Comparators

CHAPTER 7
COMPARATORS
Presentation Prepared by
Prof. Naman M. Dave
Assistant Prof. (Mechanical Dept.)
Gandhinagar Institute of Technology

INTRODUCTION
• A comparator is a precision instrument used
for comparing the dimensions of a
workpiece with a working standard or
master setting which represents the basic
size.
• A comparator works on relative
measurements, it does not measure the
actual dimension but indicates how much it
differs form the basic dimension. The
indicated difference is normally.

• In case of mass production, the identical component
parts are produced on a very large scale.
• It is very difficult, expensive and may not be feasible
to measure the dimensions of all components with use
of vernier caliper, micrometer etc.
• To check dimensional variation from the standard or
basic dimension of part.
• Comparing manufactured part to the master (standard)
part.
• The comparator requires little skill for the operator,
eliminates human element for taking measurement and
gives quick and highly consistent results.
Why comparator is required ?

Characteristics of good Comparator
• The instrument must be robust design and
construction so it can withstand the effect of
ordinary uses without affecting its measuring
accuracy.
• The instrument should be very sensitive and
withstand a reasonable ill usage without
permanent harm.
• The comparator design must be such that it can be
used for wide range of operations.
• The instrument working should not be affected by
environment change in temperature, pressure and
humidity.

• Display or indicating system should display
results in least possible time.
• Sensing or contact point should be hard
enough to minimize wear effects.
• It must have high magnification in order to
measure or detect very small change in input.
• It should be free from backlash and unwanted
friction.

1. Robust design and construction : The design and construction of
the comparator should be robust so that it can withstand the effects
of ordinary uses without affecting its measuring accuracy.
2. Linear characteristics of scale : Recording or measuring scale
should be linear and uniform (straight line characteristic) and its
indications should be clear.
3. High magnification : The magnification of the comparator should
be such that smallest deviation in size of component can be read.
4. Quick in results : The indicating system should be such that the
readings are obtained in least possible time.
5. Versatility : Instruments should be designed that it can be used for
wide range of measurements.
6. Minimum wear of contact point : The measuring plunger should
have hardened steel contact or diamond to minimize wear effects.
Further the contact pressure should be low and uniform.

7. Free from oscillations : The pointer should come rapidly, to
rest and should be free from oscillations.
8. Free from back lash : System should be free from back lash
and unnecessary friction.
9. Quick insertion of workpiece : Means should be provided for
lifting the plunger for quick insertion of work.
10. Adjustable Table : The table of the instrument should,
preferably, be adjustable in a vertical sense.
11.Compensation from temperature effects : The indicator should
be provided with maximum compensation for temperature
effects.
12.Means to prevent damage : Suitable means should be provided
for preventing damage of the instrument in the event of the
plunger moving through a greater distance than that
corresponding to the range of its measuring scale.

Applications of comparators :
• The comparators are used to check the parts in mass
production at a very fast rate.
• They are used to inspect newly purchased gauges.
• They are worked as a laboratory standards from
which working or inspection gauges are set and
correlated.
• They are used as final inspection gauges in selective
assembly of parts where parts are graded in three
groups depending upon their tolerance.
• They are used for checking parts received form
outside sources.

Applications of comparators :
• The various ways in which comparators can be used are :
1. Laboratory Standards : Comparators are used as laboratory
standards from which working or inspection gauges are set and
correlated.
2. Working Gauges: They are also used as working gauges to
prevent work spoilage and to maintain required tolerance at all
important stages of manufacture.
3. Final Inspection Gauges : Comparators may be used as final
inspection gauges where selective assembly, of production parts is
necessary.
4. Receiving Inspection Gauges : As receiving inspection gauges
comparators are used for checking parts received from outside
sources.
5. For checking newly purchased gauges : The use of comparators
enables the checking of the parts
6. Components in mass production at a very fast rate.

Classification of Comparators :

Mechanisms
Rack and pinion Cam and gear train Lever with toothed gear.
Compound levers
Twisted taut strip
Lever combined

Dial Indicator
• The dial indicator or dial gauge amplifies the length or
displacement and translate it into rotational motion of
pointer over a circular scale. It is used for pointer over
a circular scale.

Dial Indicator
Applications
(l) It can be used to compare two heights or
distances between narrow limits.
(2) It is used for determining errors in
geometrical forms (ovality, out of roundness,
taper etc.)
(3) It is used for testing the alignment,
roundness and parallelism of work piece.
(4) The dial indicators are most common
instrument for quality control and inspection
work

Johansson Mikrokator
This comparator works on
principle that when a tension is
applied to the twisted strip, this
causes the strip to untwist
resulting in rotation of pointer
which attached at center of
twisted strip.
The two halves of the strip from
the center are twisted in opposite
directions so that any tension or
pull on the strip will cause the
center to rotate.

Johansson Mikrokator
One end of the strip is fixed to
the adjustable cantilever and
other end is attached to bell crank
lever which pivoted at the frame
of comparator, and their other
end contacts with top of the
plunger.
Plunger moves upward due to
contact with the workpiece, the
bell crank lever pulls the twisted
strip, and hence the pointer
which is attached at center of
twisted strip rotates.
The rotation of pointer is
proportional to the change in
length of strip and plunger
movement.

Reed type Mechanical Comparator
• A fixed block is rigidly fastened to
the gauge head case and floating
block B which carries measuring
spindle end connected horizontally
to block A by reeds C.
• Vertical reeds D a attached to each
block with upper ends joined
together. Beyond this joint extends a
pointer.
• Linear motion of spindle moves the
free block B vertically causing the
vertical reed on the floating block to
slide past the vertical reed on the
fixed block. Since vertical reeds are
joined at their upper end instead of
slipping the movement causes both
reeds swing through an arc.
• The amount of reeds swing or
pointer rotation is proportional to
the distance the floating block B
moves with the spindle.

SIGMA Comparator
The sigma comparator works on leverage principle in which high
magnification is obtained entirely by mechanical means.

Mechanical measurement and metrology
Prof. Naman M. Dave
SIGMA Comparator

Mechanical Comparators
Advantages :
(l) They are cheaper compared to other types of comparators.
(2) They do not require any external electrical energy.
(3) They are normally robust, compact and easy to handle.
(4) Normally, they have a linear scale.
(5) Mechanical comparators are suitable for ordinary workshop conditions
and being portable can be issued from a store.
Disadvantages :
(1) Due to more moving parts, the friction is more and subsequently less
accuracy obtained.
(2) The accuracy of this comparator is considerably reduced due to backlash.
(3) Less sensitive due to inertia of moving parts.
(4) Errors due to parallax is possible as moving pointer moves over a fixed
scale.
(5) The range of instrument is limited as pointer moves over a fixed scale
(6) They have a less magnification compared to optical comparators.

Working principle of Optical Comparator

Mechanical-Optical Comparator
• Mechanical optical comparator
consists of plunger which is
attached with lever and a mirror
on which the light strikes from
the source.
• The light coming form source is
directed on the calibrated scale.
• The angle of tilting min or is
changed by movement of
plunger.

Advantages of Mechanical-Optical Comparators :
1.They have a less number of mechanical moving parts, hence less
friction and inertia effect, and gives higher accuracy
2.The scale can be made to move past a datum line and thus have high
range and no palallax.
3.They have high magnification and hence suitable for precision
measurements.
4.The scale in the optical comparators' illuminated and hence enables
readings to be taken irrespective of room Lighting conditions.
Disadvantages Mechanical-Optical Comparators :
1.They require the light source
2.The comparator is usually large and expensive.
3.When the scale is projected on a screen, then it is necessary to use
these instrument in a dark room in order to take reading easily.
4.Instrument is inconvenient for continuous use because the scale is to
be viewed through eyepiece.
5.Due to high magnification, heat generated from transformers and
lamp may cause the setting to drift.

Electrical Comparators
Advantages of electrical and electronic comparators
1.Few number of moving parts : The electric and electronic comparators have few
number of moving parts, and there is less friction and wear.
2. High magnification : It has a wide range of magnification.
3. Not sensitive to vibrations : The mechanism carrying the pointer is very light and
not sensitive to vibrations.
4. Easy to set up and operate.
5. Less error due to sliding friction : operation of the instrument on AC supply
reduces sliding friction errors.
5. The instrument is small and compact.
7. The indicating instrument need not be placed close to the measuring unit
Disadvantages
1. Fluctuation in the voltage or frequency of the electric supply rnay affect the results
2. Heating of coils in the measuring unit may cause zero drift and alter the calibration.
3. When measuring unit is remote from the indicating unit, reliability is lower.
4. Cost is generally more than mechanical comparator.
5. If only a fixed scale is used with a moving pointer than with htgh magnification a
very small range is obtained.

Pneumatic Comparator
When air passes through the narrow constant space: between nozzle and the
workpiece the pressure of air remains constant. If the space between nozzle
and workpiece changes, it causes change in the pressure of the air. This
change in pressure is utilized to measure the deviation of the dimension being
measured from the standard dimension.

• The pneumatic comparator is normally used to determine the roundness of
the job.
• The system designed in order to supply air at constant pressure to the
measuring jets.
• If the pressure of the air supplied is higher than the predefined pressure,
some air will bubble out from the bottom of the dip tube and air moving
to control orifice will be at predefined constant pressure.
• The constant pressure air then passes through control orifice and escape
from the rneasuring jets when there is no restriction provided by
workpiece to the escape of air, the level of water in the manometer tube
will same as level of water in cylinder.

But when restriction is provided at the measunirg jets, resulting increases the
back pressure and level of water in the manometer fall down which vary the
height of water in manometer tube, a calibrated scale being set beside this
tube. In other words the restriction to the escape of air depends upon the
variations in the dimensions to be measured.

Pneumatic ComparatorAdvantages of pneumatic Comparators
1. It is possible to obtain high degree of magnification (30,000 : 1) or more coupled with good
stability and readability.
2. The gauging member does not come in contact with the part to be measured and hence
practicality no wear takes place on gauging member,
3, It has few number of moving parts and in some cases none. Thus the accuracy obtainable
is more due to absence of friction and less inertia.
4. Measuring pressure is very small and the jet of air helps in cleaning the dust if any, from
the part to be measured-
S. The indicating instrument can be remote from the measuring unit.
6. It is very suitable for measuring diameter of holes whose the diameter is small compared
with the length.
7. It is probably the best method to determine the ovality and taperness of circular holes.
8. Single or number of dimensions can be inspected simultaneously.
Disadvantages
1. Limited range of measurement is available with these comparators
2 It gives low speed of response compared with electrical magnification system.
3. It requires elaborate auxilary equipment such as accurate pressure regulator.
4.The scale is generally not uniform.
5. When indicating device is the glass tube, then high magnification is necessary in order to
avoid the meniscus errors.
6. The apparatus is not easily portable.
7 Different gauging heads are required for different dimensions.

Comparators

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