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Metrology
Comparators
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Chapter Outline
Comparators
Introduction to comparators
Characteristics
Uses of Comparators
Classification of comparators
Mechanical comparators
Dial indicator
Johnson Mikrokator
Sigma comparators
Optical comparators
Principles,
Zeiss ultra optimeter,
Electric and electronic comparators principles,
LVDT,
Pneumatic comparators,
Back pressure gauges,
Solex comparators.
Comparators can give precision measurements, with consistent accuracy by
eliminating human error. They are employed to find out, by how much the dimensions of the
given component differ from that of a known datum. If the indicated difference is small, a
suitable magnification device is selected to obtain the desired accuracy of measurements. It is
an indirect type of instrument and used for linear measurement. If the dimension is less or
greater, than the standard, then the difference will be shown on the dial. It gives only the
difference between actual and standard dimension of the workpiece. To check the height of
the job H2 ,with the standard job of height H1
Metrology
Comparators
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Initially, the comparator is adjusted to zero on its dial with a standard job in position
as shown in Figure(a). The reading H1is taken with the help of a plunger. Then the standard
job is replaced by the work-piece to be checked and the reading H2 is taken. If H1and H2 are
different, then the change i~ the dimension will be shown on the dial of the comparator. Thus
difference is then magnified 1000 to 3000 X to get the clear variation in the standard and
actual job.
In short, Comparator is a device which
(1) Picks up small variations in dimensions.
(2) Magnifies it.
(3) Displays it by using indicating devices, by which comparison can be made with some
standard value.
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.
Characteristics of Good Comparators:
1. It should be compact.
2. It should be easy to handle.
3. It should give quick response or quick result.
4. It should be reliable, while in use.
5. There should be no effects of environment on the comparator.
6. Its weight must be less.
7. It must be cheaper.
8. It must be easily available in the market.
9. It should be sensitive as per the requirement.
10. The design should be robust.
11. It should be linear in scale so that it is easy to read and get uniform response.
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12. It should have less maintenance.
13. It should have hard contact point, with long life.
14. It should be free from backlash and wear.
Mechanical Comparator:
It is self controlled and no power or any other form of energy is required. It employs
mechanical means for magnifying the small movement of the measuring stylus. The
movement is due to the difference between the standard and the actual dimension being
checked
The method for magnifying the small stylus movement in all the mechanical
comparators is by means of levers, gear trains or combination of these. They are available of
different make and each has it's own characteristic. The various types of mechanical
comparators are dial indicator, rack and pinion, sigma comparator, Johansson mikrokator.
a. 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. They are designed for use on a wide
range of standard measuring devices such as dial box gauges, portal dial, hand gauges, dial
depth gauges, diameter gauges and dial indicator snap gauge.
Corresponds to a spindle movement of 1 mm. The movement mechanism of the
instrument is housed in a metal case for it's protection. The large dial scale is graduated into
100 divisions. The indicator is set to zero by the use of slip gauges representing the basic size
of part.
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Requirements of Good Dial Indicator:
1. It should give trouble free and dependable readings over a long period.
2. The pressure required on measuring head to obtain zero reading must remain constant
over the whole range.
3. The pointer should indicate the direction of movement of the measuring plunger.
4. The accuracy of the readings should be within close limits of the various sizes and ranges
5. The movement of the measuring plunger should be in either direction without affecting
the accuracy.
6. The pointer movement should be damped, so that it will not oscillate when the readings
are being taken.
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 determine positional errors of surfaces such as parallelism, squareness and alignment.
5. To check the alignment of lathe centers by using suitable accurate bar between the
centers.
6. To check trueness of milling machine arbours and to check the parallelism of shaper arm
with table surface or vice.
b) Johansson Mikrokator :
This comparator was developed by C.F. Johansson.
Principle:
It works on the principle of a Button spring, spinning on a loop of string like in the case of
Childrens toys.
Construction:
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.
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When the plunger moves by a small distance in upward direction the bell crank lever
turns to the right hand side. This exerts a force on the twisted strip and it causes a change in
its length by making it further twist or untwist. Hence the pointer at the centre rotates by
some amount. Magnification up to 5000X can be obtained by this comparator
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.
Disadvantages:
1. Range is limited as the pointer moves over a fixed scale.
2. Pointer scale system used can cause parallax error.
3. There are number of moving parts which create problems due to friction, and ultimately
the accuracy is less.
4. The instrument may become sensitive to vibration due to high inertia.
c) Mechanical - Optical Comparator:
Principle:
In mechanical optical comparator, small variation in the plunger movement is
magnified: first by mechanical system and then by optical system.
Construction:
The movement of the plunger is magnified by the mechanical system using a pivoted
lever. From the Figure the mechanical magnification = x2 / x1. High optical magnification is
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possible with a small movement of the mirror. The important factor is that the mirror used is
of front reflection type only.
The back reflection type mirror will give two reflected images as shown in Figure,
hence the exact reflected image cannot be identified.
Advantages:
1. These Comparators are almost weightless and have less number of moving parts, due to
this there is less wear and hence lessfriction.70
2. Higher range even at high magnification is possible as the scale moves past the index.
3. The scale can be made to move past a datum line and without having any parallax errors.
4. They are used to magnify parts of very small size and of complex configuration such as
intricate grooves, radii or steps.
Disadvantages:
1. The accuracy of measurement is limited to 0.001 mm
2. They have their own built in illuminating device which tends to heat the instrument.
3. Electrical supply is required.
4. Eyepiece type instrument may cause strain on the operator.
5. Projection type instruments occupy large space and they are expensive.
6. When the scale is projected on a screen, then it is essential to take the instrument to a dark
room in order to take the readings easily.
d) Sigma Comparator:
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)
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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.
If l = Distance from hinge pivot to the knife edge
L = Length of y-arm
R = Driving drum radius
D Length of the pointer
Then the total magnification = (L/l) *(D/R)
Electrical Comparators:
Electrical comparators give a wide range of advantages. As we know, components
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
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Electrical comparators have no moving parts. Thus a high degree of reliability is
expected from these instruments. Generally there are two important applications of electrical
comparators:1. Used as measuring heads2. Used for electrical gauging heads, to provide
usual indication to check the dimensions within the limits laid down. 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. 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
Advantages:
1. Measuring units can be remote from indicating units.
2. Variable sensitivity which can be adjusted as per requirement.
3. No moving parts, hence it can retain accuracy over long periods.
4. Higher magnification is possible as compared to mechanical comparator.
5. Compact sizes of probes arc available.
Disadvantages:
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. Heating of coils can cause zero drifts and it may alter calibration.
4. It is more expensive than mechanical comparator
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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.
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
Figure (b)
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. Moderate and constant supply pressure is
required to have the high sensitivity of the instrument.
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Advantages:
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.
7. The method is self cleaning due to continuous flow of air through the jets and this
makes the method ideal to be used on shop floor for online controls.
Disadvantages:
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.
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