Vernier Instruments (Metrology)

The principle of vernier is that when two scales or divisions slightly different in size are used,
the difference between then 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 posi-
tive contact members (the jaws of the
caliper gauges). The datum of the meas-
urement can be made to coincide precise-
ly 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 refer-
ence marks on the slide with a particular
scale value, significantly reduces the ex-
tent of read-out alignment errors.
Vernier caliper.
Fig. 2.86. Vernier caliper.
A sliding jaw which moves along the guiding surface provided by the main scale is coupled
to a vernier scale. The sliding jaw at its 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 the adjusting screw (Fig. 2.86). 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
slidingjaw to move, as the adjusting screw rotates on a screw which is in a way fixed to the movable
jaw. After final adjustment has been made, the locking nut A is also tightened and the 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., 1/25 x 24 x 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 to 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 form the zero on the main scale. Note
down the vernier division which coincides with a scale
division and add to previous reading the number of
thousands of a unit indicated by the vernier divisions e.g.
reading in the scale shown in Fig. 2.87 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 measuringjaw
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. 2.88, 2.89 and 2.91 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.
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 for 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.
Type B Vernier Calipertmp10-62_thumb
Fig. 2.89. Type B Vernier Caliper.
Beam and Vernier for Vernier Calipers having least count of 0.05 mm.
Fig. 2.90. Beam and Vernier for Vernier Calipers having least count of 0.05 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.
In Fig. 2.92, Graduations on beam are at every 1/2 mm and every alternate mm lines are
extended and numbered 2, 4, 6, 8. On vernier scale, there are 10 divisions within a distance of 9.5
mm and 9.5 mm = 19 divisions of main scale.
Type C Vernier Caliper
Fig. 2.91. Type C Vernier Caliper.
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 of200 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 arrange-
ment is provided on the sliding jaw in order to effective-
ly clamp it on the beam. When the sliding jaw is
clamped to the beam at any position within the measur-
ing 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.


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 pig-
ment. 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. 2.93) 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. 2.93
should not exceed 0.1 mm.
The graduations on the beam and the vernier for least count 0.05 mm are illustrated in Figs.
2.90 and 2.92. In fact, various types of markings for each least count are possible.
Beam and Vernier for VernierCaliper having least count of 0.05 mm.
Fig. 2.92. Beam and Vernier for Vernier
Caliper having least count of 0.05 mm.
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
0.1 mm
0.05 mm
0.02 mm
± (75 + 0.05^) urn
± (50 + 0.05/j) (im
± (20 + 0.02^) |im

where l± = 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 measurements with vernier calipers.

Errors are usually made in
measurements with vernier calipers from manipulation of vernier caliper and its jaws on the
workpiece. For instance, in measuring an outside diameter, one should be sure that the caliper bar
and the plane of the caliper jaws are truly perpendicular to the workpiece’s longitudinal centre
line ; i.e. one should be sure that the caliper is not canted, tilted, or twisted. It happens because the
relatively long, extending main bar of the average vernier calipers so readily tips in one direction
or the other.
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. 2.94 (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.
 Various jaw conditions which result in inaccurate caliper measurements
Fig. 2.94. Various jaw conditions which result in inaccurate caliper measurements.
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. 2.94 (b).
Where a vernier caliper is used mostly for measuring inside diameters, the jaws may become
bowlegged as in Fig. 2.94 (c) or its outside edges worn down as in Fig. 2.94 (d).

Care in use of vernier calipers.

These should not be treated or used as a wrench
or hammer because these are not rugged instruments. They should be set down gently, preferably
in the box and not dropped or tossed aside. They must be wiped free from grit, chips and oil. These
should be brought to the workpiece.
The workpiece should not be clamped in the caliper jaws and waived in air.

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 slidingjaw. 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 short-comings 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 measure-
ments. 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.
Vernier caliper must always be held at short leg of main scale and jaws never pulled.

Digital Caliper


Easy readings of digital display with
large 4,7 mm high characters with decimal point,
minus sign.
Zero setting at any position within
measuring range by conveniently placed button.
One-hand operation only two function
No misreadings, due to extremly high
measuring speed.
Max. speed 1.5 m/s.
Low power consumption. Battery has
life of 1 year continuous operation.
Data exit. For connection of printers and
computers for evaluation and processing of
measuring results. Data acquisition is initiated
by button on slide or by foot switch.
Positive locking screw for slide. This
is of advantage for many applications.
. Digital caliper
Fig. 2.95. Digital caliper.
2.28.8. Applications of Digital Caliper
Measuring of deviations
Measuring of deviations
Set nominal size, press zero button,
perform comparison measurement.
Measuring of clearance
Measuring of clearance
e.g. between shaft and bore
Measure shaft, press zero button,
measure bore and read clearance.
Measuring of center distances
Measuring of center distances
of bores with equal diameter
Measure one bore, press zero button,
measure largest distance and read
center distance.
Measuring of distances
Measuring of distances
e.g. of a bolt from an edge.
Measuring of positions with difficult access
Measuring of positions with difficult access
e.g. width of recesses in bores. When
locking screw cannot be reached,
take measurement, press zero button,
close measuring jaws and read results on
closed caliper.
Scribing of components
Scribing of components
Fig. 2.96. Applications of digital caliper.

Electronic Digital Caliper.

Electronic Digital Caliper is the most advanced
electronic digital measuring instrument for fast, accurate and reliable measurements and yet can
be used by operators—(semi skilled and non-technical) on the shopfloor. It measures outside and
inside diameters, depth and steps etc. In it is incorporated the capacitive measurement system and
with the revolutionary new chip permitting interface with computer print-out facilities. The
Electronic digital caliper.
Fig. 2.97. Electronic digital caliper.
powerful large scale integration (LSI) micro chip gives full scale computer functions to measure,
monitor, and memorise. The clear easily read LCD reads upto 0.01 mm.
Salient features of the Caliper
— Error free accurate and fast
— True inch/metric conversion.
— Floating zero allows compara-
tive measurements.
— Display memory facilitates :
— ‘HOLD’ function freezes dis-
play reading.
— No vernier scale—rack or
pinion, and there is no back-
— Unaffected by oil or dust and
ideally suited for shopfloor use.
— Low power consumption (one
button cell of 1.5 V) for one year
continuous use.
The frame, jaws and depth rod are
of hardened stainless steel precision
Deviation from reference size A
Deviation from reference size A
Comparisons between plug and hole
Comparisons between plug and hole
Measurement of centre distance between two identical holes
Measurement of centre distance between two identical holes
Measurement of centre distance between two identical holes
Measurement of end section
Fig. 2.98. Typical Application of Electronic Digital Caliper.

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