Comparators are instruments used to measure small differences between unknown dimensions and a standard length. They work by detecting and amplifying displacements of a sensing probe. There are different types of comparators classified by their amplification method, including mechanical, optical, pneumatic, and electrical. Mechanical comparators like the reed comparator amplify motions through a system of linked reeds and pointers. The sigma comparator uses flat steel springs and a hinged assembly to provide frictionless movement and amplification of the measuring contact. While robust, mechanical comparators have limitations due to friction and wear in their moving parts.

2. INTRODUCTION
Comparators are the instruments calibrated by means of
end standards to measure unknown dimensions. The
purpose of a comparator is to detect and display the small
differences between the unknown linear dimensions and
the length of the standard. The difference in lengths is
detected as a displacement of a sensing probe. The
important and essential function of the instruments is to
magnify or amplify the small input displacement so that it
is displayed on an analog scale. Comparators are classified
on the basis of type of the amplification method used.
Accordingly comparators are of following types or hybrid
thereof.
(a) Mechanical comparators, (b) Optical Comparators.
(c) Pneumatic comparators, (d) Electrical comparators.

3. Essential Parts of a Comparator
1. Fixed surface from which all measurements are taken
2. A very sensitive indicator which will show the
movement of a sliding piece usually terminating in
an anvil with curved surface
3. Some means of setting the curved face
Arrangement of above parts is used to measure the
difference between the length or diameter of a
component & a standard of length, usually made up
of slip gauges

4. Desirable Features…
1. Recording scale should be linear
2. Maximum rigidity
3. Maximum compensation of temperature effect
4. Neither lag nor backlash in the movement of the
plunger & recording mechanism
5. Indicator should be constant in its return to zero
6. Measurement pressure should be low & constant
7. Instrument should be capable of a wide measuring
range
8. Should be able to record variations of 0.0025mm.

5. Uses of Comparators…
1. To inspect newly purchased gauges
2. In mass production, where components are to be
checked at a very fast rate
3. As laboratory standards from which working or
inspection gauges are set & correlated
4. As working, to prevent work spoilage & to maintain
required tolerance at all important stages of
manufacture

6. MECHANICAL COMPARATORS
Conventional mechanical methods to obtain
magnification are not suitable in construction of
mechanical comparators as it causes backlash and
friction. Also they require a large input force. Let us
understand the mechanical comparators by studying a
reed comparator which is strictly a mechanical
comparator. The diagram represents a REED type
mechanical comparator.

7. REED COMPARATOR

8. REED COMPARATOR
In mechanical comparator, the gauging head
is usually a sensitive, high quality, dial
indicator mounted on a base supported by a
sturdy column.
The reed mechanism is frictionless device
for magnifying small motions of spindle.
It consists of a fixed block A which is
rigidly fastened to the gauge head case.
A floating block B, which carries the
gauging spindle and is connected
horizontally to the fixed block by reeds
C.
A vertical reed is attached to each block
with upper ends joined together. These
vertical
reeds are shown in the figure by letter D.
Beyond this joint extends a pointer or
target.

9. REED COMPARATOR
A linear motion of the spindle moves the free block vertically
causing the vertical reed on the floating block to slide past the
vertical reed on the fixed block.
However, as these vertical reeds are joined at the upper end,
instead of slipping, the movement causes both reeds swing
through an arc and as the target is merely an extension of the
vertical reeds, it swings through a much wider arc.
The amount of target swing is proportional to the distance the
floating block has moved but of course very much magnified.
The scale may be calibrated by means of gauge block (slip
gauges) to indicate any deviation from an initial setting.
Comparators using this type of linkage have sensitivities of the
order of 0.25 micron per scale division.
The mechanical amplification is usually less than 100, but it is
multiplied by the optical lens system.

10. SIGMA COMPARATOR

11. SIGMA COMPARATOR
The vertical beam is mounted on flat
steel springs A connected to fixed
members, which in turn are screwed
to a back plate.
The assembly provides a frictionless
movement with a restraint from the
springs.
The shank B at the base of the vertical
beam is arranged to take a measuring
contact, selecting from the available
range.
The stop C is provided to restrict
movement at the lower extremity of
the scale.
Mounted on the fixed members, is the
hinged assembly D carrying the
forked arms E.

12. SIGMA COMPARATOR
This assembly incorporates a
hardened fulcrum (provided with
means for adjustment of controlling
the ratio of trans-mitted motion)
operative on the face of a jeweled
insert on the flexible portion of the
assembly.
The metal ribbon, attached to the
forked arms, passes around the
spindle G causing it to rotate in
specially designed miniature ball
bearings.
Damping action to the movement is
affected by a metal disc, mounted on
the spindle, rotating in a magnetic
field between a permanent magnet
and a steel plate.
The indicating pointer H is secured
to a boss on the disc.

13. SIGMA COMPARATOR
The instrument is available with vertical capacities of 150
mm, 300 mm and 600 mm and magnifications of
500,1000,1500,3000 and 5000. The scales are graduated in
both English and Metric systems.
The least count which one division represents is of the
order of 0.25 microns.
Advantage :
It has got a bold scale and larger indicating pointer.
Disadvantages :
Due to motion of the parts, there is wear in the moving parts.
It is not as sensitive as optical or other types of comparators
due to friction being
present in the moving parts.

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