IMTS Mechanical Eng. (Machine Design & Drawing)

I ns t i t ut eofManagement & Techni calSt udi es MACHI NEDESI GN& DRAWI NG

PLOMAI NMECHANI CAL ENGI NEERI NG `500 DI

www. i mt s i ns t i t ut e. com

IMTS (ISO 9001-2008 Internationally Certified)

MACHINE DESIGN & DRAWING

MACHINE DESIGN & DRAWING

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MACHINE DESIGN & DRAWING CONTENTS: UNIT-1

01-26

MACHINE DESING AND MANUFACTURING CONDITIONS IN DESIGN Definition.Classifications of Machine Design.,General Considerations in Machine Design,General Procedure in Machine Design. Introduction.Manufacturing Processes.

Casting. Casting Design ,Forging. Forging Design,Mechanical Working of Metals,Hot,Working,Hot Working Processes,Cold Working. ,Cold Working Processes,Interchangeability.Important Terms Used in Limit System. Fits ,Types of Fits,Basis of Limit System,Indian Standard System of Limits and Fits,Calculation of Fundamental Deviation for Shafts,Calculation of Fundamental Deviation for Holes,Surface Roughness and its Measurement,Preferred Numbers.

UNIT-II

27-36

SECTIONAL VIEWS Introductions ,Need for sectioning ,Hatching ,Inclination of hatching lines ,Spacing hatching lines ,Hatching of larger areas ,Hatching of adjacent parts ,Sketch and of ful1 section, ,Half sections ,Types of half section,Partial or local sections, ,Revolved or super imposed, ,Removed sections ,Offset sections.

UNIT – III

37-55

LIMITS, FITS AND TOLERANCES Introduction Definition of various term used in limits Hole basis system, Shaft basis system ,Types of fits, fits Shaft and Hole –Terminology Clearance ,classification of fits, Selection of fit and applications ,Types of Tolerances,Form and position,Indication of tolerance and fits on the drawing IMTSINSTITUTE.COM

UNIT-IV

56-74

KEYS AND SURFACE FINISH Introduction ,Classification of keys,Sunk key,Saddle key,

Flat

key,Gib

head

key,Feather key,Peg key,Single head key,Double head key,Spline shaft,Woodruff key,Pin key,Cone key,Definitions,Nominal surface,Roughness,Waviness,Lay,Sampling length,Production method and surface quality,Symbol for lay,Indication of surface roughness for various machining operations,Surface texture symbol with all the characteristics,System of Indication surface texture symbols on drawing

UNIT-V

75-92

SCREW THREADS AND THREADED FASTENERS Introduction,Nomenclature of

Screw threads,Basic profiles or forms of screw

threads,Left hand thread,Right hand thread,Internal thread,External thread,V-Thread – whit worth thread,Square thread,Designation of threads,Bolt and nut,Drawing of hexagonal bolt and nut,Drawing of square head bolts,Riveted head,Types of rivet heads

UNIT QUESTIONS:

93-98

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UNIT-I MACHINE DESIGN AND MANUFACTURING CONDITION IN DESIGN Definition

The subject Machine Design is the creation of new and better machines and improving the existing ones.

A new or better machine is one which is more economical in the overall cost of production and operation.

The process of design is a long and time consuming one. From the study of existing ideas, a new idea has to be conceived. The idea is then studied keeping in mind its commercial success and given shape and form in the form of drawings. In the preparation of these drawings, care must be taken of the availability of resources in money, in men and in materials required for the successful completion of the new idea into an actual reality. In designing a machine component, it is necessary to have a good knowledge of many subjects such as Mathematics, Engineering Mechanics, Strength of Materials, Theory of Machines, Workshop Processes and Engineering Drawing.

Classifications of Machine Design

The machine design may be classified as follows : 1. Adaptive design. In most cases, the designer’s work is concerned with adaptation of existing designs. This type of design needs no special knowledge or skill and can be attempted by designers of ordinary technical training. The designer only makes minor alternation or modification in the existing designs of the product.

2. Development design. This type of design needs considerable scientific training and design ability in order to modify the existing designs into a new idea by adopting a new material or different method of manufacture. In this case, though the designer starts from the existing design, but the final product may differ quite markedly from the original product.

3. New design. This type of design needs lot of research, technical ability and creative thinking. Only those designers who have personal qualities of a sufficiently high order can take up the work of a new design. The designs, depending upon the methods used, may be classified as follows :

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(a) Rational design. This type of design depends upon mathematical formulae of principle of mechanics.

(b) Empirical design. This type of design depends upon empirical formulae based on the practice and past experience.

(c) Industrial design. This type of design depends upon the production aspects to manufacture any machine component in the industry.

(d) Optimum design. It is the best design for the given objective function under the specified constraints.

It may be achieved by minimising the undesirable effects.

(e) System design. It is the design of any complex mechanical system like a motor car.

(f) Element design. It is the design of any element of the mechanical system like piston, crankshaft, connecting rod, etc.

(g) Computer aided design. This type of design depends upon the use of computer systems to assist in the creation, modification, analysis and optimisation of a design.

General Considerations in Machine Design

Following are the general considerations in designing a machine component :

1. Type of load and stresses caused by the load. The load, on a machine component, may act in several ways due to which the internal stresses are set up. The various types of load and stresses are discussed in chapters 4 and 5.

2. Motion of the parts or kinematics of the machine. The successful operation of any ma- chine depends largely upon the simplest arrangement of the parts which will give the motion required. The motion of the parts may be :

(a) Rectilinear motion which includes unidirectional and reciprocating motions. (b) Curvilinear motion which includes rotary, oscillatory and simple harmonic. (c) Constant velocity.

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(d) Constant or variable acceleration.

3. Selection of materials. It is essential that a designer should have a thorough knowledge of the properties of the materials and their behaviour under working conditions. Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc. The various types of engineering materials and their properties.

4. Form and size of the parts. The form and size are based on judgement. The smallest prac- ticable cross-section may be used, but it may be checked that the stresses induced in the designed crosssection are reasonably safe. In order to design any machine part for form and size, it is necessary to know the forces which the part must sustain. It is also important to anticipate any suddenly applied or impact load which may cause failure.

5. Frictional resistance and lubrication. There is always a loss of power due to frictional resistance and it should be noted that the friction of starting is higher than that of running friction. It is, therefore, essential that a careful attention must be given to the matter of lubrication of all surfaces which move in contact with others, whether in rotating, sliding, or rolling bearings.

6. Convenient and economical features. In designing, the operating features of the machine should be carefully studied. The starting, controlling and stopping levers should be located on the basis of convenient handling. The adjustment for wear must be provided employing the various take- up devices and arranging them so that the alignment of parts is preserved. If parts are to be changed for different products or replaced on account of wear or breakage, easy access should be provided and the necessity of removing other parts to accomplish this should be avoided if possible.

The economical operation of a machine which is to be used for production, or for the processing of material should be studied, in order to learn whether it has the maximum capacity consistent with the production of good work.

7. Use of standard parts. The use of standard parts is closely related to cost, because the cost of standard or stock parts is only a fraction of the cost of similar parts made to order. The standard or stock parts should be used whenever possible parts for which patterns are already in existence such as gears, pulleys and bearings and parts which may be selected from regular shop stock such as screws, nuts and pins. Bolts and studs should be as few as possible to

Design considerations play important role in the successful avoid the delay

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caused by changing production of machines. drills, reamers and taps and also to decrease the number of wrenches required.

8. Safety of operation. Some machines are dangerous to operate, especially those which are speeded up to insure production at a maximum rate. Therefore, any moving part of a machine which is within the zone of a worker is considered an accident hazard and may be the cause of an injury. It is, therefore, necessary that a designer should always provide safety devices for the safety of the operator. The safety appliances should in no way interfere with operation of the machine.

9. Workshop facilities. A design engineer should be familiar with the limitations of his employer’s workshop, in order to avoid the necessity of having work done in some other workshop. It is sometimes necessary to plan and supervise the workshop operations and to draft methods for casting, handling and machining special parts.

10. Number of machines to be manufactured. The number of articles or machines to be manufactured affects the design in a number of ways. The engineering and shop costs which are called fixed charges or overhead expenses are distributed over the number of articles to be manufactured. If only a few articles are to be made, extra expenses are not justified unless the machine is large or of some special design. An order calling for small number of the product will not permit any undue expense in the workshop processes, so that the designer should restrict his specification to standard parts as much as possible.

11. Cost of construction. The cost of construction of an article is the most important consideration involved in design. In some cases, it is quite possible that the high cost of an article may immediately bar it from further considerations. If an article has been invented and tests of hand made samples have shown that it has commercial value, it is then possible to justify the expenditure of a considerable sum of money in the design and development of automatic machines to produce the article, especially if it can be sold in large numbers. The aim of design engineer under all conditions, should be to reduce the manufacturing cost to the minimum.

12. Assembling. Every machine or structure must be assembled as a unit before it can function.

Large units must often be assembled in the shop, tested and then taken to be transported to their place of service. The final location of any machine is important and the design engineer must anticipate the Car assembly line.exact location and the local facilities for erection.

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1.4 General Procedure in Machine Design

In designing a machine component, there is no rigid rule. The problem may be attempted in several ways.

However, the general procedure to solve a design problem is as follows :

1. Recognition of need. First of all, make a complete statement of the problem, indicating the need, aim or purpose for which the machine is to be designed.

2. Synthesis (Mechanisms). Select the possible mechanism or group of mechanisms which will give the desired motion.

3. Analysis of forces. Find the forces acting on each member of the machine and the energy transmitted by each member.

4. Material selection. Select the material best suited for each member of the machine.

5. Design of elements (Size and Stresses). Find the size of each member of the machine by considering the force acting on the member and the permissible stresses for the material used. It should be kept in mind that each member should not deflect or deform than the permissible limit.

6. Modification. Modify the size of the member to agree with Fig. 1.1. General procedure in Machine Design. the past experience and judgment to facilitate manufacture. The modification may also be necessary by consideration of manufacturing to reduce overall cost.

7. Detailed drawing. Draw the detailed drawing of each component and the assembly of the machine with complete specification for the manufacturing processes suggested.

8. Production. The component, as per the drawing, is manufactured in the workshop. The flow chart for the general procedure in machine design is shown in Fig. 1.1.

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Manufacturing Considerations in Machine Design

introduction In the previous chapter, we have only discussed about the composition, properties and uses of various materials used in Mechanical Engineering. We shall now discuss in this chapter a few of the manufacturing processes, limits and fits, etc.

Manufacturing Processes

The knowledge of manufacturing processes is of great importance for a design engineer. The following are the various manufacturing processes used in Mechanical Engineering.

1. Primary shaping processes.

The processes used for the preliminary shaping of the machine component are known as primary shaping processes. The common operations used for this process are casting, forging, extruding, rolling, drawing, bending, shearing, spinning, powder metal forming, squeezing, etc.

2. Machining processes.

The processes used for giving final shape to the machine component, according to planned dimensions are known as machining processes. The common operations used for this process are turning, planning, shaping, drilling, boring, reaming, sawing, broaching, milling, grinding, hobbing, etc.

3. Surface finishing processes.

The processes used to provide a good surface finish for the machine component are known as

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surface finishing processes. The common operations used for this process are polishing, buffing, honing, lapping, abrasive belt grinding, barrel tumbling, electroplating, superfinishing, sheradizing, etc.

4. Joining processes.

The processes used for joining machine components are known as joining processes. The common operations used for this process are welding, riveting, soldering, brazing, screw fastening, pressing, sintering, etc.

5. Processes effecting change in properties.

These processes are used to impart certain specific properties to the machine components so as to make them suitable for particular operations or uses. Such processes are heat treatment, hotworking, cold-working and shot peening. To discuss in detail all these processes is beyond the scope of this book, but a few of them which are important from the subject point of view will be discussed in the following pages.

Casting

It is one of the most important manufacturing process used in Mechanical Engineering. The castings are obtained by remelting of ingots* in a cupola or some other foundry furnace and then pouring this molten metal into metal or sand moulds. The various important casting processes are as follows:

1. Sand mould casting. The casting produced by pouring molten metal in sand mould is called sand mould casting. It is particularly used for parts of larger sizes.

2. Permanent mould casting. The casting produced by pouring molten metal in a metallic mould is called permanent mould casting. It is used for casting aluminium pistons, electric iron parts, cooking utensils, gears, etc. The permanent mould castings have the following advantages:

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(a) It has more favourable fine grained structure.

(b) The dimensions may be obtained with close tolerances.

(c) The holes up to 6.35 mm diameter may be easily cast with metal cores.

3. Slush casting. It is a special application of permanent metal mould casting. This method is used for production of hollow castings without the use of cores.

4. Die casting. The casting produced by forcing molten metal under pressure into a permanent metal mould (known as die) is called die casting. A die is usually made in two halves and when closed it forms a cavity similar to the casting desired. One half of the die that remains stationary is known as cover die and the other movable half is called ejector die. The die casting method is mostly used for castings of non-ferrous metals of comparatively low fusion temperature. This process is cheaper and quicker than permanent or sand mould casting. Most of the automobile parts like fuel pump, carburettor bodies, Aluminium die casting component horn, heaters, wipers, brackets, steering wheels, hubs and crank cases are made with this process. Following are the advantages and disadvantages of die casting :

Advantages

(a) The production rate is high, ranging up to 700 castings per hour.

(b) It gives better surface smoothness.

(c) The dimensions may be obtained within tolerances.

(d) The die retains its trueness and life for longer periods. For example, the life of a die for

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zinc base castings is upto one million castings, for copper base alloys upto 75 000 castings and for aluminium base alloys upto 500 000 castings.

(e) It requires less floor area for equivalent production by other casting methods.

( f ) By die casting, thin and complex shapes can be easily produced.

( g ) The holes up to 0.8 mm can be cast.

Disadvantages

(a) The die casting units are costly.

(b) Only non-ferrous alloys are casted more economically.

(c) It requires special skill for maintenance and operation of a die casting machine.

5. Centrifugal casting. The casting produced by a process in which molten metal is poured and allowed to solidify while the mould is kept revolving, is known as centrifugal casting. The metal thus poured is subjected to centrifugal force due to which it flows in the mould cavities. This results in the production of high density castings with promoted directional solidification. The examples of centrifugal castings are pipes, cylinder liners and sleeves, rolls, bushes, bearings, gears, flywheels, gun barrels, piston rings, brake drums, etc.

Casting Design

An engineer must know how to design the castings so that they can effectively and efficiently render the desired service and can be produced easily and economically. In order to design a casting, the following factors must be taken into consideration :

1. The function to be performed by the casting, 2. Soundness of the casting, 3. Strength of the casting, 4. Ease in its production, 5. Consideration for safety, and

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6. Economy in production.

In order to meet these requirements, a design engineer should have a thorough knowledge of production methods including pattern making, moulding, core making, melting and pouring, etc. The best designs will be achieved only when one is able to make a proper selection out of the various available methods. However, a few rules for designing castings are given below to serve as a guide:

1. The sharp corners and frequent use of fillets should be avoided in order to avoid concentration of stresses.

2. All sections in a casting should be designed of uniform thickness, as far as possible. If, however, variation is unavoidable, it should be done gradually.

3. An abrupt change of an extremely thick section into a very thin section should always be avoided.

4. The casting should be designed as simple as possible, but with a good appearance.

5. Large flat surfaces on the casting should be avoided because it is difficult to obtain true surfaces on large castings.

6. In designing a casting, the various allowances must be provided in making a pattern.

7. The ability to withstand contraction stresses of some members of the casting may be improved by providing the curved shapes e.g., the arms of pulleys and wheels.

8. The stiffening members such as webs and ribs used on a casting should be minimum possible in number, as they may give rise to various defects like hot tears and shrinkage, etc.

9. The casting should be designed in such a way that it will require a simpler pattern and its moulding is easier.

10. In order to design cores for casting, due consideration should be given to provide them adequate support in the mould.

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11. The deep and narrow pockets in the casting should invariably be avoided to reduce cleaning costs.

12. The use of metal inserts in the casting should be kept minimum

13. The markings such as names or numbers, etc., should never be provided on vertical surfaces because they provide a hindrance in the withdrawl of pattern.

14. A tolerance of Âą 1.6 mm on small castings (below 300 mm) should be provided. In case more dimensional accuracy is desired, a tolerance of Âą 0.8 mm may be provided.

Forging

It is the process of heating a metal to a desired temperature in order to acquire sufficient plasticity, followed by operations like hammering, bending and pressing, etc. to give it a desired shape. The various forging processes are :

1. Smith forging or hand forging

2. Power forging,

3. Machine forging or upset forging, and

4. Drop forging or stamping

The smith or hand forging is done by means of hand tools and it is usually employed for small jobs. When the forging is done by means of power hammers, it is then known as power forging. It is used for medium size and large articles requiring very heavy blows. The machine forging is done by means of forging machines. The drop forging is carried out with the help of drop hammers and is particularly suitable for mass production of identical parts. The forging process has the following advantages :

1. It refines the structure of the metal.

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2. It renders the metal stronger by setting the direction of grains.

3. It effects considerable saving in time, labour and material as compared to the production of a similar item by cutting from a solid stock and then shaping it.

4. The reasonable degree of accuracy may be obtained by forging.

5. The forgings may be welded.

It may be noted that wrought iron and various types of steels and steel alloys are the common raw material for forging work. Low carbon steels respond better to forging work than the high carbon steels. The common non-ferrous metals and alloys used in forging work are brass, bronze, copper, aluminium and magnesium alloys. The following table shows the temperature ranges for forging some common metals.

Table 3.1. Temperature ranges for forging.

MATERIAL

FORGING TEMP.

MATERIAL

FORGING TEMP.

900-1300 Wrought iron

Mild steel

940-1180 Stainless steel

750-1300 750-1250

Aluminium magnesium alloys

Medium carbon steel

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350-500

MACHINE DESIGN & DRAWING

High carbon and alloy

13

COPPER, BRASS AND

800-1150

600-950

BRONZE

steel

Forging Design

In designing a forging, the following points should always be considered.

1. The forged components should ultimately be able to achieve a radial flow of grains or fibres.

2. The forgings which are likely to carry flash, such as drop and press forgings, should preferably have the parting line in such a way that the same will divide them in two equal halves.

3. The parting line of a forging should lie, as far as possible, in one plane.

4. Sufficient draft on surfaces should be provided to facilitate easy removal of forgings from dies.

5. The sharp corners should always be avoided in order to prevent concentration of stress and to facilitate ease in forging.

6. The pockets and recesses in forgings should be minimum in order to avoid increased die wear.

7. The ribs should not be high and thin.

8. Too thin sections should be avoided to facilitate easy flow of metal.

Mechanical Working of Metals The mechanical working of metals is defined as an intentional deformation of metals plastically under the action of externally applied forces.

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The mechanical working of metal is described as hot working and cold working depending upon whether the metal is worked above or below the recrystallisation temperature. The metal is subjected to mechanical working for the following purposes :

1. To reduce the original block or ingot into desired shapes,

2. To refine grain size, and3. To control the direction of flow lines.

Hot Working

The working of metals above the *recrystallisation temperature is called hot working. This temperature should not be too high to reach the solidus temperature, otherwise the metal will burn and become unsuitable for use. The hot working of metals has the following advantages and disadvantages :

Advantages

1. The porosity of the metal is largely eliminated.

2. The grain structure of the metal is refined.

3. The impurities like slag are squeezed into fibres and distributed throughout the metal.

4. The mechanical properties such as toughness, ductility, percentage elongation, percentage reduction in area, and resistance to shock and vibration are improved due to the refinement of grains.

Disadvantages

1. It requires expensive tools.

2. It produces poor surface finish, due to the rapid oxidation and scale formation on the metal surface.

3. Due to the poor surface finish, close tolerance cannot be maintained.

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Hot Working Processes

The various *hot working processes are described as below :

1. Hot rolling. The hot rolling process is the most rapid method of converting large sections into desired shapes. It consists of passing the hot ingot through two rolls rotating in opposite directions at the same speed. The space between the rolls is adjusted to conform to the desired thickness of the rolled section. The rolls, thus, squeeze the passing ingot to reduce its cross-section and increase its length. The forming of bars, plates, sheets, rails, angles, I-beam and other structural sections are made by

Hot Rolling : When steel is heated until it glows bright red, ithot rolling. becomes soft enough to form into elabrate shapes.

2. Hot forging.

It consists of heating the metal to plastic state and then the pressure is applied to form it into desired shapes and sizes. The pressure applied in this is not continuous as for hot rolling, but intermittent. The pressure may be applied by hand hammers, power hammers or by forging machines.

3. Hot spinning. It consists of heating the metal to forging temperature and then forming it into the desired shape on a spinning lathe. The parts of circular cross-section which are symmetrical about the axis of rotation, are made by this process.

4. Hot extrusion. It consists of pressing a metal inside a chamber to force it out by high pressure through an orifice which is shaped to provide the desired form of the finished part. Most commercial metals and their alloys such as steel, copper, aluminium and nickel are directly extruded at elevated temperatures. The rods, tubes, structural shapes, flooring strips and lead covered cables, etc., are the typical products of extrusion.

5. Hot drawing or cupping. It is mostly used for the production of thick walled seamless tubes and cylinders. It is usually performed in two stages. The first stage consists of drawing a cup out of a hot circular plate with the help of a die and punch. The second stage consists of reheating the drawn cup and drawing it further to the desired length having the required wall thickness. The second drawing operation is performed through a number of dies, which are arranged in a descending order of their diameters, so that the reduction of wall thickness is gradual in various stages.

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6. Hot piercing. This process is used for the Cold Rolled Steel : Many modern prod- manufacture of seamless tubes. In its operation, the heated ucts are made from easily shaped sheet cylindrical billets of steel are passed between two conical metal. shaped rolls operating in the same direction. A mandrel is provided between these rolls which assist in piercing and controls the size of the hole, as the billet is forced over it.

Cold Working The working of metals below their recrystallisation temperature is known as cold working. Most of the cold working processes are performed at room temperature. The cold working distorts the grain structure and does not provide an appreciable reduction in size. It requires much higher pressures than hot working.

The extent to which a metal can be cold worked depends upon its ductility. The higher the ductility of the metal, the more it can be cold worked. During cold working, severe stresses known as residual stresses are set up. Since the presence of these stresses is undesirable, therefore, a suitable heat treatment may be employed to neutralise the effect of these stresses. The cold working is usually used as finishing operation, following the shaping of the metal by hot working. It also increases tensile strength, yield strength and hardness of steel but lowers its ductility. The increase in hardness due to cold working is called work-hardening.

In general, cold working produces the following effects :

1. The stresses are set up in the metal which remain in the metal, unless they are removed by subsequent heat treatment.

2. A distortion of the grain structure is created.

3. The strength and hardness of the metal are increased with a corresponding loss in ductility.

4. The recrystalline temperature for steel is increased.

5. The surface finish is improved.

6. The close dimensional tolerance can be maintained.

Cold Working Processes

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The various cold working processes are discussed below:

1. Cold rolling. It is generally employed for bars of all shapes, rods, sheets and strips, in order to provide a smooth and bright surface finish. It is also used to finish the hot rolled components to close tolerances and improve their toughness and hardness. The hot rolled articles are first immersed in an acid to remove the scale and washed in water, and then dried. This process of cleaning the articles is known as pickling. These cleaned articles are then passed through rolling mills. The rolling mills are similar to that used in hot rolling.

2. Cold forging. The cold forging is also called swaging. During this method of cold working, the metal is allowed to flow in some pre-determined shape according to the design of dies, by a compressive force or impact. It is widely used in forming ductile metals. Following are the three, commonly used cold forging processes :

(a) Sizing. It is the simplest form of cold forging. It is the operation of slightly compressing a forging, casting or steel assembly to obtain close tolerance and a flat surface. The metal is confined only in a vertical direction.

(b) Cold heading. This process is extensively used for making bolts, rivets and other similar headed parts. This is usually done on a cold header machine. Since the cold header is made from unheated material, therefore, the equipment must be able to withstand the high pressures that develop. The rod is fed to the machine where it is cut off and moved into the header die. The operation may be either single or double and upon completion, the part is ejected from the dies.

After making the bolt head, the threads are produced on a thread rolling machine. This is also a cold working process. The process consists of pressing the blank between two rotating rolls which have the thread form cut in their surface.

c) Rotary swaging. This method is used for reducing the diameter of round bars and tubes by rotating dies which open and close rapidly on the work. The end of rod is tapered or reduced in size by a combination of pressure and impact.

3. Cold spinning. The process of cold spinning is similar to hot spinning except that the metal is worked at room temperature. The process of cold spinning is best suited for aluminium and other soft metals. The commonly used spun articles out of aluminum and its alloys are processing kettles,

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cooking utensils, liquid containers, and light reflectors, etc.

4. Cold extrusion. The principle of cold extrusion is exactly similar to hot extrusion. The most common cold extrusion process is impact extrusion. The operation of cold extrusion is performed with the help of a punch and die. The work material is placed in position into a die and struck from top

5. Cold drawing. It is generally employed for bars, rods, wires, etc. The important cold drawing processes are as follows:

a) Bar or rod drawing. In bar drawing, the hot drawn bars or rods from the mills are first pickled, washed and coated to prevent oxidation. A draw bench, is employed for cold drawing. One end of the bar is reduced in diameter by the swaging operation to permit it to enter a drawing die. This end of bar is inserted through the die and gripped by the jaws of the carriage fastened to the chain of the draw bench. The length of bars which can be drawn is limited by the maximum travel of the carriage, which may be from 15 metres to 30 metres. A high surface finish and dimensional accuracy is obtained by cold drawing.

The products may be used directly without requiring any machining.

(b) Wire drawing. In wire drawing, the rolled bars from the mills are first pickled, washed and coated to prevent oxidation. They are then passed through several dies of decreasing diameter to provide the desired reduction in size. The dies are usually made of carbide materials.

c) Tube drawing. The tube drawing is similar to bar drawing and in most cases it is accomplished with the use of a draw bench.

6. Cold bending. The bars, wires, tubes, structural shapes and sheet metal may be bent to many shapes in cold condition through dies. A little consideration will show that when the metal is bend beyond the elastic limit, the inside of the bend will be under compression while the outside will be under tension. The stretching of the metal on the outside makes the stock thinner. Usually, a flat strip of metal is bend by roll forming. The materials commonly used for roll forming are carbon steel, stainless steel, bronze, copper, brass, zinc and aluminium. Some of its products are metal windows, screen frame parts, bicycle wheel rims, trolley rails, etc. Most of the tubing is now-a-days are roll formed in cold conditions and then welded by resistance welding.

7. Cold peening. This process is used to improve the fatigue resistance of the metal by setting up compressive stresses in its surface. This is done by blasting or hurling a rain of small shot at high

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velocity against the surface to be peened. The shot peening is done by air blast or by some mechanical means. As the shot strikes, small indentations are produced, causing a slight plastic flow of the surface metal to a depth of a few hundreds of a centimetre. This stretching of the outer fibres is resisted by those underneath, which tend to return them to their original length, thus producing an outer layer having a compressive stress while those below are in tension. In addition, the surface is slightly hardened and strengthened by the cold working operation.

Interchangeability

The term interchangeability is normally employed for the mass production of indentical items within the prescribed limits of sizes. A little consideration will show that in order to maintain the sizes of the part within a close degree of accuracy, a lot of time is required. But even then there will be small variations. If the variations are within certain limits, all parts of equivalent size will be equally fit for operating in machines and mechanisms. Therefore, certain variations are recognised and allowed in the sizes of the mating parts to give the required fitting. This facilitates to select at random from a large number of parts for an assembly and results in a considerable saving in the cost of production.

In order to control the size of finished part, with due allowance for error, for interchangeable parts is called limit system.

It may be noted that when an assembly is made of two parts, the part which enters into the other, is known as enveloped surface (or shaft for cylindrical part) and the other in which one enters is called enveloping surface (or hole for cylindrical part).

Notes: 1. The term shaft refers not only to the diameter of a circular shaft, but it is also used to designate any external dimension of a part.

Important Terms used in Limit System

The following terms used in limit system (or interchangeable system) are important from the subject point of view:

1. Nominal size. It is the size of a part specified in the drawing as a matter of convenience.

2. Basic size. It is the size of a part to which all limits of variation (i.e. tolerances) are applied

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to arrive at final dimensioning of the mating parts. The nominal or basic size of a part is often the same.

3. Actual size. It is the actual measured dimension of the part. The difference between the basic size and the actual size should not exceed a certain limit, otherwise it will interfere with the interchangeability of the mating parts.

4. Limits of sizes. There are two extreme permissible sizes for a dimension of the part as shown in Fig. 3.1. The largest permissible size for a dimension of the part is called upper or high or maximum limit, whereas the smallest size of the part is known as lower or minimum limit.

5. Allowance. It is the difference between the basic dimensions of the mating parts. The allowance may be positive or negative. When the shaft size is less than the hole size, then the allowance is positive and when the shaft size is greater than the hole size, then the allowance is negative.

6. Tolerance. It is the difference between the upper limit and lower limit of a dimension. In other words, it is the maximum permissible variation in a dimension. The tolerance may be unilateral or bilateral. When all the tolerance is allowed on one side of the nominal size, e.g. 20 – 0.004 , then it is said to be unilateral system of tolerance. The unilateral system is mostly used in industries as it permits changing the tolerance value while still retaining the same allowance or type of fit.

Fits

The degree of tightness or looseness between the two mating parts is known as a fit of the parts.

The nature of fit is characterised by the presence and size of clearance and interference.

The clearance is the amount by which the actual size of the shaft is less than the actual size of the mating hole in an assembly as shown in Fig. 3.5 (a). In other words, the clearance is the difference between the sizes of the hole and the shaft before assembly. The difference must be positive.

The interference is the amount by which the actual size of a shaft is larger than the actual finished size of the mating hole in an assembly as shown in Fig. 3.5 (b). In other words, the interference is the arithmetical difference between the sizes of the hole and the shaft, before assembly. The difference must be negative.

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Types of Fits

According to Indian standards, the fits are classified into the following three groups : 1. Clearance fit. In this type of fit, the size limits for mating parts are so selected that clearance between them always occur, as shown in Fig. 3.5 (a). It may be noted that in a clearance fit, the tolerance zone of the hole is entirely above the tolerance zone of the shaft.

In a clearance fit, the difference between the minimum size of the hole and the maximum size of the shaft is known as minimum clearance whereas the difference between the maximum size of the hole and minimum size of the shaft is called maximum clearance.

The clearance fits may be slide fit, easy sliding fit, running fit, slack running fit and loose running fit.

2. Interference fit. In this type of fit, the size limits for the mating parts are so selected that interference between them always occur, as shown in Fig. 3.5 (b). It may be noted that in an interference fit, the tolerance zone of the hole is entirely below the tolerance zone of the shaft.

In an interference fit, the difference between the maximum size of the hole and the minimum size of the shaft is known as minimum interference, whereas the difference between the minimum size of the hole and the maximum size of the shaft is called maximum interference, as shown in Fig. 3.5 (b).

The interference fits may be shrink fit, heavy drive fit and light drive fit.

3. Transition fit. In this type of fit, the size limits for the mating parts are so selected that either a clearance or interference may occur depending upon the actual size of the mating parts, as shown in Fig. 3.5 (c). It may be noted that in a transition fit, the tolerance zones of hole and shaft overlap.

The transition fits may be force fit, tight fit and push fit.

Basis of Limit System

The following are two bases of limit system: 1. Hole basis system. When the hole is kept as a constant member (i.e. when the lower deviation of the hole is zero) and different fits are obtained by varying the shaft size, as shown in Fig. 3.6 (a), then the limit system is said to be on a hole basis.

2. Shaft basis system. When the shaft is kept as a constant member (i.e. when the upper deviation

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of the shaft is zero) and different fits are obtained by varying the hole size, as shown in Fig. 3.6 (b), then the limit system is said to be on a shaft basis.

Indian Standard System of Limits and Fits

According to Indian standard [IS : 919 (Part I)-1993], the system of limits and fits comprises 18 grades of fundamental tolerances i.e. grades of accuracy of manufacture and 25 types of fundamental deviations indicated by letter symbols for both holes and shafts (capital letter A to ZC for holes and small letters a to zc for shafts) in diameter steps ranging from 1 to 500 mm. A unilateral hole basis system is recommended but if necessary a unilateral or bilateral shaft basis system may also be used.

The 18 tolerance grades are designated as IT 01, IT 0 and IT 1 to IT 16. These are called standard tolerances. The standard tolerances for grades IT 5 to IT 7 are determined in terms of standard tolerance unit (i) in microns, where

Calculation of Fundamental Deviation for Shafts

We have already discussed that for holes, the upper deviation is denoted by ES and the lower deviation by EI. Similarly for shafts, the upper deviation is represented by es and the lower deviation by ei. According to Indian standards, for each letter symbol, the magnitude and sign for one of the two deviations (i.e. either upper or lower deviation), which is known as fundamental deviation, have been determined by means of formulae given in Table 3.7. The other deviation may be calculated by using the absolute value of the standard tolerance (IT) from the following relation: ei = es – IT

It may be noted for shafts a to h, the upper deviations (es) are considered whereas for shafts j to Zc, the lower deviation (ei) is to be considered.

Example 1. The dimensions of the mating parts, according to basic hole system, are given as follows : Hole : 25.00 mm 25.02 mm

Shaft : 24.97 mm 24.95 mm

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Find the hole tolerance, shaft tolerance and allowance

Solution. Given : Lower limit of hole = 25 mm ; Upper limit of hole = 25.02 mm ; Upper limit of shaft = 24.97 mm ; Lower limit of shaft = 24.95 mm

Hole tolerance

We know that hole tolerance = Upper limit of hole – Lower limit of hole = 25.02 – 25 = 0.02 mm Ans.

Shaft tolerance We know that shaft tolerance = Upper limit of shaft – Lower limit of shaft = 24.97 – 24.95 = 0.02 mm Ans

Allowance We know that allowance = Lower limit of hole – Upper limit of shaft = 25.00 – 24.97 = 0.03 mm Ans.

Example 2. Calculate the tolerances, fundamental deviations and limits of sizes for the shaft designated as 40 H8 / f7.

Solution. Given: Shaft designation = 40 H8 / f 7

The shaft designation 40 H8 / f 7 means that the basic size is 40 mm and the tolerance grade for the hole is 8 (i.e. I T 8) and for the shaft is 7 (i.e. I T 7).

Tolerances Since 40 mm lies in the diameter steps of 30 to 50 mm, therefore the geometric mean diameter,

= 38.73 mm

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We know that standard tolerance unit,

i = 0.45 D + 0.001 D 0.45 * 8.73 + 0.001 Ă— 38.73=1.55793

=.00156

mm

Surface Roughness and its Measurement

A little consideration will show that surfaces produced by different machining operations (e.g. turning, milling, shaping, planing, grinding and superfinishing) are of different characteristics. They show marked variations when compared with each other. The variation is judged by the degree of smoothness.

A surface produced by superfinishing is the smoothest, while that by planing is the roughest.

In the assembly of two mating parts, it becomes absolutely necessary to describe the surface finish in quantitative terms which is measure of micro- irregularities of the surface and expressed in microns.

In order to prevent stress concentrations and proper functioning, it may be necessary to avoid or to have certain surface roughness. There are many ways of expressing the surface roughness numerically, but the following two methods are commonly used :

1. Centre line average method (briefly known as CLA method), and

2. Root mean square method (briefly known as RMS method).

The centre line average method is defined as the average value of the ordinates between the surface and the mean line, measured on both sides of it. According to Indian standards, the surface finish is measured in terms of ‘CLA’ value and it is denoted by Ra.

Landing Gear : When an aircraft comes in to land, it has to lose a lot of energy in a very short time. the landing gear deals with this and prevents disaster. First, mechanical or liquid springs absorb energy rapidly by being compressed. As the springs relax, this energy will be released again, but in a slow controlled manner in a damper-the second energy absorber. Finally, the tyres absorb energy, getting hot in the process.

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n where, y1, y2, ...yn are the ordinates measured on both sides of the mean line and n are the number of ordinates.

The root mean square method is defined as the square root of the arithmetic mean of the squares of the ordinates. Mathematically,

y12 R.M.S. value (in microns) = n According to Indian standards, following symbols are used to denote the various degrees of surface roughness :

SymbolSurface roughness (Ra) in microns ∇ 8 to 25 ∇ ∇ 1.6 to 8 ∇ ∇ ∇ 0.025 to 1.6 ∇ ∇ ∇ ∇ Less than 0.025

The following table shows the range of surface roughness that can be produced by various

manufacturing processes.

Preferred Numbers

When a machine is to be made in several sizes with different powers or capacities, it is necessary to decide what capacities will cover a certain range efficiently with minimum number of sizes. It has

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been shown by experience that a certain range can be covered efficiently when it follows a geometrical progression with a constant ratio. The preferred numbers are the conventionally rounded off values derived from geometric series including the integral powers of 10 and having as common ratio of the following factors:

10, 10 10, 20 10 and 40 10 These ratios are approximately equal to 1.58, 1.26, 1.12 and 1.06. The series of preferred numbers are designated as *R5, R10, R20 and R40 respectively. These four series are called basic series. The other series called derived series may be obtained by simply multiplying or dividing the basic sizes by 10, 100, etc. The preferred numbers in the series R5 are 1, 1.6, 2.5, 4.0 and 6.3. Table 3.12 shows basic series of preferred numbers according to IS : 1076 (Part I) – 1985 (Reaffirmed 1990).

Notes : 1. The standard sizes (in mm) for wrought metal products are shown in Table 3.13 according to IS : 1136 – 1990. The standard G.P. series used correspond to R10, R20 and R40.

2. The hoisting capacities (in tonnes) of cranes are in R10 series, while the hydraulic cylinder diameters are in R40 series and hydraulic cylinder capacities are in R5 series.

3. The basic thickness of sheet metals and diameter of wires are based on R10, R20 and R40 series.

Wire diameter of helical springs are in R20 series.

UNIT - II SECTIONAL VIEWS

INTRODUCTION

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The orthographic views viz., front, top and side views, the visible edges and faces are indicated by continuous lines, while its interior hollow portions, and invisible outer edges and faces are indicated by dashed lines. If the interior construction of the object is complex, there will be a network of mass of dashed lines in the orthographic views as shown in fig 1.1.in order to avoid this complication and to remove the hidden lines; one or more views are represented “in section�.

Fig1.1 Network of Dashed Lines

In section, the object is imagined as cut apart by planes so as to expose its interior. This imaginary process of cutting the object is called sectioning. The imaginary plane which cuts the object is called the section plane or cutting plane.

NEED FOR SECTIONING The sectional views are necessary 1. To show the internal features more clearly.

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2. To remove hidden lines.

3. To avoid complication and ambiguity.

4. For ease of understanding.

HATCHING

Fig1.2 Sectioning

The sectional views of an object comprises of both sectioned and unsectioned surfaces. To differentiate between the sectioned and the unsectioned surfaces on the sectional views, a series of thin inclined lines,called section lines, parallel to themselves and inclined usually at 45ยบ to the horizontal, or to the main axis of the object, are drawn within the region of the cut surface as shown in fig 1.2. this process of executing parallel section lines is called hatching. The section lines, sometimes are also called as hatching lines.

INCLINATION OF HATCHING LINES

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The simplest form hatching, which will be usually adequate for general purposes, involves drawing of continuous thin parallel lines inclined at 45ยบ to the outlines as shown in

fig 1.3

Fig 1.3 Other type of Hatching Lines

When the out line of the sectioned surfaces are themselves inclined at 45ยบ, the 45ยบ hatching lines will become parallel to the outlines and thus over shadows the cut surface. In such cases, hatching lines are drawn horizontal as shown

in fig.

SPACING BETWEEN HATCHINE LINES

Fig 1.4 Spacing between Hatching Lines

The hatching must be done in such a way that the sectioned surface does not appear as dark as the outlines of the object. This can be accomplished by drawing hatching hatching lines as thin lines with

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the appropriate spacing between them. A spacing of 2mm between the hatching lines will be appropriate for the general work.

HATCHING OF LARGER AREAS Hatching of larger areas should be done only at the outer lines forming the boundary leaving blank space at the middle as shown in fig 1.5.

Fig1.5 Hatching of a Large Area

Hatching of adjacent parts

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Fig 1.6 Hatching more than Two Adjacent Parts

When two different parts are joined together as in the case riveted lap joint, then these 2 parts are to be cross hatched at 45ยบ but in opposite directions as shown in fig 1.6.

When three are more parts in contact such as riveted butt joints are to sectioned, then two of the adjacent parts should be cross hatched at an angle of 45ยบ, but in opposite directions and the remaining parts are to be hatched at different angles such as 30ยบ or 60ยบ and at different spacing .when thin sections like sheet metal, Gasket, washer etc., are to be shown in section, they are shown totally black leaving thin space between adjacent parts.

SKETCH AND OF FULL SECTION When a section plane passes through the object so as to cut it completely, the sectional view is called full sectional view or simply sectional view.

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Fig 1.7 Full Section

HALF SECTION When an object is symmetrical about one its axis, the drawing of its full sectional view involves repetition of hatching work on both the sides of its symmetrical axis which requires considerable labour and time. In such cases, only one half of the object is shown in section while its other half will be shown as unsectioned. Such a view which shows one half in section and the other half as unsectioned is called half sectional view.

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Fig1.8

TYPES OF HALF SECTION The different types of half section about its horizontal or vertical axis depending on the portion of the interior details of the object to be shown in section.

1. Front view with right half in section 2. front view with top half in section top view with section 3. top view with front half in section 4. left view with front half in section

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Fig 1.9 Half Section

PARTIAL OR LOCAL SECTION In local section, only a small portion of the object surrounding the feature is assumed to be removed and the boundary of the section is shown by irregular lines, as showing in fig 1.10

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Fig 1.10 Partial or Local Sections

REVOLVED OR SUPERIMPOSED SECTION Revolved section is obtained by revolving the section through 90ยบ, so that the section is made to align with the longitudinal view of the object as shown in fig 1.11

Fig 1.11 Revolved Section

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REMOVED SECTION When the section is to be shown on enlarged scale for the purpose of dimensioning, then the section cannot be aligned with any of the views. Instead, the section is drawn away from the view along the extension of the cutting plane line as shown in fig 1.12

Fig 1.12 Removed Section

OFFSET SECTION The section planes are usually assumed to pass through the axis of symmetry or the principal axis of the object. But when it becomes necessary to show still more details about the object, then the cutting plane is made to offset, so that it passes through different locations as shown in fig 1.13

Fig 1.13 Offset Section

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UNIT – III LIMITS, FITS AND TOLERANCES

Introduction The maximum and minimum permissible sizes within which the actual machined size lies are called limits. The functional relationship between the two adjacent parts achieved by the specified tolerance is called fit. The amount of variation permitted for a basic size is called tolerance.

Definition of various terms used in limits

Fig 2.1 illustrates the various terms

Basic size It is defined as the theoretical size of a part, derived from the design after rounding off to the nearest whole millimeter. The tolerances are always specified to the basic size. In fig 2.1 the dimension 30mm is the basic size. The basic size is also represented as “zero line”.

Actual size

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It is defined as the size actually obtained by machining. It is found by actual measurement using measuring instruments. In fig 2.1 the actual size of the diameter of the shaft is Ă˜29.925mm.

Limits The two extreme permissible sizes between which the actual size lies are called limits.

Maximum limits It is the allowable maximum size of the component, which lies above the Basic size.

Minimum limit It is the allowable minimum size of the component, which lies below the Basic size.

Tolerance It is defined as the amount of variation permitted to a basic size. The difference between the maximum and minimum limits of a basic size is called tolerance.

Tolerance = maximum limit – minimum limit

Deviation It is defined as the difference between the actual size or limit sizes, either maximum or minimum, and the corresponding basic size.

Actual deviation It is the algebraic difference between the actual measured size and the corresponding basic size.

Upper deviation It is defined as the algebraic difference between the maximum limit of size and the corresponding basic size.

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Lower deviation It is defined as the algebraic difference between the minimum limit of size and the corresponding basic size.

Tolerance zone It is the zone or area that lies between the upper limit and the lower limit. The actual size of the component that lies in the tolerance zone alone will be accepted.

Hole basis system In hole basis system, the basic size of a hole is kept constant and size of the shaft is varied above or below the zero line, so as to get a desired class of fit.

This system is popular in industries, as the standard tools such drills, reamers etc., are available for producing holes. For the holes, the lower deviation is zero and the minimum size of hole is equal to the design size, which is taken as the base for computing all the other limit dimensions. The limit dimensions on the hole and the shaft are computed by selecting suitable clearances and tolerances on the shaft and the hole.

Fig shows the clearance and transistion fits in the basis system. fig 2.2 A shows the tolerance zone for the hole having its lower limit equal to the basic size. The zero line is drawn through the lower limit since the lower deviation is zero. Both the limit dimensions of the shaft lie below the zero line for the clearance fit as shown in fig 2.2 B while they are above the zero line for the interference fit as shown in fig 2.2 C.

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Fig 2.2 Hole Basis System

Shaft basis system In this system, the different types of fits are obtained by associating holes of varying limit dimensions with a single shaft, whose upper deviation is zero. When the upper deviation of the shaft is zero, the maximum limit of the shaft will be equal to its basic size, which is taken as the base for computing all other limit dimensions. The limit dimensions on the hole and the shaft are computed by selecting suitable clearance and tolerances on the shaft and the hole.

Fig shows the clearance and transition fits in the shaft basis system. Fig 2.3 A shows the tolerance zone for the shaft having its maximum limit equal to the basic size. The zero line is drawn through the maximum limit since its upper deviation is zero. Both the limit dimensions of the hole lie above the zero line for the clearance fit as shown in fig 2.3 B. While they are below the zero line for the interference fit as shown in

Fig 2.3 C.

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Fig 2.3 Shaft Basis System

TYPES OF FITS FITS A machine is built by assembling all its constituting parts. During assembling sometimes a part may be required to be filled into another part. In such cases, and during the working of the machine they may or may not be intended to have a relative motion between them. If there should be a relative motion between the two parts, they must be filled loose, or tight otherwise. The fitting of one part in to the other, either loose or tight depends on the relationship existing between their mating surfaces which in turn depends on the dimensional differences between the parts. The relationship existing between the mating surfaces of the parts because of the differences in their dimensions is called fit.

SHAFT AND HOLE — TERMINOLOGY In mechanical engineering practice, generally a rod of circular cross section and a circular hole are termed as shaft and hole respectively. In the system of fits and tolerances, for the sake of simplicity even the non circular sections and also the space containing or contained by the two parallel faces of any part such as, the thickness of a key and the width of a keyway or a slot, are also referred as ‘shaft’ and ‘hole’ respectively.

CLEARANCE It is defined as the difference between the dimensions of the hole and the shaft assigned intentionally to obtain a particular type of a fit. It may be positive or negative. When the shaft size is smaller than the hole size it will be positive and will be negative when the shaft size is bigger than the hole size. The value of the clearance will be maximum when the hole size is maximum and the shaft size is minimum. It will be minimum when the shaft size is maximum and the hole size is minimum.

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CLASSIFICATION OF FITS A fit is established when one part is inserted into the other, The type of fit Obtained between the two parts is governed by the dimensional deviations assigned for the basic size of the shaft and the hole.

For a given basic size, the deviations assigned and the performance are interdependent. But since the performance is the ultimate objective, the deviations assigned for a basic size must satisfy the performance intended. But the performance itself is of varied type like, a shaft fitting tightly into a hole, or capable of just rotation, or sliding loosely in it. So, for a given basic size, we can have different performances. Therefore to obtain different performances we need to fix different deviations for the basic size of the shaft and the hole. Each set of deviations for the given basic size results in a particular type of performance For example, for the shaft to rotate in a hole, obviously its dimensions should be less than the hole. Alternately, when a shaft is to be held rigidly in a hole, its sizes should be greater than that of the hole, so that when the shaft is driven in to the hole, the outer surface of the shaft interferes with the inner surface of the hole, In the former, since there is a positive clearance between the two sizes, the fit is called clearance fit, while in the latter, because the surfaces interfere, the fit is called interference fit. A fit resulting due to the variations in the dimensions between that of the clearance fit and the interference fit is called transition fit.

Thus the types of fits are 

Clearance,



Interference and



Transition fits.

CLEARANCE FIT It is defined as the fit established when a positive clearance exists between the hole and the shaft. It is obtained by selecting the maximum and minimum limits of the shaft and the hole so that the clearance due to the difference between the dimensions of the smallest possible hole and the largest possible shaft is always positive. There are different classes in this type of fit depending on the clearance and the specific operating conditions of the given mating parts. They vary with the shaft speed, shaft bearing load, lubricating oil grade, temperature and the length of the mating surfaces.

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Fig 2.4

Figure 2.4 shows a clearance fit. The clearance between largest possible shaft is

the smallest possible hole and the

=  29.95 —  29.90 = 0.05 mm. Figure shows the conventional

representation of a clearance fit, where the tolerance zone of the hole lies above that of the shaft.

INTERFERENCE FIT It is defined as the fit established when a negative clearance exist between the sizes of the hole and the shaft. It is obtained by selecting the maximum and minimum limits of the shaft and the hole so that there is an interference of the surfaces and the clearance due to the difference between the dimensions of the largest possible hole and the smallest possible shaft is always negative. Interference fits are obtained by several methods, for instance, a shaft may be driven into the hole with a considerable force, or heating the part having the hole in order to increase the diameter of the hole, or by cooling the shaft and thus decreasing its diameter.

Fig 2.5

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Figure 2.5 shows an interference fit. The difference between the dimensions of the largest possible hole and the smallest possible shaft is =  30.25 —  30.30 = — 0.05mm. Figure shows the conventional representation of an interference fit, where the tolerance zone of the hole lies entirely below that of the shaft.

The interference fit is obtained by driving a shaft into the hole with a considerable force. When the force applied is heavy the interference fit is called heavy force fit, and when a lighter force is used to drive the shaft into the hole, it is called light force fit.

The interference fit can also be obtained by heating and subsequent cooling. The part containing the hole is heated so that the diameter of the hole will increase due to material expansion, and then after inserting the shaft in the hole, on cooling the hole will shrink to hold the shaft rigidly.

TRANSITION FIT It is defined as the fit established when the dimensions of the hole and the shaft are such that there exists a positive clearance or a negative clearance when the shaft is fitted into the hole. It is obtained by selecting the maximum and minimum limits for the shaft and the hole such that there exists a positive clearance when the smallest possible shaft is fitted into the largest possible hole, or a negative clearance when the largest possible shaft is forced into the smallest possible hole.

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Figure 2.6 shows a transition fit. Figure shows the fitting of the smallest possible shaft of  30.55 mm in the largest possible whole of  30.60mm allowing a positive clearance of  30.60 —  30.55 = 0.05mm. Figure 2.6 shows the fitting of the largest possible shaft of  30.65mm in the smallest possible whole of  30.50mm gives an interference fit

of  30.50 — 30.65 = — 0.15mm. Figure2.6 shows

conventional representation of transition fits in which the tolerance zones of the hole and the shaft overlap.

SELECTION OF FITS AND APPLICATIONS A wide range of fits may be obtained by various combinations of tolerance grades and fundamental deviations for both the shafts and the holes. But many of the possible combinations may not be of practical use. Majority of common engineering requirements may be satisfied on the basis of restricted selection of tolerance grades resulting in economy and ease of standardisation, yet leading to universally applicable and recommended fits. IS : 2709 – 1982 offers a comprehensive guide for the selection of fits. The most commonly used fits for general classes of work are given in the following tables.

Commonly used type of fits I. For Holes Type of Fit

Class of Shaft

H7

H8

H9

-

-

-

-

c

-

-

-

d

-

d8*

d8*, d9, d10

e

e7*

e8

e8, e9*

Easy running fit

f

f6*

f7

f7, f8*

Normal running fit

g

g5*

g6

-

h

h5*

h6

h7, h8*, h9

js

js5*

js6

js7*

Push fit for very accurate location with easy assembly and disassembly

k

k5*

k6

k7*

Light keying fit (true transition) for keyed shafts, non-running locked pins, etc.,

m

m5*

m6*

m7*

Medium keying fit

Clearance

b

H11

Remarks

H6*

a

Transition

With Holes a11 b11

Large clearance fit and widely used

-

c11

Slack running fit

d8*, d9, d10

d9

Loose running fit

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Close running fit or sliding fir, also spigot and location fit h11

Precision sliding fit. Also fine spigot and location fit.

MACHINE DESIGN & DRAWING

n

Interference

P

n5*

P5*

r

r5*

46

n6

P6

r6

n7*

Heavy keying fit (for tight assembly mating surfaces)

P7*

Light press fit with easy dismantling for non-ferrous parts. Standard press fit with easy dismantling for ferrous and non-ferrous parts assembly

r7*

Medium drive fit with easy dismantling for ferrous parts assembly Light drive fit with easy dismantling for ferrous parts assembly

s

s5*

s6

s7*

Heavy drive fit for ferrous parts permanent or semipermanent assembly standard press fit for nonferrous parts

t

t5*

t6*

t7*

Force fit on ferrous parts for permanent assembly

u7*

Heavy force fit or shrink fit

u * Second preference fits.

Commonly used type of fits

II. For Shafts

Clearance

Type of Fit

Class of Shaft

With Shafts H5*

H6

H7

H8*

H9

H11

Remarks

A

A11

B

B11

Large clearance fit and widely used

C

C11

Slack running fit

D11*

Loose running fit

D

D9*

D10

D10

E

E8*

E8*

E9

Easy running fit

F

F7*

F8

F8*

Normal running fit

G

H

G6*

H6*

G7

Close running fit or sliding fir, also spigot and location fit

H7

Precision sliding fit. Also fine spigot and location fit.

H8

H8

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H11

MACHINE DESIGN & DRAWING

Transition

js

js7

js8*

Push fit for very accurate location with easy assembly and disassembly

K

K6*

K7

K8*

Light keying fit (true transition) for keyed shafts, non-running locked pins, etc.,

M

M6*

M7*

M8*

Medium keying fit

N8*

Heavy keying fit (for tight assembly mating surfaces)

N

P

Interference

js6*

47

R

S

T

N6*

P6*

R6*

S6*

T6*

N7

P7

Light press fit with easy dismantling for non-ferrous parts. Standard press fit with easy dismantling for ferrous and nonferrous parts assembly

R7

Medium drive fit with easy dismantling for ferrous parts assembly Light drive fit with easy dismantling for ferrous parts assembly

S7

Heavy drive fit for ferrous parts permanent or semi-permanent assembly standard press fit for non-ferrous parts

T7

Force fit on ferrous parts for permanent assembly

* Second preference fits.

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TYPES OF TOLERANCES Straightness Tolerance This type of form tolerance shown in Figure 2.7 refers to the axis of the cylindrical part, because the tolerance frame is connected to the dimension line which indicates the diameter of the cylindrical part.

The tolerance value ď Ś 0.08 mm means that the axis of the cylinder must be contained in a cylindrical zone of diameter 0.08 mm as shown in Figure 2.7.

Fig 2.7 Flatness Tolerance This type of form tolerance shown in Figure 2.8 refers to surface, because the leader line connecting the tolerance frame has its arrow resting on a surface. The tolerance value 0.08 mm means that the indicated surface should be contained between two parallel planes 0.08 mm apart.

Fig 2.8 Circularity Tolerance This type of form tolerance shown in Figure 2.9 refers to circularity, because the leader line connecting the tolerance frame has its arrow resting on the conical surface of the part. The tolerance value 0.1 mm means that the circumference of each cross section should be contained between two coplanar concentric circles 0.1 mm apart.

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Fig 2.9 Cylindricity Tolerance This type of form tolerance shown in Figure 2.10 refers to the cylindricity, because the leader line connecting the tolerance frame has its arrow resting on the cylindrical surface of the part. The tolerance value 0.1 mm means that the considered cylindrical surface should be contained between two coaxial cylinders 0.1 mm apart.

Fig 2.10

Profile Tolerance of a Line This type of form tolerance shown in Figure 2.11 refers to a profile because the leader line connecting the tolerance frame has its arrow resting on the profile of a surface. The tolerance value 0.04 mm means that the considered profile must be contained between two lines enveloping circles of diameter 0.04 mm, the centres of which are situated on a line having the correct geometrical profile.

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Fig 2.11

Profile Tolerance of Any Surface This type of form tolerance shown in Figure 2.12 refers to the profile of a double curved surface because the leader line connecting the tolerance frame has its arrow resting on the profile of a double curved surface. The toleranced value

0.02 mm means that the considered surface should be

contained between two double curved surfaces enveloping spheres of diameter 0.02 mm, the centres of which are situated on a surface having the correct geometrical form.

Fig 2.12

Parallelism Tolerance This type of tolerance refers to the orientation of the axis of a feature with reference to a datum linc. The tolerance shown in Fig.2.13 refers to the orientation of the axis of the upper hole with reference to the axis of the lower hole which is considered as datum, The upper axis should be contained In a cylindrical tolerance zone of diameter 0.03 mm parallel to the lower axis A.

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Fig 2.13

Perpendicularity Tolerance This type of tolerance refers to the orientation of the axis (or a line) of a feature Perpendicular to a datum surface. The axis of the cylinder to the dimension of which the tolerance frame is connected as shown in Figure should be contained between two Parallel straight lines 0.1 mm apart, lying in a plane perpendicular to the datum plane as shown in Figure 2.14.

Fig 2.14

FORM & POSITION The tolerance for the sizes, called linear tolerances, are specified only to ensure that the actual manufacturing sizes are well within the acceptable limits. However, they have no control either over the geometry of the surface or its location, i.e., for example, a shaft may have its diameter well within the specified limits of size, but may not be truly circular. Similarly, a square slot may not have its surfaces exactly perpendicular, or, a hole may not have its centre correctly located. Thus it necessitates to specify the permissible deviations not only for its sizes but also for the geometrical variations in the form of the

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surfaces and the variations for their locations. The geometry variations are called form variation and the location variations are called position variation. Specifying of the permissible variations for both form and position using symbols and letter is called geometrical tolerancing.

According to the characteristic form of the feature to be toleranced and the manner in which it is dimensioned, the tolerance zero that must be considered will be from any one of the following : (a) Area within a circle. (b) Area between two concentric circles. (c) Area between two parallel lines. (d) Space within a sphere. (e) Space within a cylinder or between coaxial cylinders. (f) Space between two parallel surfaces. (g) Space within a parallelepiped.

The geometrical tolerances are indicated by specifying the form tolerances using symbols shown in Table and tolerance value in numerical values in the same unit of linear dimensions and positional tolerances referring the datum feature identified by a letter symbol. These indications are written in a rectangular frame which is divided into tow, sometimes, three compartments as shown in Figure. These compartments are filled in from left to right in the following order. (a) The symbol for the characteristic to be toleranced as shown in Table. (b) Tolerance value in the same unit used for linear dimensions. This value may be preceded by the shape identification symbols such as , , R, S, S, SR if required. (c) The letter identifying the referred datum feature.

TABLE Geometric Characteristic Symbols

FORM OF TOLERANCES

Characteristics to be Toleranced Straightness Form of Single Features

Flatness Circularity (Roundness)

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Symbol

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Cylindricity Profile of any line Profile of any surface Parallelism Perpendicularity (Squareness) Orientation of Related Features

Angularity

POSITIONAL TOLERANCES

Runout Position Position of Related Features

Concentricity and Coaxiality Symmetry

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INDICATION OF TOLERANCE AND FITS ON THE DRAWINGS The different methods of indication of tolerance and fits on the drawings by using letter symbols and numerical values are shown in Figures.

Indications of Fits by Letter and Grade Symbols When it is required to indicate the fits by letter and grade symbol, the tolerance symbol for the hole must be placed before that of the shaft as shown in Figure2.15 A , or the tolerance symbol for the hole must be placed above that of the shaft as shown in Figure 2.15 B, and the symbols being preceded by the basic size indicated once only.

Fig 2.15

When it is also necessary to specify the numerical values of the deviations, they should be written in brackets as shown in Figure2.15 C.

Indication of Fits by Numerical Values The methods of indicating the fits by the numerical values are shown in Figure 2.16 A. The dimensions for each of the components of the assembled parts should be preceded by the name as shown in Figure 2.16 B, or item reference as shown in Figure, the dimension for the hole being placed in both the cases above the shaft.

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Fig 2.16

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UNIT IV KEYS AND SURFACE FINISH

Introduction The common method to connect a shaft and a part is to drive a small piece of metal, known as key between the shaft and the hole made in the part mounted over it. The key will be driven such that it sits partly into the shaft and partly into the part mounted on it. To introduce the key, axial grooves, called key ways are cut both in the shaft and the part mounted on it as shown in the fig 3.1. The key is fitted between the shaft and the part mounted over it as shown in fig 3.1 . While transmitting the power, the key will be subjected to shear and crushing forces.

Keys are extensively used to hold pulleys, gears, couplings, clutches, sprockets, etc., and the shafts rigidly so that they rotate together. They are also used to mount the milling cutters, riding wheels, etc., on their spindles.

Fig 3.1

Classification of Keys Keys are classified into two types, (i) Taper keys and (ii) Parallel or feather keys.

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Taper key: A taper key is of rectangular cross section having uniform width and tapering thickness. The taper keys are used to transmit only the turning moment between the shaft and the hub without any relative rotational and axial motion between them.

The examples of tapered keys are, i.

Taper sunk key

ii.

Saddle key,

iii.

Flat key and

iv.

Gib-head key.

Parallel Key (or) Feather key : A parallel key or feather key is also of rectangular cross section of uniform width and thickness throughout. Parallel keys are used to transmit the turning moment between the shaft and the hub along with the provision to allow a small sliding axial motion between them wherever required.

The examples of the parallel keys are, (i) Parallel sunk key, (ii) Peg key, (iii) Single head key, (iv) Double head key and (v) Spline shaft.

The woodruff key, cone key and pin key are the special purpose keys are used for specific applications.

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Sunk Taper Key A sunk taper key shown in fig 3.2 is of rectangular or square cross section of uniform width having its bottom surface straight and top surface tapered. The key is driven between the shaft and the hub with half of its thickness to fit in the flat key way made in the shaft and the other half having the tapered surface to fit in the tapered key way made in the hub. This type of key is generally used to transmit heavy loads. The proportions of the key are as follows.

Fig 3.2

If D = diameter of the shaft in mm, W = width of the key and T = thickness of key, Width of key = 0.25 D+ 2mm Nominal Thickness = 0.66 W Standard Taper = 1:100

Hollow Saddle Key

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A hollow saddle key is of uniform width but tapering in thickness having its upper side flat and the underside hollow so as to sit on a shaft as shown in fig 3.3. Since the saddle key holds the shaft and the part mounted on it only by friction, it is not suitable for heavy loads. This key is used when there is frequent alterations in the position of the key on the shaft is expected.

Fig 3.3

The proportions of the keys are as follows: If D = diameter of the shaft in mm, W=width of the key and T = thickness , Width of key = 0.25 D+ 2mm Nominal Thickness = 0.33 W Standard Taper = 1:100

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Flat Saddle Key A flat saddle key is similar to a hollow saddle key, except that it’s underneath surface is flat. The key sits over the flat surface formed on the shaft and fits into the key way in the hub as shown in fig 3.4.

When the shaft rotates, the key will be wedged between the flat surface on the shaft and the key way in the hub, and thus holds them to rotate together. This key cannot be used for heavy loads and will not be suitable for shafts which frequently change their direction of rotation.

Fig 3.4 The proportions of this key are as follows: If D = diameter of the shaft in mm, W=width of the key and T = thickness of the key, Width of key = 0.25 D+ 2mm Nominal Thickness = 0.33 W Standard Taper = 1:100

Gib – Head key When a tapered sunk key is used, it can be removed by striking at its exposed thin end. If this end is not accessible, a head called gib is provided integral with the sunk taper key at its thicker end as shown in fig 3.5 . When a gib –head key is to be removed, a wedge is forced vertically in the gap between the head of the key and the vertical face of the hub.

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Fig 3.5

The proportions of the key are as follows.

If D = diameter of the shaft in mm, W=width of the key and T = thickness of key,

Width of key = 0.25 D+ 2mm Height of Gib-Head =1.75 T Nominal Thickness = 0.66 W Width of Gib- Head = 1.5 T Standard Taper = 1:100

Feather Key or Parallel Key A feather key or parallel key permits an axial sliding movement for the wheel over a shaft when both of them are rotating together. This facility will be required in several power transmission applications, such as , for example, in gear boxes, loose pulleys, clutches, universal and flexible types of coupling, etc. in a gear box, for example, any one of the driven gears have to be moved axially over the driven shaft so as to engage with the driving gear to obtain different speeds.

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Fig 3.6

A feather key is rectangular or square cross section with uniform width and thickness as shown in fig 3.6. The ends of a feather key are usually rounded and the key will be sunk into the shaft for half of its thickness so that it fits snugly into the key way recess in it with a press fit. The press fit prevents the key from moving axially over the shaft when the driven wheel slides on it. In cases of higher power transmission, the feather key instead of press fit will be secured to the shaft by countersunk set screws.

The proportions of the key are as follows: If D = diameter of the shaft in mm, W=width of the key and T = thickness of key, Width of key = 0.25 D+ 2mm Nominal thickness = 0.66 W

Peg Key A peg key is a feather type of key having a peg provided in the centre of the top face of the key as shown in

Fig 3.7. The peg fits in the hole drilled in the key way in the hub. The key is a sliding fit

in the key way of the shaft. The proportions of the key are as follows.

If D = diameter of the shaft in mm, W=Width of the key and T = Thickness of key,

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H= Thickness of peg, Width of key = 0.25 D+ 2mm Thickness of peg = 0.5 T Nominal thickness = 0.66 W

Fig 3.7

Single Head Key A single head key is also a feather type of key provided with a gib head at one of its ends as shown in fig 3.8 . The key is connected to the hub by a screw. The key is a sliding fit in the shaft. The proportions of the key are as follows.

If D = diameter of the shaft in mm, T = Thickness of key, h = height of the head, b= width of the head.

Width of key = 0.25 D+ 2mm Height of the head = 1.75 T Nominal thickness = 0.66 W Width of the head = 1.5 T

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Fig 3.8 Single Head Key

Double Head Key A double head key is also a feather type of key having integral gib head at its ends as shown in fig 3.9 . It fits tight in the hub and slides along with it in the key way in the shaft. Its proportions are as follows.

Fig 3.9 If D = diameter of the shaft in mm, T = Thickness of key, H = height of the head, B = width of the head.

Width of key = 0.25 D+ 2mm Height of the head = 1.75 T Nominal thickness = 0.66 W

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Width of the head = 1.5 T

Spline Shaft A spline shaft has a series of lengthwise rectangular grooves extending for a small portion of its length leaving an equal number of feathers in between them as shown in fig 3.10. These feathers engage with corresponding recesses provided in the hub. As compared to a keyed joint, a splined joint offers the following advantage; transmission of heavier loads, accurate centering of hub, increased strength of the joint.

Fig 3.10 Spline Shaft

Woodruff A woodruff key shown in fig 3.11 differs from those dealt earlier in that, it is not primarily intended to withstand shear forces and is used in light classes of work for holding the hub over the shaft so as to prevent it from slipping. It has a uniformly thick curved-base disc of shape somewhat less than a semicircle. It fits into a similarly shaped key way in the tapered shaft or the spindle.

Fig 3.11 Woodruff Key

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The hub of the wheel has a tapered bore to suit the tapered shaft. The wheel is driven on the tapered shaft until it fits tightly over it. When a nut is then tightened –up hard against the outer face of the hub, the key grips the hub by the wedging action and locks it on the tapered shaft. The pressure exerted by the nut therefore relieves the shear stress. The proportions of the key are as follows.

If

D= diameter of the shaft in mm, d = diameter of the key, h= height of the key, b= width of the key.

Width of key = 0.25 D Diameter of key = 4 W Height of key = 1.75 W

Pin Key

Fig 3.12 Pin Key

A pin key shown in the fig 3.12 is either a plain or a tapered rod driven in the hole partly drilled in the shaft and partly in the hub. Pin keys are used generally to hold small toothed wheels, hand wheels, levers, etc., on the spindles to prevent them from slipping. Sometimes a pin key is also used with shrunkon wheel hub. In such cases, the hub of the wheel is board with a hole equal to or less than the diameter of the shaft. The hub is then heated to expand slightly and is driven on the shaft when it is still hot. As the hub cools, it contracts and grips the shaft. To provide an extra positive hold a pin key is also used. The proportions of the key are as follows.

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If D= diameter of the shaft, d = diameter of the pin, Diameter of pin = 0.2 D Taper 1:50

Cone Key Cone key are used when pulleys having holes larger than shaft are to mounted on them. A cone key consists of three segments of a hollow conical bush as shown in fig 3.13. The hub of the pulley will have a tapered bore to suit that of the cone key. The segments of the cone key are driven between the shaft and the hub so as to hold them from slipping by the friction grip.

Fig 3.13 Cone Key

DEFINITION Nominal Surface The surface of an object is its exterior boundary. A surface that has been finished by any one of the machining processes contains numerous small peaks and valleys that deviate from the theoretical geometrically perfect surface, called nominal surface, Fig 3.14 .

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Fig 3.14

Roughness The cutting edges of the cutting tools and the abrasive grains of the grinding tools cause the texture of the surface to consist of surface irregularities. The spacing and the size of these valleys and peaks of the surface irregularities depend on the degree of sharpness of cutting tools and fineness of the abrasive grain. Roughness, as shown in Fig 3.14 B, is the microirregularity-of a surface produced by the cutting action of the edges of the tool or abrasive grains.

Waviness The waviness, as shown in Fig 3.14 C, is the surface undulations of larger magnitudes, i.e., surface irregularities with larger sizes and spacing than the roughness. Waviness may result from machine or work deflections, vibrations, warping, strains, or other causes. Roughness may be Considered as being superposed on a wavy surface as shown Figure.

Lay Lay is the predominant direction of tool marks that make a characteristic pattern on a machined Surface. The direction of lay is determined by the production method employed.

Sampling Length Sampling Length is a particular length of the profile decided for the evaluation of the surface irregularities on any chosen portion of the machined surface. This length is also known as cut-off length.

The sampling length is selected depending upon the type of machining process as indicated in Table. It

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is recommended to choose smaller value for the finer grade and larger value for the coarser grade for a given machining process when more than one values are given.

Production Method And Surface Quality The value of surface roughness which is the arithmetical mean deviation from the mean line of the profile, is expressed in micrometer. Table shows the recommended values of surface roughness. it indicated in the specified place in the surface texture symbol. if the surface roughness is obtained by any production method other than machining the value surface roughness ,say 12.5 micrometer, is indicated in the basic symbol as shown in fig3.15 . if the surface roughness is obtained by removing the material by machining, the value of surface roughness, say 12.5 micrometer, should be indicated as shown in fig3.15 .if the surface roughness is to be obtained without the removal of the material or when it results from the previous production process, the value of surface roughness say 12.5 micrometer, should be indicated as shown in fig 3.15

Fig 3.15 Indication of Surface Roughness Values in Surface Texture Symbol

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Table 3.1 Surface Roughness Expected from Different Manufacturing Process

SYMBOL FOR LAY The direction of lay is represented in the symbol form, from the following series recommended by the Bureau of Indian Standards : =

┴XMCR

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Fig 3.16

Table illustrates the interpretation of these recommended symbols. The lay is indicated on to the right of the surface texture symbol as shown in Fig 3.16. roughness is measured across the direction of lay.

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INDICATION OF SURFACE ROUGHNESS FOR VARIOUS MACHINING OPERATIONS When it s required to produce the final surface texture by a particular machining method, this should be indicated in words like MILLED, REAMED, LAPPED, HONED, etc., on the horizontal extension of the longer leg of the symbol as shown in Fig 3.17.

Fig 3.17

When the surfaces are to receive additional treatment or coating, the type of treatment or coating like, CHROME PLATED, ENAMELLED, OXIDISED, CARBURISED, SAND BLASTED, etc., should be indicated on the same extension line as shown in Fig 3.17. Unless otherwise stated, as mentioned the numerical value of roughness applies to the surface texture after treatment of the coating.

When it is necessary to indicate the surface texture both before and after treatment, it should be explained by a suitable note or by specifying the value or grade of the surface texture before coating on the finished surface and the name of the coating and the value or grade of surface texture after coating on the symbolic thick chain line which represents the additional treatment as shown in Fig 3.17. The surface roughness grade N10 shown in Fig 3.17 is the texture required before coating and the surface texture of Nil is required after chrome plating.

SURFACE TEXTURE SYMBOL WITH ALL THE CHARACTERISTICS

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Fig 3.18 shows a surface texture symbol with all the characteristics of the surface roughness

number may be indicated.

Fig 3.18

SYSTEM OF INDICATION SURFACE TEXTURE SYMBOLS ON DRAWING The surface texture symbol along with the inscriptions should be orientated such that they may be read from the bottom or right hand edge of the drawing sheet as explained in aligned system of dimensioning, which is the general system used for dimensioning the drawings.

Fig 3.19 shows the different methods of indicating the surface texture symbol on the drawing. The surface texture symbol may be placed with its apex touching the line representing the surface, or on the leader line terminating in an arrow on the surface, or on the extension line of the surface as shown in Figure.

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Fig 3.19 If it is not practicable to orient the surface texture symbol so as to be read from the bottom or right hand edge of the drawing sheet, it may be drawn in any position provided it does not carry any indication of machining method or machining allowances as shown in Fig. 3.19. Note that the roughness values are written so as to be read from the right hand edge of the drawing sheet.

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UNIT-V

INTRODUCTION A Screw is a cylinder or a conical rod with a helical groove cut on it. Its function is to transform the input motion of rotation into output motion of translation. A screw thread is a continuous helical ridge formed by cutting a helical groove on a cylinder or conical shank. Basically screw threads are used to fasten the parts together, or to transmit motion and so power, or for the relative adjustments of the adjacent parts. To perform these specific functions, threads of different profiles are used.

SCREW THREAD TERMINOLOGY The following definitions refer to the various terms used in screw threads. The various elements of a screw thread are shown in fig.4.1. The external thread is the thread cut on the outer surface of a rod.

The internal thread is the thread cut on the inner surface of a hole.

Fig 4.1

ROOT: It is the bottom portion of the surface of a thread, either flat or rounded which joins the sides of the adjacent threads.

CREST:

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It is the top portion of the surface of a thread, either flat or rounded which joins the sides of the same thread.

FLANK: It is the surface of a thread that connects the crest with the root and also it offers the surface contact with its counterpart.

ANGLE OF THE THREAD: It is the angle included between the sides of the two adjacent threads and is measured on an axial plane.

DEPTH OF THE THREAD: It is distant between the crest and the root of a thread which is measured normal to the axis on an axial plane. It is designated as h3.

NOMINAL DIAMETER: It is the diameter of the cylindrical rod on which the threads are cut. This diameter specifies the size of the screw.

MAJOR DIAMETER: It is the diameter of an imaginary coaxial cylinder which bounds the crests of an external thread or the roots of an internal thread. D and d denote the major diameters of the internal and external threads respectively.

MINOR DIAMETER: It is the diameter of an imaginary coaxial cylinder which bounds the roots of an external threads, or the crests of an internal threads. D1 and d3 denote the minor diameters of the internal and external threads respectively.

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PITCH DIAMETER: It is the diameter of the thread at which an imaginary coaxial cylinder that can passed so as to cut the thread so that the width of the cut thread will be equal to the width of the groove.D2 and d2 denote the pitch diameters of internal and external threads respectively.

HEIGHT OF THE FUNDAMENTAL TRIANGLE: The imaginary equilateral triangle which bounds a V-thread is called a fundamental triangle. Its height, H is measured normal to the axis on an axial plane.

PITCH: It is the distance from a point on a screw thread to a corresponding point on the next thread, measured parallel to the axis. It may be indicated as the distance from crest to crest, or from root to root, but former is the convention.

LEAD: It is the axial distance advanced by a nut for its one full turn over a threaded rod. It is also defined as the product of the pitch and number of starts.

Basic profiles or forms of screw threads The profile of a screw thread is based on whether it functions as a fastening device or a power transmission element. The profile is triangular, known as V thread in the former, and square or its modified in the latter. Thus the two basic profiles, or forms of screw threads are: (i) V threads (ii) Square thread.

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V-Threads Bureau of Indian Standards(BIS) adopts ISO metric thread profile as a basic profile of screw threads. The V-threads is in the form of symmetrical “V” ,the angle of thread is 60˚. The roots of both the internal and external threads are rounded, while the crests are parallel to the axis of the screw.

Fig 4.2

Square thread The square thread has its flanks at right angle to the axis, so that they are parallel to each other.

For the same nominal diameter, the pitch of the square thread is more than that of the V-thread, hence the square thread provides more axial movement. The depth and the thickness of the thread are equal and equal to half the pitch.

The square thread is generally used for high power transmission such as screw jack, vice, lead screw of lathe etc.,

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Fig 4.3 Left hand threads A left hand thread is one which advances into the nut, when turned in a counter clockwise direction, and the slope of the lines representing the thread will be downward from left to right. An abbreviation LH is used to indicate the left hand thread. unless otherwise specified, a thread should be considered as a right hand thread.

Practical application of these threads is made in coupler-nut or turn-buckle.

Fig 4.4

Right hand threads:

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Fig 4.5

A right hand threads is one which advances into the nut, when turned in a clockwise direction. It can be seen from the above fig. that when the axis of screw is vertical, the lines representing the thread will have slope downwards from right to left.

Internal Thread The internal thread is a continuous helical ridge formed by cutting a helical groove on inside of a cylinder or conical shank. The conventional representation of the Internal Thread shown in Fig.

External Thread The external thread is a continuous helical ridge formed by cutting a helical groove on outside of a cylinder or conical shank. The conventional representation of the External Thread shown in Fig.

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4.8 V-Threads

Fig 4.6 Whitworth thread

This thread was introduced by Sir Joseph Whitworth and was standardized as British Standard thread, abbreviated as BSW. The profile of this thread with standard proposition is shown in fig 4.6. It has a thread angle of 55째 and is rounded off at the crest and root which renders it less liable to damage than a sharp V thread.

Square Thread The basic form of a square thread is shown in fig 4.7. The flanks or the sides of this thread are perpendicular to the axis of the thread. The depth and the thickness of the thread is equal to half the pitch.

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Fig 4.7

Standard form of square thread:

Fig 4.8

The profile of the square thread adopted by the Bureau of Indian Standards in fig 4.8 . The depth of the external thread is equal to half the pitch. The sharp corners at the root of the external threads are rounded off to the radius R-0.25 mm The depth of the internal threads is equal to 0.5P+0.25mm. The sharp corners at the crest of the internal threads are chamfered to 0.25mm*45째.

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DESIGNATION OF THREADS Threads are designated by

indicating the type of thread, the

major diameter and the pitch.

V-threads of ISO profile are

designated pitch,

by the

the

letter

two

M

followed by the major diameter and

the

being

separated by x symbol. For example,

V-threads of major diameter 10

mm and pitch 1.25 mm is designated

as M10 x l.25 as shown in Fig

4.9.

Sometimes

when

the

V-

threads are designated without

indicating the pitch, for example Ml0

as shown in Figure, it will mean

that the threads are cut with coarse

pitch, whose value is to be

obtained from the relevant Indian Standard fig 4.9 Codes.

Fig. 4.9

Multistart threads are designated by specifying the number of starts as shown in Figure.

The screw threads are always considered as right hand unless otherwise specified. When left hand threads are to be designated the abbreviation LH must be used as shown in Figure.

The square threads are designated by the ď ż symbol as shown in Fig 4.10. The trapezoidal threads are designated by the letters Tr as shown in Fig 4.11.

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Fig 4.10

Fig 4.11

BOLTS AND NUTS A bolt is a fastening element comprising of a head at one end and a threaded portion over its cylindrical shank at the other end. The parts to be fastened temporarily by bolts and nuts, admit the bolts through the holes in them having suitable clearances. The projected threaded end of the bolt in turn now admits the washer and nut, an internally threaded member, which after sufficient turn offers necessary clamping grip. Bolts and nuts of various shapes are used for different purposes but the hexagonal head and square head are very common. Although the square shape provides a better spanner grip than the hexagon, but needs one-fourth of a turn to bring it into the same position for inserting spanner again, whereas a hexagon need only one-sixth of a turn and hence preferred. Although an octagonal shape would require one-eighth of a turn, the spanner grip reduces a it is more liable to slip. In general, the hexagonal bolts and nuts are preferred for fastenings in machines wherever the space and other conditions permit. The square head bolts are used instead of hexagonal bolts when frequent loosening and tightening is required, for example, on job holding devices like, vices, tools posts in machines, etc.

DRAWING OF HEXAGONAL BOLT AND NUT In machine drawing practice, hexagonal bolts and nuts are drawn all most in all the drawings.

Drawing of these to the actual dimensions involves laborious drafting work arid unnecessary time. Since the bolts and nuts are standard items, they need not be drawn in the assembly drawings to accurate sizes, instead they are drawn conventionally to empirical proportions listed in Table. All the. proportions are expressed in terms of the major diameter of the bolt and nut.

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Step by Step Procedure: I Step : Draw the shank of the bolt equal to the given diameter d and length. The thickness of bolt head equal to 0.8d and the thickness of nut equal to 0.9d are marked. Measure the width across corners equal to 2dand complete the three faces of the bolt head and the nut in thin lines.

The right view of the bolt and nut assembly is drawn as follows. With any point C 1 on the axis as centre and radius equal to d, draw a thin circle. Draw the vertical diameter 1-2 of this circle. With 1 and 2 as centres and radius equal to d cut the circle on either side of the vertical axis and inscribe the hexagon. The chamfer circle is drawn as a thick circle with the centre C 1 and radius C1E. TABLE 4.1 Empirical Proportions of Hexagon and Square Head Bolt & Nut

Detail

Proportion

Nominal Diameter

d = Size of Bolt or Nut, mm

Width Across Flats

s = 1.5d + 3 mm

Width Across Corners

e = 2d

Thickness of Bolt Head

k = 0.8 d

Thickness of Nut

m = 0.9 d

Root Diameter

d1 = d – (2 X Depth of Thread) or = d (4 X Thickness of Lines) Or = 0.9 d (approximate)

Length of Bolt

l = As specified

Thread Length

b = 2d + 6 mm (for l < 150 mm) = 2d + 12 mm (for l > 150 mm)

Radius of Bolt End

r = d (for spherical ends)

Chamfer of Bolt End

z = Depth of Thread X 45 or

O

= 0.1 d (Approximate) Chamfer Angle of Bolt Head &

O

= 30

Nut

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II Step : The chamfer arcs in the three face view of bolt head and nut are drawn as follows.

O

Through the corner B, draw a line at 30 to the axis of the bolt or nut to cut it at O1, With O1 as centre and radius O1A draw the chamfer arc in the centre face. The chamfer arcs on the two side faces are drawn as follows. Draw the perpendicular bisector of BC to cut BO 1 at O2. With O2 as centre and radius O2D draw the chamfer arc. Repeat the construction on the other side face.

III Step : The chamfer lines on he side faces of the three face views of the bolt head and nut are O

drawn as follows. Through the points P and Q draw lines inclined at 30 to the flat face of the bolt head O

or nut. The end of the bolt is chamfered to 0.1 d X 45 .

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The threaded portion of the shank is indicated by drawing two thin lines at a distance equal to d 1 = 0.9d.

The root circle in the right view is represented by a thin three-fourth of a circle drawn with centre CT and diameter O.9d.

The two face view of the bolt head and the nut is projected from the side view. If the side view is not drawn, then the distance across the flats is measured equal to 1.5d + 3mm. The chamfer arcs in the two face view are drawn as follows. Project P to get X. Mark Y the midpoint of FG. Draw the

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perpendicular bisector of XY and FG to intersect each other at 0 3. With centre 03 and radius 03Y draw the chamfer arc. Repeat the construction on the other face.

DRAWING OF SQUARE HEAD BOLT The square head bolt and nut are drawn to the proportions shown in Table.

I Step: Draw the shank of the bolt equal to the given diameter d and the length of the bolt. The thickness of the bolt head equal to 0.8d and the thickness of the nut equal to 0.9d are marked. The right view of the bolt and the nut assembly is drawn as follows. With any point C1 on the axis as centre and diameter equal to 1.5d + 3mm draw the chamfer circle. Draw a square circumscribing the chamfer circle with its sides inclined at 45째 to the axis. Project the corners 1 and 2 to get points P. Draw a thick circle with diameter to indicate the nominal diameter.

II Step: The chamfer arcs in the view across the corners of the bolt and nut are drawn as follows.

Through the corner P, draw a line inclined at 30째 to the axis. Draw the perpendicular bisector of PQ to intersect the 30째 line at O. With O as centre and radius. OR draw the chamfer arc. Repeat the construction on the other face.

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III Step: The chamfer line is drawn at 30째 to the flat face of the bolt head and nut. The threaded portion on the shank of the bolt is indicated conventionally by drawing two thin lines spaced at a distance equal to the root diameter d, O.9d. The root circle in the right view is represented by a thin three-fourth of a circle drawn with centre C1 and diameter 0.9d. The end of the bolt is chamfered to 0.1d x 45째.

RIVETED HEAD A riveted joint is a permanent type of fastener used to join the metal plates or rolled steel sections together. Riveted joints are extensively used in structural works such as bridges and roof trusses and in the construction of pressure vessels such as storage tanks, boilers, etc. Although welded joints are best suited to several of these applications than the riveted joints, however, riveted joints are ideal in cases where the joints will be subjected to pronounced vibrating loads. Riveted joints are also used when a nonmetallic plate and a metallic plate are to be connected together. They are also used when the joints are not expected to be heated while joining as in welding, which may cause warping and tempering of the finished surfaces of the joints.

The disadvantages of riveted joints are (i) more metal is removed while making of the holes, which weakens the working cross sections along the line of centres of the rivet holes, and (ii) weight of the rivets increases the weight of the riveted members.

Differences between a Bolt and a Rivet As a fastener, a rivet resembles a bolt, but differs from it in the shape and the application as well.

Although the shape of a rivet is similar to that of a bolt, unlike the bolts, its shank end is not threaded.

With regards to the application, it is used as a Permanent fastener to withstand shear forces acting perpendicular to its axis, whereas a bolt is used as a temporary fastener to Withstand axial tensile forces.

Rivet A rivet is a round rod made either from mild steel or nonferrous materials such as copper, aluminium, etc., with a head of any one of the shapes shown in Figure, formed at one end during its manufacture and its tail end being slightly tapered as shown in Figure. The length of the shank of the rivet must be sufficient enough to accommodate the connecting plates and also provide enough material for forming a head at its shank end. In general, the length of the shank of the rivet will be equal to sum of the thicknesses of the connecting plates plus 1.5 to 1.7 times the diameter of the rivet.

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l = length of the shank of the rivet d = diameter of rivet t = thickness of each of the connecting plates

then, l =ď&#x192;Ľ

TYPES Various types of rivet heads for use in general engineering work and boiler work as recommend by the Bureau of Indian Standards are shown in Figure. The different proportions of these rivet heads are given in terms of the nominal diameter d of the rivet. The rivet heads to be used for general purposes for diameters below 12 mm are specified in the Indian Standard Code IS: 2155-1962 and for diameters between 12 and 48 mm are specified in the Indian Standard Code IS: 1929-1961. The rivet heads to be used for boiler work are specified in the Indian Standard Code IS: 1928-1961. The rivet heads to be used for ship building are specified in the Indian Standard Code IS : 4732-1968.

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UNIT QUESTIONS UNIT 1 QUESTIONS 1.Enumerate the various manufacturing methods of machine parts which a designer should know.

2,Explain briefly the different casting processes.

3,Write a brief note on the design of castings?

4,State and illustrate two principal design rules for casting design.

5.List the main advantages of forged components.

6.What are the salient features used in the design of forgings? Explain.

7.What do you understand by ‘hot working’ and ‘cold working’ processes? Explain with examples.

8.State the advantages and disadvantages of hot working of metals. Discuss any two hot working processes.

9.What do you understand by cold working of metals? Describe briefly the various cold working processes.

10.What are fits and tolerances? How are they designated?

11.What do you understand by the nominal size and basic size?

12.What is the difference in the type of assembly generally used in running fits and interference fits?

13.State briefly unilateral system of tolerances covering the points of definition, application and advantages over the bilateral system.

14.What is meant by ‘hole basis system’ and ‘shaft basis system’? Which one is preferred and why?

15.Discuss the Indian standard system of limits and fits.

16.What are the commonly used fits according to Indian standards?

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17.What do you understand by preferred numbers? Explain fully.

UNIT-II QUESTIONS SELF â&#x20AC;&#x201C;ASSESMENT QUESTIONS:I

1.

2.

3.

4.

What are the different types of half section?

What is the spacing between hatching lines?

What is the need for sectioning?

The another name of revolved section is -----------------

UNIT QUESTIONS 1. Explain the hatching of larger areas.

2. Define hatching.

3. What is offset section?

ANSWER OF SELF ASSESSMENT QUESTIONS 1. Front view with right half in section Front view with top half in section top view with section Top view with front half in section Left view with front half in section 2. A spacing of 2mm between the hatching lines will be appropriate for the general work.

3. NEED FOR SECTIONING o

To show the internal features more clearly.

o

To remove hidden lines.

o

To avoid complication and ambiguity.

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95

For ease of understanding.

4. Superimposed sectioning

UNIT-III QUESTIONS SELF â&#x20AC;&#x201C;ASSESSMENT QUESTIONS:I Answer the following 1. What are the different types of fits?

2. Define limits 3. Sketch the symbols for the following charesteristics used for form tolerances. (a)

Straightness

(b)

Flatness

(c)

Circularity

(d)

cylindricity

UNIT QUESTIONS 1. Define fits and tolerances 2. What is transition fit?

3. Define shaft basis system.

4. Sketch the symbols of the following and classify them into : (1) Form of single feature (2) Orientation of related features and (3) Position of related features (i) Concentricity (ii) Straightness (iii) Circularity (iv) Perpendicularity (v) Cylindricity

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(vi) Angularity

ANSWERS OF SELF â&#x20AC;&#x201C;ASSESSMENT QUESTIONS

I 1. (a) Clearance fit (b) Transition fit (c) Interference fit 2. The maximum and minimum permissible sizes within which the actual machined size lies are called limits.

3. (a)

Straightness

(b)

Flatness

(c )

(c)

Circularity

Cylindricity

UNIT-IV QUESTIONS

SELF-ASSESSMENT QUESTIONS:I

1. What is the use of key?

2. Define lay.

3. What are the proportions of pin key?

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UNIT QUESTIONS: 1. Define waviness.

2. Classify the keys.

3. What is the method to indicate the surface roughness for various machining operations?

ANSWER OF SELF-ASSESSMENT QUESTIONS: 1.

Keys are extensively used to hold pulleys, gears, couplings, clutches, sprockets, etc., and the shafts rigidly so that they rotate together. They are also used to mount the milling cutters, riding wheels, etc., on their spindles

2. Lay is the predominant direction of tool marks that make a characteristic pattern on a machined Surface 3. The proportions of the pin key are as follows.

If D = diameter of the shaft, d = diameter of the pin, Diameter of pin = 0.2 D Taper 1:50

UNIT-V QUESTIONS SELF â&#x20AC;&#x201C;ASSESMENT QUESTIONS:I

1.

2.

3.

4.

What is left hand thread?

What is the difference between bolt and rivet?

Draw the neat sketch of rivet head which is used for boiler .

Draw a neat sketch to differentiate the internal thread and external thread.

UNIT QUESTIONS 1. How will you designate the threads.

2. Explain the square thread with neat sketch.

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3. Define bolt and nut with neat sketch 4. What is the use of rivets.

ANSWER OF SELF ASSESSMENT QUESTIONS 1. A left hand thread is one which advances into the nut, when turned in a counter clockwise direction, and the slope of the lines representing the thread will be downward from left to right. An abbreviation LH is used to indicate the left hand thread.

2. Rivet is used as a Permanent fastener to withstand shear forces acting perpendicular to its axis, whereas a bolt is used as a temporary fastener to Withstand axial tensile forces.

3. The flat counter sunk riveted head is used for boiler works. The proportion of the flat counter sunk rivet is shown in fig

4. In the diagram the part (A) refers the external thread, and part (B) refers the internal thread

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