Comparator
Introduction :
 A comparator is a precision instrument
employed to compare the dimension of a
given component with a working standard
(usually slip gauges).
 It thus does not measure the actual
dimension but indicate how much it differs
from the basic dimension.

In mass production identical component
parts are produced on a very large scale.
 To produced interchangeability these parts
produced very close dimensional
tolerance.
 As a result, inspection is often more
concerned with the dimensional variation
from the standard parts.

The micrometer and vernier calliper are
not more feasible because skill involve and
time required to measure the dimension.
 In such case comparator is more suitable
for the measurement.

Basic principle of operation
The comparator first adjust on zero on its
dial gauge with a gauge block in position.
 The gauge block is of dimension which the
work piece should have.
 The work piece to be checked is then
placed in position and comparator gives
the difference in dimension in relation to
the gauge block.

Dimension may be greater, less or equal
than the standard dimension.
 If the dimension greater or equal than the
standard gauge its indicate on dial gauge.
 Thus comparator does not give the
dimension of the work piece but just give
the difference b/w work piece and
standard gauge.

Mechanical optical comparator
Sigma comparator
It is mechanical
comparator providing
magnification in the
range of 300 to 5000.
 It consist of a plunger
mounted on two flat
steel string.
 This provide
frictionless linear
movement.

The plunger carries
the knife edge which
bear upon the face of
mounting blocks of
cross strip hinge.
 The moving block
carries a light metal
y- forked arms and
wrapped around the
smalldrum.


A thin phosphors
bronze ribbon is
fastened to the end of
the forked arms and
wrapped around the
small drum, mounted
on the spindle
carrying the pointer.
Any vertical movement movement of the
plunger and hence that of the knife edge
makes the moving block of the cross strip
lever to pivot.
 This cause the rotation of the y-arms.
 The metallic band attached to the arms
makes the driving drum hence the pointer
to rotate.


Total magnification of the instrument is
L/a = l/r