate.

5. Work held on mandrel.

6. Work held in turning fixtures.

Work held between centres (By catch plate and carriers) Long shafts are generally held between centres. Here a driving plate or catch plate is screwed to the nose of the head stock spindle. The live centre is inserted in the head stock spindle. The tailstock carries the dead centre. The workpiece is supported between centres. The driving dog is clamped to the workpiece by a screw. The tail of the dog (carrier) is attached to the catch plate. When the spindle rotates, the workpiece will rotate through the catch plate and carrier. The live centre will revolve with the work. The dead centre will support the right end of the work. Work held in chucks Works of short length, works of large diameter and works of irregular shape can be held in chucks. Work which cannot be mounted between centres can be mounted in chucks. The chuck is attached to the spindle of the lathe. The work is clamped between the jaws of the chuck and the jaws are tightened. The right end of the work may be supported by the dead centre if needed. (i) Three jaw self centering chuck It has three jaws. When the chuck key is turned, all the jaws will move equal distance at the same time. The work can be centered automatically and quickly. This chuck is used for holding round, hexagonal and other regular shaped workpiece. (ii) Four Jaw Independent Chuck It has four jaws. These jaws may be made to slide within the slots in the body of the chuck. Each jaw can be moved independently. When the chuck key is turned in the slot that particular jaw only will move. So irregular jobs can be held in this chuck by moving each jaw to the required amount. The jaw can be

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reversed for holding the job of larger sizes. Concentric circles are inscribed on the face of the chuck for quick centering. Four jaw chuck is used for holding large jobs and irregular jobs.

(iii) Magnetic chuck This is used for holding thin jobs. When the pressure of jaws on the jobs is to be prevented, this chuck is used. The chuck gets magnetic magnetic power from an electro-magnent. Only magnetic materials can be held on this chuck.

Work held on face plate Face plate is circular plate. It is screwed to the lathe spindle. There are many holes and slots on the face of the face plate by â&#x20AC;&#x153;Tâ&#x20AC;&#x2122; bolts, clamps and nuts. When the spindle rotates, the face plate will rotate and so the work will rotate. Large, irregular, heavy jobs which cannot be held between centres or in chucks can be held on face plates.

Work held on angle plate Angle plate has two accurately machined faces at right angles. There are holes and slots on each face. The workpiece is clamped to one face of the angle plate by bolts and nuts. The other face of the angle plate is attached to the face plate. The face plate is screwed to the spindle of the lathe. When the spindle rotates, the face plate also rotates. So the angle plate and job also will rotate. The angle plate is used for holding jobs like elbow pipes which cannot be held by chucks and on face plates directly for machining.

Work held on mandrel Mandrel is used for holding jobs. The workpiece is mounted over the mandrel and the mandrel is rotated between centres. The outside diameter of mandrel should be equal to the inside diameter of job. Different types of mandrels are used for different types of jobs. (Refer figure 4.45 and 4.46).

Lathe tools. Different types of lathe tools are shown below: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Left hand turning tool Right hand turning tool Round nose turning Wide face square nose finishing turning tool Form tools Left hand facing tool Right hand facing tool R.H chamfering tool Necking or parting tool External threading tool

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11. Boring tool 12. Internal threading tool 13. Knurling tool

Operations done on lathe: Different operations carried out on a lathe are: straight turning, taper turning, eccentric turning, chamfering, facing, parting off, drilling, boring, reaming, tapping, knurling, forming, grooving, polishing, spinning, thread cutting (internal and external) Spring winding, milling and grinding can be done by using special attachments.

Straight turning. In straight turning, cylindrical surface is produced. The workpiece is rotated about the lathe axis. The tool is fed parallel to the lathe axis. The workpiece is held between the lathe centres or held in chuck. A right hand turning tool is clamped on the tool post. For light cuts, the tool is inclined towards the head stock. For heavy cuts, the tool is inclined towards the tailstock. Hand feed or automatic feed can be used for straight turning. For rough turning the rate of feed of the tool is fast and depth of cut is heavy. (Feed is the rate of travel of tool; it is expressed in mm per revolution of the job). For rough turning, rough turning tool is used. In rough turning the depth of cut will be from 2 mm to 6 mm. The feed rate will be from 0.3 to 1.5 mm per revolution of the work. For finish turning the cutting will be high, feed will be small and depth of cut will be very small. A finish turning tool is used in finish turning. The depth of cut will be from 0.5 to 1 mm. The feed will be from 0.1 to 0.3 mm per revolution of the workpiece.

Facing

In facing operation, the ends of the workpiece are made flat. It is sometimes called squaring. The work may be held between centres or in a chuck. The workpiece is rotated about lathe axis. A facing tool is fed perpendicular to the axis of lathe. The tool is slightly, inclined towards the workpiece end. The feed can be given by hand or it can be automatic. Shoulder turning Finishing the side of shoulders of the workpiece having different diameter is known as shoulder turning. The finishing of a shoulder is similar to facing operation. There are four kinds of shoulders.

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a) Square Shoulder

b) Angular or beveled shoulder.

c) Radius or filleted shoulder

d) Undercut shoulder

Chamfering Chamfering is beveling or turning a slope at the end of the workpiece. This is done to remove the burrs. Chamfering is done for jobs after knurling, rough turning and thread cutting. The ends of threads must be always chamfered. The workpiece is rotated about the lathe axis. A chamfering tool is perpendicular to the workpiece.

Knurling

Knurling is done to give a good gripping surface on the work piece. It is also done to slightly increase the diameter of the workpiece. A knurling tool has two hardened steel rollers. The rollers have teeth out on their cylindrical surface. The teeth may be fine, medium or coarse. The knurling tool is held in tool post and pressed against the rotating workpiece. The tool is moved parallel to the lathe axis. Automatic feed is given to the tool; very slow cutting speed is used. The knurling tool does not cut the

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workpiece. But it produces impression on the work surface. Many knurling cuts may be necessary to get the full impression.

Forming

In form turning, a concave, convex or any irregular shape is made on the workpiece, by a form tool. The cutting edge of the tool is ground to the required form. Form tool is cross fed against the rotating workpiece. Form turning is also done be combining cross and longitudinal feeds. By tracing or copying a template, forming operation can be done.

Grooving and parting off

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Grooving is also known as recessing, undercutting or necking. Grooving is generally done at the end of the threaded portion and the edge of the shoulder. The workpiece is held in a chuck. The grooving tool is fed crosswise against the rotating workpiece. After the workpiece is machined, it is cut-off. This is called parting off. Parting off is done by parting tool. While parting off, the carriage is locked. The tool is fed cross-wise. Spinning.

It is a process of forming a thin sheet metal to the required shape. Cup shaped parts are produced by spinning. Round shaped sheet metal is held between a form block and tailstock centre. The sheet metal is rotated at high speed. A long round nose forming tool presses the rotating sheet metal against the form block.

Eccentric turning:

Eccentric turning is useful for turning a crank-shaft or an eccentric on a lathe. To turn the eccentric surface, two sets of centre holes are drilled at the ends of workpiece. The centre holes are off set from the normal axis of the workpiece. The amount of off set is equal to half of the eccentricity required. First the workpiece is mounted on its true centre. The journal potion of the workpiece is turned. Then, the workpiece is mounted on the offset centre to turn the eccentric portion.

Drilling

For drilling, the workpiece is rotated in a chuck or face plate. The tailstock spindle has a standard taper. The drill is fitted into the tailstock spindle directly or through drill chuck or socket. The tailstock is moved over the bed and clamped on the bed near the work. When the job rotates, the drill bit is moved into the work by turning the tailstock hand wheel. Enough coolant is applied during drilling operations. The tailstock spindle has markings to know the length of hole drilled.

Boring Boring is enlarging a hole. Boring is used when correct size drill is not available. Boring removes tool marks. Boring cannot make a hole. The workpiece is rotated in a chuck or face plate. The boring tool is fitted on the tool post. The boring tool is a single piece forged tool for small holes. For large holes, a boring bar with a tool bit is used. The depth of cut is given by the cross slide. Longitudinal feed is given by moving the carriage parallel to the lathe axis by turning the apron hand wheel.

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Taper turning

A taper is defined as the uniform change in the diameter of a workpiece measured along its length. It is expressed as a ratio of the difference in diameter to the length. Taper is also expressed in degrees of half the included angle. Taper turning is producing a conical surface on the work piece on lathe.

Taper turning can be done in the following methods:

1. By a form tool

2. By a tailstock set over

3. By swiveling the compound rest

4. By taper turning attachment

5. By combining longitudinal feed and cross feed.

1. Form tool method

Taper turning by form tool is shown in figure. The form tool is ground to the required taper angle. The tools is fed perpendicular to the lathe axis. This method is used to turn short lengths of taper only. The taper length should be less than the tool cutting edge length.

2. Tailstock set over method

Set-over

S=L x {(D-d)/2xl} mm

If taper is turned on the entire length of the workpiece then l=L Set-over

S=(D-d)/2 mm

The body of tailstock is moved crosswise (perpendicular to the lathe axis). This is done be turning the set-over screw. The work piece is held between the live centre and the dead centre (which is off-set). The job is inclined due to the set over given to the tailstock. Now, the tool is moved parallel to the lathe axis

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and taper will be turned on the workpiece. This method is used for turning very slight (small) tapers on long workpiece. 3. Compound rest method.

In this method, the workpiece is rotated parallel to the lathe axis and the tool is moved at an angle to the lathe axis. The compound rest is mounted on a circular base. The base can be swivelled and fixed at an angle. The compound rest base is swivelled and set at half taper angle. The tool is moved by the compound rest hand wheel. The work is rotated in a chuck or face plate or between centres. This method issued for turning steep tapers on small lengths of job. The compound test can be swivelled upto 45degree on both sides. The tool should be moved only by hand. 4. By taper turning attachment

A taper turning attachment is fitted by a bracket (1) at the back of the bed. There is a guide bar (3) pivoted at its centre. It has graduations in degrees. It can be swivelled to both sides at an angle to lathe axis as shown in figure. There is a guide block (2) which is connected to the rear end of the cross slide (4). The guide block will move on the guide bar. The cross slide is made free from its screw by removing the binder screw (5). When the longitudinal feed is engaged, the tool will move at an angular path as the guide block moves at an angle on the guide bar. Depth of cut is given by compound rest hand wheel. The guide bar is set at half taper angle. By taper turning attachment, steep tapers, long tapers, accurate tapers and internal tapers are produced. The setting of work is not disturbed. 5. Combined feed method

By combining both the longitudinal and cross feeds at the same time, a taper can be turned. This is not a very accurate method. This method requires high skill. Thread cutting The thread cutting principle is shown in the figure 4.37. When the job is rotated. The tool is automatically moved by the lead screw in the longitudinal direction. The longitudinal feed should be equal to the pitch of the thread to be cut per revolution of the work. The lead screw has a fixed pitch. So, a ratio between the rotation of headstock spindle (workpiece) and the longitudinal feed is found out. The leadscrew and spindle are connected by change gears. The leadscrew is connected by carriage by engaging the half nut lever. So, when the headstock spindle (work) rotates, the leadscrew rotates at the same speed, the pitch of work will be equal to the pitch of leadscrew. So, for obtaining different pitches on the work, the speed of the leadscrew can be changed by fixing proper change gears between headstock spindle and leadscrew. The change gear calculation is made as follows: Driver teeth --------------Driven teeth

teeth on spindle gear =

-----------------------------teeth on leadscrew gear

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pitch to be cut on work =

---------------------------Pitch of leadscrew

The spindle gear is the driver gear. Leadscrew gear is the driven gear. The speed of leadscrew gear can be changed by suitable change gears. The lathe is supplied with a set of gears from 20 to 120 teeth in steps of 5 and a 127 teeth gear. Two types of gear trains are used for getting the required ratio between the driver and driven. A simple gear train and a compound gear train are shown in figure 4.38 and 4.39. While cutting right hand thread, the leadscrew rotates in clockwise direction. That is, leadscrew rotates in the same direction of the lathe spindle. The tool (carriage) moves towards headstock end. For cutting left hand thread, the leadscrew rotates in anti clock wise direction. That is, leadscrew rotates in a direction opposite to lathe spindle. The tool moves towards tailstock end. Lathe accessories: There are a number of devices which can be used in a lathe to improve its performance. They are called lathe accessories. The various lathe accessories are: 1. 2. 3. 4. 5. 6. 7. 8.

Lathe centres Dogs of carriage Catch plates or driving plates Chucks Face plates Angle plates Mandrels Steady rest and follower rest

4.5.1. Lathe centres The workpiece can be rotated in a lathe between two centres - live centre and dead centre. Live centre rotates with the work and the dead centre supports the right end of the work. It does not revolve with the work. Various types of lathe centres are in use. The shanks of centres have standard Morse taper. Refer fig.4.42. 1. The ordinary centre is used for most general work.

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2. The ball centre is used to reduce wear and strain on the ordinary centre during taper turning by set over method.

3. Rotating centre is used for supporting heavy work at high speed. The centre revolves on the ball and roller bearings. The rotating centre is also known as frictionless centre.

4. Half centre is similar to ordinary centre, but half of the centre is ground away. Using the centre, facing operations can be done without removing the centre.

5. The tipped centre has a hard alloy tip, brazed into an ordinary steel shank. This tip resists wear and strain.

6. The insert type centre is made of high speed steel. After the insert wears out, the insert is replaced instead of the whole centre.

7. The pipe centre is used for supporting the open end of pipes and shells.

Dog or carrier This is a device used to transfer the motion from the rotating driving plate to the work held between centres. The eye of the dog is attached to the end of the workpiece with clamping screws. Its leg engages with the driving pin fitted to the driving plate. Different types of dog are shown in the figure 4.43

Catch plate or driving plate It is used to drive the workpiece through a dog when the workpiece is held between centres. This is a plain disc of steel or cast iron. This has a central boss projecting from it. The boss is threaded internally. This thread fits on the head stock spindle. A driving pin is fitted to the plate. This pin transfers the motion to the dog fitted with the workpiece.

Chucks Face plates Angle plates These have been explained under the topic work holding devices already. Mandrel This is a device used for holding hollow workpiece between centres. Some of the common types of mandrel used in lathe works are shown in figure 4.45 and 4.46.

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1. Solid mandrel This is also known as plain mandrel. It is most commonly used in shop. The body of the mandrel has a slight taper. The taper is given for gripping of the workpiece. The difference in diameter will be 1 to 2 mm per 100 mm length. This type of mandrel is available in various sizes. (55 mm to 450 mm). This type of mandrel is used for holding only workpieces with the same bore diameters. For workpieces with different bores different mandrels are used.

2. Gang mandrel This type of mandrel has a fixed collar at one end. The other end of mandrel has a threaded portion. At this threaded end there is a movable collar. This movable collar is adjusted to the required position by a nut. A set of hollow workpieces can be held between the two collars.

3. Expanding mandrel This mandrel has a tapered arbor. A bush is mounted over this arbor. This bush has a tapered hole and a cylindrical outside surface. Three longitudinal slots are cut on the bush. Two slots are cut nearly through. Third slot is cut for the full length. By this construction, it is possible to expand the split bushing within a limit. Therefore various workpieces of different diameter holes can be held in this expanding mandrel. A particular bushing can hold workpieces of hole diameters ranging from 0.5 to 2 mm . Using bushed of different sizes, this range can be increased. 4. Cone mandrel The cone mandrel has two conical ends. One end has a solid cone. The other end has got threads. At the threaded end there is a sliding cone. This sliding cone can be adjusted by a nut. This come mandrel is used for holding workpiece of different hole diameters. The workpieces are held between the two cones as shown. 6. Screwed mandrel This mandrel is threaded at one end. It has a collar. The screwed mandrel is used to hold workpieces having threaded holes. The workpiece is screwed on to the mandrel against the collar. 7. Step mandrel

Step mandrel has steps of different diameters. Therefore it is used to hold workpieces having different sizes of holes.

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8. Collar mandrel

This mandrel has two collars. The collars fit firmly into the hole of workpieces. The collar mandrel is lighter in weight. It is used for holding workpieces having larger diameter holes (above 100 mm). Steady rest and follower rest Rest is a device which supports long workpieces when machined between centres or by a chuck. Rest prevents vibration and bending of the workpiece. It supports the work at convenient points in between headstock and tailstock. Rest should be used when the length of the workpieces is more than 10 to 13 times the diameter. The two types of rest used as: 1. Steady rest 2. Follower rest 1. Steady rest It has a cast iron base. The base can slide on the lathe bed ways. It can be clamped at any position on the bed. The figure 4.47 shows a steady rest. The upper portion of the rest is hinged at one end. This helps the removal of the job without disturbing the steady rest. There are three jaws in the steady rest. Two are in the lower portion. One is in the upper portion. The jaws can be adjusted redially. They act as bearing surfaces to the workpiece. After setting the jaws over the workpiece, the rest is clamped to the lathe bed. For longer workpieces two or more steady rests can be used. 2. Follower rest This is shown in figure 4.47. It has a C type casting. It has two adjustable jaws to support the workpiece. The rest is bolted to the back end of the carriage. The rest moves with the carriage. The jaws always follow the tool. Thus it gives continuous support to the workpiece. Cutting speed. Cutting speed is the peripheral speed of the work past the cutting tool. It is expressed in metres/minute. It is the speed at which metal is removed by the tool from the work. Cutting speed = (iidN)/1000 Metre/minute Feed is the distance the tool advances for each revolution of the workpiece. It is expressed in mm/revolution. Depth of Cut It is the perpendicular distance measured from the machined surface to the uncut surface of work. It is expressed in mm Depth of cut

(d1-d2)/2

d1 = dia of work before machining d2 = dia of work after machining

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Lathe attachments Milling attachments Milling operation can be done on the lathe using a milling attachment. It is done by any of the two methods.

(i)

(ii)

The milling cutter is held in a chuck and rotated. The work is supported on the cross slide by a special attachment. The work is fed against the rotating milling cutter. The depth of cut is given by vertical adjustment of work given by the attachment. This method is used for cutting keyways or grooves. The work is supported between centres and does not rotate. The milling attachment is mounted on the carriage. The cutter is driven by a separate motor. The feeding is given by the carriage. The cutter can be moved vertically in the attachment.

Grinding attachment

A special grinding attachment is mounted on the cross-slide of the lathe. Both the grinding wheel and job are rotated during grinding. For grinding external surface, the workpiece is revolved between centres. For internal grinding, the workpiece is revolved in a chuck or plate, longitudinal feed is given by the carriage. Depth of cut is given by cross slide.

Machining calculations on lathe. Screw cutting calculations: Modern lathes are fitted with change gear boxes. Using these gear boxes we can select the required r.p.m. for the lead screw for thread cutting. But it ordinary centre lathes these gear boxes are not available. So it is necessary to calculate and arrange the change gears to get the required r.p.m. for the lead screw. Change gear calculations, for thread cutting has been already explained in this chapter. Anyhow some more practical examples are given below:

Taper turning calculations: METAL CUTTING In engineering industry, components are made of metals in different shapes, sizes and dimensions. Metals are shaped to the required forms by various processes. These processes can be generally divided into two groups. They are: 1. Non-cutting shaping process.

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2. Cutting shaping process.

In non-cutting shaping, the metal is shaped under the action of heat, pressure or both. Here there is no chip formation. This group includes operations like forging, drawing, spinning, rolling, extruding etc. In cutting shaping, the required shape of metal is obtained by removing the unwanted material from the workpiece in the form of chips. A few important processes are turning, boring, milling, drilling, shaping, broaching etc. These operations are known as machining or metal cutting operations. The metal cutting is done by a relative motion between the workpiece and the hard edge of the cutting tool. The relative motion between the workpiece and the cutting tool may be obtained by: 1. 2. 3. 4.

Rotation of the work against the tool (Turning) Rotation of the tool against the work (Milling & Drilling) Linear movement of the work against the tool (Planning) Linear movement of the tool against the work (Shaping).

Metal cutting can be done either by single point cutting tools or by multi point cutting tools. Cutting tool materials. Various materials are used for making cutting tools. The selection of cutting tool material will depend upon the following factors:

1. The volume of production.

2. The type of machining process.

3. The tool design.

4. Physical properties of work material.

5. General condition of the machine.

Properties of cutting tool material. A satisfactory tool material must possess the following properties. 1. Hot hardness It is the ability of the cutting tool to withstand high temperature without loosing its cutting edge. The tool must remain harder than the work material at high temperature. Addition of chromium, molybdenum, tungsten or vanadium to the tool material will increase not hardness.

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2. Wear resistance. It is the ability to resist wear. Constant abrasion of the workpiece causes wear in the tool. If the tool is not sufficiently were resistant, cutting edge will fail quickly. This will lead to poor surface finish. Addition of cobalt increases the wear resistance of the tool. 3. Toughness. Toughness is the combined property of strength and ductility. The material must have sufficient toughness to withstand shock and vibrations. This property limits the hardness of the tool. High hardness tool will be brittle and weak in tension. Addition of molybdenum and nickel increases toughness. This property is important when interrupted cutting is done. 4. Low friction. The co-efficient of friction between the tool and the workpiece must be low. This will reduce friction, heat and tool wear. 5. Low cost. The material should be available at low cost. It should be easy to manufacture the tool from the material. The Different cutting tool materials are: 1. Carbon steel. It has 0.8 to 1.15% carbon. It has low heat resistance and low wear resistance. Hardness is lost at 250 C. It is cheap, easy to forge and simple to harden. It is used for making cutting tools operated at low cutting speeds. That is, for hand tools like chisels, files, hacksaw etc.

2. Medium alloy steels. These are carbon steels alloyed with small quantities of tungsten, molybdenum, chromium and vanadium. It has carbon upto 1.5% and the different alloying elements upto 5 %. Alloying improves hot hardness, strength, wear resistance, shock resistance and corrosion resistance. Hardness is lost at 350 C. It is used for making taps, reamers, dies, punches, knives etc.

3. High speed steel (H.S.S) This tool steel cuts the metal effectively even at high speeds. It has superior hot hardness and high wear resistance. The cutting speeds can be 2 to 3 times higher than carbon steels. This tool steel maintains its hardness even upto 900 C. Carbon content is upto 0.8%. the main alloying elements in highspeed steel are tungsten, molybdenum, cobalt, chromium and vanadium. These high speed steels are used to make drills, turning tools, broaches, taps, dies and milling cutters.

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There are three types of high speed steels:

1. 18-4-1 High speed steel. 2. Molybdenum High speed steel. 3. Cobalt High speed steel.

1. 18-4-1 High Speed Steel. It has 18% Tungsten, 4% Chromium and 1% Vanadium. It has about 0.75% Carbon. It is an all purpose tool steel. Most of the cutting tools are made of this steel. Lathe, planer and shaper tools, drill bits, milling cutters etc. are made from this steel. 2. Molybdenum high speed steel. This steel has 6% Molybdenum 5% Tungsten, 4% Chromium and 2% Vanadium. It has high toughness and cutting ability.

3.Cobalt high speed steel. It has 12% Cobalt, 20% Tungsten, 4%Chromium and 2% Vanadium. It is also known as super high speed steel. This steel is used for heavy duty rough cutting tool like planer tools, lathe tools and milling cutters.

4. Stellities. It is a non-ferrous cast alloy. It has cobalt, chromium and tungsten. (Cobalt 45%, Chromium 35%, Tungsten 15%, Carbon 2%) Hardness is retained upto 1000 C Cutting speed is 2 times higher than high speed steel. Stellite is brittle and can not be forged to shape. Small tips of satellite are brazed to tool shank. Stellites are used for cutting rubber and plastics. Here the cutting loads are gradually applied and the support to the cutting edge is very strong.

5. Carbides. Cemented carbides mainly have carbon mixed with other elements. Tungsten powder 94% is mixed under high heat (1500 C) with pure carbon, 6% by weight. This new compound is tungsten carbide. Tungsten carbide is then mixed with cobalt and pressed at very high pressure in blocks. The blocks are cut and ground to shape and then sintered by high temperature heating. One composition is 82% tungsten carbide, 10% titanium carbide and 8% cobalt. Carbide tips are brazed to steel tool holder. The tool can withstand higher temperatures upto 1000 C. Its cutting speed is 6 times higher than the of high speed steel. But it is brittle and has low resistance to shock. It must be supported very strongly to prevent cracking. Carbides are used for machining very hard steels and for machining brittle materials like cast iron and bronze.

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6. Ceramics. Ceramic tool material consists of aluminium oxide. Aluminium oxide powder is pressed in moulds at high pressures and sintered at 2200 C. Ceramic tools are made in tips and clamped on the metal shanks of tools. Ceramic tools have high hot hardness and high compressive strength. But they are brittle. They cannot be used for operations where there is vibration and heavy chip removal. They can withstand temperatures upto 1200 C. Cutting speed is 40 times of high speed steel. They are used for single point tools for machining cast iron and plastics. No coolant is needed; but the tool must be very strongly supported. 7. Diamond It is the hardest cutting material. Cutting speed is 50 times higher than high speed steel. It can resist temperatures upto1250 C. Diamond conducts heat quickly. It has low coefficient of friction. Diamonds tipped tools are used for machining very hard materials like abrasive wheels, glass, plastic and ceramics. Maximum depth of cut is only 0.125 mm. Methods of metal cutting The two basic methods of metal cutting using a single point cutting tool are:

1) Orthogonal cutting

2) Oblique cutting

If the cutting face of the tool is at 90 to the direction of the tool travel the cutting action is called orthogonal cutting. If the cutting face of the tool is inclined at less than 90 to the path of the tool, the cutting action is called Oblique cutting (Refer figure 4.50).

The differences between orthogonal cutting and oblique cutting are given below:

S.No.

Orthogonal Cutting

Oblique Cutting

Cutting edge of tool is perpendicular to the direction of tool travel.

The cutting edge is inclined as an angle less than 90degree to the direction of tool travel.

The direction of chip flow is perpendicular to the cutting edge.

The chip flows on the tool face making an angle.

The chip coils in a tight flat spiral.

The chip flows side ways in a long curl.

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For same feed and depth of cut, the force which shears the metal acts on a smaller area. So the tool life is less.

The cutting force acts on larger area and so tool life is more.

Produces sharp corners.

Produces a chamfer at the end of cut.

Smaller length of cutting edge is in contact with the work.

For the same depth of cut greater length of cutting edge is in contact with the work.

Generally parting off in lathe, broaching and slotting operations are done in this method.

This method of cutting is used in almost all machining operations.

Cutting tool nomenclature.

Naming the various parts and angles of a cutting tool is known as nomenclature of cutting tool.

The important parts of a single point cutting tool are:

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1. Shank - It is the body of the tool which is ungrounded.

2. Face

- It is the surface over which the chip slides.

3. Flank - It is the surface of the tool facing the workpiece. There are two flanks namely and flank end side flank.

4. Base

It is the bottom surface of the shank.

5. Cutting edge-It is the junction of the face and the flanks. There are two cutting edges namely side cutting edge and end cutting edge.

6. Nose

- It is the junction of side and end cutting edges. The Important angles of a single point cutting tool are:

1. Top rake angle It is also called back rake angle. It is the slope given to the face or surface of the tool. This slope is given from the nose along the length of the tool.

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2. Side rake angle. It is the slope given to the face or top of the tool. This slope is given from the nose along the width of the tool (side ways). The rake angles help easy flow of chip.

3. Clearance angle or relief angle. These are the slopes ground downwards from the cutting edges. These are two clearance angles namely, side clearance angle and end clearance angle. This is given to the tool to prevent rubbing of the job on the tool. 4. Cutting edge angles. There are two cutting edge angles namely side cutting edge angle and end cutting edge angle. Side cutting edge angle is the angle, the side cutting edge makes with the axis of the tool. End cutting edge angle is the angle, the end cutting edge makes with the width of the tool.

5. Lip angle. It is also called cutting angle. It is the angle between the face and end surface of the tool.

6. Nose angle. It is the angle between the side cutting edge and end cutting edge.

Chip formation The chip formation is shown in figure 4.53. When the tool advances into the workpiece, the metal in front of the tool is severely stressed. The cutting tool produces internal shearing action in the metal. The metal below the cutting edge yields and flows plastically in the form of chip. Compression of the metal under the tool takes place. When the ultimate stress of the metal is exceeded, separation of metal takes place. The plastic flow takes place in a localized area called shear plane. The chip moves upward on the face of the tool. The process is continued and a continuous chip formation takes place. The grains of metal in front of the tool cutting edge start elongating along line AB.

This elongation continues until the grains are completely deformed along the line CD. The region between the lines AB and CD is called shear zone. After passing the shear zone, the deformed metal slides along the tool face due to the velocity of cutting tool.

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1. Types of chip

There are three types of chips. 1. Continuous chip 2. Discontinuous or segmental chips 3. Chips with built-up edge.

It is in the form of a long continuous ribbon of metal. This long curl of chip will be produced continuously from the work. This chip is considered a good one because, there is low friction at the tool-chip interface. It gives good surface finish and long tool life. Conditions for the formation of continuous chip are: 1. 2. 3. 4. 5.

Machining ductile metal like mild steel; copper and aluminium. High cutting speed and line feed. Large rake angle. Sharp cutting edge. Efficient lubrication.

2. Discontinuous chips

These will be produced as small pieces of separate segmental chips. Chips will not come in continuous form. Discontinuous chips are produced in machining brittle metals like cast iron and bronze. Cast iron and bronze will rupture during plastic deformation and form separate small pieces of chip. When these chips are produced, the cutting edge smoothes over their regularities. A good finish is obtained. Tool life is good and power consumption is low. But, if discontinuous chips are produced on ductile metals, tool wear will take place. Tool life will be less and surface finish will be bad. The conditions for producing discontinuous chips are brittle metals, excessive depth of cut, low cutting speed and small rake angle. 3. Chip with built-up edge It has a small built up edge sticking to the nose of the tool. The built up edge occurs with continuous chips. It occurs due to high temperature and high pressure at the chip tool interface. A small particle of metal from the workpiece welds on to the tool face. When this weld builds upl it affects the cutting action of the tool. Then it breaks off and portions of chips stick to the chip and work. The work surface becomes rough. Sometimes the breaking off takes away a portion of tool and produces a crater in the face of the tool. Built up edge is caused below cutting speed, low rake angle high speed and high friction between chip and tool face. It can be avoided by taking light cuts at high speeds and by using a coolant.

Chip breakers During machining, long and continuous chip will affect machining. It will spoil tool, work and machine. It will be difficult to remove metal and also dangerous to safety. The chip should be broken into small pieces for easy removal, safety and to prevent damage to machine and work. This is very important in automatic machines and machines which run at high speeds. Chip breakers are used to break the long continuous chip into small pieces. The chip breaker is provided on the cutting tool. Different types of chip breakers

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used on a cutting tool are (1) Step type (2) Groove type and (3) Clamp type. In the step type, a step is ground in the tool face behind the cutting edge. The step will break the chip. In the groove type, a groove on the tool face behind the cutting edge will break the chip. In the clamp type, a thin chip breaker is clamped or screwed on the face of the tool to break the chip.

Tool life and tool failure Tool life is an important factor in cutting tool performance. A tool can not cut effectively for an unlimited period of time. It has a definite life.

Tool life is the time a tool will operate satisfactorily until it becomes blunt. It is the time between two successive grinds. Cutting tool may gradually fail due to the following.

1. Face wear

2. Flank wear

3. Nose wear

4. Loss of hardness

5. Spalling or Crumbling or chipping.

1. Face wear The face (top) of the tool is always in contact with the chip. The chip slides over the faces of the tool. Due to the pressure of the sliding chips the tool face wears our gradually. A cavity is formed on the tool face. The cavity is called crater. This wear is known as face wear or crater wear. This type of wear takes place when cutting ductile material. This wear weakens the tool. Cutting temperature is increased. Friction and cutting forces also increase. When the crater becomes larger, the tool will totally fail.

2. Flank wear. This is also called edge wear. Flank wear is a flat portion worn out behind the cutting edge. The worn out region of the flank is known as wear land. This wear occurs because of friction and abrasion between the tool and workpiece. This wear takes place when machining brittle material like cast iron. This also occurs when the feed is less than 0.15 mm/revolution.

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Flank wear reduces the clearance in the tool. A tool with flank wear will give poor surface finish on the workpiece.

3. Nose wear. Nose wear takes place at the nose of the tool. When the nose of the tool is rough, abrasion and friction between the tool and workpiece will be high. Due to this, too much heat is generated. Also more cutting force will act on the tool. As a result, the nose of the tool will wear out quickly. This is called nose wear.

4. Loss of hardness. In metal cutting, due to very high heat generated, the tool looses its hardness. It becomes soft. The tool can not cut further. Different tool materials will loose their hardness at different temperatures.

5. Spalling or Crumbling or Chipping. The cutting edge may crumble due to improper relief angle excess clearance and insufficient support to the tool. This may also happen if the workpiece is hard.

Factors influencing tool life. The life of a cutting tool is affected by the following factors.

1. 2. 3. 4. 5. 6. 7.

Cutting speed Feed and depth of cut Tool geometry Tool material Cutting fluid Work materials Rigidity of work, tool and machine.

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UNIT-IV METROLOGY PRESS WORKING & NON-CONVENTIONAL MACHINING PROCESSES

METROLOGY

Metrology is the science of measurements. Measurements are necessary to inspect the quality of a product. The quality of a product is specified by its shape, size, surface finish etc. These quality characteristics are measured using various types of measuring instruments.

Depending upon the accuracy required in measurements, various measuring instruments are selected. Some instruments measure the exact dimensions of the component. Vernier caliper, micrometer etc. belong to this category. Some devices are used to check whether the size of the component is within the specified limits. They are called limit gauges. Some instruments compare the size of the workpiece with the standard and show deviations. These instruments are known as comparators.

Measuring Instruments.

1. Vernier Caliper

Vernier caliper is used for measuring both inside and outside diameter of shaft, thickness of workpieces etc. The accuracy of the vernier caliper is 0.02 mm. Figure 5.01 shows a vernier caliper. The caliper has a main beam called rule. It is graduated in millimeters. It has a fixed jaw at its end. There is a vernier head with a sliding jaw. It has a vernier scale marked on it. The sliding jaw slides over the rule and can be locked in any position. The vernier head is fitted with a fine adjustment. The inside face of the fixed and sliding jaws are perfectly parallel. These faces are used for measuring outside

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diameters and thicknesses. The outside faces of the jaws have curved surfaces. They are used while measuring inside diameters and internal dimensions. For measuring the outside diameter, the workpiece is placed between the inner faces of jaws. The vernier head is locked and reading is noted.

For measuring the inside diameter, the jaws are introduced in the bore of the workpiece. The outer curved faces of the jaws touch the inner bore of the workpiece. The reading is noted. The thickness of the jaws (specified by the manufacturer) is added to this reading to get the diameter of bore.

The enlarged view of the main scale and vernier scale is shown in figure 5.02. In the main scale, 1 cm is divided into 20 equal divisions. So each division is equal to 0.5 mm. The vernier scale has got 25 divisions. The length of 25 divisions in the vernier is equal to the length of 24 divisions or 12 mm on the main scale.

So each vernier division = 12/25 = 0.48 mm

Least count = Length of one main scale division â&#x20AC;&#x201C; length of one vernier scale division

= 0.50 mm - 0.48 mm

0.02 mm

To read the vernier, note down the main scale reading against the zero line of the vernier scale (in cm, mm and 0.5 mm). Note down the vernier scale division which exactly coincides with the main scale division.

Then the reading = Main scale reading + 0.02 x Vernier scale reading

Two examples of vernier readings are shown in figure 5.03. In the first example, zero of the vernier has th passed the main scale division of 32 mm (30 + 2). 16 vernier scale division coincides with a main scale division.

So the reading = 32 + (16 x 0.02) = 32.32mm

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Similarly for the second example, the reading=18.5+(5x0.02) = 18.60 mm 2. Vernier height gauge Vernier height gauge is used to measure the height of the workpiece. It is also used for precision marking on the workpiece. The accuracy of the vernier height gauge is 0.02 mm. The height of the workpiece is measured after placing the workpiece and the height gauge on a surface plate. The height gauge has a solid base. A vertical beam is mounted over the base. The vertical beam has main scale graduations. A slider slides up and down along the beam. The slider has a vernier scale. The slider can be locked using a clamping screw. The slider can be adjusted accurately using a fine adjustment screw. The slider has the measuring jaw integral with it. A scriber is clamped to the measuring jaw.

The workpiece is placed on the surface plate. The slider is adjusted so that the lower face of the scriber touches the top surface of the job. The readings are noted in the vernier and main scales. The main scale is graduated in 0.5 mm. The least count of the vernier is 0.02 mm. Therefore the reading = Main scale reading + 0.02 x vernier scale reading. With this reading, the height of the base of the vernier height gauge is added. This is specified in the height gauge itself. It may be 5 cm or 9 cm depending upon the size of the height gauge. To know the height of workpiece, this 5 cm or 9 cm has to be added to the reading.

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3. Gear tooth vernier

Gear tooth Vernier is used to measure the chordal thickness of a gear tooth. This vernier has one horizontal and one vertical slide. Each slide can be moved independently using screws. For measuring the chordal thickness, first the vertical slide is set to the height ‘h’ is the vertical height measured from the chord at pitch point to the crest of the tooth. This height ’h’ varies according to the module of the tooth and the number of teeth on the gear. The distance ‘h’ is obtained from standard tables. It may be also calculated as follows:

h = m[1+z(1-cos90/z)/2] where m= module of the gear z= number of teeth After setting the height ‘h’ in the vertical slide, the slide is locked in position. Then the slide is made to rest over the crest (top) of the gear tooth. The horizontal slide is adjusted to touch the side of the gear tooth. Now the chordal thickness is measured from the horizontal vernier scale.

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4. Micrometer

The micrometer shown in figure 5.06 is used to measure the external dimensions like diameter of shafts, thickness of workpieces etc. So this is also called External Micrometer. The micrometer has a C shaped frame. A hardened anvil is fitted at the left end of the frame. A barrel is fitted to the right end of the frame. The main scale is graduated on the barrel. It has a horizontal datum line. The main scale is graduated in 0.5 mm. The barrel has threaded bore. The pitch of the thread is 0.5 mm. A screw of 0.5 mm pitch passes through this bore. Measuring spindle is connected to the left end of the screw. The thimble is connected to the right end of the screw. The thimble is a tubular cover and moves with the screw and spindle. The thimble has a beveled edge. The vernier scale is graduated on this beveled edge. The vernier scale has 50 equal division. There is a small extension at the right end of the thimble. This is the ratchet. The ratchet slips when the pressure on the screw exceeds a certain limit. There is spindle clamp fitted at the right side of the frame. This is used for locking the spindle. The workpiece is introduced between the anvil and the spindle. The ratchet is screwed until the spindle face touches the work surface. At this point the ratchet will slip. Reading The main scale is graduated in 0.5 mm divisions. One rotation of thimble moved the spindle by 0.5 mm (as the pitch of the screw is 0.5 mm). As the thimble is divided into 50 equal divisions, each division is equal to 1 x 0.5/50 mm = 0.01mm

The reading = Main scale reading + (vernier scale division coinciding With the datum line x 0.01 mm)

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Referring to figure 5.06,

The reading = 11.0 + (30x0.01)

= 11.30 mm

Referring to figure 5.07,

The reading

= 11.0 + (34x0.01)

= 11.34 mm

5. Depth micrometer Depth micrometer is used to measure the depth of blind holes, slots and grooves. The accuracy of the micrometer is 0.01 mm. The micrometers can be used only in places where proper seating available for the head of the micrometer. The bottom of the head is placed over the seating of the workpiece. It has a sleeve. The main scale is graduated on the sleeve. It is graduated in 0.5 mm divisions. The bore of the sleeve is threaded with 0.5 mm pitch. A screw passes through the bore. The spindle is attached to the bottom end of the screw. The thimble is attached to the top of the screw. The thimble has a beveled edge. The vernier scale is graduated on the beveled edge. The vernier scale has 50 equal divisions. One vernier scale division = 0.5/50 = 0.01 mm. The thimble has ratchet. The main scale divisions are graduated in decreasing order from the bottom of the sleeve. Depth micrometers are available with various measuring ranges.

6.Inside micrometer Inside micrometer is used for measuring the internal dimension. Working principle is similar to external micrometer. The minimum dimension that can be measured in an internal micrometer is 50 mm. The use of the internal micrometer is illustrated in figure 5.09. The micrometer has an anvil at one end. The spindle at the other end moves when the thimble is rotated. The micrometer is held in position with the help of the handle. The spindle can be moved to the maximum of 13 mm. As such the micrometer can measure from 50 mm to 63 mm. For measuring larger diameters, the anvil is removed and suitable extension rod is fitted in its place. The accuracy of the internal micrometer is 0.01 mm.

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7. Thread micrometer

Thread micrometer is used to measure the pitch diameter of the thread accurately. The construction is similar to ordinary external micrometer. The spindle has conical end. The anvil has a V shaped end. Conical point and the V shaped anvil have the same form of the thread to be measured. For different threads, different sets of conical points and V anvils are used. These are inserted into the holes in the micrometer while measuring. While measuring , the conical point and the V anvil contact the flanks of the thread. The pitch diameter is directly read from the micrometer. The graduations of the main scale on the sleeve and the vernier scale on the thimble are similar to that of external micrometer. 8. Three wire method For accurately measuring the pitch diameter of a screw thread, three wire method is used. In this method, an ordinary micrometer and three wires are used. These wires are of equal diameters. They are made of hardened steel and are accurately finished by lapping. The size of the wire is selected according to the size of the thread to be measured. A set of two wires is fitted in one holder. A single wire is fitted in another holder. The holder are freely connected to the micrometer as shown in the figures 5.11. The method of measurement is shown in the figure 5.12. When measuring, all the three wires contact with the flanks of the thread. The distance over wires (W) is read from the micrometer. Using the following formula, the pitch diameter is calculated.

Pitch diameter

Where, W

W-d{1+sin /2}

is the distances over the wire read from the micrometer

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is the diameter of the wires

is the included angle of the thread.

9. Bevel protractor

It is a device used for measuring and testing angles. By this measurement angles can be measured with an accuracy of 5 mts. The instrument has a stock beam. The stock is integral with a circular disc is pivoted as its centre on a circular dial. Main scale is graduated in this circular dial. It is graduated in degrees along an arc of 180 degree (0 to 90o from each end). A vernier scale is fitted to the disc. There is a movable blade. It can slide length wise in a groove on the dial. The movable blade can be locked to the dial using a clamp. A locking screw is available for locking the circular disc on the dial. The stock can be adjusted to any angular position with reference to the movable blade. Measuring the angle of a lathe centre is illustrated in figure 5.13. The centre is placed between the movable blade and stock. The stock is locked in this position.

Reading The main scale is graduated in degrees. The length of 23 division in the main scale is divided into 12 divisions in the vernier scale. One vernier scale division = 23/12 = (1 11/12)

Let us make the zero marking of the vernier scale coincide with the zero marking of the main scale. Now the difference between the first graduation line of the vernier and the nearest graduation of the main scale is 1/12 or 5 minutes. So the least count is 5 mts.

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Gauges 1. Feeler gauges Feeler gauges are used to measure to the clearance between two surfaces. For example feeler gauges are used for checking the clearance between piston and cylinder, shaft and journal, guide and guide ways etc. Feeler gauges are narrow strips of steel sheets. These are made in various thickness in steps of 0.001, 0.05 or 0.1 mm. By combining two or more blades the required thickness of the strip can be obtained. These strips are flexible and can be inserted in the clearance easily. The feeler gauge should not be forced in the clearance. They should not also slide freely. A set of blades are assembled in a protective cover as shown in the figure 5.15. 3. Limit gauges In mass production, a large number of identical components are manufactured. Components are manufactured as per the specified tolerance limits, (viz) upper limit and lower limit. The dimension of the components should be within these upper and lower limits. If the dimensions of components are outside these limits, the components are rejected. If we use micrometer, vernier or other measuring instruments to check the dimensions, it will be time consuming. Moreover, it is not necessary to know the amount the error in the dimension. It is enough if we know whether the size of the component is within the limits or not. For this purpose we can make use of gauges. These gauges are known as limit gauges. The common types of limit gauges are: 1. Plug gauges 2. Ring gauges 3. Snap gauges

1. Plug gauges Plug gauges are used for checking holes. A double ended plug gauge is shown in figure 5.16 (A). It has two cylindrical ends. The ends are hardened and accurately finished by grinding. One end is the GO end. The other is NO GO end. The diameter of the GO end will be equal to the lower limit size of the hole. The diameter of the NO GO end will be equal to the upper limit size of the hole.

If the side of the hole is within the limits, the GO end should go inside the hole and NOGO end should not go.

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If the GO end does not go, the hole is under size and hence the component is rejected. If the NO GO end goes into the hole, the hole is over size and hence the component is rejected. Progressive type of plug gauge is shown in figure 5.16 (B). In this type both the GO end and NO GO end are arranged in the same side by the plug. So GO end and NO GO and can be used progressively one after another while checking the hole. This saves time. In plug gauges, the GO end is made longer than the NO GO end.

2. Taper plug gauge

Taper plug gauges are used to check tapered holes. This plug has two check lines. One is a GO line and another NO GO line. These GO and NO GO lines are at a certain distances from the end face of the gauge. The GO portion of the gauge corresponds to the lower limit size of the taper hole. The NO GO portion of the gauge corresponds to the upper limit size of the taper hole. When checking the correct taper hole the GO check line should go inside the hole and the NO GO check line should remain outside the hole.

3. Ring gauges

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Ring gauges are used for checking the diameter of shafts. The ring gauge is in the form of a ring having a central hole. The hole is accurately finished by grinding and lapping after hardening. The periphery of the ring is knurled. Two ring gauges, GO ring gauge and NO GO ring gauge are used to check the diameter of shaft. The hole of the GO ring gauge is made to the lower limit size of the shaft. The hole of the NO GO ring gauge is made to the upper limit size of the shaft. While checking a shaft of a correct size, the GO ring gauge will pass through the shaft. Figure 5.18 shows a GO ring gauge. A NO GO ring gauge will have a groove on its periphery or a red mark for its identification.

4. Snap gauge Snap gauges are used for checking external dimensions. They are also called gap gauges. Diameters of shafts are mainly checked using snap gauges. The different types of snap gauges are: 1. Double ended snap gauge. 2. Progressive snap gauge. 3. Adjustable snap gauge.

a) Double ended snap gauge This gauge has two ends. One end has GO anvils. The other end has NO GO anvils. The gap between

the GO anvils is equal to the lower limit size of the shaft diameter. The gap between the NO GO anvils is equal to the upper limit size of the shaft diameter. This gauge is also called solid snap gauge. A double ended snap gauge is shown in figure 5.19

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b) Progressive snap gauge

This gauge is also called Caliper Gauge. It is used for checking large. Diameters of over 100 mm. The gauge has both GO and NO GO anvils at the same end. The GO end will be at the front and NO GO end will be at the rear. These ends are used progressively for checking the diameters. This gauge is made of horse shoe shaped frame with I section. This gauge is shown in figure 5.20

c) Adjustable snap gauge

Adjustable snap gauges are used for checking large size shafts. The gauge is made of horse shoe shaped frame with I section. It has one long fixed anvil and two small adjustable anvils. The adjustable anvils are fitted to the frame by means of screws. The anvils can be adjusted for specified limits of sizes and tightened. The gaps between the fixed anvil and adjustable anvil are set with the help of slip gauges.

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3. Slip gauges

Slip gauges are measuring blocks. Slip gauges are also called precision gauge blocks. They are made of hardened alloy steel of rectangular cross section.

Their surfaces are finished to a high degree of accuracy. The distance between the two opposite faces gives the size of the gauge. Slip gauges are used for accurate measurement of distances. They are used in comparators and sine bars. For measuring or testing, slip gauges of different sizes can be combined to get the required distance. Slip gauges are combined by sliding or wringing. Wringing means, sliding the measuring face of one gauge with the measuring face of the other gauge. The two gauges will stick together by adhesion. This is because of the very high degree of surface finish of the measuring faces. Different sets of slip gauges are available. A normal set of slip gauges has 55 pieces as shown in figure 5.23. The slip gauges are stored carefully in a box.

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Sine Bar

Sine bar is a simple device used for accurately measuring or checking angles of tapered surfaces. The angle can be measured with an accuracy of 2 seconds. The most commonly used sine bar is shown in figure 5.24. It is made of steel of rectangular cross section. It is accurately finished in all surfaces by lapping. It has steps at the ends. Rollers are fitted to these ends. The rollers are of equal diameter and finely finished. The distance between the centres of rollers is the size of the sine bar. The size varies from 100 mm to 300 mm. The centre line of roller centres are parallel with the top surface and bottom surface of the sine bar. Use of sine bar An arrangement for measuring the angle of the tapered surface of a workpiece is shown in figure 5.25. The workpiece is placed over a surface plate. The bottom surface of the sine bar is placed over the tapered surface of the workpiece. One roller of the sine bar rests on the surface plate. The slip gauges are selected such that there is no gap between the sine bar and the work surface. The angel of inclination 0 is calculated as follows:

Sin0 = h/l

Where, h = is the height of the slip gauge blocks l = is the length of the sine bar.

Comparators Comparators are devices which are used to compare the size of workpiece with standard precision gauge blocks. Comparators normally will not show the actual dimension of the workpiece. They will show only the deviation in size.

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All comparators have magnifying devices. The magnifying device magnifies how much a dimension deviates from the standard size. According to the method of magnification, the comparators are classified as follows: 1. 2. 3. 4.

Mechanical comparators. Electrical comparators. Pneumatic comparators. Optical comparators.

1. Mechanical comparators

A dial gauge is used as a mechanical comparator. The comparator has a work table. The top surface of the work table is finely finished. A vertical post is fitted on the work table. The dial gauge is held in the vertical post. The dial gauge can be adjusted vertically and locked in position by a screw.

Let us assume that the required height of the component is 35.35 mm. First this height of 35.35 mm is built up with slip gauges as shown in figure 5.26. The slip gauge blocks are placed under the stem of the dial gauge. The pointer in the dial gauge is adjusted to zero. The slip gauges are removed. The component to be checked is introduced under the stem of the dial gauge. If there is any deviation in the height of the component, it will be indicated by the pointer. The dial of the gauge is graduated into 100 division. Each division represents 0.01 mm.

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Figure 5.27 shows the mechanism of a dial gauge. The stem has rack teeth. A set of gears engage with the rack. The pointer is connected to a small pinion. The small pinion is independently hinged. (It is not connected to the stem). The vertical movement of the stem is transmitted to the pointer through the set of gears. A spring (not shown in the figure) gives a constant downward pressure to the stem.

2. Electrical comparator

The comparator has a solid base. A vertical post is mounted on its one end. A casing is fitted to the vertical post. The casing can be adjusted vertically along the post and locked in position. Inside the casing a soft iron armature is hinged to one end. There are two electromagnets placed inside the casing. One magnet is placed above the armature. The other magnet is placed below the armature. A measuring plunger is fitted to the armature. The coils of the electro magnets are connected to a Wheatstone bridge circuit. First, a standard specimen of correct size or a set of slip gauge blocks is placed between the plunger and the table. The resistance of the Wheatstone bridge is adjusted so that the meter of the Wheatstone bridge indicates zero reading. The specimen is removed. The workpiece is introduced under the plunger. If there is any variation in the height of the workpiece, the plunger moves the armature, up or down. This changes the distance between the armature and the electro-magnets. This causes an unbalance in the Wheatstone bride circuit. The meter connected to the Wheatstone bridge shows the variations in dimension in microns (0.001 mm). The meter is calibrated in division of 0.001mm. The comparator is very accurate. It has high magnification.

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3. Pneumatic comparator

The pneumatic comparator has a water tank. A manometer is fitted to the water tank. A scale is fitted vertically by the side of manometer. A dip tube is immersed in the water tank. Restriction jet is provided at the top of the dip tube. The dip tube is connected to a flexible pipe through a control jet. The top of the manometer tube is connected to the flexible pipe. Compressed air enters the dip tube through the restriction jet. The restriction jet reduces the velocity of air. Then the air passes through the control jet to the flexible pipe. From the flexible pipe, the air goes to a measuring head and escapes to the atmosphere. An internal measuring head is used to check the size of holes in the workpiece. External measuring head is used to check the height of workpiece. When the air flows freely through the measuring head, the water levels in the manometer tube and in the water tank will be same. When there is some restriction to the flow of air in the measuring head, there will be a pressure difference between the flexible pipe and the dip tube. This pressure difference is indicated by the water level in the manometer (h).

1) Internal measurements An internal measuring head is connected to the flexible pipe. The internal Measuring head is cylindrical in shape. It has two openings through which air can escape. First the measuring head is introduced into the hole of a standard specimen (ring gauge). The reading in the manometer is marked. This is called zero setting. Then the measuring head is introduced in the workpiece. When the hole of the workpiece is smaller, the restriction for air flow will be more. So, the water level in the manometer will go down from the zero setting. When the hole of the work piece is larger, the restriction for air flow will be less. So, the water level in manometer will rise above the zero setting. The scale of the manometer is suitably calibrated in the units of 0.001 mm to indicate the difference in diameters.

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2) External measurements An external measuring head is connected to the flexible pipe. It is used for checking the thickness or height of the workpieces. Zero setting is done by keeping slip gauge blocks under the measuring head. The gap between the workpiece and the measuring head varies according to the size of the workpiece. These variations are indicated in the manometer. Pneumatic comparator is very accurate and fast. It has very high magnification. There is no wear and tear for measuring head. Deep bores can be checked easily using pneumatic comparators.

4. Optical comparator

An optical comparator is shown in the figure. This comparator has a lamp. The light rays from the lamp are condensed by a condensing lends. The condensed light rays fall on the projection lens. The light rays from the projection lens fall on the mirror. The mirror reflects the light rays on a screen. The workpiece is placed on the table between these condensing and projecting lenses. The profile of the workpiece is magnified by the projection lens. The mirror again magnifies this image. The magnified image falls on the screen. The screen is a semi transparent glass. A magnified master drawing of the workpiece profile is placed over the screen. The projected image of the workpiece profile is compared with the master drawing. This type of optical comparator is used for inspecting small parts like screw threads, gear teeth, saw teeth, cutting tools needles, cam profiles etc. Different magnifications can be obtained by adjusting the projection lens and the position of the mirror. Profile Measurements 1. Optical flats Optical flats are used for testing the flatness of surfaces of workpiece-gauge blocks, micrometer anvils etc. Optical flats are made from natural quartz.. Optical flats are circular in section. They are made in various diameters and thickness. The faces of the optical flat are very accurately polished to make it truly flat. The optical flat works under the principle of interference of light.

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The principle of interferometry is illustrated in figure 5.31. The optical flat is placed over the surface of the workpiece with a slight inclination of * A ray of light having same wave lengths (mano chromatic) falls on the optical flat at point A. It travels through the optical flat along the path A-B and reaches the work surface at C. The light is reflected by the work surface along the path C-D-E-F. The light is also reflected from the point B on the bottom surface of the optical flat. It travels along B-G-F. From the figure it can be seen that the ray B-C-D-E-F is longer than the ray B-G-F. The difference in length is B-C-D. If this distance B-C-D is equal to one or any whole number of wave lengths of the light (*, , , , â&#x20AC;Ś.where * is the wave length of light) the two reflected rays reinforce each other. A bright band of light is seen by the observer. The bright and dark bands are called interference pattern. The interference band pattern depend upon the gap between the work surface and the optical flat. Figure 5.32.shows different interference band patterns. 1. If the bands appear regularly spaced, the surface is flat. 2. If the bands are curved towards the contact edge, the surface is concave.

3. If the bands are curved away from the contact edge the surface is convex.

Use of optical flat: Figure 5.33 illustrates the use of an optical flat in checking the diameter of a workpiece. A correct size gauge block and the workpiece are placed over the work table with a distance L between them. The optical flat is placed over the workpiece and the gauge block. If the workpiece is not of correct size the optical flat will be slightly tilted. When a monochromatic light is passed over the optical flat, interference bands can be seen. The difference in height H between the workpiece and the gauge block is calculated as shown below:

H = (*/2)N(L/W) N = Number of dark bands W = Width of gauge block L = Distance between the contacts

Profilo Meter This is an instrument used to find out directly the surface roughness of the work surface. The profilometer has a tracer head. The tracer has a magnet. A stylus is fitted at the middle of the magnet. The stylus is held in position by a spring. The stylus has a coil wound on it. The coil is positioned

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in the magnetic flux. The coil is connected to a roughness meter through an amplifier. The tracer head is moved along the surface of the workpiece at a constant speed. The stylus rises and falls according to the roughness of the work surface. So the coil would in stylus moves the magnetic field. This generates a current. This current is proportional to the surface roughness of the workpiece. This current is amplified and fed into the roughness meter. The roughness meter directly indicates the roughness of workpiece surface. The roughness meter is calibrated din microns.

PRESS WORKING Press working is the process of pressing metals into the required shape by using a machine known as “press”. The metal can be pressed in cold condition or in hot condition, In cold condition the metal is stretched beyond its elastic limit. This cold working can be done only on ductile metals the machine used for pressing metal in cold condition is called ,”Cold Working Press” In hot working, metal is heated to recrystallisation temperature. At this temperature, metal is pressed to get the required shape. The press used for pressing metal in hot condition is called “Hot Working Press”. In this chapter we discuss only about cold working presses Types of Presses Presses maybe classified as i) ii)

Manually operated (fly) presses Power operated presses

i) Manually operated presses This is the fly press which we commonly use for producing small components in workshop. The arm of the press is rotated manually to move the ram up and down. ii) Power operated presses Power presses can be classified as, a) Mechanical Presses b) Hydraulic Presses c) Pneumatic Presses

Mechanical Presses Mechanical power presses are driven by electrical motors. Various types of mechanical driving mechanisms are used for transferring power from electric motor to the ram. In a press, the ram reciprocates vertically. For this movement of the ram, power drive is given. This type of drive depends upon the length of stroke needed and load on the ram.

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Different driving mechanisms used are: 1. 2. 3. 4. 5. 6. 7.

Crank and connecting rod drive Eccentric drive Knuckle joint drive Cam drive Toggle lever drive Screw drive Rack and pinion drive

Types of Drives In a press, the ram reciprocates vertically. For this movement of the ram, power drive is given. The type of drive depends upon the length of stoke needed and the load on the arm. The various types of drives are explained below. i) Crank and connecting rod drive This is the simplest and most common method of drive. Here the crank shaft is driven by a motor. The ram is connected to the crank by a connecting rod. So when the crank shaft rotates, the ram reciprocates. This type of drive is used to get long stroke lengths of ram. (Refer figure 5.35.). ii) Eccentric drive An eccentric is made integral with the driving shaft. One end of the connecting rod has a housing. This housing fits on the eccentric. The other end of the connecting rod is connected to the ram. The driving shaft rotates with the eccentric. So the ram reciprocates. (Refer figure 5.35). This type of drive is used for shorter stroke lengths of ram. iii) Knuckle joint drive The knuckle joint drive has two knuckle levers and one connecting lever. One end of the upper knuckle lever is pivoted to the crown of the machine. One end of the lower lever is connected to the ram. The other ends of the knuckle levers and one end of the connecting levers are joined by a pin. The other end of the connecting lever is connected to the crank of the crank shaft. When the crankshaft rotates, the ram reciprocates. At the bottom of the stroke, the knuckle levers become vertical. So the complete load acts on the crown through the knuckle levers. The crank shaft is not stressed much. So heavy loads can be applied in this press.(Ref. fig.5.35) This drive is used for coining and squeezing operations. iv) Cam drive In this press, the driving shaft has a cam. The upper end of the connecting rod has a housing. The housing fits over the cam. The other end of the connecting rod is connected to the ram. When the driving shaft rotates, the cam actuates the ram to reciprocate. The cam profile is so designed that the ram remains idle for some time at the bottom of the stroke. (Refer figure 5.36). The cam drive gives short stroke length for the ram. This drive is used for actuating the outer ram of a double acting press.

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v) Toggle lever drive In this drive, there are two toggle levers. These lever are pivoted at their centres to the machine frame. The bottom of the toggle levers are connected to the ram through small links. The upper part of the toggle lever and one end of the connecting lever are joined together by a pin. The other end of the connecting lever is fitted to the crank. The crank is rotated by the driving shaft. When the crank rotates, the ram is actuated to move up and down. At the end of the stroke, the ram will remain stationary for sometime. This toggle drive is used for actuating the outer slide of the double acting press. This drive gives a shorter stroke length for the ram. (Ref.fig.5.36) vi) Screw drive Here the ram is loosely connected to a screw. The screw passes through a stationary nut. The top end of the screw is connected to a fly wheel. The periphery of the fly wheel is in contact with a friction disc. When the friction disc rotates the screw also rotates. The screw moves down. So the ram moves down. It does not rotate. When the ram reaches the bottom of the stroke, the friction disc reverse and the ram moves up. This screw drive is used for longer stroke length. The movement of ram is uniform in this drive. vii) Rack and pinion drive In this drive, the ram is connected to a long rack. The rack is in mesh with a position. The pinion rotates about a fixed axis. When the pinion is rotated, the rack moves down the ram. At the end of the stroke, the pinion reverses. So, the ram moves up Rack and pinion drive is used when a long stroke length is required. Typical mechanical presses using the above types of drives are 1) OBI Press 2) Adjustable bed press.

1. OBI press OBI press means Open Back Inclinable press. The press has a frame pivoted to the base. The frame can be adjusted to any inclined position and clamped. There is a gap in the frame. This gap is meant for the removal of the parts. The parts will slide down through the gap due to gravity. The dies fitted to the bottom of the frame. The punch is connected to the bottom of the ram. Ram slides along the guide ways fitted to the frame. A crank shaft is fitted to the top of the frame. The ram is connected to the crank shaft by means of a connecting rod. The crank shaft is driven by a motor. The crank shaft carries a fly wheel at the end. The rotation of the crank shaft makes the slide to move up and down. The fly wheel stores energy and makes the ram to move at constant speed.

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2. Adjustable bed press

In this type of press the bed can be adjusted vertically. This is done by an elevating screw. The bed is lowered or raised to suit the different heights of the work and the dies. The press is not very rigid. The adjustable bed is known as knee, so this press is also called knee press. Hydraulic presses Hydraulic presses are driven by a hydraulic drive. In the hydraulic drive, the ram is connected to a piston through a piston rod. The piston moves up and down in a hydraulic cylinder. The oil at high pressure is pumped through the top side of the piston. The ram moves downwards. At the end of the stroke, the oil is pumped through the bottom side of the piston. The ram moves up. The hydraulic drive is used when a very heavy pressure is required on the ram. This is used for drawing and forming operations. Hydraulic drive gives a noiseless and smooth operation. The pressure on the ram can be easily changed. The pressure applied is uniform. The ram movement takes place at uniform speed. The following types of presses are operate by hydraulic drive. 1. Straight side press 2. Pillar type press

1. Straight side press The straight side press has two vertical frames. The frames are mounted on to the two sides of the base. The two vertical frames are connected by means of a crown at the top. The ram is fitted to the crown. The frames and the crown are rigid and absorb the heavy load. So this press is suitable for heavy work. The sheet metal cannot be fed from the sides. It can be fed only from the front. 2. Pillar type press Refer figure 5.42. This press has four pillars. The pillars are mounted vertically on four corners of the square base. The four pillars are connected by crown at the top. The pillars guide the movement of the ram. This type of press is always of hydraulic type. The sheet metal can be fed from all four sides. So this press is also known as open frame press. Advantages of hydraulics presses The main advantages of hydraulics power presses are: 1. Stroke length can be adjusted â&#x20AC;&#x201C; short strokes and long strokes can be applied.

2. Full force is applied throughout the length of the stroke.

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3. Force exerted by the press can be adjusted even to about 20% of its Maximum capacity â&#x20AC;&#x201C; this can be done by adjusting the relief valve setting. 4. Stroke position can be adjusted to the required position. 5 Speed of travel can be adjusted. 6 There is no shock loading â&#x20AC;&#x201C; hence die life increases. 7 This presses have lesser moving parts and hence they require lesser

maintenance.

Comparison of Mechanical and Hydraulic Presses Characteristics

Mechanical Presses

Hydraulic Presses

1. Force Applied

Not uniform through out the stroke length

Uniform and constant through the stroke length

2. Length of stroke

Limited

Longer strokes are possible

3. Slide Speed

Higher speed. Maximum at mid stroke.

Slow pressing speed. Rapid advance during return stroke

4. Capacity

6000 tons maximum

Up to 50,000 tons

5. Control

Stroke cannot be reversed in the middle

Slide movement can be reversed at any position

6. Application

Operations requiring maximum pressure near the bottom of stroke-Eg Blanking, Forming & Drawing

Operation requiring steady pressure. Eg. Straightening, filling of mould cavities.

Principle of operation of a press Every press has got certain basic units. They are : bed, frame, sliding ram, drive for the ram and power source. (Refer figure 5.38) Base or bed is the lower part of the press frame. A thick plate called bolster plate is placed on the top of the bed. A die is fitted on the top of the bolster plate. The driving mechanism is mounted to the frame. The frame has got guide ways for sliding movement of the ram.

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The driving mechanism is connected to the ram. The punch is fitted at the bottom of the ram. The die and punch are correctly aligned. The work piece is the form of sheet metal. It is fed over the die. When the ram comes down, the punch presses the sheet metal. The required operation is carried out.

For doing various operations in a press, different types of dies and punches are used.

Specification and capacity of press

1. Capacity: It is the Maximum force that the press ram is able to exert on the workpiece – capacity is also called tonnage. The capacity is expressed in tons.

2. Maximum : Stroke length of the ram.

3. Die Space : The area available for mounting the die on the bed to the Bottom of the ram when the ram is at the bottom most Position of the stroke.

4. Type of drive.

5. Type of frame.

6. Number of slides – single acting – double acting.

7. Weight and floor area.

Press Tools

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Die and punch are known as press tools. Die is the lower part of the press tool. It is clamped on the bolster plate of the press. It remains stationary during the operation. The die has a cavity to receive the punch. The cavity may be with a clearance or without clearance. Punch is the upper part of the press tool. It is attached to the lower end of the ram of the press. It slides with the ram during the operation and is forced into the die cavity. Die and Punch must be in perfect alignment for proper operation. Die and Punches are always used together. Dies are classified according to either the type of construction or the operation to be performed. High Speed Steel (HSS), satellite or cemented carbide are the materials used for making dies and punches. The die material selected, depends on the type of production, operation, sheet metal thickness and accuracy. Press Accessories Press Accessories (also known as the die accessories) are essential for production work to facilitate easy and quick operation. These accessories are essential for easy and fast removal of finished material and for the correct location of the blank in between the punch and die. The different press accessories with their specific functions are listed below.

S.No.

Accessories

Function

Stop

To stop the work material at the correct length

Pilot

For correct location of work piece

Stripper

For removal of work piece from the punch or die after blanking or punching operation.

Knock-out

For forcing out the cut out blank from the die opening.

Pressure pad

To hold the sheet in its position while drawing. This is to prevent wrinkling of work piece surface.

1. Stops Stops are used to stop the sheet metal at the correct length. This prevents the wastage of sheet and reduces scrap. The stops may be made in the form of button or lever or pin. These stops also help to speed up the operation. 2. Pilot Pilot is fitted to the lower end of the punch concentrically.

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The function of the pilot is to locate the blank accurately with respect to the previously pierced hole for succeeding blanking and other operations.

The correct spacing and position of the sheet is obtained without using stops. The pilot will have a sliding fit with the already pierced hole in the sheet. 3. Strippers. The main function of the stripper is to remove the work piece from the punch or die after a blanking or piercing operation. Strippers may be of fixed type or spring operated type. Fixed strippers are rigidly attached to the die block. The spring operated strippers travel up and down with the shank of the punch. When, the punch moves down, the springs compress the stripping plate after it comes into contact with the sheet metal. So the stripper plate holds the sheet. After the end of the operations, the punch goes up. The work piece is stripped off from the punch cutting edge by the stripper plate. This prevents the workpiece from being lifted along with the punch. Thus the strippers knock out the work or scrap and speed up the operation. 4. Knockouts The function of the knockout is similar to that of a stripper. It forces the cut blank out of the die. Knockout is used to kick out the flanged shell after drawing operation which can not pass through the die. Knockout may be spring loaded or actuated by an air cylinder. 5. Pressure pad Pressure pad is used for applying sufficient pressure on the metallic sheet and to hold it in its position. It prevents wrinkling during drawing process. The pressure may be applied by springs, pneumatic or hydraulic means. When the punch moves downward, the springs are compressed and hence holds the sheet with sufficient pressure. This results in ironing of the metal as it flows plastically between the punch an d die and eliminates wrinkling. To maintain the flat surface at the bottom, similar spring loaded or hydraulic loaded pad is used at the bottom of the cup. Press working operation Different Press working operations are described below. Bending operations Bending is a non cutting operation, performed on a press. It is a process of plastically deforming a metal sheet along a line. The surface area remains the same even after bending. Bending can be done using die and punch or rolls. Bending should be done perpendicular to the direction of the grains. It bending is done parallel to the grains, cracks may develop. Bending operation can be done only on ductile material.

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Bend parts will spring back slightly after bending. This factor should be considered while designing bending dies.

Different bending operations are:

1. Angle bending

2. Curling

3. Roll forming

4. Plate bending

5. Seaming.

1. Angle bending Angle bending is the operation of bending a sheet metal to a sharp acute angle. The punch and die are shaped to the required angle. The angle of the die and punch is selected taking into account the spring back of the sheet metal. 2. Curling Curling is the operation of rolling the edges of the sheet metal to a small circular form. Curling is done to strengthen the edges. By curling, the sharp edge of the sheet metal is avoided. The lower die is held stationary. The upper die or punch has got curved from at its end. When it moves down, the edge of the work piece is curled into a circle. Curling is used in making sheet metal vessels and containers. 4. Seaming Seaming is the process of inter locking sheet metal product such as drums, cans etc. First, the edges of the sheet metal are folded. Then they are inter locked and pressed with rollers as shown in the figure 5.50. The interlocked bents from the seam joint. Generally, two types of seam joints are used. They are lock seam and compound seam. The lock seam is used for longitudinal joints, where more tightness is not important. Compound seams are used in box sections. These are much stronger and tighter. Shearing Operations In shearing operation, the punch moves down and pressed the work piece In the opening of the die. There is a clearance between the die opening and punch. The space between the die opening and the punch is the clearance. The amount of clearance depends upon the type and thickness of material. By this pressure, the work piece metal is deformed plastically. The plastic

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deformation takes place in a small area between the cutting edges of punch and die. So the metal in this area is highly stressed. When the stress exceeds the ultimate strength of the material, fracture takes place. The cutting edge of the die starts the fracture in the metal from the bottom. The cutting edges of the punch start the fracture of the sheet metal from the top. These fractures meet at the centre of the plate. As the punch continues to move down, the metal under the punch is completely cut off from the sheet metal. The cut out portion to the die opening of the metal drops down through the die opening. To make the metal to drop down freely, a die relief is given. 1. Punching Punching is a piercing operation. It is the operation of producing a circular hole in a sheet metal using a punch and die. The material pushed out through the die is the scrap. The punch and die set-up for punching is shown in figure 5.52. The punching size will be exactly the same as the size of the hole to be pierced. The die opening is made slightly larger to get the clearance. The sheet metal is introduced between the punch and die through the stripped. The punch pierces the hole in the metal when it down. The pierced out metal (slug or scrap) drops down through the die opening. After piercing, the punch moves up. He sheet metal in the die may stick with the punch surface. The stripper strips off the sheet metal from the punch. 2. Blanking Blanking is the operation of cutting off a flat sheet of required shape. The metal blanked out through the die is the required product. The sheet metal left on the die is the scrap. The blank is further processed-bending or drawing is done on the blank. The size of the blank depends on the size of the die. So the size of the die opening is equal to the blank size. Clearance is given to the punch. 3. Cutting off Cutting off is the operation of cutting a piece from a sheet metal. The cut is made along a straight line or a curve. The lower blade is fixed to the machine frame. The upper blade is connected to the ram. It slides vertically. The work piece is placed between the two blades. When the upper blade moves down, it cuts off the sheet metal . A slight clearance is given between the cutting edges of the blades. The clearance depends upon the thickness of the work piece. In cutting off, there is no-scrap. 4. Parting Parting is the operation of cutting the sheet metal into two pieces. There are two lower blades fixed to the machine frame. A gap is provided between the cutting edges of the lower blades. This gap depends upon the thickness of sheet metal. The gap can be adjusted. The upper blade is connected to the ram. The work piece is placed between the upper and lower blades. The upper blade moves down and cuts the sheet metal. Some scrap is removed in this operation.

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5. Notching It is the operation of cutting small notches at the edge of the sheet metal. A notching die and punch are used. 6. Slitting It is the operation of cutting a sheet metal in straight line. The cut takes place along the length of the sheet metal. 7. Lancing Lancing is the operation of cutting a sheet metal through a small length and then bending the cut portion. 8. Trimming During drawing operations, the blank is held over the die by a holder. Impressions are formed on the area gripped by the pressure pad or holder. This unwanted metal surface should be cut off. Cutting off of the excess metal edge is called trimming. Dies and punches similar to blanking dies are used for trimming operation. 9. Shaving The edges of components produced in sheet metal operations May not be smooth. They may have burrs and irregularities. These edges are finished by a shearing operation. This operation is called shaving. The component to be finished is located over a die. A shearing punch is used to shear off the burrs. SPECIAL MACHINING PROCESSES In recent years many new materials have been developed. These include Titanium alloys, Hastalloys, Nimonic alloys, etc. These material are used in space crafts, nuclear reactors,, special cutting tools, turbines, injectors, etc. These new material cannot be accurately machined by the conventional machining processes like Tturning and milling. Moreover, producing complex shapes in these materials is very difficult and takes much time. Therefore, new methods of machining have been developed to machine these new materials. These new methods of machining are known as unconventional machining process. Classification Unconventional machining processes are classified according to the type of energy used for machining. The following unconventional machining processes are generally used: 1. Ultrasonic machining (USM)

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2. Chemical machining (CHM)

3. Electro chemical machining (ECM)

4. Electro chemical grinding (ECG)

5. Electrical discharge machining (EDM)

6. Plasma arc machining (PAM)

7. Laser beam machining (LBM)

Unconventional Machining

Mechanical Energy

Chemical Energy Energy

a) Ultrasonic

Chemical

machining (USM)

Machining

b) Abrasive jet machining (AJM)

(CHM)

c) Water jet machining (WJM)

Electro Chemical Energy

Thermo Electrical Energy

a)ElectroChemical machining(ECM) b) ElectroChemical Grinding (ECG)

a) Electrical discharge machining (EDM) B) Plasma arc machining (PAM)

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ULTRASONIC MACHINING

In ultrasonic machining, the tool vibrates at very high frequency i.e.20 KHz. Per second. Abrasive slurry is applied between the tool and work piece. By this, very minute particles of work piece are removed by erosion and abrasion. The tool is slowly fed on the work piece. The tool shape is reproduced in work piece. The ultrasonic equipment has a transducer, tool holder and the tool. The transducer has nickel laminations wound with a coil. A high frequency current is supplied to the coil of the transducer. The transducer converts this high frequency current into mechanical vibration. These mechanical vibrations are transmitted to the tool through the tool holder. The tool vibrates axially with amplitude of 0.05 mm and a frequency of about 20 KHz. Abrasive slurry is applied between the work surface and the tool. The slurry is made of a mixture of water and fine grains (800 to 1000 grit size) of aluminum oxide, silicon carbide or boron carbide. Because of the tool vibration, the abrasive grains in the slurry are hammered into the work surface. Hence by abrasion and erosion, the metal is removed from the work as minute particles. By this action, the shape of the tool is reproduced in the work. The tool is fed downwards very slowly. Maximum feed rate is 0.1 mm/sec. The tool is made 0.01 mm smaller than the required hole. The tool is made of soft ductile material such as soft steel, copper or brass. The tool holder is made of stainless steel. Dimensional accuracy up to 0.05 mm is possible. Applications: Ultrasonic machining process is applied:

1. for drilling holes in hard and brittle materials like ceramics, glass, boride, ferrite, carbides, carbides, precision stones like diamond and hardened steel.

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2. for making wire drawing dies in tungsten carbide or diamond.

3. for engraving, die sinking, slicing and broaching of hard materials.

4. for machining both conducting and non-conducting materials.

5. for machining precision stones and ceramics.

Advantages: 1. Very hard and brittle materials like carbide and tungstenare easily These materials cannot be machined by the ordinary process.

machined by this process.

2. Set up the machine is simple.

3. Less skilled operator is sufficient.

4. It is possible to make holes of any shape (circular and non circular) for which a tool can be made.

5. Set up time is less.

6. Production cost is less.

7. It can be used for machining both conducting and nonconducting material.

8. Accuracy is more.

Disadvantages:

1. Very slow metal removal; hence machining time is more.

2. High power consumption.

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Not suitable for heavy stock removal.

Chemical Machining In chemical machining, some chemicals are used to remove material from the required portions of the work piece. This process is also called chemical milling. The figure 5.61 shows the process of chemical milling. Chemical milling is done in the following steps.

1. Cleaning The work piece surface is thoroughly cleaned.

2. Masking The portions of the work piece which do not require machining is covered with masking sheets. The sheet is cut and removed from the area where machining is required. Templates are used for this purpose. If the entire area of the work piece is to be machined, masking is not necessary. Usually vinyl, neoprene and rubber based materials are used as mask sheets. 3. Etching After masking, the work piece is submerged in a hot chemical solution. This solution is called the etchant. Caustic soda is used as enchant for Aluminum. Acids are used for steel, magnesium and titanium alloys. The etchant removes the metal from work piece by chemical action. The rate of metal removal is about 0.025 mm per minute. The rate of metal removal depends upon the concentration and the temperature of the enchant. Higher the concentration and the temperature, more is the rate of metal removal. The amount of metal removal also depends upon the time duration for which the work piece is immersed in the etchant. 4. Demasking After etching, the work piece is taken out from the etchant. The work piece is cleaned in water. Then the marking is removed.

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Applications:

1. Chemical machining is effectively used for removing metal from a curved or irregular surface.

2. It is used for machining on very thin surface.

3. It is used to produce special profiles in aeroplane parts, automobile parts, electronic equipments and instruments.

4. Sheets having taper on its surface can be produced in this process.

5. Chemical machined parts are used in tape-recorders, computers, cameras, T.V. sets, electric motors, timers, telephones, medical instruments, etc.

Advantages: 1. Very low operating costs.

2. Low skill of the operator.

3. Parts of any profile can be machined.

4. The process does not produce any stress in the workpiece.

The process can be used for any metal.

6. Parts of large size as well as thin sections are machined in the process.

7. Uniform metal removal in all surface.

8. Metal removal is easily controlled.

9. All the slides of the workpiece are machined at the same time.

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Disadvantages:

1. Very slow process.

2. Heavy stock material removal is not possible.

3. Chemical vapours are injurious to health.

4. Larger floor space is required.

Electro Chemical Machining Electro chemical machining process is the reverse of electro plating process. The workpiece becomes anode (+ive) and the tool becomes cathode (-ive). Therefore the workpiece looses metal. Normally, the metal will flow through the electrolyte and get deposited on the cathode. Here the tool is the cathode. Before the metal gets deposited on the cathode, the dissolved metal is pumped away with the electrolyte.

The workpiece is held in a suitable fixture inside a tank. The workpiece is connected to the +ve terminal (anode) of a 20 V D.C. supply. The tool is held in position over the workpiece. The tool is hollow one. It is connected to the â&#x20AC;&#x201C;ve (cathode) terminal of the supply. The shape of the tool depends on the shape to be produced on the workpiece. A small gap of about 0.2 mm is maintained between the workpiece and the tool. The sides of the tool are insulated. So the sides of the tool will not machine the workpiece. This prevents taper in the hole machined. An electrolyte, usually sodium chloride, sodium nitrate or sodium chlorate is passed through the hollow tool. When the D.C. supply is given, the current flows through the circuit. Electrons are removed from the surface of the workpiece (anode). These ions will try to reach the cutting tool (cathode). But these ions are carried away by the fast flowing electrolyte. The tool is fed toward the workpiece automatically to maintain the gap between the workpiece and tool surface.

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The machining rate and surface finish are directly proportional to the current. The electrolyte is filtered and recruited using a pump. The temperature of the electrolyte is maintained between 25 to 60 degree C.

Applications:

1. Used for machining gas turbine blades, aircraft frame components and pump impellers.

2. Used for die sinking.

3. Used to produce complex shapes in very hard materials.

4. Used to machine blind holes, through holes, irregular shaped holes and complex external shapes.

5. Used for rough machining of heavy forgings.

6. Used for machining rock boring bits, gears etc.

Advantages:

1. Simple and faster method.

2. There is no thermal and mechanical stress on the workpiece.

3. Very thin sections can be machined.

4. It can be used for all metals irrespective of their mechanical properties (strength, hardness, etc.)

5. Any complex profile can be machined.

6. Very good surface finish is obtained (up to 0.4m)

7. There is no tool wear. Hence longer tool life.

8. Faster metal removal.

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9. Dimensional accuracy of 0.01 mm can be achieved.

10. No sharp corners and burrs in finished component.

Disadvantages:

1. Very high power consumption.

2. It cannot be used for machining non-conducting materials.

3. It leads to corrosion in machine parts.

4. Rigid fixtures are required to withstand high flow of electrolyte.

5. It is an expensive process.

Electro Chemical Grinding (ECG)

The figure 5.63 shows the process of electro chemical grinding. Electro chemical grinding. Electro chemical grinding is also called electrolytic grinding.

Metal is removed from the surface of the workpiece by electro chemical action and also by abrasive action of a grinding wheel. 90% of metal is removed by electro chemical action and 10% of metal is removed by the abrasive action of the grinding wheel.

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The equipment has a metal bonded grinding wheel. Brass, bronze and copper are bonded with abrasive grains in the grinding wheel. Diamond abrasive is used for grinding tungsten. Aluminium oxide abrasives is used for other metals. The wheel is held in a horizontal spindle. The spindle is supported on insulated bearings. The workpiece is held in a fixture against the grinding wheel. A gap of about 0.01 mm is maintained between the wheel and the surface of the workpiece. The workpiece is connected to the +ve terminal of a D.C. supply. The grinding wheel is connected to the â&#x20AC;&#x201C;ve terminal. 4 to 16V, 300 to 1000Amps D.C. supply is applied. A mixture of sodium chlorite, sodium chlorate or sodium nitrate and water is used as the electrolyte. The electrolytic solution is made to flow between the workpiece and the grinding wheel. Electro chemical action takes place. Metal from surface of the workpiece is removed in small particles. In addition to this, the rotating grinding wheel also removes metal from the work surface by abrasion. The small particles of metal removed from the workpiece are carried away by the electrolyte. The electrolyte is collected in a reservoir. It is filtered and recirculated by a pump. Electrolyte also acts as a coolant. The workpiece is slowly fed towards the grinding wheel maintaining a constant gap between the workpiece and the grinding wheel. Applications: 1. Used for machining hard materials which are conductive to electricity.

2. Used for grinding of tungsten carbide tool tips and hard steels.

3. Used to grind thin section.

4. Cylindrical grinding, form grinding, plunge grinding and surface grinding operations are done using this process.

5. Used for machining refractory materials, high strength steel, nickel and cobalt base alloys etc.

Advantages:

1. Very fine finish is obtained (0.2 to 0.4 microns can be obtained)

2. Suitable for machining very hard materials like carbides. Carbides are difficult to machine by other process.

3. No heat is generated during the process.

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4. No distortion to the workpiece

5. No burrs are produced.

6. Fast operation.

7. Thin materials can be ground without deflection as the grinding wheel does not press the workpiece.

8. Wheel wear is drastically reduced.

9. No heat is generated so there is no danger of burning or heat distortion.

10. Tolerance of about +_0.02 mm can be obtained.

Disadvantages: 1. This process can be used to machine only metals which are conductive.

2. Sharp corners of the workpiece cannot be machined.

3. Electrolytic solution is corrosive.

4. Initial cost of the equipment is high when equipped with larger power supplies.

5. Intricate shapes may not be formed.

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Electric Discharge Machining (EDM)

Electric discharge machining is also called spark erosion or electro erosion machining. Here the metal is removed from the workpiece surface due to erosion. The erosion is caused by an electric spark produced between the workpiece and the tool. The workpiece is held in a fixture and placed inside a tank. The tank contains a di-electric fluid. Kerosene, mineral oil, white spirit, and paraffin are some for the di-electric fluid used. The di-electric fluid does not conduct electricity. The workpiece is connected to the +ve terminal of a D.C. supply (50 to 440V). the tool is held vertically over the workpiece. The tool is made of copper, brass, tungsten or graphite. The tool is hollow. It is connected to the â&#x20AC;&#x201C;ve terminal of the D.C. supply. The tool becomes the cathode. A gap of 0.005 to 0.05 mm. is obtained between the workpiece and the tool. The di-electric fluid is forced with pressure through the gap between the tool and the workpiece. When the D.C. supply is given, an electric spark is produced in the gap between the tool and workpiece. Because of the spark, a high heat (of about 1200o C) is produced in the gap. This heat melts a small area of the metal of the workpiece. Thousands of sparks occur per second across the gap. The forces due to the sparks tear the particles of molten metal from the workpiece. The tool is connected to the â&#x20AC;&#x201C;ve terminal to minimize wear on the tool. A servo mechanism is used to feed the tool and maintain a constant gap between the tool and workpiece. The di-electric fluid acts also as a coolant. It carries away the eroded metal particles. The fluid acts as an insulating medium. Applications: 1. Used for die sinking.

2. Used for precision drilling of very small holes, slots, etc. in diesel fuel injection nozzles.

3. Used for producing profiles and cavities on very hard and brittle materials like tungsten carbide and stellites.

4. Used for machining intricate shapes.

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5. Used for blanking of parts from sheets.

6. Used to cut off rods.

7. Used for sharpening of tools, cutters and broaches.

8. Using wire electrodes small holes up to 0.1 mm diameter can be produced.

Advantages:

1. Any electrically conductive material can be machined.

2. Mechanical properties of the workpiece (strength, hardness, etc) do not affect the machining.

3. Surface finish with an accuracy of 0.2 micron is possible.

4. No stress on the workpiece as there is no cutting force. So thin workpieces can be maintained.

5. Any complicated shape that can be made on the tool can be produced in the workpiece.

6. It takes lesser time.

7. High accuracy (normally 0.05 mm, special cases 0.03 mm)

8. The hardness of the workpiece is not a factor. As long as the material can conduct current, it can be machined.

Disadvantages: 1. High power consumption.

2. Only electrically conductive materials can be machined.

3. Perfectly square corners cannot be produced.

4. Re-dressing of tools is necessary for deep holes.

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5. It is slow when compared to conventional or even Electro Chemical Machining.

6. Excessive tool wear.

7. Machining heats the workpieces and affects the metallurgical properties of workpiece.

Plasma arc machining (PAM)

When a flowing gas is heated to a very high temperature (about 1600oC) it becomes partially ionized. This ionized gas is known as plasma. In Plasma Arc Machining, metal is removed from the workpiece surface by means of high temperature plasma. Metal is also removed due to electron bombardment. The equipment has a gas chamber with a copper nozzle at the bottom. A tungsten electrode is held vertically in the gas chamber. The tungsten electrode is connected to the â&#x20AC;&#x201C;ve terminal of a 400 V, 200KW D.C. supply. The nozzle is connected to the +ve terminal when the supply is given, an arc is produced between the tungsten electrode (cathode) and the copper nozzle(anode). A di-atomic gas, usually or is passed through the gas chamber. The gas passes through the arc. It is heated and gets ionized by the arc because of the high temperature. The ionised gas flows out of the nozzle as plasma flame. The plasma flame is forced on the work surface. Because of the high temperature of the gas, the surface of the workpiece gets melted. Due to the bombardment of ions on the surface of the workpiece, the metal is eroded. The rate of metal removal can be increased by increasing the gas flow. Nozzle is water cooled. In due course, the tungsten electrode tip gets eroded due to high temperature. So the position of the electrode has to be adjusted. Applications: 1. Used for cutting stainless steel and aluminium alloys.

2. Used for profile cutting and slitting in hard materials such as super alloy steels.

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Advantages: 1. This process can be used to cut any metal.

2. The cutting action is very fast.

3. It is possible to cut thick material upto 150 mm.

Disadvantages: 1.

Because of the high heat, metallurgical change takes place on the workpiece.

The process is unsafe. Safety precautions are necessary.

Safety Precautions: 1. The plasma flame gives out ultra violet and infra red rays. These rays are highly harmful to eyes and skin. Therefore projective eye glasses, suitable gloves and protective dresses should be used.

2. They should be sufficient ventilation in the room for the exit of poisonous gases.

3. There will be lot of noise while machining. So ear plugs should be used to protect ears.

Laser Beam Machining (LBM)

The word LASER means Light Amplification by Stimulated Emission of Radiation. Laser is an electro magnetic radiation. It is a beam of light having the same wave length (monochromatic light). This beam can be focused by a lense on a very small spot on a workpiece. The laser beam emits high heat which can melt and vapourise any material in the world.

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The equipment has a ruby crystal. The crystal is placed inside a flash lamp coil (1000W) as shown in figure 5.66. The flash lamp a filled with Xenon gas. When the flash lamp is switched on, it gives high intensity light. The ruby crystal is stimulated and it emits the laser beam. By using a lense, the beam is focused on the workpiece. The workpiece is fed past the beam. The portion of the metal is melted and vaporized. Applications: 1. Laser beam machining is a micro machining method. It is used for producing very fine and minute holes (0.005mm) dia.

2. It is used for drilling in small nozzle, orifices in very hard materials.

3. Laser beams can be used in surgery.

4. Used for drilling holes in surgical needles, oil or gas orifices and relief holes in pressure plunges.

5. Used for cutting complex profiles in thin and hard materials like ceramics.

Advantages: 1. It can melt and vapourise any known material.

2. The machining operation is localized.

3. Precision location can be achieved.

4. Welding dissimilar metals can be done using laser beam.

5. There is no tool wear.

6. Can be used for machining materials that are less sensitive to heat-like ceramics.

7. No mechanical force on the workpiece.

8. It can machine both metals and non-metals.

9. Holes can easily be drilled on curved or angular surfaces.

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10. The holes are burr free.

11. The holes are drilled automatically and may be programmed by the addition of automatic or numerically controlled equipment.

12. No direct tool contact with workpiece, so heat is extremely localized.

Disadvantages:

1. High cost of operation.

2. Large amount of metal removal is not possible.

3. It cannot be used for cutting metal of high heat conductivity or high reflectivity (e.g. Al, Cu, and their alloys)

4. Machined holes will not be perfectly round

5. Life of flash lamp is short.

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