Calipers (Metrology)

2.4.
For the parts which can’t be measured directly with the scale, assistance of calipers can be
taken. Calipers thus act as accessories to scales. The caliper consists of two legs hinged at top, and
the ends of legs span the part to be inspected. This span is maintained and transferred to the scale.
It would be noted that calipers easily sense diameter (i.e. maximum distance) and transfer
the distance between the faces to the rule in such a way as to reduce sighting errors and increase
the reading accuracy.
Calipers can be either spring type or firm-joint type. Again under spring calipers we can
have outside and inside calipers and under firm-joint calipers we have outside, inside, transfer and
hermophrodite calipers. In spring calipers, spring tension holds the caliper legs firmly against the
adjusting nut. These are more accurate and permit accurate sense of touch in measuring.
Firm joint calipers work on the friction created at the junction of legs. These become loose
after certain use. But they are easier to adjust and are particularly suitable for larger work.
The various types of calipers could thus be classified as firm joint calipers and spring calipers.
2.4.1.

Firm Joint Calipers.

These are the devices for comparing measurements against
known dimensions. In the case of firm joint calipers, two legs (which are made from carbon and
alloy steel containing not more than 0.05 per cent sulphur and 0.05 per cent phosphorus) and
working ends are suitably hardened and tempered to a hardness of 400 to 500 HV and measuring
Firm joint Outside Caliper
Fig. 2.7. Firm joint Outside Caliper.
Firm joint Inside Caliper
Fig. 2.8. Firm joint Inside Caliper.
faces are hardened to a hardness of 650 ± 50 HV exactly identical in shape with the contact points
and equally distant from the fulcrum, the legs are joined together by a rivet. The legs are set correctly
so that the working ends meet evenly and closely when brought together. In case the two legs are
joined together by screw, nut and washer instead of rivet, then the threads of screw and nut should
be full and true and the joints made such that these function freely with even tension without any
undue play or stiffness. The component parts of the calipers should be free from seams, cracks,
flaws and must have smooth bright finish.
The firm joint calipers can be designed for inside measurement as well as outside measure-
ments depending upon the shape of the legs. The distance between the roller centre and the extreme
working end of one of the legs is known as nominal size and these calipers are available in the
nominal size of 100,150, 200 and 300 mm. The capacity of the caliper is the maximum dimension
which can be measured by it. The capacity of the caliper should not be less than its nominal size.
The legs of these calipers are made of rectangular cross-section.
For assuring that the calipers function satisfactorily, it is essential to ensure that each caliper
works smoothly and retains its size when adjusted to three test pieces of the sizes at extreme (lower
and upper) ends and middle of the instrument range. The accurate use of calipers depends upon
sense of feel of operator. For this purpose, caliper should be held gently, and held square to the
work, and only light gauging pressure should be applied.
2.4.2.


Firm Joint Divider.

These are used for scribing arcs and circles. The distance
between fulcrum roller centre and the extreme working end of one of legs is known as the nominal
size and these are available in the size of 100, 200 and 300 mm. The working ends of the divider
legs (upto 10 mm from extreme tips) are suitably heat treated in order to impart a hardness which
will make it capable of scribing a clearly defined line across the face of a block (having hardness of
620 to 660 HV) and still the working ends should be sharp and visual examination should not reveal
any sign of damage or failure. The working legs are made
very sharp so that the trace on the test block does not show
any sign of damage or failure. The fulcrum roller is hardened
and tempered to a hardness of 625 HV and joint made such
that the divider shall be capable of smoothly retaining its
size. The rigidity of the leg and the proper functioning of the
divider is judged by clamping securely one leg of the divider
in a vice at a point about 13 mm from the fulcrum and
opening the other leg slightly such that both the legs are in
a horizontal plane. A test load of 4.5 kg is then gradually
applied by suspension from the free leg as close to the
working end as possible. On removal of the load, the joint
should not be displaced and the legs should not show any
signs of permanent set.
Firm Joint Divider
Fig. 2.9. Firm Joint Divider.
2.4.3.

Spring Calipers.

The legs of spring calipers are made from suitable alloy steel,
measuring faces properly heat-treated and hardened to a hardness of 650 ± 50 HV. These should
be free from seam, cracks, flaws etc. and all component parts should be smoothly finished. The
spring which is made from carbon spring steel is properly hardened and tempered to a hardness of
470 to 520 HV and all components are manufactured and assembled in such a way as to achieve
and maintain the smooth adjustment and efficient operation.
The working ends of the legs of each spring caliper should be identical in shape and have
contact points equally distant from the fulcrum. The measuring ends of each pair are so shaped
that all sizes within its capacity may be readily transferred from measuring scales or gauges. The
cross-section of the legs may be either of rectangular or circular shape. The calipers are adjusted
to a set dimension by means of either a knurled solid nut or a knurled quick-action release nut
operating on a finely threaded adjusting screw, the latter one being preferred. The top portions of
Outside Spring Caliper
Fig. 2.10. Outside Spring Caliper.
ide Spring Caliper
Fig. 2.11. Inside Spring Caliper.
the legs are located in a flanged fulcrum roller and held in position by a spring in order to maintain
the alignment of the working ends. The spring provides sufficient tension to hold the legs rigid at
all points of the adjustment. The friction between the adjusting nut and the leg is minimised by
using a separate washer under the nut. One end of the adjusting screw is securely hinged to a leg
of the caliper and a steel ball positively fixed to the free end of the screw for the purpose of retaining
the adjusting nut. The inside and outside spring calipers are available in sizes of 75,100,150, 200,
250 and 300 mm. A good caliper should be able to retain its sizes when adjusted to three pieces near
the lower and upper extremes of its range and in the middle.
According to other considerations, calipers could be classified as :
(i) Outside calipers, (ii) Inside calipers,
{iii) Transfer calipers, (iv) Hermophrodite calipers.
2.4.4.

Outside Calipers.

These are designed to measure outside dimensions. The accuracy
in caliper measurement depends upon the inspector’s sense of feel. When a caliper has to be set
from the scale to control the dimension of any component, then leg of the caliper must be held firmly
against the end of the proper dimension by adjusting nut with the thumb and forefinger.
Outside Spring Caliper
Fig. 2.12. Outside Spring Caliper.
Outside Firm Joint Caliper
Fig. 2.13. Outside Firm Joint Caliper.
When caliper is used to measure the outside diameter of a cylinder, then it must be held in
hand with its nut between fore-finger and thumb, and one end of leg touching with the surface of
cylinder. The other leg is moved to and fro on the opposite surface so that other leg just passes
through the cylinder. The caliper reading is transferred to the scale by holding one leg of the caliper
against the scale firmly and the reading taken at the other end of leg.
For accurate settings, the distance between the outside calipers may be set by slip gauge or
by micrometer anvils. Sometimes it may be required to transfer this dimension to caliper.
2.4.5.

Inside Calipers.

While using inside calipers, the use of hook scale to measure the
distance between its legs is desirable. With ordinary scale, one leg of inside caliper must be coincided
with a vertical brace and second leg adjusted accordingly. (Refer Fig. 2.15) It will be best if the legs
of the inside caliper are adjusted with a micrometer.
2.4.6.

Transfer Calipers.

These are used for measuring recessed areas from which the legs
of the calipers can’t be removed directly but must be collapsed after the dimension has been
measured. In the transfer calipers, an auxiliary arm is provided to preserve the original setting
after the legs are collapsed.
2.4.7.

Hermophrodite Calipers

(Refer Fig. 2.14).
It is also known as odd leg caliper and is a scribing tool
consisting of one divider and one caliper leg. Thus it is used
in layout work. It can have two types of legs, viz., notched
legs or curved legs
In order to avoid excessive wear of nut while closing
the legs of spring calipers, the legs must be first collapsed.
The caliper must be held square to the work when
checking (Refr Fig. 2.16. Also when measuring the outside
diameter, a caliper should not be forced upon the work.
Forcing the caliper upon the work results in springing of
legs and the false readings.
For checking the inside diameters, one leg of caliper
is held in contact with the bore while, the other is rotated
through arc to get the correct “feel”, using no force.
Hermophrodite caliper
Fig. 2.14. Hermophrodite caliper.
 Setting Caliper
Fig. 2.15. Setting Caliper
to Scale.
Setting of caliper to a plug gauge
Fig. 2.16. Setting of caliper to a plug gauge (Note the
perpendicularity in different planes).
2.4.8.

Dial Calipers.

(Refer Fig. 2.17) These are typical of all hand-held calipers. These
provide a typical direct-reading capability of 0.02 mm. These units are quite flexible, offering a
typical measurement range of 150 mm with accessories available for range extensions, as well as
specific measurement accessories.
These use mechanical comparators. The translation of the linear form of the workpiece is
accomplished by referencing the caliper jaws to a measurement scale by mechanical means. The
workpiece variation is assessed by a gear-train and lever (mechanical) translation technique that
Dial Calipers
Fig. 2.17. Dial Calipers.
utilises principles of amplification. Calipers utilising electronic technology to assess and display
dimensions are also available.
Many modern calipers are fitted with microprocessor-based direct reading devices. These
are capable of direct hook up or interface with computerised storage and display unit; which can
chart and interpret assessed variation over time.

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