types of comparators - metrology

1. comparators
presented by K.J.C.BOSE @ VLITS

2. comparator
• A comparator is a precision instrument
employed to compare the dimension of a
given component with a working standard
(usually slip gauges).
• Thus, It does not measure the actual
dimension but it indicates how much
difference from the basic dimension.

3. application
• 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 caliper are not more
feasible because skill involve and time required to
measure the dimension.
• In such case comparator is more suitable for the
measurement.

4. Basic principle of mechanical comparator
• The comparator first adjust to 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.
• 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.

5. Types of comparators
1. Mechanical comparators
2. Electrical comparators
3. Electronic comparators
4. Pneumatic comparators
5. Optical comparators

6. Mechanical comparators
• Mechanical comparators are further classified
into 4 types
1. Dial Gauge Comparator
2. Johansson Mikrokator Comparator
3. Reed Type Comparator
4. Sigma Comparator.

7. Dial guage mechanical comparator

8. • The simplest type of mechanical comparator
• It consists of a base with a rigid column rising
from its rear
• An arm mounted on this column and it carries a
dial gauge at its outer end
• The indicator is set at zero by the use of slip
gauges
• The part to be checked is placed below the
plunger
• The linear movement of the plunger is magnified
by mechanical means to a sizable rotation of the
pointer

9. • This type is generally used for inspection of
small precision machined parts
• The dial gauge with various attachments are
commonly used
• This comparator is ideal for the checking of
components with a tolerance of + 0.005 mm

10. Johansson Mikrokator

11. Working of the comparator
• A very thin metal strip at the centre carries a light
pointer made up of glass .
• One end of the strip is connected to the adjustable
cantilever strip and the other end is to the spring
elbow, in turn connected to the plunger.
• The slight movements of the plunger will make the
bell crank lever to rotate.
• This rotation will create tension in the strip and
causes the strip to rotate thereby the strip start to
untwist resulting in the movement of the point

12. • The spring ensures that the plunger returns when the
contact is removed
• The length of the cantilever can be varied to adjust
the magnification
• The mechanical magnification is obtained by
“Abramson Movement”

13. Reed Type Comparator

14. Working of Mikrokator
• 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, and floating block B, which carries
the gauging spindle and is connected horizontally to the
fixed block by reeds C

15. • 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.
• 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.

16. • The amount of target swing is proportional to the
distance the floating block has moved.
The scale may be calibrated by means of gauge block
(slip gauges) to indicate any deviation from an initial
setting.
• Sensitivity is in 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. It is
available in amplifications ranging from x 500 to x
1000.

17. Sigma Comparator

18. • The plunger is mounted on a pair of slit diaphragms
in order to have frictionless linear movement. A knife
edge is mounted on it and bears upon the face of the
moving member of a cross strip hinge.
• The cross strip hinge consists of the moving
component and a fixed member which are connected
by thin flexible strips alternately at right angle to each
other.
• Thus if an external force is applied to the moving
member ; it will pivot, as would a hinge, about the line
of intersection of the strips.
To the moving member an arm of Y shape and having
effective length I is attached.

19. Advantages of mechanical comparators
1.Cheaper than all the other type of comparators
2.Does not require any external source of power or
air supply
3.These comparators use a linear scale that can be
easily understood.
4.Usually these comparators are robust and compact
but are very easy to handle
5.These are small in size and can are portable from one
place to other very easily without much difficulty

20. Electrical comparator
• Unlike Mechanical comparators, Electrical
comparators are not have moving parts like
levers, gears, racks and pinions, activate
mechanical devices.
• The accuracy and life of the instruments are
affected as they are subjected to wear and
friction

22. • Because of no moving parts Electrical
comparators are expected to give a high
degree of reliability from these instruments.
• Generally there are two important
applications of electrical comparators:
1. Used as measuring heads
2. Used for electrical gauging heads, to
provide Visual indication to check the
dimensions within the limits laid down.

23. • The first application is very important when
there is a requirement for precise
measurement for e.g. Checking or comparison
of workshop slip gauges against inspection slip
gauges.
• The second application is used to indicate
with a green light if a dimension is within the
limits. A red lamp indicates an undersize
dimension; a yellow lamp indicates an
oversize dimension.

24. • So the operator is not required to be aware of
the actual tolerances on the dimension. After
setting the instrument correctly, all that
needs to be done is to place the component
under the plunger of the gauging head.
• The signal lamps provide in standard positive
indication of the acceptability of the
dimension under test.

25. ADVANTAGES OF ELECTRICAL COMPARATOR
• Measuring units can be remote from indicating units.
• Variable sensitivity which can be adjusted as per requirement.
• No moving parts, hence it can retain accuracy over long
periods.
• Higher magnification is possible as compared to mechanical
comparator.
• Compact sizes of probes available.

26. 1
• The accuracy of working of these comparators is likely
to be affect due to temperature and humidity.
2
• It is not a self contained unit; it needs stabilized
power supply for its operation.
3,4
• Heating of coils can cause zero drifts and it may alter
calibration.
• It is more expensive than mechanical comparator
DIS-ADVANTAGES OF ELECTRICAL COMPARATOR

27. ELECTRONIC COMPARATOR

28. • Electronic comparator is more complex,
advances in integrated circuits have reduced
in size and power consumption of the
equipment.
• Electronic gauges are more accurate and
reliable.
• Most significant advantage is speed of
response, a measurement rate of 500 per
minute is easily accomplished by an electronic
comparator this makes the instrument a
dynamic measurement.

29. advantages
• High accuracy and reliability
• High sensitivity in all ranges
• High speed of response
• Versatile in design
• Easy integration into an automated system

30. Pneumatic Comparators (Solex Gauge)
Principle:
• It works on the principle of pressure
difference generated by the air flow.
• Air is supplied at constant pressure through
the orifice and the air escapes in the form of
jets through a restricted space which exerts a
back pressure.
• The variation in the back pressure is then used
to find the dimensions of a component.

32. Working:
1. the air is compressed in the compressor at high pressure
which is equal to Water head H.
The excess air escapes in the form of bubbles. Then the
metric amount of air is passed through the orifice at the
constant pressure.
2. Due to restricted area, at A1 position, the back pressure is
generated by the head of water displaced in the manometer
tube.
3. To determine the roundness of the job, the job is rotated
along the jet axis, if no variation in the pressure reading is
obtained then we can say that the job is perfectly circular at
position A1.

33. 4. Then the same procedure is repeated at various positions
A2, A3, A4, position and variation in the pressure reading is
found out.
5. Also the diameter is measured at position A1 corresponding
to the portion against two jets and diameter is also
measured at various position along the length of the bore.
6. Any variation in the dimension changes the value of h, e.g.
Change in dimension of 0.002 mm changes the value of h
from 3 to 20 mm.
7.Moderate and constant supply pressure is required to have
the high sensitivity of the instrument.

34. ADVANTAGES OF PNEUMATIC COMPARATORS
1. It is cheaper, simple to operate and the cost is
low.
2. It is free from mechanical hysteresis and wear.
3. The magnification can be obtained as high as
10,000 X.
4. The gauging member is not in direct contact
with the work.
5. Indicating and measuring is done at two
different places.
6. Tapers and ovality can be easily detected.

35. 1. They are very sensitive to temperature and
humidity changes.
2. The accuracy may be influenced by the surface
roughness of the component being checked.
3. Different gauging heads are needed for different
jobs.
4. Auxiliary equipments such as air filters, pressure
gauges and regulators are needed.
5. Non-uniformity of scale is a peculiar aspect of air
gauging as the variation of back pressure is linear,
over only a small range of the orifice size
variation.
DIS-ADVANTAGES OF PNEUMATIC COMPARATORS

36. Optical Comparators
• Optical comparators are based on the
principle of projection of image.
• The arrangement consists of mechanical
system which causes a plane reflector to tilt
about an axis so that the image of an index is
projected on scale on the inner surface of a
ground glass screen.

38. • The actual difference x between the two
dimensions is amplified by a lever to give an
angular displacement ‘X’ of a pivoted mirror.
• The reflected ray is deflected through an angle
2 X from the original line and gives a reading
of X on the scale.
• The main advantage of an optical comparator
is that it is capable of giving higher degree of
magnification due to reduction of moving
members and better wear resistance qualities.