What is a Comparator in Metrology ? | Types of Comparators January 5, 2018 by In Metrology, The comparator is a Precision Instrument, which is used to compare the dimensions of the given component with the actual working standard. The Comparator is an indirect type of precision measurement because it will not measure the dimension, it will indicate the difference in measurement between the given component and working standard, and another magnification instrument is needed to measure this difference with accuracy. Still Didn’t get it? Let’s talk about The principle Principle and operation of Comparator The comparator (which will have a dial indicator) will be used along with the gauge blocks. Gauge blocks are also known as Slip Gauge (You already knows right?) Now you need to Arrange the slip gauges to a dimension of which the workpiece should have. The Slip gauges will have the working standard dimension, but the workpiece will have a deviation from this working standard. The workpiece dimension may be less than or greater than this(Slip gauge) dimension What is a Comparator in Metrology ? | Types of Comparators January 5, 2018 by In Metrology, The comparator is a Precision Instrument, which is used to compare the dimensions of the given component with the actual working standard. The Comparator is an indirect type of precision measurement because it will not measure the dimension, it will indicate the difference in measurement between the given component and working standard, and another magnification instrument is needed to measure this difference with accuracy. Still Didn’t get it? Let’s talk about The principle Principle and operation of Comparator The comparator (which will have a dial indicator) will be used along with the gauge blocks. Gauge blocks are also known as Slip Gauge (You already knows right?) Now you need to Arrange the slip gauges to a dimension of which the workpiece should have. The Slip gauges will have the working standard dimension, but the workpiece will have a deviation from this working standard. The workpiece dimension may be less than or greater than this(Slip gauge) dimensionWhat is a Comparator in Metrology ? | Types of Comparators January 5, 2018 by In Metrology, The comparator is a Precision Instrument, which is used to compare the dimensions of the given component with the actual working standard. The Comparator is an indirect type of precision measurement because it will not measure the dimension, it will indicate the difference in measurement between the given component and working standard, and another magnification instrument is needed to measure this difference with accuracy. Still Didn’t get it? Let’s talk about The principle Principle and operation of Comparator The comparator (which will have a dial indicator) will be used along with the gauge blocks. Gauge blocks are also known as Slip Gauge (You already knows right?) Now you need to Arrange the slip gauges to a dimension of which

1. COMPARATORS
 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.
 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.

2. Classification:
1. Mechanical Comparator: It works on gears pinions, linkages,
levers, springs etc.
2. Pneumatic Comparator: Pneumatic comparator works by using
high pressure air, valves , back pressure etc.
3. Optical Comparator: Optical comparator works by using lens,
mirrors, light source etc.
4. Electrical Comparator: Works by using step up, step down
transformers.
5. Electronic Comparator: It works by using amplifier, digital signal
etc.
6. Combined Comparator: The combination of any two of the above
types can give the best result.

3. MECHANICAL COMPARATORS
 Some of the comparator which belong to this class are:
1. Dial indicator
2. Reed type comparator
3. Sigma Comparator
4. Johnson Mikrokrator

4. Dial Indicator:
 It operates on the principle, that a very slight upward pressure on the
spindle at the contact point is multiplied through a system of gears
and levers.
 It is indicated on the face of the dial by a dial finger.
 Dial indicators basically consists of a body with a round graduated
dial and a contact point connected with a spiral or gear train so that
hand on the dial face indicates the amount of movement of the
contact point.
 The indicator is set to zero by the use of slip gauges representing the
basic size of part.

5. DIAGRAM

6. Dial Indicator: (different uses)

7. Applications:
1. Comparing two heights or distances between narrow limits.
2. To determine the errors in geometrical form such as ovality,
roundness and taper.
3. For taking accurate measurement of deformation such as intension
and compression.
4. To check the alignment of lathe centers by using suitable accurate
bar between the centers.
5. To check trueness of milling machine arbours and to check the
parallelism of shaper arm with table surface or vice.

8. Johansson Microkator :
Principle:
 It works on the principle of a Button spring, spinning on a loop of string like
in the case of Children’s toys.
 The method of mechanical magnification is shown in Figure.
 It employs a twisted metal strip. Any pull on the strip causes the centre of
the strip to rotate.
 A very light pointer made of glass tube is attached to the centre of the
twisted metal strip.
 The measuring plunger is on the slit washer and transmits its motion
through the bell crank lever to the twisted metal strip.
 The other end of the twisted metal strip is fastened to the cantilever strip.
The overhanging length of the cantilever strip can be varied to adjust the
magnification of the instrument.
 The longer the length of the cantilever, the more it will deflect under the
pull of the twisted metal strip and less rotation of the pointer is obtained.

9. DIAGRAM

10. Reed Type Mechanical Comparator

11. HOW IT WORKS
 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.
 The floating block B 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, as shown in the figure . Beyond this joint extends a pointer
or target.
 A linear motion of the spindle moves the free block vertically
causing the vertical reed to slide.
 This movement causes the pointer or target to swing through an arc
over the scale proportional to the distance traveled by the spindle and
of course very much magnified. Comparators of this type may have
sensitivities of the order of 0.25 micron per scale division.

12. Mechanical - Optical Comparator:

13. Principle:
 In mechanical optical comparator, small variation in
the plunger movement is magnified first by
mechanical system and then by optical system

14. HOW IT WORKS
 In mechanical optical comparator
small displacement of the measuring
plunger are amplified first by
mechanical system consisting of
pivoted lavers.
 The amplified mechanical
movement is further amplified by a
simple optical system involving the
projection of an image.
 Mechanical system causes a plane
reflector to tilt about an axis and the
image of an index is projected on a
scale on the inner surface of a
ground glass screen.
 Optical magnification provides high
degree of measuring precision.

15. IMPORTANT
 In this system,
Mechanical amplification = x2 / x1
Optical amplification = (x4 / x3 ) * 2
 It is multiplied by 2, because if
mirror is tilted by an angle δθ, then
image will be tilted by
2 x δθ.
 Thus over magnification of this
system = 2 (x2 / x1) *(x4 / x3)
 Advantages of Optical
Comparators:-
 1. It has small number of moving
parts and hence a higher accuracy.
 2. In the optical comparators, the
scale can be made to move past a
datum line and thus have high
range and no parallax errors.
 3. It has very high magnification.
 4. Optical lever is weightless.

16. Sigma Comparator:

17. HOW IT WORKS
 The plunger is attached to a bar which is supported between the
bending plates at the top and bottom portion as shown in Figure (a)
 The bar is restricted to move in the vertical direction. A knife edge is
fixed to the bar.
 The knife edge is attached to the sapphire plate which is attached to the
moving block.
 The knife edge extorts a force on the moving block through sapphire
plate. Moving block is attached to the fixed block with the help of
crossed strips as shown in Figure (b).
 When the force is applied on the moving block, it will give an angular
deflection.
 A Y-arm which is attached to the moving block transmits the rotary
motion to the driving drum of radius r.
 This deflects the pointer and then the reading is noted.

18. Advantages of Mechanical Comparator
 1. They do not require any external source of energy.
 2. These are cheaper and portable.
 3. These are of robust construction and compact
design.
 4. The simple linear scales are easy to read.
 5. These are unaffected by variations due to external
source of energy such air, electricity etc.

19. Disadvantages:
• Range is limited as the pointer moves over a fixed
scale.
• Pointer scale system used can cause parallax error.
• There are number of moving parts which create
problems due to friction, and ultimately the accuracy
is less.
• The instrument may become sensitive to vibration due
to high inertia.

20. 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.

21. How It Works
Working:
 As shown in Figure (a) 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.
 Due to restricted area, at A1 position, the back pressure is generated by the
head of water displaced in the manometer tube.
 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.
 Then the same procedure is repeated at various positions A2, A3, A4,
position and variation in the pressure reading is found out.
 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

22. Metrological Advantages of
Pneumatic Comparators
 Reasons for Pneumatic Inspection of Small Holes:
 1. Pneumatic gaging elements can be very small. This permits precision
measurement of smaller inside diameters that is possible by other
means of measurement.
 2. Deeper holes can be inspected because depth has little effect on ease
of measurement.
 3. Greater accuracy is obtained because rocking and centralizing are
unimportant.
 4. Nominal gaging force permits thin-walled and yielding materials to
be measured.
 5. Absence of metal-to-metal contact protects finely finished surfaces.

23. Disadvantages of Pneumatic
Comparators:
 i)They require elaborate auxiliary equipment such as
accurate pressure regulation.
 ii) The scale is generally not uniform.
 iii) When indicating device is the glass tube, then high
magnification is necessary in order to avoid the
meniscus errors.
 iv) The apparatus is not easily portable and is rather
elaborate for many industrial applications.
 v) Different gauging heads are required for different
dimensions.