The principle of vernier is that when two scales or divisions slightly different in size are used, the difference between them can be utilised to enhance the accuracy of measurement. The vernier caliper essentially consists of two steel rules and these can slide along each other. One of the scales, i.e., main scale is engraved on a solid L-shaped frame. On this scale cm graduations are divided into 20 parts so that one small division equals 0 05 cm. One end of the frame contains a fixed jaw which is shaped into a contact tip at its extremity.
The three elements of vernier caliper, viz. beam, fixed jaw, and sliding jaw permit substantial improvements in the commonly used measuring techniques over direct measurement with line graduated rules. The alignment of the distance boundaries with the corresponding graduations of the rule is ensured by means of the positive contact members (the jaws of the caliper gauges). The datum of the measurement can be made to coincide precisely with one of the boundaries of the distance to be measured. The movable jaw achieves positive contact with the object boundary at the opposite end of the distance to be measured. The closely observable correspondence of the reference marks on the slide with a particular scale value, significantly reduces the extent of read-out alignment errors.
A sliding jaw which moves along the guiding surface provide
by the main scale is coupled to a vernier scale. The sliding jaw at iis left extremity contains another measuring tip. When two measuring tip surfaces are in contact with each other, scale shows zero reading. The finer adjustment of the; movable jaw can be done by I he adjusting screw (Fig. 2.73). First the whole movable jaw assembly is adjusted so that the two measuring tips just touch the part to be measured. Then lock nut B is tightened. Final adjustment depending upon the sense of correct feel is made by the adjusting screw. The movement of adjusting screw makes the part containing locking nut A and sliding jaw to move, as the adjusting screw rotates oil a screw which is in a way fixed to the movable jaw. After final adjustment has been made, the locking n:it A is also tightened and (he reading is noted down. The measuring tips are so designed as to measure inside as well as outside dimensions.
Reading the Vernier Scale. For understanding the working of vernier scale let us assume that each small division of the main scale is 0.025 unit. Say, the vernier scale contains 25 divisions and these coincide exactly with 24 divisions of main scale. So now one vernier division is equal to 1/25 of 24 scale divisions, i.e., l/25×24×0.025=0.024 unit. Therefore, difference between one main scale small division and one vernier division (least count of the instrument) equals 0.025—0.024, i.e. 0.001 unit. It means if the zero of main scale and zero of vernier coincide, then the first vernier division will read 0.001 unit less than the 1 small scale division. Second vernier division will read 0.002 unit less than 2 small scale divisions and so on. Thus if zero vernier scale lies in between two small divisions on main scale its exact value can be judged by seeing as lo which vernier division is coinciding with main scale division.
Thus to read a measurement from a vernier caliper, note the units tenths and fortieths which the zero on the vernier has moved from the zero on the main scale. Note down the vernier division which coincides with a scale division and add to previous reading I lie number of thousands of a unit indicated by the vernier divisions
e.g, reading in the scale shown in Fig. 2.74 is 3 units +0.1 unit +0.075 unit +0.008 unit=3.183 units. When using the vernier caliper for internal measurements the width of the measuring jaws must be taken into account, (Generally the width of measuring jaw is 10 mm for Metric System).
Types of vernier calipers. 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 upto 2000 mm with vernier accuracy of 0.02, 0.05 and 0.1 mm. The three types are called types A, B, C, and have been shown in Figs. 2.75, 2.76 and 2.79 respectively. 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 0-125, 0-200, 0-250, 0-300,0-500, 0-750, 0-1000, 750-1500 and 750-2000 mm.
Graduations on beam at every 1 mm and each 5 mm line
extended and each 1 cm line is numbered. On Vernier scale
there are 20. divisions within a distance of 19 mm and 19
mm=19 divisions of main scale.
On type A, scale serves for both external and internal measurements, whereas in case of types B and C, the main scale serves for external measurements and for marking purposes 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 jaws in case of types B and C calipers. The combined width should be uniform throughout its length to within 0 .01 mm.
On vernier scale, there are 10 divisions within a distance of 9.5 mm and 9.5 mm=19 divisions of main scale.
The beam for all the types is made flat throughout its length to within the tolerances of 0 05 mm for nominal lengths upto 300 mm, 0.08 mm from 900 to 1000 mm, and 0.15 mm for 1500 and 2000 mm sizes,, and 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 tine ground finish. The portions of the jaws between the beam and the measuring faccs 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 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 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 the their combined width.
Graduations. All graduations should be clearly engraved so that they are legible. Sometimes to facilitate reading, the surfaces of the beam and the vernier may be given a matt finish and the graduation lines filled with black pigment. For clarity sake, the length of visible portion of graduations on main scale on beam and vernier scale lines (dimension h) should be about 2-3 times the width of interval between adjacent lines (Refer Fig. 2.80) and the distance from the graduated face of the beam to the edge of the graduated bevelled face of the vernier (dimension s) in Fig. 2.80 should not exceed 0.1 mm.
The graduations on. the beam and the vernier for least count 0.05 mm arc illustrated in Figs. 2.77 and 2.78. In fact, various types of markings for each least count are possible.
The error in reading the vernier caliper should not exceed the values obtained by the following formulae :
Vernier with least count Permissible error in reading
0.1mm ±(75+0.05 h) μm
0.05mm ±(50+0.05 h) μm
0.02mm ±(20+0 .02 h) μm
where h =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 and disposed in such a way that each measurement calls into play a different vernier graduation line.
Errors in Calipers. 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 position, the caliper is held against a light source. If there is wear, spring or warp, a knock-kneed condition as shown in Fig. 2.81 (a) 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. 2.81 (b).
Where a vernier caliper is used mostly for measuring inside diameters, the jaws may become bowlegged as in Fig. 2.81 (c) or its outside edges worn down as in Fig. 2.81 (d):
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 workpiece.
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 workpiece’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.
In general, the vernier caliper should be gripped near or opposite the jaws ; one hand for the stationary jaw and the other hand generally supporting the sliding jaw. The instrument should not be held by the over-hanging “tail” formed by the projecting main bar of the caliper.
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?o 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.
Vernier caliper must always be held at short leg of main scale and jaws never pulled.