CHAPTER I Mechanical Metrology 1.1 DEVICES BASED ON VERNIER SCALE 1.1.1 Vernier Principle The Vernier principle of measuring was named for its inventor, Pierre Vernier (1580-1637). He was a French mathematician. Vernier is an additional scale sliding against main scale, Fig. 1.1. Vernier is divided into (n) division, which correspond to (n-1) divisions on the main scale. To clarify the principle, suupose we take 9 divisions on the main scale and divide it to 10 divisions on the vernier scale, Fig. 1.1. Accordingly, the length of the division on the vernier scale is shorter than the main scale division by 0.1 mm. The graduation line on the vernietr is leading the first line in the main scale by 0.1 mm, the second line leads by 0.2, and so on. Consequently, If the vernier slide 0.1 mm, the first line on the vernier is coincide with the graduation line on the main scale. If the vernier slide 0.2 mm, the second line is aligned with the main scale, and so on. Noting the vernier line No. that aligned with the marks of the main scale, the sliding distance could be determined. Page 1
Fig.1.1 Vernier scale of resolution 0.1 1.1.2 Resolution (Scale Value) Of the Vernier Scale The resolution of the vernier may be determined by the relation sm Sv n where Sv is the vernier resolution, sm is the main scale revolution, and n is the No. of vernier divisions. Vernier are commonly made with resolution of 0.1 mm,fig. 1.1, 0.05 mm, Fig. 1.2a, and 0.02 mm, Fig. 1.2b. Page 2
Fig. 1.2 Vernier scale of, (a)resolution 0.05, (b) resolution 0.02 1.1.2 Reading of the Vernier Scale To read vernier reading. Firstly, note the reading of the main scale up to the 0 line on the vernier scale. Secondly, Note down the No of the vernier scale mark that aligned with the main scale. Finally, multiply the line No. with the vernier resolution and add the result to the main scale reading, Figs. 1.32 and 1.4. Fig.1.3. Vernier reading 83.6 mm Page 3
Fig.1.4. Vernier reading 105.7 mm 1.1.3 Vernier Caliper. According to IS:3651 1974 (Specification for vernier caliper), three types of vernier calipers have been specified to meet-the various needs of external and internal measurements up to 2000 mm with vernier resolution of 0.02, 0.05 and 0.1 mm. The three types are called types A, B, C, and have been shown in Fig. 1.5. All the three types are made with only one scale on the front of the beam for direct reading. Type A has jaws on both sides for external and internal measurements, and also has a blade for depth measurements. Type B is provided with jaws on one side for external and internal measurements. Type C has jaws on both sides for making the measurements and for marking operations. All parts of the vernier calipers are made of good quality steel and the measuring faces hardened to 650 H.V. minimum. The recommended measuring ranges (nominal sizes) of vernier calipers as per IS 3651-1974 are 0125, 0-200, 0-250, 0-300, 0-500, 0-750, 0-1000, 750-1500 and 7502000 mm. On type A, scale serves for both external and internal measurements, whereas in case of types B and C, the main scale Page 4
serves for external measurements, and for marking purpose also in type C, but on types B and C internal measurements are made by adding width of the internal measuring jaws to the reading on. the scale. For this reason, the combined width of internal jaws is marked on the jaw in case of types B, and C. The combined width should be uniform throughout its length to within 0.01 mm. The beam for all is made flat throughout its length to within the tolerances of 0.05 mm for nominal lengths up to 300 mm, 0.08 mm from 900 to 1000 mm, and 0.15 mm for 1500 and 2000 mm sizes. Guiding surfaces of the beam are made straight to within 0.01 mm for measuring range of 200 mm and 0 01 mm every 200 mm measuring range of larger size. The measuring surfaces are given a fine ground finish. The portions of the jaws between the beam and the measuring faces are relieved. The fixed jaw is made an integral part of the beam and the sliding jaw is made a good sliding fit along with the beam and made to have seizure-free movement along the bar. A suitable locking arrangement is provided on the sliding jaw in order to effectively clamp it on the beam. When the sliding jaw is clamped to the beam at any position Page 5
Fig. 1.5 Types of vernier caliper. within the measuring range, the external measuring faces should remain square to the guiding surface of the beam to within 0.003 mm per 100 mm. The measuring surfaces of the fixed and sliding jaws should be coplanar to within 0.05 mm when the sliding jaw is clamped to the beam in zero position. The external measuring faces are lapped flat to within 0.005 mm. The bearing faces of the sliding jaw should preferably be relieved in order to prevent damage to the scale on the Page 6
beam. Each of the internal measuring surface should be parallel to the corresponding external measuring surface to within 0.025 mm in case of type B and C calipers. The internal measuring surfaces are formed cylindrically with a radius not exceeding one-half of their combined width. Errors in Calipers. The error in reading the vernier caliper should not exceed the values obtained by the following formulae : Vernier with resolution Permissible error in reading 0.1 mm ±(75+0.05 l1) µm 0.05mm ±(50+0.05l1) µm 0.02 mm ±(20+0.02 l1) µm where l1= upper limit of the measuring range in mm. The error in reading is found by placing slip gauges at right angles to the longitudinal direction of the measuring faces ; the readings being taken at three different points along the length of the jaws and same pressure applied to the sliding jaw each time. (The error measured in this way will include errors in, the flatness and parallelism of the measuring jaws). The check should be repeated at a number of points distributed over the range of measurement. The accuracy of the measurement with vernier calipers to a great extent depends upon the condition of the jaws of the caliper. The accuracy and the natural wear, and warping of vernier caliper jaws should be tested frequently by closing them together tightly or setting them to the 0-0 point of the main and vernier scales. In this Page 7
Fig. 1.6 Various jaw conditions which result in inaccurate caliper measurements position, the caliper is held against a light source. If there is wear, spring or warp, a knock-kneed condition as shown in Fig. 1.6a, will be observed. If measurement error on this account is expected to be greater than 0.005 mm the instrument should not be used and sent for repair. When the sliding jaw frame has become worn or warped so that it does not slide squarely and snugly on the main caliper beam, then jaws would appear as shown in Fig. 1.6b. Where a vernier caliper is used mostly for measuring inside diameters, the jaws may become bowlegged as in Fig.1.6c or its outside edges worn down as in Fig.1.6d. Page 8
Precautions in the Use of Vernier Caliper. No play should be there between the sliding jaw on scale, otherwise the accuracy of the vernier caliper will be lost. If play exists then the gib at the back of jaw assembly must be bent so that gib holds the Jaw against the frame and play is removed. Usually the tips of measuring jaws are worn and that must be taken into account. Most of the errors usually result from manipulation of the vernier caliper and its jaws on the workpicce. In measuring an outside diameter it should be insured that the caliper bar and the plane of the caliper jaws are truly perpendicular to the workplace's longitudinal centre line. It should be ensured that the caliper is not canted, tilted or twisted. The stationary caliper jaw of the vernier caliper should be used as the reference point and measured point is obtained by advancing or withdrawing the sliding jaw.the accuracy in measurement primarily depends on two senses, viz; sense of sight and sense of touch (feel). The shortcomings of imperfect vision can however be overcome by the use of corrective eye-glass and magnifying glass. But sense of touch is an important factor in measurements. Sense of touch varies from person to person and can be developed with practice and proper handling of tools. One very important thing to note here is that sense of touch is most prominent in the finger-tips, therefore, the measuring instrument must always be properly balanced in hand and held lightly in such a way that only fingers handle the moving and adjusting screws etc. If tool be held by force, then sense of feel is reduced. Page 9
1.1.4 Bevel Vernier Protractor The simple protractor shown in Fig.1.7, utilize the vernier principle. The main scale divided into degrees. The vernier scale has 12 divisions correspond to 23 deg. on the main scale, Figs. 1.8. and 1.9. each division of the vernier equal to 1deg. 55 min. So, the resolution of the protractor is 5 min. Fig. 1.7 bevel protractor Fig. 1.8 Reading of 45 deg, and 35 min. Page 10
1.2 Devices Based on Screw and Nut (Micrometers). 1-2 Micrometer 1-2-1 Working Principle The principle of the micrometer is based on a very accurate made screw thread that rotates in a fixed nut, Fig.1.9. Fig.1.9. Micrometer principle Refereeing to Fig.1.10, The screw thread is cut on the spindle, and is attached to the thimble. The spindle is advanced or recedes from the Fig. 1.10. External micrometer, (1) anvil, (2) contact tips, (3) spindle, (4) sleeve, (5) tumble, (6) ratchet, (7) main scale, (8)locking arm, (9) frame, (10) isolator Page 11
anvil by rotating the thimble. A graduation is engraved in the sleeve with a division equal to the thread pitch. The beveled edge of the thimble is divided into a number of divisions each of them represents a fraction value of the spindle pitch. In case that, the thread pitch is 0.5 mm and the length of the threaded part is about 25 mm, a length of 25 mm on the sleeve is divided into 50 divisions. Each division is equal to 0.5 mm. Every 5 mm are numbered and indicated by a longer line. The beveled edge of the thimble is divided into 50 divisions, each represents (0.5/50)=0.01 mm. 1-2-2 Micrometer Construction Fig. 1.11 reveals the internal parts of the micrometer lock nut is provided for locking a dimension by preventing motion of the spindle. Ratchet stop is provided at the end of the thimble cap to maintain sufficient and uniform measuring pressure so that standard overriding clutch held by a weak spring. W..sn the spindle fs consists brought into contact with the work at the correct measuring conditions of Page 12
Fig.1.11 sectional view of external micrometer of measurement are attained. Ratchet stop consists of an overriding clutch held by a week spring. When the spindle is brought into contact with the work at the correct measuring pressure, the clutch starts slipping and no further movement of the spindle takes place by the rotation of ratchet. In the backward movement it is positive due to shape of ratchet. Frame. The frame of the micrometer is shaped as to permit measurements of the cylinder of diameter equal to the measuring range of micrometer, and the stiffness of the frame should be such that a test load of 1 kg weight does not alter the distance between them by more than 1.5 µm for range 0 to 25 mm, 2 µm for range 25 to 50 mm and so on. It should be neatly and evenly black, enameled or treated by other means to prevent corrosion and rusting. The frame is generally made of steel, cast steel, malleable cast iron or light alloy. The use of light alloys is recommended, for sizes over 300 mm in order to facilitate the handling of large micrometers. But it has to be understood that coefficient of expansion of light alloys is appreciably greater than that of steel or iron and thus the temperature changes during handling of the micrometer frame may result in appreciable changes in the zero reading. It is, therefore, Desirable features, this instrument should have Proper plating of all parts for resistance to strains, corrosion and wear. Faces of anvils lapped to mirror finish for accurate measurements. The measuring face of the anvil is so designed as to have a hardness Page 13
of about 800 HV (63'5 HRC). It is desirable that the measuring face be tipped with tungsten carbide or other suitable hard material or be faced with a deposit of hard chromium. Ring type lock-nut for locking spindle and The main nut is fitted tightly into the sleeve or barrel which is integral with the frame. The thimble is permanently secured to the screw and is knurled on the outer surface The spindle Is one piece having extra hard threads of extreme lead accuracy for lasting precision. It having the measuring face at its end is tightly fitted into the screw. The spindle is thus rotated by rotating the thimble. It will be observed that the thimble extends over the end of the barrel so that threaded portions of the screw and the main nut are all the time completely enclosed. The nut and the thread portion of the screw are so proportioned as to ensure full length engagement in all positions of spindle. The fixed index line is marked upon the barrel and the angular graduations around the left hand chamfered end of the thimble. It will be noted that in addition to the main nut there is a shorter nut also by its side with which the screw also engages. The adjacent faces of the main nut and the shorter nut are provided with small V-shaped teeth which form a clutch and prevent the shorter nut from rotating with the screw. In the recess between these two nuts is housed a light coil spring, the tendency of which is to force the two nuts apart and in this way any slight backlash between the threads in the nuts and the screw is automatically taken Page 14
up. Spindle locking arrangement. In order that any setting of the micrometer be definitely retained, it is desirable to have features of locking the spindle in any position temporarily. One way of achieving this is shown in Fig.1.11, in which a sliding pin is located in a hole in the frame and is pressed against the side-of the spindle by a cam. The cam is fitted with dished and knurled heads on both sides of the frame. The shape of the cam is such that when a slight turning movement is imparted to it by gripping the dished and knurled heads on both sides of the frame between finger and thumb, the pin is forced against the spindle and a rigid lock is.obtained. In another design shown in Fig. 1.12, the micrometer is slotted, to receive a split ring which surrounds the spindle and is prevented from rotating. The split ring is surrounded by a knurled outer ring. A cam slot is provided on the periphery of the split ring in which a roller moves up and down. When the outer ring is turned in one direction the roller rides flip (he cam surface thereby tending to close the split ring and securely clamping the spindle. The grip is released on turning the outer ring in opposite direction. Page 15
Fig. 1.12 Locking arrangement Ratchet stop mechanism. The object of the ratchet stop is to ensure that a certain maximum torque on the spindle is not exceeded and the sense of the feel of operator is eliminated and consistent readings are obtained. By providing this arrangement, when the spindle has engaged the work with a certain pressure, further rotation causes, the ratchet merely to slip, no additional movement being imparted to the spindle. The ratchet stop mechanism is incorporated in the knurled extension provided at the end of the thimble. The mechanism is shown in Fig. 1.11. The knurled extension is free to rotate on its retaining stud. The inner face of this is provided with fine ratchet teeth, and through these teeth and the spring loaded pawl the movement is transmitted to the spindle. As soon as the resistance to the motion of the latter reaches a certain value, the pawl is forced back against the pressure of the spring and ratchet slips. Micrometer Types Micrometer are commercially available with different configurations to be adapted to different application. Besides the external micrometer, the internal and depth micrometers shown in Fig. 1.13 are the most common types. Page 16
Fig.1.13 micrometer types Micrometer Resolution The resolution of the micrometer is equal to the resolution of the scale engraved on the sleeve divided by the number of the divisions engraved on the thimble. Micrometers are commercially available of resolutions of 0.01 mm, 0.02 mm, and 0.001 mm, Fig. 1.14. Fig. 1.14 Micrometer Scale, (a) 0.01mm, (b) 0.001mm Page 17
Micrometer Reading To read a micrometer, add the total reading in millimeters visible on the sleeve to the reading in hundredths of millimeter indicated by the graduation on the thimble which coincide with the longitudinal line on the micrometer sleeve. It must be noted that the scales of the depth, and internal micrometers are inverted, so hidden reading is the considered one,fig. 1.15. Also, lengths of extension rod and collar must be added to the mike reading. Fig. 1.15. Micrometer readings, (a) 11.34, (b) 12.57, (c) 13.27 mm Use of the Micrometer To measure a part with the micrometer,close the micrometer and take the zero reading. Place the work in position and rotate the thimble until the ratchet slips and prevent further tightening. This ensures consistent accurate measurement by limiting the spindle Page 18
pressure on the work to a definite amount, even when different machinists use the same mike. Lock the micrometer with the lock lever, and take the micrometer reading. Correct the reading by subtracting the value of the zero reading. In case round part is measured,on contact the surface must always be moved sideways up and down in order to ensure that diameter is being measured and not chord. The micrometer is then removed and reading taken. Differential Screw Micrometer. Fig. 1.16 Differential screw micrometer As shown in Fig. 1.16, a very high degree of accuracy can be obtained in the micrometer screw gauges utilizing the principle of differential screw on the operating spindle. In such a micrometer, the screw has two types of pitches, one smaller and one larger, instead of one uniform pitch as in conventional micrometer. Both the screws are right-handed and the screws are so arranged that the rotation of the thimble member moves one forward and the other backward. If Page 19
the larger screw has a pitch of 1.25 mm and smaller screw of 1.00 mm pitch, then the net result would be a total forward movement of 1.25-1.00=0.25 mm per revolution. Thus if thimble has 100 divisions and vernier scale be engraved on the sleeve, then the least count of the instrument will be 00025 mm. It will be noted that this will have appreciably smaller total range of linear movement, although the main spindle's travel is larger. This is because the main spindle which is attached to the moving portion gets only differential movement. Page 20
1.3 Metrology Devices Based on Gear Magnification Dial Indicator 1-3-1 Working Principle Dial gauge indicator is a measuring instrument where gears are employed to convert and magnify the small linear displacement of a plunger to a large angular displacement of a pointer, Fig.1.17. Fig. 1.17 Gear magnification Page 21
1-3-2 Resolution of Dial Indicator The resolution of the indicator is usually written on it. Indicators are commercially available of resolutions of 0.01 mm, 0.02 mm, and 0.001 mm 1-3-3 Construction of Dial Indicator Fig. 1.18 shows the different parts of dial indicator Fig. 1.18. Construction of Dial indicator, 1-3-4 Reading of Dial Indicator The reading of the indicator could be determined by adding the value of the number of revolutions indicated by the revolution counter into the pointer reading, Fig. 1.19. Page 22
Fig. 1.19. Dial indicator reading, (a) 3.48 mm, (b) 7.14 1-3-2 Use of Dial Indicator Dial indicators are used to compare the size of inspected parts with relative to a standard size. The dial is held in a suitable stand, Fig. 1.20.. Part to be checked is located under the indicator plunger. Indicator is adjusted so that contacts the surface of the a standard block gauge(hst), and its reading is observed and recorded (Rst), ( Usually adjusted to be zero). Standard block is then replaced by the inspected part, and the reading of the indicator is taken (R w). The height of the part (Hw) is then determined by; Hw= [Rw Rst]+Hst Fig. 1.20 Mechanical comparator. Page 23
1.4 Metrology Devices Based on Lever Magnification. 1-4-1 Lever Magnification Fig. 1.21 Lever magnification Errors in-the lever-type movements result from variable or inaccurate ratios. As shown in Fig. 1.21a is the diagram of a measuring lever which transmits a rectilinear displacement to a reading indicator. Such a transmission is free of error only if the centers of the spherical measuring ends of the lever and its axis of rotation are in a straight line, and also if the arms of the lever are equal. With the lever in question, the displacement S being measured and angle φ of lever rotation, as taken in reference to its zero position, are related as follows: Page 24
S=l sin φ Hence, levers having spherical measuring surfaces are called "sine levers". Figure 1.21 b shows a lever with flat measuring surfaces. The ratio of such a lever is constant only if its axis of rotation and the centers of spherical surfaces of the measuring plungers interacting with the lever are on a straight line parallel to the measuring surfaces of the lever. The displacement S being measured and angle of φ lever rotation are related by S = l tan φ Levers with flat measuring surfaces are called "tangent levers". Measuring levers are usually made as combinations of the sine and tangent levers, Fig. 1.21c. The spherical measuring end of such a combination lever ensures the correct point contact with parts of any configuration, and its flat end, with the spherical point of a reading indicator. The ratio of this lever always depends on the angle of rotation and is close to its nominal value in the zero position of the lever. To reduce the measuring error, the axis of lever rotation and the centers of the measuring points of the lever and of the reading indicator should be in a straight line. Both sine levers, Fig. 1.21d and tangent levers, Fig. 1.21e and their combinations can be used to transform linear displacements into angular ones. In all these instances, the basic (theoretical) relationship between the displacements of the driven and driving members will be the same: Page 25
St = l φ where St = theoretical linear displacement of measuring plunger with lever turned through angle φ l= length of lever arm With a sine lever, actual displacement Sa of the measuring plunger will be Sa =l sin φ Using the relationship, - we obtain Error in a sine-lever transmission will be the difference between the actual and theoretical displacements of the measuring plunger With a tangent lever, actual displacement sa of the measuring plunger is Sa = l tan φ Using the relationship, - Page 26
Error in a tangent-lever transmission will be the difference between the actual and theoretical displacements of the measuring plunger 1-4-2 Part Revolution Dial Gauge As shown in Fig. 1.22 the displacement of the plunger is transmitted to a gear sector fixed to a magnification lever. The gear sector turns a pinion on which the pointer is fixed. Fig. 1.22 Part Revolution Dial Gauge Page 27
1-4-3 Passimeter Gauge Fig. 1.23 Passimeter gauge The passimeter shown is used for internal comparison. Three contact Page 28
points are provided, one of them is movable and the other two arc fixed. The movable contact is provided with a flat surface that makes contact with the spherical end of a long lever mounted in the instrument tube. The lever has knife edge bearings and its other end actuates a dial gauge. The scale value of the dial gauge of the passimeter is either 0.01, 0.002 or 0.001 mm, according to the measuring range required. With the aid of exchangeable measuring pins, the measuring capacity of the passimeter can be increased. The passimeter is provided with standard ring gauges for the purpose of setting the instrument to zero. The scale is also provided with tolerance marks. 1.5 Twisted Band Comparator. This instrument was first devised by the british engineer C. F. Johansson and therefore it may be called Johansson Mikrokator. It uses a twisted strip to convert small linear movement of a plunger into a large circular movement of a pointer. It uses the simplest method for obtaining the mechanical magnification designed by H, Abramson which is known as Abramson s movement. A twisted thin metal strip carries at the centre of its length a very light pointer made of thin glass. As illustrated in Fig. 1.24b, for very small linear movement of the twisted chord in the direction of the arrows the disc rotates at a considerable speed. One end of the strip is fixed to the adjustable cantilever strip and the other end is anchored to the spring elbow, one arm of which is carried on measuring plunger. The spring elbow acts as a bell crank lever. The construction of such a Page 29
comparator is shown in Fig. 1.26. Fig. 1.25 Twisted strip comparator Fig. 1.26 Construction of twisted strip comparator Slight upward movement of plunger will make the bell crank lever to Page 30
rotate. Due to this a tension will be applied to the twisted strip in the direction of the arrow. This causes the strip to untwist resulting in the movement of the pointer. The spring will ensure that the plunger returns when the contact pressure between the bottom tip of the plunger and the workpiece is not there, that is when the workpiece is removed from underneath the plunger. The length of the cantilever can be varied to adjust the magnification. In order to prevent excessive stress on the central portion, the strip is perforated along the centre 1ine by perforation as shown in Fig.1.25. The magnification of the instrument is approximately equal to the ratio of rate of change of pointer movement to rate of change in length of the strip. The magnification of the instrument could be expressed as follows:- where, Q = twist of mid point of strip with respect to the end L = length of twisted strip measured along its neutral axis w= width of twisted strip and, n = number of turns. It is thus obvious that in order to increase the magnification of the instrument a very thin rectangular strip must be used, Page 31
1.6 Sine Bar. 1-6-1 Sine bar construction Fig. 1.27 Sine bar As shown in Fig. 1.27, sine bar is a rectangular cross sectional steel bar, having two accurately ground rollers of equal diameters, one at each end. The axes of the rollers are separated by a nominal distance usually 100 mm or 250 mm as stamped on the bar. Sine bar must be made with some requirements to assure accurate measuring results. The upper surface be flat, and the roller muse be made of equal diameters, their axes must be parallel to each other, and parallel to the upper surface of the bar, the distance between the axes of the rollers must be accurately set. Page 32
1-6-2 Sine bar working principle Fig. 1.28 The principle of sine bar h =(h2+r)-(h1+r)= θ = sin-1( h/l) where h is the difference between the over rollers or under rollers readings, L is the length of the sine bar (distance between rollers axes) 1-6-3 Source of error in Sine bar. Any measuring results encountering some errors. The main source of error in determining the angle by the use of sine bar is the error arise from measuring the size over or under the roller and the error in the Page 33
length of the sin bar. It is clear that, in case of using the sine bar, the raised error is a function of tan θ. This results in highly increase of the error value as the measured angle value exceed 45 degrees, Fig. 1.29. For this reason, sine bar is most frequently used to measure angles less than 45 degrees. For angles greater than 45 degrees, the complement angle is measured, Fig. 1.30 & 31. Page 34
Fig. 1.29 error in sine bar Fig 1.30 (a) correct setting, (b) wrong setting Fig. 1.31 measuring the complement angle Page 35
1-6-2 Sine bar measuring technique. Figures 1.32 show the measuring technique by the use of the sine bar. The bar may be placed on the measured surface such that its upper surface is laid on the measured surface. In this case readings over the rollers may be taken by height gauge, or dial indicator. Other technique is that, the bar is clamped to the workpiece such that its upper surface is aligned with the measured surfaces. The roller under sizes are then measured by the use of block gauges, inside micrometers, or dial indicators. Another technique is to place the workpiece on the sine bar. The bar is gradually tilted till the upper surface of work being parallel to the reference base. This may be checked by the aid of a dial indicator. On reaching this position the tilting angle of the bar is determined. Page 36
Fig. 1.32 Sine bar measuring technique Page 37
1.7 Sensitive (spirit) Level. 1.7.1 Construction of Sensitive Level Sensitive (spirit) levels are used to check the tilting of planes with respect to the horizontal or vertical planes. Horizontal planes is defined as the plane parallel to a plane tangential to the earth s curvature. Sensitive level may be flat or square as shown in Fig. 1.33. Flat type are used to check horizontal planes, and square types are used to check either horizontal or vertical planes. Fig. 1.33 Flat and square levels. Sensitive level is consists of a curved glass vial (small tube) enclosing a liquid in which is an air bubble. A scale is marked on the vial outside surface, Fig. 1.34. The glass tube is fitted to a metal block in a manner that when the lower surface of the block is horizontal,fig. 1.35. The air bubble will center itself at the highest point of the tube, i.e. in the middle of the vial Page 38
Fig. 1.34 A schematic of the vial Fig. 1.35 detail construction of the level 1-7-2 Operating principle of the level. The operating principle of the level is that the level of the liquid is always horizontal, and during vial tilting the air bubble is displaced to the highest point ( this is due to gravitational attraction). The reading is taken from the scale by the margin of the bubble., Fig, 1.36. Page 39
Fig. 1.36. Operating principle of the level. l nd nc R R nc where n is the number of division displaced by the bubble, d is the scale division, R is the radius of the vial curvature c=d/r is the level constant, always expressed as mm/m Level setting Most of the levels have an adjustment screw, which aligns the vial to the base. Levels must be adjusted before used, the procedure is as follows:firstly, orient the level on a flat surface until the bubble is centered, then clamp a straight edge to the surface along one side of the level, Fig. 1.37. Secondly, reverse the level and take its reading. Using the Page 40
adjustment screw, Correct the level by half of the reading. Reorient the level to center the bubble and repeat the procedure till the bubble does not displaced as the level is reversed. Fig. 1.37. Level adjustment procedure Page 41
1.8 Other Mechanical Metrology Devices 1-8-1 Ball Type Plug Gauge Fig. 1.38 Ball-type plug gauge. This gauge is used for measurement of diameter of bores. This also detects the ovalily and taper effect and also indicates the surface finish. This gauge is based on the principle of three equally spaced balls of same size which are moved outwards by a spring-loaded cone as shown in Fig.1.38. Thus when this gauge is inserted into any bore, the cone moves down under the action of the spring till all the three balls come in exact contact with the bore. The axial movement of the cone is transmitted to a dial indicator through a rod. The three balls move freely on the lapped surface of this plug gauge. The arrangement obviates the use of levers and cranks and their bearings Page 42
as in the case of Keilpart gauge. To measure the diameter of any bore first the gauge is inserted into a ring gauge of nearly the same size and dial reading is set at zero by adjusting the bevel ring. The gauge is then inserted into the hole and the plus or minus reading noted. 1-8- Bore Gauge (Keilpart) The Keilpart bore gauge, Fig. 1.39, is used as an internal dial comparator for measuring the diameter of deep holes at different positions along the depth. It employs two spring loaded hinged members, a fixed anvil and a movable plunger whose horizontal displacement is converted to a vertical displacement by means of a magnification lever to a dial gauge fitted in the tube of the instrument. The vertical bar which slides in the tube carries a mark which indicates the horizontal displacement of the movable anvil against a scale engraved on the tube. The mark can be seen through a slot cut in the tube. The fixed pin can be replaced by exchangeable rods with different lengths to enable the measuring capacity of the gauge to be increased. The measuring range and the scale value of the instrument depend upon those of the dial gauge used. Usually, the bore gauge has a scale value raf 0.01 mm, a measuring range of 10 mm, and its capacity ranges between 35 to 60 mm. Page 43
Fig. 1.39. Keilpart Page 44