metrology doc

2 STANDARDS OF MEASUREMENT As described in chapter one for every kind of quantity to be measured there must be a unit to express the result of measurement and a standard to permit making the measurement by maintaining the'uniformity through dut the world.

A standard is defined as something that is set up and established by authority as a rule for the measurement of quantity, weight, extent, value or quality etc.

Any system of measurement must be related to a known standard otherwise the measurement has no meaning. Industry, commerce, international trade and in fact modern civilization itself would be impossible" without a good system of standards. The role of standards is to support the system which makes uniform measurement throughout the world and helps to maintain interchangeability in mass production.

Systems of Measurement A measuring system is based on few fundamental units, e.g., length, mass, time, temperature, etc. All the physical quantities can be expressed in terms of these fundamental units. The following systems of measurement are in use in different countries. (a) F.P.S. System: In this system unit of length is yard, unit of mass, weight or force is pound, unit of time is seconds and unit of temperature is degree Fahrenheit.

This system being inconvenient is steadily loosing its popularity.

(6) Metric System : Metric system is the predominant system in the world. It is a decimal system of weight and measure. It is based on metre as a unit of length, kilogram as the unit of mass and kilogram force (kgf) as the unit of weight or force, unit of temperature is degree centigrade. This system is simple for calculation purposes than F.P.S. system. (c) S.I. System : This system is extension and refinement of the metric system. It is more convenient and superior to other systems. This S.I. (System International) like traditional metric system is based on decimal arithmetic. It provides only one basic unit for each physical quantity. It is comprehensive, because its seven basic units cover all disciplines. These seven basic units are as follows: (29) Derived S.I. Units Units that are a combination of two or more quantities and which usually requires a compound word to name them are called compound or derived units. Some of the derived units are as given below :

S.I. system of units is now being adopted throughout the world. This

system rationalises the main metric units of measurement and standardizes their names and symbolic representation. The main feature of this is that, the newton, the unit offeree is independent of the earth's jgravita-* tion and 'g' need not be introduced i n equations.

STANDARDS OF MEASUREMENTS 31 Development of Material Standard The need for establishing standard of length was arised primarily for determining agricultural land areas and for the erection of buildings and monuments. The earliest standard of length was established in terms of parts of human body. The Egyptian unit was called a cubit. It was equal to the length of the forearm (from the elbow to the tip of the middle figure).

Rapid advancement made in engineering during nineteenth century were due to improved materials available and more accurate measuring techniques developed. It was not until 1855 that first accurate standard, was made in England. It was known as imperial standard yard. This was followed by International Prototype metre made in France in the year 1872.

These two standards of lengths were made of material (metal alloys) and hence they are called as material standards in contrast to wavelength standard adopted as length standard later on.

Imperial Standard Yard The imperial standard yard is made of 1 inch square cross-section bronze bar (82% copper, 13% tin, 5% zinc) 38 inches long. The bar has two 1/2 inch diameter X 1/2 inch deep holes. Each hole is fitted with ~^th inch diameter gold plug. The top surface of these plugs lie on the neutral axis of the bronze bar. ' The purpose of keeping the gold plug lines at neutral axis has the following advantages.

- Due to bending of beam the neutral axis remains unaffected - The plug remains protected from accidental damage.

38" 36'at 62°F Neutral axis »/2DIA:CGOLD PLUG) # Enlarged view of-Gold plug showing engraved line — (Actual diameter 1/10") Fig. 2.1. Imperial Standard Yard.

32 METROLOGY The top surface of the gold phigs are highly polished and contains three lines engraved transversely and two lines longitudinally.

The yard is defined as the distance between two central transverse lines on the plugs when, (i) the temperature of the bar is constant at 62°Fand, (ii) the bar is supported on rollers in a specified manner to prevent flexure.

This standard was legalised in 1853 and remained legal standard until 1960.

International Standard Metre (Prototype) This standard was established originally by International Bureau of Weights and Measures in the year 1875. The prototype meter is made of platinum-irredium alloy (90% platinum and 10% irredium) havinga crosssection as shown in Fig. 2.2. The upper surface of the Web is highly polished and has two fine lines engraved over it. It is inoxidisable and can have a good finish required for ruling good quality of lines. The bar is kept at 0°C and under normal atmospheric pressure. It is supported by two rollers of at least one cm diameter symmetrically situated in the same horizontal plane.

The distance between the rollers is kept 589 mm so as to give minimum deflection. The web section chos'en gives maximum rigidity and economy of costly material. The distance between the centre portions of two lines engraved on the polished surface of this bar of platinum-irredium alloy is taken as one metre.

T6 mm Neutral axis 16 mm Lines engraved Length = 100cm.

Fig. 2.2. International Prototype metre cross-section According to this standard, the length of the metre is defined as the straight line distance, at 0°C between the centre portions of pure platinumirredium alloy (90% platinum, 10% irredium) of 102 cm total length and having a web cross-section as shown in Fig. 2.2.

TANDARDS OF MEASUREMENTS 33 The metric standard when in use is supported at two points which are 58.9 cm apart as calculated from Airy's fof mula, according to which the best distance between the supporting points is given by hL .where, L = total length of bar (assumed uniform) b = distance between points n =. number is supports 102 For prototype metre, b = —r— = 58.9 cm.

V(2)2-l This reference was designated as International Prototype Metre - M in 1399. It is preserved by (BIPM) at Sevres i n France. The B I P M is controlled by the International Committee of Weights and Measure.

The imperial standard yard was found to be decreasing in length at die rate of one-millionth of an inch for the past 50 years'when compared with internal standard meter. The prototype meter is quite stable. Therefare, yard relationship had to be defined in terms otj^etre as 1 yard = 0.9144 metre, or inch = 25.4 mm.

Disadvantages of Material Standard 1. The material standards are influenced by effects of variation of environmental conditions like temperature, pressure, humidity and ageing etc., and it thus changes in length.

2. These standards are required to be preserved or stored under security to prevent their damage or distruction.

3. The replica of these standards were not available for use somewhere •rise.

4. These are not easily reproducible.

5. Conversion factor was to be used for changing over to metric working.

6. Considerable difficulty is experienced while comparing and verifying the sizes of gauges. .

Airy Points When straight bars above 125-200 mm length are supported horizona l l y for measurements by two supports at its end, they w i l l sag i n the niddle. If the supports are provided towards the centre, then the ends w i ll Dend down. Both these extremes would result i n an error i n measurement.

This error can be minimized by providing the two supports at such a distance.that the slope at the ends is zero and the end faces of the bar are mutually parallel. Sir G.B. Airy showed that this condition was obtained when die distance between the supports is, , . where n = number of supports, Vn2-.l fc = length of the bar. For a simply supported beam, the expression becomes 34 :l3 = 0.577 L. 1(2)' -1 These points of supports are known as Airy points. Thus ÂŁ| are achieved when the distance between two supports is 0.577 x| bar. Airy points are marked for lengths above 125-200 mm length, the length bars, can be used unsupported. Airy points length standards as already described. For prototype meter marked at a distance of 58.9 cm.

Wavelength Standard The major drawback with the metalic standards meter that their length changes slightly with time. Secondly, consider ty is experienced while comparing and verifying the sizes of gai; material standards. This may lead to errors of unac'ceptal magnitude. It therefore became necessary to have a stand which will be accurate and invariable. Jacques Babim philosopher suggested that wavelength of monochromatic lig] as natural and invariable unit of length. In 1907 the Angstrom (A) unit was defined in, terms of wavelength of red dry' air at 15°C (6438.4696 A = 1 wavelength of red cadm: General Conference of Weights and Measures approved in 1 tion of standard of length relative to the metre in terms of the red cadmium as an alternative to International Prototyp

Orange radiation of isotope krypton-86 was chosen for i of length in 1960, by the Eleventh General Conference of Measures. The committee decided to recommend that.Krypa most suitable element and that it should be used in a hot-ca lamp maintained at a temperature of 63° kelvin According to this standard meter was defined as equal Wavelengths of the red orange radiation of Krypton isotope ' The standard as now defined can be reproduced to q about 1 part in 10 .

The metre and yard were redefined in terms of wave 1« Kr-86 radiation as, 1 metre = 1650763.73 wavelengths, and 1 yard = 0.9144 metre = 0.9144 x 1650763.73 wavelengths = 1509458.3 wavelengths.

Metre as of Today Although Krypton-86 standard served well, technologi demands more accurate standards. It was through that a i on the speed of light would be technically feasible and vantageous. Seventeenth General Conference of Weights 34 METROLOGY These points of supports are known as Airy points. Thus Airy points are achieved when the distance between two supports is 0.577 x length of bar. Airy points are marked for lengths above 125-200 mm, below 125 mm length, the length bars, can be used unsupported. Airy points are used for length standards as already described. For prototype meter airy points are marked at a distance of 58.9 cm.

Wavelength Standard The major drawback with the metalic standards meter and yard is that their length changes slightly with time. Secondly, considerable difficulty is experienced while comparing and verifying the sizes of gauges by using material standards. This may lead to errors of unacceptable order of magnitude. It therefore became necessary to have a standard of length which will be accurate and invariable. Jacques Babinet a French philosopher suggested that wavelength of monochromatic light can be used as natural and invariable unit of length. In 1907 the International Angstrom (Aj unit was defined in terms of wavelength of red cadmium in dry air at 15°C (6438.4696 A = 1 wavelength of red cadmium). Seventh General Conference of Weights and Measures approved in 1927, the definition of standard of length relative to the metre in terms of wavelength of the red cadmium as an alternative to International Prototype metre.

Orange radiation of isotope krypton-86 was chosen for new definition of length in 1960, by the Eleventh General Conference of Weights and Measures. The committee decided to recommend that Krypton-86 was the most suitable element and that it should be used in a hot-cathode discharge lamp maintained at a temperature of 63° kelvin.

According to this standard meter was defined as equal to 1650763.73 Wavelengths of the red orange radiation of Krypton isotope 86 gas.

The standard as now defined can be reproduced to an accuracy of Q about 1 part i n 10 .

The metre and yard were redefined in terms of wave length of orange Kr-86 radiation as, 1 metre = 1650763.73 wavelengths, and 1 yard = 0.9144 metre " = 0.9144 x 1650763.73 wavelengths = 1509458.3 wavelengths.

Metre as of Today Although Krypton-86 standard served well, technologically increasing demands more accurate standards. It was through that a definition based^ on the speed of light would be technically feasible and practically advantageous.

Seventeenth General Conference of Weights and Measures DARDS OF MEASUREMENTS 35 ÂŤd to a fundamental change in the definition of the meter on 20th iber 1983.

Accordingly, metre is know defined as the tength of the path travelled ght i n vacuum in 1/299792458 seconds. This can be realised i n practice ugh the use of an iodine-stabilised helium-neon laser.

The reproducibility is 3 parts in 101 1 , which may be compared to luring the earth's mean circumference to an accuracy of about 1 mm.

With this new'definition of metre, one standard'yard will be, the length rfthe path travelled by light travelled in 0.9144 x * sec. i.e., in 299792458 3 x 10~9 seconds.

Advantages of Wavelength Standard The advantages of wavelength standard are : 1. It is not a material standard and hence it is not influenced by effects of variation of environmental conditions like temperature, pressure, humidity and ageing.

2. It need not be preserved or stored under security and thus there is no fear of being destroyed as in case of metre and yard.

3. It is not subjected to destruction by wear and tear.

4. It gives a unit of length which can be produced consistently at a l l the times i n a l l the circumstances, at all the places. In otherwords it is easily reproducible and thus identical standards are available with a l l .

5. This standard is easily available to all standardising laboratories and industries.

6. There is no problem of transferring this standard to other standards meter and yard.

7. It can be used for making comparative measurements of very high accuracy. The error of reproduction is only of the order of 3 parts i n 101 1.

Subdivision of standards The international standard yard and the international prototype meter cannot be used for general purposes. For practical measurement there is a hierarchy of working standards. Thus depending upon their importance of accuracy required, for the work the standards are subdivided into four grades; these are : 1. Primary standards 3. Territory standards 2. Secondary standards 4. Working standards.

Primary Standards For precise definition of the unit, there shall be one, and only one material standard, which is to be preserved under most careful conditions.

36 METROLOGY It is called as primary standard. International yard and International metre are the examples of primary standards. Primary standard is used only at rare intervals (say after 10 to 20 years) solely for comparison with secondary standards. It has no direct application to a measuring problem encountered in engineering.

Secondary Standards Secondary standards are made as nearly as possible exactly similar to primary standards as regards design, material and length. They are compared with primary standards after long intervals and the records of deviation are noted. These standards are kept at number of places for safe custody. They are used for occasional comparison with tertiary standards whenever required.

Tertiary Standards The primary and secondary standards are applicable only as ultimate control. Tertiary standards are the first standard to be used for reference purposes in laboratories and workshops. They are made as true copy of the secondary standards. They are used for comparison at intervals with working standards.

Working Standards Working standards are used more frequently in laboratories and workshops. They are usually made of low grade of material as compared to primary, secondary and tertiary standards, for the sake of economy. They are derived from fundamental standards. Both line and end working standards are used. Line standards are made from H-cross-sectional form.

Neutral plane Fig. 2.3. Wooing line standard Most of the precision measurements involves the distance between two surfaces and not with the length between two lines. End standards are suitable for this purpose. For shorter lengths upto 125 mm slip gauges are used and for longer lengths end bars of circular cross-section are used. The distance between the end faces of slip gauges or end bars is controlled -to ensure a high degree of accuracy. f Some times the standards are also classified as : (a) Keference standards- Used for reference purposes.

(6) Calibration standards - Used for calibration of inspection and working standards.

STANDARDS OF MEASUREMENTS 37 (c) Inspection standards - Used by inspectors. (d) Working standards - Used by operators, during working.

Line and End Measurements A length may be measured as the distance between two lines or as the distance between two parallel faces. So, the instruments for direct measurement of linear dimensions fall into two categories.

1. Line standards.

2. End standards.

Line Standards. When the length is measured as the distance between centres of two engraved lines, it is.called line standard. Both material standards yard and metre are line standards. The most common example of line measurements is the rule with divisions shown as lines marked on Characteristics of Line Standards 1. Scales can be accurately engreved but the engraved lines themselves possess thickness and it is not possible to take measurements with high accuracy.

2. A scale is a quick and easy to use over a wide range.

3. The scale markings are not subjected to wear. However, the leading ends are subjected to wear and this may lead to undersize measurements.

4. A scale does not possess a "built in" datum. Therefore it is not possible to align the scale with the axis of measurement.

5. Scales are subjected to parallax error.

6. Also, the assistance of magnifying glass or microscope is required if sufficient accuracy is to be achieved.

End standards : When length is expressed as the distance between two flat parallel faces, it is known as end standard. Examples : Measurement by slip gauges, end bars, ends of micrometer anvils, vernier callipers ac. The end faces are hardened, lapped flat and parallel to a very high iegree of accuracy.

Characteristics of End Standards 1. These standards are highly accurate and used for measurement of dose tolerances in precision engineering as well as in standard Mboratories, tool rooms, inspection departments etc. .

2. They require more time for measurements and measure only one dfeension at a time.

3. They are subjected to wear on their measuring faces.

4. Group of slips can be "wrung" together to build up a given size; faulty •ringing and careless use may lead to inaccurate results.

5. End standards have built in datum since their measuring faces are 2a: and parallel and can be positively locked on datum surface.

6. They are not subjected to parallax effect as their use depends on feel.

The accuracy of both these standards is affected by temperature change and both are originally calibrated at 20 ¹ ^°C. It is also necessary

to take utmost case in their manufacture to ensure that the change of shape with time, secular change is reduced to negligible.

-"ANDARDS OF MEASUREMENTS .19 Classification of Standards and Traceability In order to maintain accuracy and interchangeability in the items manufactured by various industries in the country, it is essential that the standards of units and measurements followed by them must be traceable to a single source, i.e., the National Standards of the country. Further, the National Standards must also be linked with International Standard to maintain accuracy and interchangeability in the items manufactured by the various countries. \ The national labroatories of well-developed countries maintain close tolerance with International Bureau of Weights and Measures, there is assurance that the items manufactured to identical dimensions in different countries w i l l be compatible.

Application of precise measurement has increased to such an extent that it is not practicable for a single national laboratory to perform directly all the calibrations and standardizations required by a large country. It has therefpre become necessary that the orocess of traceability technique needs to be followed in stages, that is, National laboratories, standardizing laboratories, etc. need to be established for country, states, and industries but all must be traceable to a single source as shown in Fig. 2.4 below.

National Standards National Reference Standards j Working Standards | 1 11Inter laboratory Standards I'I Laboratory Reference Standards  1 ' Working^Standards Reference or works standards of lower graded (shop floor standards) F i g . 2.4. Classification of standards in order •  Clearly, there is degradation of accuracy in passing from the defining standards to the standard in use. The accuracy of a particular standard depends on a combination of the number of times it has been compared with 40 METROLOGY a standard of higher order, the recentness of such comparisons, the care with which it was done, and the stability of the particular standard itself.

Transfer from Line Standard to End Standard The primary standard is the line standard, but it is highly inconvenient for general measurement applications. Therefore, the practical workshop standards are end standard which are derived from the length standards. In end standard the distance is measured between the working faces of the measuring instrument which are flat and mutually parallel.

The end standard, thus must be calibrated from a primary line standard.

In order to transfer a line standard to an end standard a composite line

standard is used. F i g . 2.5 (a) shows a primary line standard of a basic length of one metre, but whose actual length is accurately known.

Fig. 2.5. (b) is also a line standard of basic length one metre. It consists of a central length bar having a basic length of 950 mm and two end blocks of 50 mm wrung to either end of the central bar. Each end block has a central engraved line.

1 metre calibrated Composite line standard 1 metre basic length L1 •4 (a) 950 mm.: A 50mm. (b) 50mm.

Fig. 2.5. Conversion from a line standard to an end standard The composite line standard is compared with the primary and length Li is obtained as : =A + b + c The two blocks at the end are arranged i n four ways by using a l l possible combinations, and the comparisons are made w i t h the primary line stand length L x , Thus the results of four possible combinations are : Lx -A + b + c L±=A + b+d L\ +a+c and L i =A+a + d Adding the four measurements, we get 4L1 = 4A + 2a + 2b + 2c + 2d = 4A + 2(a + b) + 2(c + d) ...U) STANDARDS OF MEASUREMENTS 41 As the blocks (a + b) and (c + d) cannot be exactly of the same length.

The two are compared and let the difference between them is, as shown in Fig. 2.6 below.

We have c+d = (a + b)+x -(H) (a+b) (c+d) Fig. 2.6. Comparison of blocks Putting this value of c+d In eqn. (£), we get 4L1 = 4A + 2(a + b) + 2[(a + b)+x] . « 4L1 = 4A + 2(a + b) + 2(a + b)+x 4 L 1 = 4A + 4(a + 6) + 2x Dividing by 4, we have L 1 = A + (a + 6) + |* .

An end standard of known length can now be obtained consisting of either A f ( a + 6) or A + (c+d) as shown in Fig. 2.7. The length of id+b. F c+d = (a+b)+x -X 2

Fig. 2.7.

42 METROLOGY A + (a + b) is A + (A + b) + x less ^ x where (a + b) is the shorter of the two end blocks.

The length of A + (c + d) is A + (a + b) + ^ x plus ^ x where (c + d) is the longer of the two end blocks. The calibrated composite end standard can be used to calibrate a solid end standard of the same basic length.

Sub-divisions of the End Standard by Brookes Level Comparator Brooks Level comparator devised by A . J . C . Brookes, consist of a very accurate spirit level. The spirit level is supported on balls so that it makes only point contact on gauges. The table on which the gauges are to be compared is levelled properly by a spirit level. Then the two gauges to be compared are wrung on the table and the spirit level is lowered on them.

After recording the reading, the table is rotated through 180°, so that the position of two gauges is interchanged as shown in F i g . 2,8. The spirit; level is lowered down on gauges again and the bubble reading is noted. If the two gauges are not of equal length then the two readings will be different and the effect of interchanging the position of the gauges will be to tilt the level through an angle equal to twice the difference between the height of gauges divided by the spacing of level supports. The difference of gauges will, therefore, be equal to half of the above difference. As the distance between the two balls is fixed the bubble readings can be directly calibrated in terms of difference in height. The advantage of turning of table by 180° is to eliminate the effect of table not being levelled initially.

Spirit level Standard to be -» compared ——— standard-* Standard i - to be compared ir Fig. 2.8.

It is desirable that the room temperature must be maintained at 20°C by thermostatic control. Moreover, sufficient time must be allowed for the room and the gauges to attain steady temperature.

Comparison of an end gauge with a line standard by the displacement method: Fig. 2.9 illustrates trie principle of comparison of end gauge with a line standard by the displacement method. In this method tool room microscope i is used for small specimens and a horizontal microscope for longer j specimens.

ANDARDS OF MEASUREMENTS 43 The line standard is placed on a carrier and adjusted such that the line .A is brought under the cross wires of the- microscope. The micrometer

spindle of the microscope is rotated until it makes contact wiih the projection on the carrier and the micrometer reading is noted. The carrier is then moved along to bring the line B under the microscope cross wir :>,s. Then the end gauge is inserted as shown in the Fig. 2.9 and the micrometer reading is noted again. The length of the end gauge is equal to the length of the line standard plus the difference between micrometer readings.

Fixed microscope Line of (J rL , n< measurement-\ / "X Carrier ine standard ^^^^^^^^^^^^ X Z= L End guage â&#x20AC;˘577LFig. 2.9. Comparison of an end gauge with a line standard End and Length Bars End bars and length bars are made in sizes varying from 100 mm to 1200 mm. They are made of high quality tool steel hardened and stabilized.

Their end faces are lapped w i t h a high degree of accuracy and flatness.

The length or end bars are available i n four grades of accuracy a) Reference grade (c) Inspection grade and, (6) Calibration grade (d) Workshop grade.

The workshop and inspection grades have internally threaded ends and can be used in combination with the help of a standard stud. To maintain accuracy in measurement, the end bars should be supported at Airy points which are engaged on it. However, when the end bars are used to built up desired length by combination it becomes necessary to calculate the A i r y points for the accurate length of the combination, i f the bars are to be used And supported in a horizontal plane.

Calibration of End Bars The procedure for calibrating two bars each of basic length 500 mm with the help of one piece calibrated metre bar is as follows : 44 METROLOGY The calibrated metre bar is wrung to the surface plate and the two 500 mm bars are wrung together' to form a basic length of one metre. This combination of two 500 mm bars is then wrung to the surface plate along side the calibrated metre bar. The difference in height xh is obtained with the help of a microscope as illustrated i n F i g . 2.10. r OCt CO B T B I 3C2 Fig. 2.10.

Let LA = Length of one of the 500 mm bars.

LB - Length of the other 500 mm bar. xx = the difference between the calibrated metre bar and the combined length of barA and B„ Then from the first measurement L+x1 = LA + LB (depending upon whether LA + Lg is longer or shorter than L) From the second measurement - LA ± x2 = LQ (depending upon whether A is longer or shorter than B) Then, L ±x1 = LA +LB L±x1=LA + (LA±x2) = 2LA±x2 is 2LA = L ± x1 ± x2 L±x1±x2 LA = 1 LB=LA+x2 x Same procedure can be used for calibrating three, four or any number of length bars of the same basic size. Since the length of metals changes with varying temperature, it is necessary to maintain room temperature at 20°C to obtain accurate results.

3 LINEAR MEASUREMENT Linear measurement. Linear measurement applies to measurement of lengths, diameters, heights and thicknesses including external and internal measurements. The line measuring instruments have series of accurately spaced lines marked on them, e.g. scale. The dimension to be measured is aligned with the graduations of the scale. Linear measuring instruments are designed either for line measurements or end measurements.

In end measuring instruments, the measurement is taken between two end surfaces as in micrometers, slip gauges etc.

The instruments used for linear measurements can be classified as : (t) Direct measuring instruments (ii) Indirect measuring instruments The direct measuring instruments are of two types : , (£) graduated («) non-graduated.

The graduated instruments include rules, vernier callipers, vernier height gauges, vernier depth gauges, micrometers, dial indicators etc. The no '.-graduated instruments include callipers, trammels, telescopic gauges, surface gauges, straight edges, wire gauges, screw pitch gauges, radius gauges, thickness gauges, slip gauges etc. They can also be classified as : v (j) Non-precision instruments such as steel rule, calliper etc. (ii) Precision measuring instruments, such as vernier instruments, micrometers, dial gauges etc.

Steel Rule (Scale) Steel rule is a line measuring device. It is a part replica of them international prototype meter. It compares an unknown length to befl measured with the previously calibrated length. It is made of hardened! steel or stainless steel having series of equally spaced lines engraved on i t • Steel rule is most commonly -<sed in workshop for measuring com-B

ponents of limited accuracy. The & ks on a good class rule vary from 0.12H mm to 0.18 mm wide, so that we cannot expect to obtain a degree (J accuracy much closer than within 0.012 mm. The quickness and ease w i t l 9 which it can be used and its low. cost, makes it a popular and widely usedH measuring device.

The steel rules are manufactured in different sizes and styles. Thesr â&#x20AC;˘ are available in 150, 300, 600, or 1000 mm lengths. The scale need not bel graduated uniformly throughout its length. It may be graduated hi.^B millimeters in sqme portion1 and 1 millimeters on the other.

(52) UNEAR MEASUREMENT S3 desirable qualities of the steel rule are : It should be made of good quality spring steel.

If should be machine ground on its faces and have clearly engraved lines.

It should have graduations on both edges.

It should have minimum thickness.

It should be chrome plated to prevent corrosion and protection against staining.

Precautions while using a Steel Rule The following precautions should be taken while using a steel rule : 1. The ends of the rule are worn out due to continuous or improper . use. It should be preserved from wear, as it generally forms the basis for one end of dimension.

2. The rule should never be used for cleaning between parts or as a substitute for screw driver, for scraping Tee slots and Machine tables, otherwise its edges and ends will be damaged.

3. Rusting of the rule should be avoided by oiling it during weekends and when it is not in use. ' 4. To maintain sharpness of the graduations for easy and accurate reading, scale should be cleaned With grease dissolving fluids.

5. To have correct reading of the dimension to be measured scale should never be laid flat on the part to be measured.

6. When taking measurements with a rule, it should be so held that the graduation lines are as close as possible (preferably touching) to the faces being measured.

7. To avoid parallax error, while making measurements, eye should be directly opposite and 90° to the mark on the part to be measured.

Callipers To measure the diameter of a circular part it is essential that the measurement is made along the largest distance or true diameter. The steel rule alone is not a convenient method of measuring directly the size of the circular part. A calliper is used to transfer the distance between the faces of a component to a scale or micrometer. It thus converts an end measurement situation to the line system of the rule.

The calliper consists of two legs hinged at top and the ends of legs span

the part to be measured. The legs of the calliper_are made from carbon and alloy steels. They are exactly identical in shape, with the contact points equidistant from the fulcrum. The working ends are suitably hardened and tempered to a hardness of 400 to 500 HV, and the measuring faces to a hardness of 650 Âą 50 HV. The 1.

2.

3.

4.

5.

54 METROLOGY The accurate use of c a l l i p e r depends upon the sense of feel t h a t can o n l y be acquired by practice. W h i l e u s i n g c a l l i p e r s the f o l l o w i n g rules s h o u l d be followed : (i) hold the c a l l i p e r g e n t l y a n d n e a r t h e j o i nt (ii) hold i t s q u a r e to the work (Hi) apply only l i g h t g a u g i n g pressure (iv) handle it gently to a v o i d d i s t u r b i n g the s e t t i n g for accurate measurement.

C a l l i p e r s c a n be c l a s s i f i e d as : (i) F i r m j o i n t ( F i x e d j o i n t ) c a l l i p e rs (ii) S p r i n g type c a l l i p e r s.

F i r m j o i n t c a l l i p e r s . F i r m j o i n t c a l l i p e r s w o r k on t h e f r i c t i o n c r e a t e d> at t h e j u n c t i o n of legs. The legs m a y become loose after c e r t a i n use, b u t can be a d j u s t e d easily. These c a l l i p e r s are p a r t i c u l a r l y s u i t a b l e f o r l a r g e work.

T h e y can be designed for outside as w e l l as i n s i d e measurements. The d i s t a n c e between t h e j o i n t r o l l e r centre a n d t h e extreme w o r k i n g e n d of one of t h e l e g i s k n o w n as n o m i n a l size. T h e f i rm j o i n t c a l l i p e r s a r e a v a i l a b l e in t h e n o m i n a l sizes of 100, 150, 200, 250 a n d 300 mm. The legs of these c a l l i p e r s are made w i t h r e c t a n g u l a r cross-section.

S p r i n g c a l l i p e r s . S p r i n g c a l l i p e r s a r e i m p r o v e d v a r i e t i e s of o r d i n a ry f r i c t i o n j o i n t c a l l i p e r s . T h e legs of s p r i n g c a l l i p e r s are made f r om s u i t a b le a l l o y steel, the m e a s u r i n g faces p r o p e r l y heat t r e a t e d to a h a r d n e s s of 650 Âą 50 H V . The two legs c a r r y a c u r v e d s p r i n g at the top, f i t t e d i n the notches. A flanged p i n is i n s e r t e d between the two legs i n the c i r c u l ar grooves p r o v i d e d a l i t t l e b e l ow t h e notches. T h e c u r v e d s p r i n g i s m a d e from c a r b o n s p r i n g s t e e l . It is p r o p e r l y h a r d e n e d a n d t e m p e r e d to a h a r d n e s s of 470 to 520 H V . A screw is f i x e d i n one l e g a n d made to pass t h r o u g h the

other. It is p r o v i d e d w i t h a k n u r l e d n u t for m a k i n g adjustments. The tendency of the s p r i n g i s to force the legs a p a r t a n d the d i s t a n c e between t h em c a n be a d j u s t e d by a p p l y i n g the p r e s s u r e a g a i n s t t h e s p r i n g p r e s s u re by t i g h t e n i n g the n u t . T h u s , i n s p r i n g c a l l i p e r s the legs are h e l d f i r m ly a g a i n s t the a d j u s t i n g nut by s p r i n g t e n s i o n . S p r i n g c a l l i p e r s are more a c c u r a t e a n d p e r m i t accurate sense of t o u c h i n m e a s u r i n g . The i n s i d e a nd o u t s i d e s p r i n g c a l l i p e r s are a v a i l a b l e i n s i z e s of 75, 100, 150, 200, 250 a nd 300 m m .

C a l l i p e r s c a n also be c l a s s i f i e d a c c o r d i n g to t h e i r use as u n d e r : (i) Outside c a l l i p e r s (ii) Inside c a l l i p e rs (Hi) Transfer c a l l i p e r s (iv) Odd l e g c a l l i p e r s. (i) Outside callipers. An outside c a l l i p e r has t w o legs w h i c h a r e bent i n w a r d s as s l i o w n i n fig. 3.1. It c a n be used for m e a s u r i n g or c o m p a r i ng d i a m e t e r s , t h i c k n e s s e s a n d other outside d i m e n s i o n s by t r a n s f e r i n g the r e a d i n g s to t h e steel r u l e , v e r n i e r c a l l i p e r or micrometer.

LINEAR MEASUREMENT 55 F i g . 3.1. (a) F i r m j o i n t outside c a l l i p e r (6) S p r i n g t y p e p u t e i d e c a l l i p e r , When measuring with outside callipers they should be adjusted by tapping one leg, or by adjusting screw, untill when the work is straddled by legs, it is just possible to feel the contact between calliper and work. The contact should not be too heavy, otherwise the legs may be slightly sprung and false reading obtained. When a nice feel has been obtained on the job the size should be read on rule by resting the end of one leg on the end of the rule and taking the readings at the other. To set outside callipers to a fairly particular size they should be set from a block or gauge of the given dimension. (ii) Inside calliper. The inside calliper is made with straight legs which are bend outwards at the ends. Inside callipers are used for measur-. ing hole diameters, distance between shoulders etc. While using they should be adjusted until! they are at the largest size at which their legs can just be felt contacting the extremities of a diameter of the hole, and to find this, the joint should be held by the thumb and first finger, one leg held stationary in contact with the inside of the hole and the other leg rocked about in a small circle. The opening of inside callipers can be checked by a rule or micrometer. The inside spring calliper has the advantage that the calliper can be withdrawn from the hole by closing the legs, when the legs are released after withdrawal it stops at the measured position of legs.

F i g . 3.2. (a) F i r m j o i n t type (6) Inside c a l l i p e rs 56 (Hi) Transfer callipers. Transfer callipers are used for measuring recessed areas from which the legs of the callipers cannot be removed ttti-cctly, tout must"De collapsed, alter trie dimension rias "been measured. In these callipers, an auxiliary arm is provided to preserve the original setting after the legs are collapsed.

Fixed joint Fixed joint Lock nut Scriber - Notch Fig. 3.3. Transfer Calliper Fig. 3.4. Odd leg calliper (iv) Odd leg callipers. Odd leg callipers are also called as Hermophrodite callipers. These are scribing tools having one leg bent and the other leg equipped with a scriber. Distances from the edge of a workpiece may be scribed or measured with these callipers. They may have either friction joint or a spring arrangement. Odd leg callipers are specifically used for finding centres of circular jobs, marking a line parallel to a true edge and many other types of marking operations.

Surface Plate Surface plate forms the basis of measurement. They are extensively used in workshops and metrological laboratories where inspection is carried out. They are used as : (i) A reference or datum surface for testing flatness of surfaces.

Fig. 3.5. Surface plate.

EAR MEASUREMENT 57 (ii) Reference surfaces for all other measuring instruments haying flat Bases e.g., for mounting V-blocks, angle plates, sine bars, height gauges, i i a l gauges, comparators etc.

Surface plates are massive and highly rigid in design. They have truly fat level planes. They are generally made up of C.I. free from blow holes, delusions and other surface defects and are heat treated to relieve internal Presses. A l l the surface plates are of deep sections and properly ribbed at sbe bottom, so that they are rigid enough to carry their own weights as well as the weights of heavy objects placed on them, without appreciable deflecaon.

The top surface is scrapped to true flatness within close limits of accuracy-either by hand scrapping or by lapping. The four edges of the plate are finished smooth, straight, parallel and reasonably square to each other and to the top surface. B i g surface plates are provided with four levelling screws to adjust their top surfaces truly horizontal.

M a t e r i a l of s u r f a c e p l a te The surface plates should be manufactured from a material: - which w i l l provide high degree of rigidity - freedom from warping - capable of taking high finish and - resistant to wear and corrosion.

C. L The most commonly used material for making surface plates is pin in or alloyed, close grained C.I. of good quality, the plate should be lEowed to age either naturally or by proper heat treatment in order to relieve internal stresses. The heat treatment is carried out by keeping the surface plates in an annealing furnace and heating up to 450 to 500째C and keeping at .this temperature for about 3 hrs or more depending on its size.

The C.I. plates have the advantage of allowing a certain amount of

wringing and surface or edges are not readily chipped out jf something is dropped on it.

G r a n i t e . Granite surface plates are rustless and unaffected by dampaess and temperature variation, heat etc. There are no burns and therefore these maintain correct flatness at a l l times. It is harder than C.I. These surface plates are non-wringing. Due to non-magnetic properlies the magnetic base stand cannot be used.

G l a s s . Surface plates are also made of glass. These are available i n six szes ranging from 150 x 150 mm to 660 x 900 mm. Glass surface plates aave the following advantages : 1. These are light i n weight.

2. These plates are free from corrosion and burrs.

3. These plates maintain their accuracy for longer period. ' 4. These plates can be manufactured w i t h high accuracy.

However, glass surface plates are breakable and needs careful hanikng.

Due to non-magnetic properties magnetic base cannot be used.

Ceramic surface plates are also used nowadays.

58 METROLOGY Care of surface plates (precautions while using surface plates): 1. The surface plates are used as a reference or datum surface and needs, to be protected from damage. The measuring instruments should not be allowed to drop on its surface.

2. When not in use, they should always be kept covered with a felt lined wooden cover.

3. They should be firmly supported on the stands and properly levelled.

4. The variations in local flatness of the surface should be checked occasionally.

5. During use its top surface should be wiped clean from dust and other particles.

6. After use, the surface should be coated with a corrosion resistance liquid such as petroleum jelly or a thin film of oil, grease etc.

7. The full available working area of the surface plate should be used instead of using limited area. This will ensure equal wear, as far as possible over the whole surface.

Testing flatness of surface plates Flatness is defined as the minimum distance between two parallel planes that contain the surface. Flatness testing is similar to straightness testing except that measurements are to be done over a surface instead of a plane. Thus, the measurements of straightness arc made along a number of lines such that the whole surface is covered and then flatness error is calculated.

In testing the flatness of surface plate, it is the general practice to measure the actual deviation from the true plane at various points.

The various methods of testing of surface plate are : ' 1, Using two footed/three footed twist gauge 2. Spirit level method

3. Auto collimator 4. Beam comparator 5. Laser beam 6. By comparison with the liquid surface 7. Interference method etc.

1. Using two footed twist gauge. Two-footed twist gauge is an electronic indicator. It has a sensitivity of one-tenth micron. It is used for overall twist in surface plate. The instrument is adjusted on the surface plate i n such a manner that its base rests on the plate and the end feet rests on the diagonal corners of the plate. The instrument is set such that its measuring probe registers zero on a central gauging lapped pad. The twftt gauge is then swung 90째 to the opposite diagonal corners.

The relative difference i n heights or "twist" is registered as plus or minus reading on a gauging lapped pad.

LINEAR MEASUREMENT 59 2. Comparison with the liquid surface. In this method the surface of a liquid is used as a reference. It is the' most rapid method. It can be conveniently used for testing large surface with an accuracy ot the order of 0.005 mm.

In this test two cylinders connected by rubber tubing at thair bases as shown in F i g . 3.6 are used. They contain mercury or dilute soda solution.

Both the tubes have micrometer head having a conical point to the spindle.

For testing the flatness of a surface one of the cylinder is placed i n the centre and the other cylinder is moved at different positions on the surface to be tested.

F i g . 3.6. , At each of the position the micrometer spindles are moved till contact is m ^ l e with the liquid surface and readings are taken (for each position).

The difference in two readings when the other cylinder is moved from one position to another indicates the errors in flatness at that particular position tested.

While using this method it should be ensured that the inter connecting rubing must be free from air bubbles. A stop cock is provided in the tube which is closed while moving the second cylinder from one position to another; this prevents the flow of liquid from one cylinder to another while moving one of the cylinder at different positions.

3. Beam Comparator. The principle of this method is based on comparing the straightness of a succession of generators in the surface :mg Supporting foot Spherically ended Contact Surface plate spring plunger plunger F i g . 3.7. Beam comparator 60 METROLOGY being tested with that of a known reference straight edge, by means of a sensitive dial indicator. The comparator determines the vertical displacement of the mid-point of any particular generator with reference to a theoretical straight line joining its ends. In this manner the apparent or

concavity of each generator may be used to determine the general departure of the surface from a mean true plane.

The be^m comparator consists of a body i n the f o rm of a light beam provided with three spherically ended supporting feet, one of which is fixed at the centre of the beam but offset from the line joining the other feet. The two supporting feet at the ends may be adjusted relative to the central foot.

To prevent the beam being over turn, a fourth foot, in the form of spherically ended spring plunger, is fitted so that it comes into operation only i f the beam receives an impulse tending to roll it over.

A very sensitive dial indicator is supported in the central web, as shown in F i g . 3.7. The axis of the spherically ended contact plunger of the dial indicator lies in the plane through the axis of the two adjustable feet.

A reference surface whose straightness has already been checked by other methods and thus with known error of straightness is used for testing purpose. For testing purposes the comparator is supported upon equal slip gauges on both the reference and the test surfaces and the readings are taken along the different generators AB, AC, AD, HF etc. Fig. 3.8, i l lustrates the method adopted for testing a surface plate of size 100 cm x 600 cm. Usually, the flatness of the plate towards the edge is not bothered and thus a distance of about 5 cm from.all the edges is not considered.

Fig. 3.8. Testing of surface plate by Beam comparator The span of the adjustable feet is set i n turn for each of the different generators and observations are taken first on the reference surface, then on the surface plate and finally on the reference surface again.

Tool makers flats Tool makers flats represent the most accurate reference surface.

These are .used for measurement work of the highest precision. Tool makers flat is a circular disc of hardened steel 20 to 25 mm thick. It is made of solid steel free from inclusions which after proper heat treatment gives a hardness of about 850 H V . F i g . 3.9, shows a toolrnakers flat. It's upper and lower AR MEASUREMENT 61 aces are machined, ground and lapped smooth, polished, flat to 0.00025 . It allows slip gauges to be wrung to it.

Fig. 3.9. Tool Maker's Flat.

It exhibits straight fringes when viewed through an optical flat. Tool maker's flats are available commercially in sizes from 50 mm to 200 mm cameter. It is recommended that a shallow groove should be provided all round the periphery of larger flats (from 100 mm onwards) to facilitate handling and also to minimize the possibility of uneven hardening.

Angle plate C.I. angle plates are widely used in workshops or inspection laboratories with surface plates for measarement purposes. It has two working surfaces truly perpendicular to each Longitudinal edge other. Angle plates are generally made f jâ&#x20AC;&#x201D; interior face from close grained C.I. After being cast and rough machined, they are heat ^^JV^^N^T^-â&#x20AC;&#x201D; End face

treated to.relieve internal stresses and are then finished by scrapping. The _ material used for these plates should be sound, free from blow holes and porosity. Fig. 3.10. Aijgle plate.

No sharp edges are allowed in the plates.

Angle plates are available in two grades depending upon the accuracy.

Grade I - all exterior and interior faces and edges be finished by either grinding or hand scrapping.

Grade II - all exterior faces are finished by planning or milling jperation.

Multipurpose angle plates The multipurpose angle plates have threaded holes on the faces and . sides which enables them to be used as fixture. These plates are made with all the ten faces accurately ground and 90째 true to one another. Accurately ground step is provided on the inside of three angle plates. The outer faces have recessed grooves. All these features allow them to be used for variety of purposes.

V-Block The V-block is made of C.I. with all the faces machined true. ' V grooves are provided on two opposite sides and slots on other two faces as 62 METROLOGY s h o w n i n F i g . 3.11. G e n e r a l l y t h e angle of V i s 90째 a n d these are a v a i l a b le i n w i d e v a r i e t y o f shapes, [/-clamps a r e p r o v i d e d to secure t h e w o r k f i r m ly on the V-groove.

Clamp screw Fig. 3.11. V-block V - b l o c k s a r e m a i n l y u s e d for t h e f o l l o w i n g p u r p o s e s: 1. To h o l d t h e c y l i n d r i c a l w o r k pieces f i r m l y for m a r k i n g centres.

2. F o r c h e c k i n g out of r o u n d n e s s of c y l i n d r i c a l w o r k pieces.

3. To e s t a b l i s h p r e c i s e l y the centre l i n e or a x i s of c y l i n d r i c a l w o rk pieces.

4. T h e y m a y also be used to 'support r e c t a n g u l a r components at 45째 to t h e d a t u m surface.

5. V - b l o c k s are a l s o m a n u f a c t u r e d i n p a i r s for h o l d i n g a n d s u p p o r t i n g l o n g c i r c u l a r components p a r a l l e l to t h e d a t u m surface.

D e p e n d i n g u p o n the accuracy. Is : 2949-1964 specifies the V - b l o c k s i n two grades ; g r a d e A a n d grade B. These grades v a r y o n l y i n t h e a m o u n t of f l a t n e s s t o l e r a n c e o n t h e w o r k i n g faces of t h e blocks. V - b l o c k s h a v i n g 120째 i n c l u d e d angles between V-grooves are a l s o a v a i l a b l e .

I n u s i n g V - b l o c k , i t is v e r y e s s e n t i a l t h a t t h e c y l i n d r i c a l piece s h o u id r e s t f i r m l y o n the sides of t h e V a n d not on edges. These blocks shouloYbe checked p e r i o d i c a l l y for basic accuracy and should be prevented from rusting.

Straight edges S t r a i g h t edges are u s e d for c h e c k i n g s t r a i g h t n e s s a n d f l a t n e s s of the p a r t s i n c o n j u n c t i o n w i t h surface plates a n d s p i r i t l e v e l s . Is : 2200 covers

C . I . s t r a i g h t edges of two types of design. (a) bow shaped s t r a i g h t edges f r om 300 to 800 m m l e n g th (b) I-section s t r a i g h t edges f r om 300 to 5000 m m l e n g t h.

C . I . s t r a i g h t edges are made f r om close g r a i n e d g r e y C . I. These are w i d e l y - u s e d for t e s t i n g m a c h i n e tool s l i d e w a y s . T h e bow s h a p e d s t r a i g h t edges a r e h e a v i l y r i b b e d to p r e v e n t d i s t o r t i o n .

T h e y are p r o v i d e d w i t h feet u p o n w h i c h t h e y s h o u l d s t a n d w h e n not i n use to p r e v e n t d i s t o r t i o n o f s t r a i g h t edge u n d e r i t s o w n w e i g h t . T h e feet _;NEAR MEASUREMENT 63 - Feel for supporting when not in use Length 300 mm to 21/2 m Support points p. (a)Rectangular section straight edge ;ngr; L engraved arrows indicating support points Working surface oooooo Working surface Thickness F i g . 3.12. are provided at the point of minimum deflection. Steel or granite straight edges are available in rectangular cross-section i n lengths up to 2000 mm and have bevelled edge.

The straight edges are classified as : 1. Tool makers straight edge 2. Wide edge straight edge 3. Angle straight edge 4. Box straight edge.

Tool makers straight edges are intended for very accurate work. They are available in various cross-sections and'lengths from 75 to 300 mm. The accuracy of straight edge should be such that when placed against a well illuminated background on the tool maker's flat or plate no white light should be visible. For checking purposes usually a flat back glass test plate is also provided with straight edge.

Spirit Levels Spirit levels are used for (i) measuring small angle or inclinations ii) to determine the position of surface with respect to the horizontal position and (Hi) to establish a horizontal datum.

The spirit level consists of a sealed glass tube mounted on a base. The inside surface of the tube is ground to a convex barrel shape having large radius. The precision of the level depends on the accuracy of this radius of the tube. A scale is engraved on the top of the glass tube. The tube is nearly filled with either ether or alcohol, except a small air or vapour i n the form

of a bubble.

Principle. The bubble always tries to remain at the highest point of the tube. If the base of the spirit level is horizontal, the centre point is the highest point of the tube. So that, when the level is placed on a horizontal surface, the bubble rests at the centre of the scale. If the base of the level is tilted through a small angle, the bubble will move relative to the tube a distance along its radius corresponding to the angle. The F i g . 3.13, shows two positions of the base of the level (OAx and OA2 ) and corresponding positions of the bubble (Bh B2). When the base (OAj) is horizontal, the 64 bubble occupies position B^. Let '9' be the small angle through which the base is tilted. The bubble now * occupies the position B2- Let T be the distance travelled by the bubble along the tube and 'h' the difference i n heights between the ends of the base, then, l=RQ-andh=L9 Therefore, Q=^ = y or l = h METROLOGY Fig. 3.13. Principle of spirit level. where, R = radius of curvature of the tube L = length of base Sensitivity of spirit level is the angle of tilt i n Seconds that will cause the bubble to move through one division, (i) A 10 second level means that t i l t i n g the level through an angle of 10 seconds to the horizontal will move the bubble by one devision. (ii) A level with a sensitivity of 0.05 mm/m means that i f the level were placed one metre long straight edge, and that if one end of that straight edge was raised by 0.05 mm, then the bubble will move one division. It depends'upon the radius of curvature, length of bubble and internal radius of the tube.

Uses. Spirit levels are used (i) to measure small angles or inclinations and, (ii) to test straightness and flatness of surfaces. When using a spirit level to determine horizontal surface it should be used i n two directions at right angles to check the horizontal plane.

Combination Square (combination set) The combination set is a commonly used, versatile, non-precision instrument. It is used in layout and inspection work.

It consists of: ii) steel rule {ii) square head iiii) protractor head, and (iu) a centre head.

The steel rule is grooved a l l along its length. The sliding square is fitted i n the groove. B y setting the steel rule flush w i t h the sliding head, it may be used as a height gauge as well as depth gauge. The steel rule can be removed from the head, permitting the use of the rule and sliding head

separately.

One surface of the square head is always perpendicular to the rule andj it can be adjusted at anyplace by a locking bolt and nut. The spirit level provided i n the square head is used to test the surface for parallelism. Fo laying out dovetels an included angle is also mounted on the scale. It c also slide to any position and locked there. A scribing point is also insert into the rear of the base for scribing purposes. ^EAR MEASUREMENT 65 By substituting the centre head for the sliding head, a centre square obtained for finding the centre line of cylindrical objects. The centre head slotted in the centre so that the rule, when inserted, bisects the 90° angle. : this way the measuring surface becomes tangent to the circumference of indrical work making it possible to locate the centre. .• J • I • I • I • I • | < I • F i g . 3.14. Uses of c o m b i n a t i o n set The protector can also be fitted on the steel rule. It can also slide along e rule. It contains, a semi-circular disc graduated from 0 to 90° on either F i g . 3.15. Uses of c o m b i n a t i o n set 66 METROLOGY side of centre. It also contains a spirit level which can be used for levelling a surface or checking or measuring an included angle i n relation to the horizontal or vertical plane.

Fig. 3.15 illustrates, the various uses of a combination set: Universal Surface Gauge on Scribing Block The universal surface gauge is the most versatile instrument used in non-precision measurement. It consists of a rigid steel base ground perfectly flat on the bottom and sides. It has V-groove in the bottom for use on Fig. 3.16. Universal surface gauge cylindrical work surfaces. A spindle carrying a scriber in a universal clamp is attached to the base. The spindle can be inclined to the base i n any position and clamped i n place by tightening the spindle nut. A fine adjustment screw is provided in the base to enable the scriber to set accurately.

The scriber can be adjusted at any position on the spindle by a clamping screw.

The scribing block is mainly used for scribing lines at a given height from a datum surface. The scribing block is used in conjunction with th steel rule. The steel rule is set i n a vertical position w i t h the help of a angl plate. The bent end of the scriber is then used for transfering dimensions from the rule to the work by drawing horizontal lines. It is thus used to measure and mark vertical positions, including parallel lines, from a surface plate datum. It can also be held i n the lathe chuck.

Engineer's Square (Try square) A try square consists of two straight edges rigidly set at right angle The blade is made of good quality tool or alloy steel suitably stabilized, an lapped. It is riveted to the stock, which is of C.I. or cost-steel. Generally, th stock is thicker than blade so that it may be set against the edge of the wor -[NEAR MEASUREMENT 67

*~S'de face of blade Outer edg Of Mode -» _ . ^ * ~ * » d e fac - Inner edge of blade «- Blade - Groove or relief Inner working ock / r Inner worl / / face of st —•!

I —2x error Ii H I! (b) (o) LI11 F i g . 3.17. Engineer's square Is : 2103-1972 prescribes three grades of accuracy (grade A, grade B and grade C) for engineer's square. The standard sizes of blade length are : 50, 100, 150, 200, 300, 500, 700 and 1000 mm. For good results, it is desirable that: 1. The blade should be slightly relieved across the width at the junction of the inner faces of the stock and blade, to make a good contact with the work surface to be checked.

2. The edges of the blades and also the working faces of the stock should be parallel. The maximum permissible error in the parallelism {i.e., the difference between the maximum or minimum width) between the rdges of the blade or the working faces of stock should be ± L nicrons for grade A ' ± 5 + L 100 microns for grade 'J3' and ± 2+ 10 + 100 L 100 micron for grade 'C where L = length of blade or stock in mm.

3. The error in parallelism of the side faces of stock should not exceed 3 times that of the working faces of the stock as specified above.

4. The side faces of the stock should be truly square with the working aces of the stock.

5. Also, the outer edges of the blades should also be perfectly square with the outer working face of the stock.

6. The blades should be rigidly and permanently fixed in the stock by

rjrveting.

68 METROLOGY I n c h e c k i n g s t r a i g h t n e s s of a n edge of the workpiece place the t ry s q u a r e as i n E i g . 3.17 ( w i t h the blade p e r p e n d i c u l a r to the edge b e i ng tested). I f t h e edge is not s t r a i g h t , w e can see the l i g h t b e t w e e n the blade a n d the w o r k . W h e n m a r k i n g o u t , press the stock f i r m l y a g a i n s t the w o rk w i t h t h e left h a n d a n d m a r k w i t h the scriber. ' To t e s t t h e t r y s q u a r e for accuracy, first place the s q u a r e o n a t r u e edge or base as i n p o s i t i o n (a) i n F i g . 3.17. D r a w a l i n e a l o n g t h e b l a d e . T h e n t u rn t h e whole t r y s q u a r e over i n t o p o s i t i o n (b) a n d d r a w a n o t h e r l i n e a l o n g the b l a d e . F i g . 3.17 shows how the l i n e s appear w h e n the s q u a r e is less t h an r i g h t angle.

Engineer's Parallels E n g i n e e r ' s p a r a l l e l s are g e n e r a l l y u s e d i n tool r o om for s e t t i n g the w o r k p i e c e s at d e s i r e d d i s t a n c e i n m i l l i n g , g r i n d i n g a n d s h a p e r vices or for c h e c k i n g purposes on t h e surface p l a t e s . C o m m e r c i a l l y , t h e s e are a v a i l a b le i n t h r e e forms : (t) S o l i d type (ii) Box type, a nd (Hi) A d j u s t a b l e type.

S o l i d type engineer's p a r a l l e l s a r e a v a i l a b l e i n t w o grades A and B, d e p e n d i n g upon t h e i r accuracy. T h e y are made of s u i t a b l e steel h a r d e n ed to 550 H V . T h e y are g e n e r a l l y s u p p l i e d i n p a i r s a n d the l e n g t h of the two pieces i n p a i r s s h o u l d agree to w i t h i n 0.5 m m . T h e t h i c k n e s s a n d w i d t h of two members i n a p a i r s h o u l d be equal to w i t h i n ± 0.002 m m . A l l t h e faces s h o u l d be s t r a i g h t to w i t h i n ± 0.002 m m f r om t h e t r u e m e a n p l a n e . A l l t he four sides of e n g i n e e r ' s p a r a l l e l a r e perfectly f i n i s h e d , g r o u n d a n d lapped a n d s h a r p edges r o u n d e d off or b e r e l l e d . T h e p a r a l l e l s a r e a v a i l a b l e i n n i ne sizes v a r y i n g f r om 5 x 10 x 100 to 50 x 100 x 400 mm.

B o x type p a r a l l e l s are available i n five sizes v a r y i n g from 100 x 100 x 100 m m to 250 x 250 x 350 mm. I n order reduce the weight these are made of h o l l ow cross-section. These dimensions are made acc u r a t e to w i t h i n ± 0.02 m m . A l l t h e four w o r k i n g faces a r e finished ground.

T a p e r e d p a r a l l e l s (adjustable tvpe) consists of two pieces, one fixed member a n d other s l i d i n g membei the same size. T h e s l i d i n g m o t i o n is o b t a i n e d by a d o v e t a i l . T h e two members can be h e l d firmly i n desired p o s i t i o n by means of a l o c k i n g screw arrangement. Tapered p a r a l l e ls p r o v i d e a n o t h e r m e a n s for m e a s u r i n g i n t e r n a l diameter. T h e y are a l s o used to check the w i d t h a n d p a r a l l e l i sm o f slots a n d to p l u g h o l e s for m e a s u r i ng hole centre distance. T h e y are a v a i l a b l e i n seven sizes, a l l h a v i n g a t h i c k ness

of 8 m m a n d l e n g t h between 40 to 100 mm.

Planer Gauge â&#x20AC;˘ \ The p l a n e r gauge i s a convenient a n d q u i c k w a y of s e c u r i n g v a r i a b l h e i g h t s above a surface plate. I t consists of two t r i a n g u l a r s h a p e d bloc' w i t h s l o p i n g sides w h i c h c a n be clamped.

LINEAR MEASUREMENT 69 It's main member or base (A) is a 3 0 - 6 0 - 9 0 ° triangle as shown in Fig. 3.18, It can be rested on the surface plate.

The member (B) can be slided along the hypotenuse and clamped in any position by means of a clamping screw. This enables the surface'S' to be adjusted vertically up or down to the desired height.

The surface 'S' always remains parallel to the surface plate on which the main member of the planer gauge rests.

S (Sliding surface) C .(Clamp screw) Fig. 3.18. Planer Gauge.

Feeler Gauge Feeler gauge is used to measure/check the clearance between the two mating parts. For example, it can be used in gauging of the clearance between the piston and cylinder and also for adjusting the spark gap between the distributor points of an automobile. The feeler gauge set consists of narrow strips of sheet steel of different thickness assembled hinged) together in a holder.

Their working entirely depends upon the sense of feel. In using the strips (blades), it is essential that the strips should neither be forced between the surfaces nor slide freely. The correct strip or a combination of strips will give a characteristic 'gauge fit' type of feel.

A set of feeler gauge generally consists of a.series of blades of thickness varying from 0.03 to 1 mm. The blades are made of heat treated bright polished tool steel. The width of the blade in 12 mm at the heal and tapered for outer part of their length so that the width at the tip is approximately 6 mm. The holder protects the blade when not is used. The nominal thickness of the blade is marked on it legibly.

IS : 3179 recommends seven sets of feeler gauges with thickness from 0.03 to 1.00 mm. Each set is devised so as to permit maximum utility with minimum number of blades. Table below gives the thickness and number of blades in each set as recommend by Indian standard IS : 3179.

Fig. 3.19. Feeler gauge 70 METROLOGY Set No. No. of blades

in the set Thickness of blades in mm 1 8 0.03 to 0.01 in steps of 0.01 '2 9 0.03, 0.03, 0.04, 0.04, 0.05, 0.05, 0.06, 0.07„0.08 3 16 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 4 ' 11 0.03, 0.04, 0.05, 0.06, .0.07, 0.1, 0.15, 0.20, 0.30, 0.40, 0.50 5 14 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.20, 0.25, 0.30, p.40, 0.50, 0.75, 1.0 6 11 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.15, 0.20, 0.40, 0.75,1.0 7 11 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.8.5, 0.90, 0.95, 0.10 Screws pitch gauges Screw pitch gauge is used to check the pitch of the screw thread. They quickly determine the pitch of the thread by matching the teeth on the strips with the teeth on the work. They are available with 55° and 60° included thread angles. A typical set is shown in F i g . 3.20.

Fig. 3.20 Screw pitch gauge.

Radius gauges Radius gauges are employed for checking external and internal radii on a curved surface. Radius gauges consists of sets of blades. The corresponding radius is permanently marked on each blade. The set consists of blades with internal radius on one side and external radius on the other so that it may be suitable for checking fillets as well as radius. The passage of light between the gauge and the work allows the radius to be checked properly.

Fig. 3.21. Radius gauge aNEAR MEASUREMENT 71 Wire Gauge W i r e gauges F i g . 3.22 a r e u s e d for f i n d i n g d i a m e t e r s of w i r e s by i n s e r t i n g t h e w i r e i n t h e notches p r o v i d e d a n d f i n d i n g out w h i c h i t f i t s .The i i a m e t e r a n d t h e n u m b e r m a r k e d on t h e d i s c are r e a d off f r om t h e gauge.

The w i r e gauge h a s t h e r a n g e f r om 0.1 m m to 10 m m .

Fig. 3.22. Wire gauge Precision Linear Measurements The mass p r o d u c t i o n w h i c h i s a c h a r a c t e r i s t i c of m o d e r n e n g i n e e r i ng m a n u f a c t u r e makes i t n e c e s s a r y to m a n u f a c t u r e component p a r t w i t h close

d i m e n s i o n a l tolerances to m a k e t h em i n t e r changeable. I n t e r c h a n g e a b i l i ty eaa be a c h i e v e d only by p r e c i s i o n d i m e n s i o n a l control of t h e p a r t s b e i ng m a n u f a c t u r e d . T h u s , to measure the d i m e n s i o n s of t h e p a r t w i t h close accuracy p r e c i s i o n i n s t r u m e n t s p l a y a n i m p o r t a n t role.

Characteristics of Precision. Measuring Instruments To measure the d i m e n s i o n s of t h e m a n u f a c t u r e d part, the p r e c i s i on m e a s u r i n g i n s t r u m e n t s s h o u l d possess t h e f o l l o w i n g c h a r a c t e r i s t i c s: ({) High degree of sensitivity. The p r e c i s i o n m e a s u r i n g i n s t r u m e nt s h o u l d be s e n s i t i v e . I f t h e m e a s u r i n g i n s t r u m e n t is s e n s i t i v e , a s m a ll change i n t h e m e a s u r e d d i m e n s i o n can be e a s i l y d e t e r m i n e d . T h e i n s t r u ment s h o u l d be designed i n s u c h a m a n n e r that i t s s e n s i t i v i t y r e m a i ns constant t h r o u g h o u t the r a n g e of d i m e n s i o n to be measured. (ii) High degree of accuracy. It s h o u l d h a v e h i g h degree of accuracy so t h a t i t w i l l be a b l e to m e a s u r e the d i m e n s i o n s of t h e p a r t s close to b e t r ue values. {Hi) Precision. The i n s t r u m e n t s h o u l d give n e a r l y t h e s a m e r e a d i ng for r e p e a t e d measurements of s a m e q u a n t i t y. (iv) Proper calibration. The a c c u r a c y w i l l be h i g h i f c a l i b r a t i o n is proper a n d c l e a r. (v) Less wearing. Wear of t h e m e a s u r i n g surfaces a n d o t h e r p a r t s of the i n s t r u m e n t s s h o u l d be as m i n i m u m as possible. (vi) Minimum inertia. Inertia a n d f r i c t i o n i n m o v i n g p a r t s o f i n s t r u ment s h o u l d be m i n i m u m , so t h a t i t w i l l not be s l u g g i s h . A l l t h e i n s t r u â&#x20AC;&#x201C; 72 METROLOGY ments which depends on mechanical linkage and mechanical system, displacement of fluid, diaphragm etc. are subjected to disadvantage of inertia.

However, instruments based on the optical principle are entirely free from inertia. (vii) Good amplification. The measuring instrument should be able to amplify the very small changes in the quantity to be measured.

Principle Vernier Pierre Vernier, a Frenchman, devised principle of vernier for precise measurements in 1631.

The principle of vernier is based on the difference between two scales or divisions which are nearly, but not quite alike for obtaining small difference. It enables to enhance the accuracy of measurement.

Least Count of Vernier Instruments Vernier instruments have two scales, main scale and the vernier scale.

The main scale is fixed and the vernier scale slides over the main scale.

When zero on the main scale coincides with the zero on the vernier scale, the vernier scale has one more division than that of the main scale with

which it coincides. So, the value of a division on vernier scale is slightly smaller than the value of a diyision on the main scale. This difference is the least count.

Least count (L.C.) is the difference between the value of main scale division and vernier scale division. Thus least count of a vernier instrument = Value of the smallest division on the main scale - The value of the smallest division on the vernier scale.

Fig. 3.23, illustrates the principle of vernier scale and gives a clear idea about its least count. main scale O 10 20 30 frfl SO |»/^iff/|iUin%^l/np/ti"^W|'jsu/|^iffi|/.tiA•"" 0 5 / 0 15. ZO 25 30 35 t+o 1,5 s o vernier scale Fig. 3.23. Least count of vernier The value of smallest division on the main scale is 1 mm. F i g . 3. shows that 50 divisions on the vernier scale coincides with 49 divisions .o the main scale. Therefore, the value of smallest division on vernier seal 49 = — mm. Thus, least count = value of smallest division on main scale 50 value of smallest division on vernier scale. _ J V E A R M E A S U R E M E N T 73 L.C. 49 50" 0.02 mm.

The least count can also be calculated by the ratio of the value of iTinimnm division on the main scale to the number of divisions on the wernier scale, in this case L.C. = ~ = 0.02 mm.

50 Vernier Calliper Construction. The vernier calliper consists of two scale : one is fixed md the other is movable. The fixed scale, called main scale is calibrated on L-shaped frame and carries a fixed jaw. The movable scale, called vernier scale slides over the main scale and carries a movable jaw. The movable jaw *s well as the fixed jaw carries measuring tip. When the two jaws are closed the zero of the vernier scale coincides with the zero of the main scale. For precise setting of the movable jaw an adjustment screw is provided. Also, ir. arrangement is provided to lock the sliding scale on the fixed main scale.

-Measuring" t i p s (for internal diamension) r Main scale (fixed scale 5678 11|11 l l l l III!Mill I I I ! mi I I I I I I I ii Vernier scale (movable scale) ; C l a m p i n g s c r ew M o v a b l e jaw

Measuring tips "(for external diamension) Fig. 3.24. Vernier c a l l i p er Uses. Vernier callipers are employed for both internal and external ;asurements. It is generally used by closing the jaws on to the work rface and taking the readings from the main as well as the vernier scale, obtain the reading, the number of divisions on the main scale are first read off. The vernier scale is then examined to determine which of its ion coincide or most coincident with a division on the main scale. Fig. 3.25, rate the procedure for taking reading by using vernier instrument. / main scale, reading vernier scale rcoding l i i i i i i i i i i j 1111 Fig. 3.25 74 METROLOGY 1. Before using the instrument, it should be checked for zero error. The zero line on the vernier scale should coincide with zero on the main scale.

Then take the reading in mm on the main scale to the left of zero on sliding scale ; in the figure it is 12.5 mm.

2. Now count the number of divisions on the vernier scale from zero to a line which exactly coincides with any line on the main scale. From the figure, we observe that 17th division exactly coincides with the line on the main scale. The total reading is now = main scale reading + the number of the division which exactly coincides with a division on main scale x L.C. of vernier calliper. Thus, in our example, total reading in mm = 12.5 + 17 x 0.02 = 12.84 mm.

Vernier callipers can also be preset to a given measurement for checking the dimension of a component.

Types of Vernier Callipers According to IS : 3651 - 1974 there are three types of vernier callipers to meet the various needs of external and internal measurements upto Knife edge measuring face" for internal measurement ^•pBeeaamm ^, Main scale P .1 .4 .3 '.* 6 7 8 .9 «ih 20 Fixed jaw T Depth measuring blade 1 Vernier scale , \ [ Clamping screw Guiding surface ^Sliding jaw • Ternal measuring faces --Clamping screw Fixed jaw •/_ ?.....•'. ,2.,3. A 5 6 7 8 9 W'lf ;12 13 H 15 16 I

X Vernier scale G u , d ! n S s u r f a c e External measuring faces Sliding jaw scale External measuring faces f Knife edge face for marking purpose • Clamping screw Guiding surface Internal measuring faces Fig. 3.26. Type of vernier calliper as specified in IS = 3651-1974 NEAR MEASUREMENT 75 2000 m m w i t h v e r n i e r l e a s t count or a c c u r a c y of 0.02, 0.05, a n d 0.10 mm.

These are a v a i l a b l e i n s i z e s o f : 0 - 1 2 5 , 0 - 2 0 0 , 0 - 3 0 0 , 0 - 5 0 0 , 0 - 7 50 0 - 1 0 0 0 , 7 5 0 - 1 5 0 0 , a n d 1500 to 2000 mm.

The t h r e e types of c a l l i p e r s recommended by I n d i a n s t a n d a r d are types A , B a n d C as s h o w n i n F i g s . T h e nomenclature-as per I S 3651 is also pven.

Type A has j a w s on both sides for e x t e r n a l a n d i n t e r n a l measurements, a n d also has blade for depth measurements. T h e v e r n i e r c a l l i p e rs are made of s u i t a b l e good q u a l i t y s t e e l of h a r d n e s s 650 ± 50 H V . T h e b e am should be flat throughout its l e n g t h to 0.05 m m for 150 m m size. The r i d i n g surface of t h e b e am s h o u l d be s t r a i g h t to w i t h i n 0.01 m m . E x t e r n a l • e a s u r i n g faces a r e l a p p e d flat to w i t h i n 0.005 m m a n d i n t e r n a l m e a s u r i ng isces a r e p a r a l l e l to t h e c o r r e s p o n d i n g m e a s u r i n g faces to w i t h i n 0.025 mm.

The l e a s t count i s u s u a l l y m a r k e d on t h e v e r n i e r c a l l i p e r .

It s h o u l d be noted t h a t o n types B a n d C i n t e r n a l m e a s u r e m e n t s are • a d e by a d d i n g w i d t h o f t h e i n t e r n a l m e a s u r i n g j a w s to the r e a d i n g on the scale. ' F o r t h i s reason the combined w i d t h of i n t e r n a l j a w s is u s u a l ly s i a r k e d on t h e j a w s of types B a n d C c a l l i p e r s . T h e combined w i d t h s h o u ld ; e u n i f o rm t h r o u g h o u t i t s l e n g t h to w i t h i n 0.01 mm.

Possible E r r o r s i n V e r n i e r Instruments The e r r o r s i n the measurement c a r r i e d out by a v e r n i e r i n s t r u m e n t are u s u a l l y d u e to m a n i p u l a t i o n o r m i s - h a n d l i n g o f t h e i n s t r u m e n t a n d i ts jaws on t h e w o r k p i e c e . T h e v a r i o u s causes of e r r o r are as g i v e n below : U) E r r o r due to t h e p l a y b e t w e e n s l i d i n g j a w o n t h e scale. (ii) If t h e s l i d i n g j a w frame becomes w o r n or w a r p e d , i t w i l l not s l i de s q u a r e l y o n t h e m a i n s c a l e a n d w i l l cause e r r o r i n m e a s u r e m e n t.

(Hi) D u e to w e a r a n d w a r p i n g of t h e j a w s t h e zero l i n e o n m a i n scale may not coincide w i t h t h a t o n t h e v e r n i e r scale. T h i s i s c a l l e d as zero e r r o r.

(iv) E r r o r s are a l s o caused b y i n c o r r e c t r e a d i n g o f t h e v e r n i e r s c a l e as t h e scales are d i f f i c u l t to r e a d even w i t h the a i d of m a g n i f y i ng glass. (v) E r r o r is also i n t r o d u c e d i f the l i n e of m e a s u r e m e n t does not coincide w i t h t h e l i n e of t h e scale. (vi) Since i t is d i f f i c u l t to o b t a i n correct feel due to i t s s i z e a n d w e i g ht a n e r r o r m a y be i n t r o d u c e d due to i n c o r r e c t feel.

Precautions i n the use of v e r n i e r c a l l i p er To m i n i m i z e the errors, the f o l l o w i n g p r e c a u t i o n s s h o u l d be t a k en whileusingtheinstrument: (i) T h e l i n e of m e a s u r e m e n t m u s t coincide w i t h the l i n e o f scale. ' (ii) W h i l e m e a s u r i n g o u t s i d e d i a m e t e r s w i t h v e r n i e r c a l l i p e r t h e p l a ne i f the m e a s u r i n g t i p s o f t h e c a l l i p e r m u s t be p e r p e n d i c u l a r to t h e c e n t r e l i ne rfthe work piece. T h e c a l l i p e r s h o u l d not be t i l t e d or t w i s t e d .

76 METROLOGY (Hi) G r i p the i n s t r u m e n t n e a r or opposite to the j a w s a n d not b y the o v e r h a n g i n g projected m a i n b a r of c a l l i p e r . (iv) M o v e the c a l l i p e r j a w s on the w o r k w i t h l i g h t t o u c h . Do not a p p ly undue p r e s s u r e. (u) T h e a c c u r a c y of m e a s u r e m e n t p r i m a r i l y depends on two sense, v i z , sense of s i g h t a n d sense of feel (touch). The s h o r t c o m i n g of i m p e r f e c t v i s i on c a n be overcome by the use of corrective eye glass a n d m a g n i f y i n g glass.

The sense of t o u c h v a r i e s f r om p e r s o n to p e r s o n a n d c a n be developed w i th sufficientpracticeandproperhandling. {vi) T h e m e a s u r i n g i n s t r u m e n t m u s t a l w a y s be p r o p e r l y b a l a n c e d in h a n d a n d h e l d l i g h t l y i n s u c h a w a y t h a t o n l y fingers handle t h e m o v i n g a nd a d j u s t i n g screws.

V e r n i e r H e i g h t Gauge V e r n i e r h e i g h t gauge is s i m i l a r to v e r n i e r c a l l i p e r b u t i n t h i s i n s t r u ment the g r a d u a t e d bar is h e l d i n a v e r t i c a l position' a n d i t i s used in c o n j u n c t i o n w i t h a surface p l a t e. ' C o n s t r u c t i o n . A v e r n i e r h e i g h t gauge consists of (i) A f i n e l y g r o u nd a n d l a p p e d base. T h e base i s m a s s i v e a n d r o b u s t i n c o n s t r u c t i o n to ensure r i g i d i t y a n d s t a b i l i t y.

Vertical bar Fine adjustment screw Vernier scale Bracket Clamping screw Clamp ,

T Scribing jaw Base Fig. 3.27. Vernier height gauge NEAR MEASUREMENT 77 (ii) A vertical graduated beam or column supported on a massive base.

(Hi) Attached to the beam is a sliding verpier head carrying the vernier scale and a clamping screw. (iv) A n auxiliary head which is also attached to the beam above the sliding vernier head. It has fine adjusting and clamping screw. (v) A measuring jaw or a scriber attached to the front of the sliding vernier.

Use. The vernier height gauge is designed for accurate measurements and marking of vertical heights above a surface plate datum. It can also be used to measure differences in heights by taking the Vernier scale readings at each height and determining the difference by substraction. It can be nsed for a number of applications in the tool room and inspection department.

The important features of vernier height gauge are : - All the parts are made of good quality steel or stainless steel.

- The beam should be sufficiently rigid square with the base.

- The measuring j aw should have a clear projection from the edge of the beam at least equal to the projection of the base from the beam.

- The upper and lower gauging surfaces of the measuringjaw shall be flat and parallel to the base.

- The scriber should also be of the same nominal depth as the measuringjaw so that i t may be reversed.

- The projection of the j aw should be at least 25 mm.

- The slider should have a good sliding fit for all along the full working length of the beam.

Height gauges can also be provided w i t h dial gauges instead of vernier.

This provides easy and exact reading of slider movement by dial gauges which is larger and clear.

Precautions. When not in use, vernier height gauge should be kept in its case. It should be tested for straightness, squareness and parallelism of the working faces of the beam, measuringjaw and scriber. The springing of the measuringjaw should always be avoided.

Electronic Digital Read out Height Gauge The digital height gauge provides an immediate digital read out of the measured value without any ambiguity. It is possible to store this value in memory and use as a datum for further readings, or for comparing with given tolerances. It is also possible to provide a binary coded digit outputs to enable the results for further statistical analysis and for providing print out.

The air-bearing floatation system provides a cushion of a i r between the base of the stand and the surface plate. It helps to reduce the effort

required to move the gauge and the possibility of damage to the surface 78 METROLOGY plate. The instrument can be adjusted to zero at any position after which it will display positive or negative dimensions with reference to the datum.

Vernier Depth Gauge Vernier depth gauge is used to measure the depths of holes, slots and recesses, to locate centre distances etc. It consists of (i) A sliding head having flat and true base free from curves and waviness. (it).A graduated beam known as main scale. The sliding head slides over the graduated beam.

(HI) An auxiliary head with a fine adjustment and a clamping screw.

The beam is perpendicular to the base in both directions and its ends square and flat. The end of the sliding head can be set at any point with fine adjustment mechanism locked and read from the vernier provided on it, While using the instrument, the base is held firmly on the reference surface and lower the beam into the hole untill'it contacts the bottom surface of the hole. The final adjustment depending upon the sense of correct feel is made by the fine adjustment screw. The clamping screw is then tightened and the instrument is removed from the hole and reading taken in the same way as the vernier calliper.

While using the instrument it should be ensured that the reference surface on which the depth gauge base is rested, is satisfactorily true, flat and square.

Fine adjustment Screw Vernier scale Fig. 3.28. Vernier height gauge Micrometer The accuracy of the vernier calliper is 0.02 mm ; most engineeri precision work has to be measured to a much greater accuracy than th" especially to achieve interchangeability of component parts. To achieve tb

greater precision, measuring equipment of a greater accuracy and se sitivity must be used. Micrometer is one of the most common and m~ popular forms of measuring instrument for precise measurement with 0.0 mm accuracy. Micrometers with 0.001 mm accuracy are also availabl Micrometers can be classified as : LINEAR MEASUREMENT 79 (a) Outside Micrometer (6) Inside Micrometer (c) Screw Thread Micrometer and, (d) Depth Gauge Micrometer.

Principle of Micrometer Micrometers work .on the principle of screw and nut. We know that when a s.crew is turned through nut through one revolution, it advances by one pitch distance i.e., one rotation of screw corresponds to a linear movement of a distance equal ^o pitch of the thread. If the circumference of the screw is divided into number of equal parts say 'n', its rotation through one division w i l l cause the screw to advance through â&#x20AC;&#x201D;-â&#x20AC;&#x201D; length. Thus, the minimum length that caii be measured by such arrangement will bepitch A By reducing the pitch of the screw thread or by increasing the number of divisions on the circumference of screw, the length value of one circumferential division can be reduced and accuracy of measurement can be increased considerably.

Least Count of Micrometer Least count is the minimum distance which can be measured accurately by the instrument. The micrometer has a screw of 0.5 mm pitch, with a thimble graduated in 50 divisions to provide a direct reading of pitch" n 0.50 50 = 0.01 mm.

Least count of a micrometer is thus, the value of one division on a thimble, which is connected to the screw.

Pitch of the spindle screw L . C . of micrometer: Number of divisions on the spindle Outside Micrometer Fig. 3.28, illustrates an external micrometer. It is used to measure the outside diameter and length of small parts to an accuracy of 0.01 mm. The main parts of an outside micrometer are : Locknut does not push spindle over to one side Adjustable main nut ensures maximum number of threads in contact with screw Stop nut prevents thimble from slipping Fig. 3.29. Part sectional view of a micrometer 80 METROLOGY

1. U-shaped steel frame. The outside micrometer has 'IT shaped or 'C shaped frame. It holds all the micrometer parts together. The gap of the frame permits the maximum diameter or length of the job to be measured.

The frame is generally made of steel, cast steel, malleable C.I. or light alloy.

It is desirable that the frame of the micrometer be provided with conveniently placed finger grips of heat insulating material. ' Anvil and spindle. The micrometer has a fixed anvil protruding 3 mm from the left hand side of the frame. The diameter of the anvil is the same as that of the spindle. Another movable anvil is provided on the front of the spindle.-The anvils are accurately ground and lapped with its measuring faces flat and parallel to the spindle. These are also available with tungsten carbide faces. The spindle is the movable measuring face with the anvil on the front side. The spindle engages with the nut. It should run freely and smoothly throughout the length of its travel. There should be no backlash between the spindle screw and nut. There should be full engagement of nut and screw when the micrometer is at its full reading.

Lock nut. A lock nut is provided on the micrometer spindle to lock it when the micrometer is at its correct reading. The design of the lock nut is such that it effectively locks the spindle without altering the distance between the measuring faces. It thus retains the spindle in perfect alignment.

Sleeve or barrel. The sleeve is accurately divided and clearly marked in 0.5 mm division along its length which serves as a main scale. It is chrome plated and adjustable for zero setting.

Thimble. The thimble can be moved over the barallel. It has 50 equal divisions around its circumference. Each division having a value of 0.01 mm. .

Ratchet. The ratchet is provided at the end of the thimble. It is used] to assure accurate measurement, and to prevent too much pressure being applied to the micrometer. When the spindle reaches near the work surface to be measured the operator uses the ratchet screw to tighten the thimble.

The ratchet automatically slips when the correct (uniform) pressure ia applied and prevents the application of too much pressure.

The micrometer usually has a maximum opening of 25 mm. They arej available in measuring ranges of 0 to 25 mm, 25 to 50 mm, 125 to 150 mm) upto 575 to 600 mm.

Reading the Micrometer The procedure for measuring the dimension with the help of micrometer is as described below : 1. Select a micrometer with a desired range depending upon the size! of the workpiece to be measured. The next step is to check it for zero errorj In case of 0-25 mm micrometer the zero error is checked by contacting the faces of the fixed anvil and the spindle. While using micrometers of 25-5fl mm or 125 to 150 mm size, the zero error is checked by placing a master o i LINEAR MEASUREMENT 81 25 mm or 125 mm respectively, between the anvil and spindle faces. The zero on the thimble should coincide with the zero on the reference line on

the main scale (barrel). If this does not happen, then zero error is present in the micrometer which must be taken into account while taking the readings. A special spanner is usually provided with the micrometer for eliminating the zero error.

2. The barrel has graduations, in intervals of 1 mm above the reference line. There are also graduations below the reference line at the middle of two successive upper graduations, so as to read 0.5 mm.

3. For measuring the particular dimension, hold the work between the faces of the anvil and spindle and then . i , i . . i - i i Barret graduated move the spindle by rotating the thimble , ,â&#x20AC;&#x17E; mm and i 2 mm Th.mbie until the anvil and spindle touches the work surface. Make fine adjustment with â&#x20AC;˘ the ratchet. Now take the reading on the main scale taking into account the divisions below the reference line. Suppose, the main scale reading is 11.00 mm as, shown in Fig. 3.30. ' *"ig. 3.30. Reading the micrometer 4. Take the thimble reading which coincides with the reference line on the sleeve. In our example it is 34.

Then total reading = main scale reading + L.C. x reading on the thimble = 11.00 + 0.01 x 34 = 11.34.

Precautions to be taken while using a micrometer 1. First clean the micrometer by wiping of oil dirt, dust and grit etc.

2. Clean the measuring faces of the anvil and spindle with a clean piece of paper or cloth.

3. Set the zero reading of the instrument before measuring.

4. Hold the part whose dimension is to be measured and micrometer properly; Then turn the thimble with forefinger and thumb till the measuring tip just touches the part and find adjustment should be made by ratchet so that uniform measuring pressure is applied.

5. While measuring dimensions of circular parts, the micrometer must be moved carefully over representative arc so as to note maximum dimension only.

Procedure for setting zero Before a micrometer is used to measure the dimension of a component it is necessary to set zero (by removing zero error if it exists). To do this, first clean the measuring faces and then rotate the thimble until the two anvils are touching and the ratchet slips. At this point reading is taken ; this should read zero. If the zero mark on the sleeve does not coincide with zero on the thimble there is an zero error. Then use the adjusting spanner to

82 METROLOG correct the error with slight rotation of the barrel. A small hole in the bar is provided for this purpose.

Procedure for checking flatness and parallelism of micromet

anvil surfaces : (Testing for accuracy) Micrometer is an end measuring device. It expresses length as t distance between its anvil surfaces. The lack of flatness and parallelism the anvils will result in incorrect functioning of the instrument and \ cause errors in the measurement. Therefore, the micrometer anvil surfa should be perfectly flat andiruly parallel to each other.

The flatness and parallelism of micrometer is generally checked wf an optical flat. When checking the flatness, an optical flat is brought contact with each of the two anvils in turn and moved in such a manner t" minimum number of fringes are produced. By observing the number fringes produced, the condition of flatness of anvil surface can be de mined. Interference fringes observed on each of the two anvils should be more than two for practically all ranges of grand I micrometer.

Parallelism of the anvils of micrometer can also be checked by ur optical flat and flat parallel plate. The interference fringes produ between flat parallel plate and anvil surfaces are observed. The sum of fringes observed on the two anvil surfaces indicates the degree of p lelism between the surfaces.

For 0-25 mm micrometer the sum of the fringes should not be than 6, for 25 to 75 micrometer not more than 7 and for 25-100 microme not more than 10.

Possible sources of error in micrometers Some possible sources of error which may occur in micrometer cause incorrect readings are as follows : 1. Lack of flatness of anvil surfaces.

2. Lack of parallelism of the anvils at some or all parts of the 3. In accurate setting of the zero reading (zero error).

4. In accurate readings following the zero position.

5. In accurate readings shown by the fractional divisions o thimble.

6. Applying two much pressure on the thimble "or not using ratchet.

7. Wear of the anvil surfaces threads on spindle, due to const incorrect use. • » 8. Wear of ratchet stop mechanism, locking arrangement etc.

Types of outside Micrometers A number of different designs of outside micrometers available the specific application. Some of these are : EAR MEASUREMENT 83 1. Outside micrometer with dial indicator on the anvil side - (it es direct reading and eliminates human error).

2. Screw thread micrometer - to measure pitch of the thread.

3. Disc type micrometer. It is used for measuring distance between r teeth of spur gear or helical gear.

4. Sheet metal micrometer. These are specially designed for gauging •et metal, fibre etc.

5. Limit micrometers. It consists of two micrometer heads installed arallel i n one frame and can be adjusted to go and not go dimensions of iauge.

6. Spline micrometers. These are used for reading of the diameter of me shafts groove.

7. Point micrometer*. These are used for quick comparison in cutting ÂŤw threads, for measuring web thickness of drills, taps, small grooves d recesses where a conventional micrometer cannot be used.

8. Tube micrometers. These ai e used for measuring the wall thicks of solid or split bearings, tubings, sleeves, rings etc.

9. V-anvil micrometer. It is used f6r measuring odd fluted taps, Ling cutters, reamers etc. It's special feature is that any out of roundness dition can be quickly checked i n centreless grind and machining opera10. Indicating micrometer. It consists of a indicator built in the rometer. It can be operated as go no of gauge. The indicator of this rometer has tolerance hands which can be set as desired. erential Screw Micrometer As the name implies, this type of micrometer uses differential screw iple and hence is more accurate than ordinary micrometer. F i g . 3.31 s differential mircrometer. In this micrometer, the right hand screw J s of different pitches one smaller and one larger are provided on the spindle. (In conventional micrometer the spindle has threads of one rm pitch).

By operating the thimble moves one screw forward and other backand a differential movement is obtained.

M a i n s c r ew (larger screw has pitch of 1 2 5 mm 8. smaller ot 1.00mm Sleeve Thimble F i g . 3.31. Differential screw micrometer 84 METROLCX In case of metric micrometer the usual pitch employed for the screi are 0.4 and 0.5 mm. Therefore, for one revolution of the thimble tJ measuring anvil will advance by an amount equal to (0.5 - 0.4) = 0.1 mi The thimble periphery is graduated in 100 equal parts. Therefore, ea division on the thimble will correspond to 0.1 x = 0.001 mm of the ani movement.

Differential micrometer has appreciably smaller total range of lina movement, this is because the main spindle attached to the moving port:: gets only differential movement.

The Vernier Micrometer The vernier principle may also be applied to an outside micromeld increasing its accuracy. It gives reading with an accuracy of 0.001 mm. a vernier principle is illustrated in Fig. 3.32. The vernier scale is engraved on the micrometer barallel.

There are 10 divisions on the vernier scale, and these are equal to 9 divisions on the thimble.

Hence one division on the vernier scale is equal to x 9 : 10 that of Vernier scale ' =- 45 the thimble. But one division on thimble is equal to 0.01 mm (similar to conventional micrometer).

Fig. 3.32. Vernier micrometer Therefore one division on vernier scale = x 0-01: 0.009 mm.

The least count according to principle pf vernier will be, val smallest division on thimble - value of smallest division on vernier = 0.01-0.009 mm = 0.001 mm.

Depth Micrometer Depth micrometer as the name indicates is used for measuring the depth of holes, slots and recessed areas. It has a shoulder which acts as a reference surface. The shoulder is held firmly and perpendicular to the centre line of the hole. Extension rods in steps of 25 mm may be used for longer range of measurement.

The extension rod can easily be inserted by removing the spindle cap. When the cap is replaced, the rod is held firmly against the reference surface. The extension rods are marked with their respective capacity and are F i g 8 3 3 D e p t h m i c r o m\ EAR MEASUREMENT 85 p i a r e w i t h the base i n a n y p o s i t i o n . T h e m e a s u r i n g faces of the base a nd ids a r e h a r d e n e d . It s h o u l d be noted t h a t t h e scale of d e p t h m i c r o m e t e r is d i b r a t e d i n t h e reverse d i r e c t i o n.

Telescopic Gauge. T h e telescopic gauge is u s e d for m e a s u r i n g i n t e r t l d i a m e t e r of holes, slots a n d grooves etc. I t consists of a h a n d l e w i t h two â&#x20AC;˘is i n a t u b e at one end a n d a w o r k i n g s c r ew at t h e other end. T h e rods aving s p h e r i c a l contacts can s l i d e w i t h i n a tube a n d are forced a p a r t by : i n t e r n a l s p r i n g . T h e l o c k i n g s c r ew can lock the rods at a n y desired i s i t i o n t h r o u g h a s p r i n g . W h i l e t a k i n g measurements, the rods are â&#x20AC;˘^ssed closer a n d i n s e r t e d i n t o the hole to be measured. The rods t h en jen out to t o u c h the m e t a l s u r f a c e of the hole o n b o t h sides. T h e y a r e t h en eked i n p o s i t i o n by means of a l o c k i n g screw. T h e telescopic gauge is t h en iken out f r om the hole. The d i m e n s i o n across the tips is m e a s u r e d by i c r o m e t e r or v e r n i e r c a l l i p e r .

Locking screw Fig. 3.34. Telescopic gauge D i g i t a l Micrometer. D i g i t a l micrometers are also a v a i l a b l e now-aI d a y s . These micrometers give d i r e c t r e a d i n g to 0.001 m m a n d e m p l o y S q u id d i s p l a y o p e r a t i n g o n a l k a l i n e manganese battery.

B e n c h micrometer. It consists of a large d i a m e t e r of micrometer B e a d . It is e x t r e m e l y r i g i d i n c o n s t r u c t i o n . It has a super p r e c i s i o n d i al indicator for c o m p a r a t o r measurements upto 1 m i c r o n . I t uses a m a g n i f y i ng aechnique for m e a s u r i n g s m a l l difference i n d i m e n s i o n of p a r t s . It i s t h us •sed as a comparator.

Inside M i c r o m e t er Inside m i c r o m e t e r i s u s e d for m e a s u r i n g l a r g e r i n t e r n a l d i m e n s i o n s It consists of t h e f o l l o w i n g m a i n p a r t s : (i) M e a s u r i n g h e a d ( m i c r o m e t e r u n i t) (ii) E x t e n s i o n rods (iii) S p a c i n g c o l l a rs (iv) H a n d le The r a n g e is 50 m m to 210 m m a n d for a n y one e x t e n s i o n r o d i t s range • 20 m m . T h e m i c r o m e t e r u n i t or m e a s u r i n g h e a d consists of a b a r r e l a nd m t h i m b l e s i m i l a r to the outside micrometer. It h a s no frame a n d s p i n d l e, t h e m e a s u r i n g p o i n t s are at extreme ends a n d a d j u s t m e n t i s effected by • • V a n c i n g or w i t h d r a w i n g t h e t h i m b l e a l o n g t h e b a r r e l . A s e r i e s o f e x t e n sion rods are p r o v i d e d i n order to o b t a i n a wide m e a s u r i n g range. F or 86 METROLOGY Anvil 1 Stop _A Sleeve ITth imb l e A n v i l D Extension rod 1=3 Fig. 3.35. Inside micrometer measuring bores of comparatively small diameters, a handle is provided which can be screwed into a radial hole in the barrel.

For taking measurements with this instrument first the diameter of bore is measured approximately by a scale. Extension rod is then selected to the nearest one cm and inserted in the micrometer head. Then zero error is checked by taking the dimension on standard sized specimen. The micrometer is then adjusted at a dimension slightly smaller than the diameter of bore the micrometer head

is then held firmly against the bore and other contact surface is adjusted by moving the thimble till correct feel is sensed. One contact surface moved sideways up and down to ensure tha full diameter is measured: The micrometer is then removed and reading! taken. The lengths of extension and collar are added to the micrometer reading.

Cummulative and progressive error in micrometer (calibration micrometer).

After considerable use, due to wear of thread an error may be intro-j duced and the micrometer may give inaccurate results. The micrometer is subjected to two main sources of error - periodic error and progressiva error. It is seen that certain pattern of errors are repeated for each revol tion of the thimble and spindle. This is called the cyclic or periodic error the instrument. .

At the same time it is seen that the general trend of the errors 1 progressively away from the axis as the errors build up. This trend is call the progressive or cumulative error.

Periodic error icycllcel error) horizontal scele expended for clertty Periodic error â&#x20AC;˘ horizontal scale expended for clarity Periodic error horizontal scale expended for clarity jS Âť Âť M 015 Nominal micrometer readings in millimetres Fig. 3.36. periodic and progressive error in micrometer For getting accurate results it is necessary to determine these e and calibrate the micrometer for applying necessary correction.

LINEAR MEASUREMENT 87 The accuracy of the readings of micrometer is usually checked by :aking readings on a series of slip gauges (a standard of high accuracy than the micrometer). Slip gauges or combination of slip gauges are placed between the micrometer anvils and the reading are taken over the entire measuring range. For each reading the error is noted by the relation, error = micrometer reading - slip gauge reading. The error may be positive or negative. A graph between the slip gauge readings and corresponding error in micrometer is plotted as shown in Fig. 3.36. The following series of gauges serve for testing both progressive and periodic errors throughout its range : 2.5, 5.1, 7.7, 10.3, 12.9, 15.0, 17.6, 20.2, 22.8 and 25.0 mm for 0.25 mm micrometer.

Now, while measuring the dimension with this micrometer, the reading obtained is 15.175 mm, but the micrometer has already been proved to be reading over size by 0.002 mm at this point in its range then the actual size of the component = 15.175 mm - 0.002 mm = 15.173 mm.

For supporting the micrometer while introducing slip gauges between the anvils of micrometer a micrometer stand may be used.

Digital Measuring Instruments Digital instruments are capable of measuring the dimensions with a high degree of accuracy. These instruments provide binary coded digital output which can be fed to the computer or printer for further processing.

Bectronic digital read out system consists of a glass scale unit or transducer and the counter or digital read out unit.

The advantages of digital read out instruments are : 1. The error caused by mechanical elements such as gears, screws etc. are eliminated.

2. Human errors caused by taking readings are also eliminated.

3. Readings can be quickly taken even by unskilled person, no calculations aretiecessary.

4. Digital system gives clear reading, the error caused by vision are eliminated.

5. The instrument is compatible.

6. The display can be brought to zero.

Sip Gauges Slip gauges or gauge blocks are universally accepted end standard of length in industry. These were introduced by Johanson, a Sweedish engineer, and are also called as Johanson Gauges.

Slip gauges are rectangular blocks of high grade steel with exceptionally close tolerances. These blocks are suitably hardened through out to â&#x20AC;˘â&#x20AC;˘sure maximum resistance to wear. They are then stabilized by heating and cooling successively in stages so that hardening stresses are removed.

88 METROLOGT Fig. 3.36. Dimension of a slip ga After being hardened they are carefully finished'by high grade lapping to a high degree of finish, flatness and accuracy.

For successful use of slip gauges their working faces are made truly flat and parallel. A slip gauge is shown in fig. 3.36.

Slip gauges are also made from tungsten carbide which is extremely hard and wear resistance.

The cross-sections of these gauges are 9 mm x 30 mm for sizes upto 10 mm and 9 mm x 35 mm for larger sizes. Any two slips when perfectly clean may be wrung together. The dimensions are permanently

marked on one of the measuring faces of gauge blocks.

Gauges blocks are used for : (i) Direct precise measurement, where the accuracy of the workp demands it. (ii) For checking accuracy of vernier callipers, micrometers, and other measuring instruments.

(Hi) Setting up a comparator to a specific dimension. iiv) For measuring angle of workpiece and also for angular sett' conjunction with a sine bar. (v) The distances of plugs, spigots, etc. on fixture are often measured with the slip gauges or end bars for large dimens (vi) Tp check gap between parallel locations such as in gap gau between two mating parts.

There are many measurements which can be made with slip g either alone or in conjunction with other simple apparatus such as st edges, rollers, balls, sine bars etc.

Wringing of Slip Gauges The success of precision measurement by slip gauges depends phenomenon of wringing. The slip gauges are wrung together by through a combined sliding and twisting motion. The gap betwe wrung slips is only of the order of0.00635 microns (0.635 x 10 is negligible. mm) Procedure for Wringing (i) Before using, the slip gauges are cleaned by using a lint free a chamois leather or a cleansing tissue. . Âť (ii) One slip gauge is then oscillated slightly over the other | with a light pressure.

NEAR MEASUREMENT 89 (Hi) One gauge is then placed at 90° to other by using light pressure and then it is rotated untill the blocks one brought i n one line.

In this way air is expelled out from between the gauge faces causing the gauge blocks to adhere. The adhesion is caused partly by molecular attraction and partly by atmospheric pressure. When two gauges are wrung in this manner is exactly the sum of their individual dimensions. The wrung gauge can be handled as a unit without the need for clamping all the pieces together.

Slide F i g . 3.37. Wringing of slip gauges Indian Standard on Slip Gauges According to IS : 2984-1966, the size of the slip gauges is defined as the distance I between two plane measuring faces, are being constituted by the surface of an auxiliary body with which one of the slip gauge faces is wrung and the other by the exposed face to the slip gauge. Generally the slip gauges are made from high grade steel with coefficient of thermal

expansion (11.5 ± 1.5) x 10~6 per degree Celsius between 1°C to 30°C. The slip gauges are hardened more than 800 HV to make them wear resistant.

IS : 2984 'slip gauges' gives recommendations covering the manufacture of gauge blocks upto 90 mm in length in five grades of accuracy.

Grade II. Grade II gauge blocks are workshop grade for rough checks.

They are used for preliminary setting up of components where production tolerances are relatively wide; for positioning m i l l i n g cutters and checking mechanical widths.

Grade I. Grade I gauge blocks are used for more precise work such as setting up sine bars, checking gap gauges and setting dial test indicators to zero.

Grade O. These are inspection grade gauge blocks, used in tool room and inspection department for high accuracy work.

Grade OO. These gauges are placed in- the standard room and used for highest precision work. Such as checking Grade I and Grade II slip gauges.

Calibration Grade. This is a special grade, with the actual size of the slips calibrated on a special chart supplied w i t h a set. The chart must be referred while making up dimension.

The following two sets of slip gauges are in general use : L I N E A R M E A S U R E M E NT 91 Selection of slip gauges for required dimension (procedure for build up gauge blocks for required dimension). Standard procedure as given below should be followed while selecting the slip gauge to built up the required dimension.

Always start with the last decimal place and deduct this from the required dimension. Select the next smallest figure in the same way, find the remainder, and continue this until the required dimension is completed.

•Minimum number of slips necessary to build up the given dimension should be selected.

Let us suppose that the dimension to be build up is 29.758 mm.

For last decimal place of 0.008, select, 1.008'mm slip gauge. Now dimension left = 29.758 - 1.008 = 28.75 mm.

For second decimal place of 0.05, select 1.25 mm slip gauge.

Therefore, the remainder = 28.75 - 1.25 = 27.5 mm.

Now select 7.5 mm and 20 mm slip gauge to build up the required combination.

Thus we have 20 + 7.5 + 1.25 + 1.008 = 29.758 mm i.e., the above four slip gauges are required to build 29.758 mm.

After selecting the minimum number of slip gauges in this manner, clean them by using a lint-free cloth, a chamois leather, or a cleaning tissue :o remove dirt, oil, dust etc.

If the slip gauges have been protected with Vaseline, this may be removed by a solvent such as petroleum, ether etc.

Now, begin wringing with the largest sizes first. While using avoid touching the measuring surfaces with fingers and handle them as little as

possible place one gauge over the other at right angles, and with light pressure, twist it through untill the block are brought in one line.

When the largest gauges have been assembled, follow with others in order of decreasing size untill the required combination is build up.

When the dimension to be measured is unknown and is to be found using slip gauges, then first it must be determined to the nearest 0.1 mm size with a micrometer, calliper etc. This will reduce the time required to build up the required combination of slip gauges.

Care of slip gauges General care 1. Protect all the surfaces against climatic conditions by applying suitable anticorrosive such as petroleum jelly.

2. Keep the slip gauges in a suitable case in which there is a separate compartment for each gauge and keep the case closed when not in use.

3. Protect the gauges and their case from dust and dirt.

4. Gauges should not be magnetized otherwise they will attract metallic dust.

92 METROLOGY Preparation before use 1. Remove protective coating applied to it with petrol.

2. Clean gauges to be used with chamois leather or soft linen cloth! even if they are temporarily returned to the case uncoated.

Care in use.

1. During the actual use, the fingering of lapped faces should be] avoided.

2. Handling should be as minimum as possible to avoid transfer of] heat from hand to gauges.

3. If the gauges have been handled for some time, they should be allowed to settle down to the prevailing room temperature.

4. For highest accuracy measurement at a temp, of 20째C is necessary.

(In air condition room free from dirt and dust).

5. Actually both the work to be tested and the gauges wrung! together should be allowed to settle down to the prevailing temperature of the room before doing any test.

6. Gauges should not be held above the open case when being wru together. The required gauges should be selected and the c~ then closed.

7. Placing gauges with their working surfaces on surface plate e should be avoided.

8. While wringing gauges standard procedure as already explain should be followed.

9. If during wringing process, any sign of roughness or scratching' felt the process of wringing should be stopped and faces examin 92 METROLOGY

Preparation before use 1. Kemove protective coating applied to it w i t h petrol.

2. Clean gauges to be used with chamois leather or soft linen cloth even i f they are temporarily returned to the case uncoated.

Care in use.

1. During the actual use, the fingering of lapped faces should be avoided.

2. Handling should be as minimum as possible to avoid transfer of heat from hand to gauges.

3. If the gauges have been handled for some time, they should be allowed to settle down to the prevailing room temperature.

4. For highest accuracy measurement at a temp, of 20째C is necessary.

(In air condition room free from dirt and dust).

5. Actually both the' work to be tested and the gauges wrung together should be allowed to settle down to the. prevailing . temperature of the room before doing any test.

6. Gauges should not be held above the open case when being wrung together. The required gauges should be selected and the case then closed.

7. Placing gauges w i t h their working surfaces on surface plate etc. should be avoided.

8. While wringing gauges standard procedure as already explained should be followed.

9. If during wringing process, any sign of roughness or scratching is felt the process of wringing should be stopped and faces examined for burrs or scratches.

Care after use 1. Gauges should not be left wrung together for an unnecessary length of time.

2. Immediately after use, the gauges should be slid apart, (no^ pulled) cleaned and the measuring faces coated with suitable protective layer of jelly, grease etc. w i th a clean piece of soft line A brush should not be used as this may aerate the jelly an moisture i n the air bubbles so formed may cause rusting of t' faces.

3. Calibration - Due to handling i n the laboratory or inspectio room for a considerably long period, slip gauges are liable to we and, therefore, they should be checked or recalibrated at regul intervals.

Workshop and inspection grade gauges are calibrated by direct co; parison w i t h the calibration grade gauges in a comparator.

LINEAR MEASUREMENT 93 Adjustable S l i p Gauges A d j u s t a b l e s l i p gauges consists of two h a r d e n e d , g r o u n d a n d l a p p ed blocks. E a c h ' b l o c k has one flat a n d one tapered surface. The tapered surfaces are f i n i s h e d to w r i n g i n g q u a l i t y . A f t e r w r i n g i n g thrf two outer

surfaces w i l l be f l a t a n d p a r a l l e l to each other. T h e t a p e r e d surfaces c a n be made to s l i d e a l o n g t h e l e n g t h of e a c h other. T h i s e n a b l e to. o b t a i n d i f f e r e nt sizes w i t h i n a v e r y s m a l l range. T h e upper b l o c k has a r a c k f i x e d i n a slot on b o t t om t a p e r e d surface. A n i n d e x p i n is a l s o f i t t e d i n t o t h e l o w e r h a l f of the block. A scale w i t h d i v i s i o n s a r e m a r k e d i n t h e l o w e r block, correspondi n g to r a c k t o o t h i n t e r v a l i n u p p e r block. -A reference point i s m a r k e d an upper block. B y s l i d i n g the two tapered surfaces a l o n g the l e n g t h of each other, v a r i o u s sizes i n t h e g i v e n r a n g e can'be obtained.

Fig. 3.38. Adjustable slip gauges S l i p G a u g e Accessories The field of a p p l i c a t i o n of s l i p gauges is c o n s i d e r a b l y i n c r e a s e d by u s i n g few simple accessories s u c h as holders, j a w s etc. These accessories are m a i n l y u s e d : (i) w h e n i t is d e s i r e d to use s l i p gauge c o m b i n a t i o n of considerable l e n g t h i n o r d e r to e n s u r e more r i g i d assembly, (ii) for m e a s u r i n g p l u g a n d r i n g gauges, (Hi) for t h e i n s p e c t i o n of gauges a n d p r e c i s i o n e q u i p m e nt (iv) for m a r k i n g out purpose.

These accessories may be b u i l t up q u i c k l y a n d u s e d a t e m p o r a r y l i m it gauge.

The v a r i o u s accessories used w i t h s l i p gauges a r e : (i) M e a s u r i n g j a ws (it) S c r i b i n g a n d centre point (iii) Holders a nd (iv) Base.

These are made f r om s u i t a b l e steel free f r om defects a n d h a r d e n ed u n i f o r m l y , m i n i m u m h a r d n e s s of 800 H V .

Measuringjaws.Measuringjawsarealwayssuppliedinpairs.The se are o f t w o types : T y p e A a n d T y p e B. Type A j a w s are u s e d for b o t h i n t e r n al a n d e x t e r n a l m e a s u r e m e n t s and type B j a w s are used for e x t e r n a l measu r e m e n t only. T h e size of t h e m e a s u r i n g j a w is e n g r a v e d on t h e f r o n t end 94 METROLOGY face of t h e j a w . T h e w r i n g i n g surface of e a c h m e a s u r i n g j a w s h o u l d be flat to w i t h i n 0.00025 m m . The n o m i n a l size of m e a s u r i n g j a w s h o u l d be a c c u r a t e to w i t h i n Âą 0.0007 m m .

Size of ^,5-0 measuring ~f law dth of wringing' face 9.00 h>40i^ H- 4-5 75 . â&#x20AC;&#x201D; J * 20

width of wringing face to exceed "+ 0.001mm All dimensions are in mmType A measuring jaw L wringing face Type B Measuring <av Pair of jaws wring together Fig. 3.39. Measuring jaws W h e n the p a i r of j a w s are w r u n g together t h e e r r o r i n o v e r a l l dimen-j &ion s h o u l d not exceed Âą 0.001 m m . t h e i n t e r n a l measurement i s made between c u r v e d surfaces of the! j a w s ( F i g . 3.40). T h e j a w s a r e h e l d i n t h e h o l d e r a n d ttie c u r v e d s u r f a c e s are made i n contact w i t h t h e i n t e r n a l surface o f t h e r i n g g a u g e b e i n g m e a s u r e Measuring ja v\ Side walls ;iip gauges Type A measuring jaws being used for internal measurement measurement Fig. 3.41.

Fig. 3.40. * The e x t e r n a l m e a s u r e m e n t i s made between f l a t surfaces of t h e j as Shown i n ( F i g . 3.41). conjunction w i t t (a)) shows a k n i f e edge s c r i b i n g point.

W h i l e ( F i g . 3.42 (b)) shows a c e n t r e point.

LINEAR MEASUREMENT 95 Scnber point Sharp,centre point <2 Scriber Cenrte point Fig. 3.42. (a) and (6) Holder. These are made of suitable design for holding rigidly combination of slip gauges within their range. These are commercially available in three ranges viz. 0-60, 50-110,'and 100 to 200 mm. The surface upon which the gauges are wrung should be flat to within 0.0001 mm and parallel to the base.

Base. The base is made with the robust construction. The base is designed in such a manner that holder can be attached to it, normal with respect to the wringing surface of the base. The wringing surface of the base should be flat to within 0.0002 mm. It should be suitably relieved and an air vent should be provided. The base with the scriber used in conjunction with the holder can be used as an accurate height gauge.

Manufacture of Slip Gauges

Various manufacturing methods have been developed for the production of slip gauges. National Physical Laboratory successfully developed a method of manufacturing slip gauges, with the required qualities of accuracy and surface finish. The brief description of the manufacturing stages involved in this method is as follows : Base Fig. 3.43. Base 96 METROLOGYl 1 The high grade steel gauge blocks are hardened and after rough leaving gauges in a stable condition.

2. Batches of eight blanks of similar nominal size are mounted on eight ro-nlaner faces of a magnetic chuck. ^ 3 While mounted on the magnetic chuck one set of faces are lappe^ A,.*Y*ve. <j,a.vs.^<aÂť K t e vjosso.x>sKsKrA<a&-â&#x20AC;˘iss^.^s^exX'sipp'fe^i^aces wrung to eight faces of solid chuck which are also accurately in one plane.

9 n il 10 12 13 2 4' 9 11 10 12 98 2 15 10 7 1 16 9 8 2 15 10 7 Fig. 3.44. Arrangement of manufacturing slip gauges on lapping chucks 6. To determine whether the gauges are of the required sizes they removed from the chuck, wrung together in combination, and their gregate size compared with an appropriate sized master in a suita comparator. An advantageous magnification factor of 8 is obtained in th calibration since, as each of the 8 gauges must be identical in size, differences between the combination and master may be divided by ei and this difference appropriated to each gauge.

7. The gauges may, if necessary, be replaced on the eight facet ch to bring their individual lengths within the required accuracy. * Slip gauges can also be manufactured by : 1. The American method of manufacturing slip gauges.

2. The German (Zeiss) method of manufacturing slip gauges.

96 METROLOGY 1. The high grade steel gauge blocks are hardened and after rough grinding they are subjected to a cyclic low temperature heat treatment with the purpose of a balancing the internal stresses produced by hardening thus leaving gauges in a stable condition.

2. Batches of eight blanks of similar nominal size are mounted on eight

co-planer faces of a magnetic chuck.

3. While mounted on the magnetic chuck one set of faces are lapped truly flat.

4. The gauges are then removed and their lapped faces wrung to the eight faces of solid chuck which are also accurately in one. plane.

5. The exposed faces of gauges are now lapped flat and i n one plane.

The opposing faces of each gauge are now truly flat but not necessarily parallel. The required degree of accuracy in parallelism and equality of size is achieved by interchanging four of the eight gauges as shown in F i g ; 3.44.

They are interchanged diagonally and turned end for.end. Thus any errors in parallelism are equalized and on relapping into one plane a much higher degree of accuracy i n parallelism is achieved. In practice, this interchange need only be done once or twice before the gauges are both truly flat and truly parallel.

1 3 9 11 2 4 10 12 1 3 9 11 2 4 10 12 1 16 2 15 10 1 16 2 15 98 10 7 F i g . 3.44. Arrangement of manufacturing slip gauges on lapping chucks 6. To determine whether the gauges are of the required sizes they ar removed from the chuck, wrung.together in combination, and their aggregate size compared with an appropriate sized master in a suitable comparator. A n advantageous magnification factor of 8 is obtained in their calibration since, as each of the 8 gauges must be identical in size, the differences between the combination and master may be divided by eight and this difference appropriated to each gauge.

7. The gauges may, i f necessary, be replaced on the eight facet chuck to bring their individual lengths within the required accuracy. .

Slip gauges can also be manufactured by : 1. The American method of manufacturing slip gauges.

2. The German (Zeiss) method of manufacturing slip gauges.

5 COMPARATORS Introduction A comparator is a precision instrument employed to compare the dimension of a given component with a working standard (usually'slip gauges). It thus does not measure the actual dimension but indicates how much i t differs from the basic dimension. The indicated difference is usually small and hence suitable magnification device is provided to measure the difference with consistent accuracy.

Thus, a comparator is an indirect type of precision instrument which : - gives linear measurement - works on relative measurement - indicates only dimensional difference in relation to the basic dimension.

- has a sensing device,, magnifying or amplifying system and or display system (usually scale and pointer) to provide suitable read out.

- eliminates human element in taking measurement and gives . accurate results consistently.

Need for a comparator In mass production identieal component parts are produced on a very large scale. To achieve inter changeability these parts should be produced to a close dimensional tolerances. As a result, inspection is often more concerned with the dimensional variation from the standard or basic dimension of the. part. To this extent inspection becomes a process of comparing manufactured part to the master part envisaged by the designer.

The use of vernier calliper, micrometer etc w i l l not be feasible because of die skill involved and the time required to measure the dimension. Use pf eomparatur requires little or no skill for the operator, eliminates human dement for taking measurement and gives quick and highly consistent results.

Basic Principle of Operation i The basic principle of operation of a comparator is : The comparator is first adjusted to zero on its dial or recording device with a guage block i n position. The gauge block is of dimension which the workpiece should have. The workpiece to be checked is then placed i n * position and the comparator gives the difference i n dimension i n relation to (117)