1. COMPARATORS
Introduction:
• Comparator is an instrument employed to
compare the dimension of a given component
with a working standard.
• Purpose of a comparator, in general, is to
detect & display the small differences b/w the
unknown linear dimension & length of the
standard
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Unit-I
2. Essential Parts of Comparators:
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 b/w the length or diameter of component &
a standard of length, usually made up of slip gauges
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3. Function:
The Important & essential function of the
instrument is to MAGNIFY or AMPLIFY the
small input displacements is displayed on an
dial or recording device.
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4. Desirable Features of a Comparator:
1. It should at least be able to record variations of
0.0025mm.
2. Recording scale should be linear.
3. Maximum rigidity.
4. Maximum compensation of temperature effect.
5. Neither lag nor backlash in the movement of the
plunger & recording mechanism.
6. Indicator should be constant in its return to zero.
7. Measurement pressure should be low & constant.
8. Instrument should be capable of a wide measuring
range.
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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.
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6. Types of Comparators:
Most common commercially available
comparators can be classified into the
following types:
1. Mechanical comparators
2. Optical comparators
3. Electrical & Electronic comparators
4. Pneumatic comparators
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7. Mechanical comparator
• The mechanical comparators utilize
mechanical methods of magnifying, the
movement of the contact plunger; their
manufacture requires a high degree of skill &
accuracy.
• The usual magnifications of the mechanisms
range from about 250 to 1000.
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8. MECHANICAL COMPARATOR
• Some of the comparator which belong to this
class are:
1. Dial indicator
2. Reed type comparator
3. Sigma Comparator
4. Johnson Mikrokrator
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11. Working principle:
• Firstly the indicator is set to zero by the use of
slip gauges representing the basic size of the
part.
• The part that is to be checked is then placed
below the measuring plunger of indicator.
• The linear movement of the plunger is
magnified by means of a gear and pinion train
into sizable rotation of the pointer.
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12. Cont.…….
• The variation in dimension of the part from
basic size is indicated on the dial.
Uses:
• It is generally used for inspection of small
precision machined parts.
• With various attachments it can be used for
large number of works
• EX: with v-block attachment it can be used for
checking the roundness of a cylindrical part.
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19. working principle
• A slight upward movement of the plunger will
make the bell crank lever to rotate.
• Due to this a tension will be applied to the
twisted strip in the direction of the arrow.
• This causes the strip to untwist resulting in the
movement of the point.
• The spring will ensure that the plunger returs
when the contact pressure between the bottom
tip of plunger and w/p is not there i.e.,
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20. • When the w/p is removed from underneath
the plunger.
• The length of the cantilever can be varied to
adjust the magnification.
• In order to prevent excessive stress on the
central portion strip is perforated along the
center line by preformation.
• The magnification of the instrument is equal
to ratio of rate of change of pointer
movement to the rate of change in length of
strip i.e .,dQ /dL .
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21. • It can be shown that the magnification of the
instrument dQ/Dl proportional to L/nw2
• Where Q=twist of mid point of strip
L=length of twisted strip measured along its neutral axis.
w= width of the twisted strip.
n=no of turns
NOTE:
So it is thus obvious that in order to increase the
magnification of the instrument a very thin rectangular
strip may be used.
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23. working principle
• A reed mechanism is a frictionless device for
magnifying small motions of the spindle
• A linear motion of the spindle moves the free
block vertically causing the vertical reeds on
the floating block to slide past the vertical
reed on the fixed block.
• However as the vertical reeds are joined at the
upper end instead of slipping the movement
causes both reeds swing through an arc.
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24. • The scale may be calibrated by means of
gauge block to indicate any deviation from the
initial setting
• The mechanical amplification is usually less
than 100 but it is multiplied by optical lens
system.
• It is available in the amplification range from
500 to 1000.
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26. Working principle
• Any vertical displacement of the measuring
plunger and hence that of the knife edge
makes the moving block of the cross strip liver
to pivot.
• This causes the rotation of Y-arms.
• The metallic band attached to the arms make
the driving drum and hence the pointer to
rotate.
• Magnification range 300 to 5000.
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27. • The ratio of effective length(L) the arm and the
distance(x) of the knife edge from the pivot gives
the first stage magnification.
• The ratio of pointer length l and radius of the
driving drum gives second magnification of the
instrument.
• The total magnification of the instrument is thus
(L/a*l/r).
• The magnification of the instrument can be
varied by changing the distance (x)of knife edge
by tightening on loosening of the adjusting
screws.
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28. Advantages of mechanical comparators
• Cheaper
• No need of external agency(like electricity ,air)
• Linear scale
• Robust and compact so are easy to handle.
• Portable(for ordinary workshop conditions)
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29. Disadvantages
• Less accuracy due to more moving parts
friction is more which reduces the accuracy.
• Sensitive to vibrations(have more inertia)
• Faults magnified(backlash ,dimensional faults)
• Limited range
• Parallax error(as pointer moves over a fixed
scale)
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31. Working principle
• Small displacements of the measuring plunger
are amplified first by a mechanical system
consisting of pivoted levers.
• The amplified mechanical movement is further
amplified by a single optical system involving the
projection of an image.
• The 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 screen.
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32. • Mechanical amplification=L2/L1(by lever rule)
• Now if the movement of the plunger causes
the mirror to tilt by an angle α, then the image
will be tilted by 2α.
• Therefore optical amplification=2*L4/L3
• Thus overall magnification of this
system=2*L2/L1*L4/L3
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34. Working principle
• The optical system of Zeiss ultra optimeter
involves double reflection of light and thus give
higher degree of magnification.
• The light rays from the lamp falls on the green
filter.
• The green filter filters all and only green light
passes to a condenser, which projects it on to a
movable mirror(M1).
• It is then reflected to another fixed mirror M2
and then back again to first movable mirror M1.
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34
35. • the objective lens brings the reflected beam from
the first mirror to a focus at a transparent
graticule containing a precise scale which is
viewed by an eye-piece.
• Magnification : if the distance from the plunger
center line to the first mirror pivot is x and the
plunger moves a height h, then the angular
movement of the mirror §𝚹=h/x.
• if f be the focal length of the lens then the
movement of the scale is 2f. §𝚹 i.e.,2f h/x
• Therefore magnification=2fh/xh =2f/x
• Overall magnification=2f/x*eyepiece
magnification.
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37. Working principle
• A beam of light passes through a graticule suitable
engraved with a linear scale.
• The movement of the mirror causes this scale to move up
and down past a translucent screen inside the observing
hood of the instrument.
• The eye placed near the eye piece views the image of a
small scale engraved on the glass after the reflection
from the plunger actuated mirror and the prism.
• In the focal plane of the eye piece a fine reference line is
provided and the system of the lenses is so arranged that
the image of the scale is projected in the same focal
plane.
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38. • Thus only the movement of the scale image can
be measured with reference to the fixed line.
• The division of the scale image opposite the index
line indicates the amount of movement of the
contact plunger.
• The image of the scale and the index line could
also be viewed through a projection system.
• The overall magnification is given by 2f/d*eye
piece magnification. Where f is focal length d id
distance between the knife edge and the planer.
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40. Working principle
• An armature supported on the thin steel strips
is suspended between two coils A & B.
• When the distance of the armature surface
from the two coils is equal the whetstone
bridge is balanced and no current flows
through its galvanometer.
• Slight movement of the measuring plunger
unbalances the bridge resulting in the flow
through the galvanometer.
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41. • The scale of the galvanometer is calibrated to
give the movement of the plunger.
• These have minimum moving parts and
therefore give a high degree of reliability.
• Magnification of the order X3000 are as
possible.
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42. Advantages:
• Fewer number of moving parts
• High magnification
• Easy to set up and operate
• Less error due to sliding friction
• Instrument is small and compact
• Indicating instrument need not be placed near
to the measuring units.
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43. disadvantages
• Fluctuation in the voltage or frequency of the
electric supply may effect the results.
• Heating of coils in the measuring unit may cause
zero drift and alter the calibration.
• When measuring unit is remote to the indicating
unit the reliability is lower.
• Cost is generally more than the mechanical
comparator.
• If only a fixed scale is used with the moving
pointer then with high magnification a very small
range is obtained.
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45. advantage
• It is possible to obtain high degree of
magnification(30000:1) or more coupled with
the good stability and reliability.
• The gauging member does not come with
contact with the part to be measured and
hence practically no wear takes place on the
gauging member.
• It has few number of moving parts and in
some cases none.
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46. • Measuring pressure is very small and the jet of air
helps in cleaning the dust if any from the part to
be measured.
• The indicating instrument can be remote to the
measuring unit.
• It is very suitable for measuring the diameter of
holes whose diameter is small compared with the
length.
• It is probably the best method to determine the
ovality and taperness of the circular holes.
• Single and no of dimensions may be inspected
simultaneously.
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47. disadvantages
• Limited range of measurement is available
• It gives low speed of response compared with
electrical magnification system.
• It requires elaborate auxiliary equipment such as
accurate precision regulator.
• The scale is generally not uniform
• The apparatus is not easily portable
• Different gauging heads are required for different
dimensions.
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