Comparator :
A comparator works on the relative measurement
It gives only dimensional differences in relation to a
basic dimension
A comparator has to compare the unknown dimensions
of a part with some standard (Basic Size)
These are commonly used for linear measurement in
mass production
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Some of the comparators in use are
Dial indicator
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Sigma Comparator
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Optical Comparator
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Pneumatic Comparator
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Classification
Depending
on
the
amplification
system,
Comparators are classified as
1. Mechanical Comparators
2. Electrical Comparators
3. Optical Comparators
4. Pneumatic Comparators
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Mechanical Comparators :
Mechanical comparators are extremely convenient
to use
It does not require any external power for its
operation
The measuring head consists of the work contact
plunger(Stylus)
The movement due to size difference between
standard and work piece operates a pointer
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Mechanical Comparators :
The pointer moves over a dial
This movement of the plunger is magnified by means
of the gear train or levers
The magnification of mechanical comparators is
limited to 1000
These are cheap and intended to measure external
surfaces
However the mechanical devices are subjected to
wear
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Types of Mechanical Comparators
1. Dial indicator
2. Reed type mechanical comparator
3. The Sigma comparator
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Dial indicator :
The Dial indicators are used to test,inspect
the size and trueness of finished work
The measured accuracy is up to 0.01mm
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10
Constructional Details
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Dial indicator
11
Constructional Details :
The
dial indicator consists of a base,
column,arm,dial indicator and stylus(work contact
plunger)
The arm can be adjusted at required height along
the column
The work table is mounted on the base to
provide a reference surface
The work is placed on
the work table during
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Mechanism
Plunger
13
Mechanism :
Thus a turn of the hand by one scale division
represents a spindle travel of 0.01mm
The movement of stem is transmitted by means of a
toothed rack through a gear train
This movement is obtained around a dial face
The required measuring pressure is provided by small
springs incorporated in the mechanism
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14
Mechanism :
The indicator is adjusted zero by turning the rim of
the dial
The dial indicators are also available for ranges of
measurement of 0-3,0-5,0-10mm.
These measurements indicate the movement (left) of
the plunger
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15
Reed type Mechanical Comparator
The instrument used to measure linear displacement
of spindle with the help of the reed mechanism is
known as reed type Mechanical comparator.
It is a device magnifying small motions of spindle
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Constructional details
Reed type Mechanical Comparator
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Constructional details :
The reed mechanism is frictionless device for magnifying
small motions of the 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
The floating block is also connected horizontally to the
fixed block by reeds `C`
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Constructional details :
A vertical reed is attached to each block with upper
ends jointed together
These vertical reeds are shown in fig by letter `D`
A pointer is attached to this joint
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Working principle
A linear motion of the spindle moves the floating block
vertically upwards
This vertical movement causing the vertical reed on the
floating block to slide past the vertical reed on the fixed
block
The movement causes both reeds swing through an arc
As the point is nearly an extension of the vertical reeds `M`
it swings through a much wider arc
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Sigma Comparator
The Sigma Comparator is a mechanical comparator in which
the amplification is obtained by means of a compound lever
Advantages: It has got a bold scale and larger indicating
pointer
Dis Advantages: Due to motion of parts ,there is wear in the
moving parts
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Constructional details
Sigma Comparator
22
Constructional details :
The vertical beam is mounted on flat steel springs
connected to fixed members
The vertical beam which in turn are screwed to a back
plate
The assembly provides a frictionless movement with
restraint from the springs
The hinged assembly carrying the forked arms
incorporates a hardened fulcrums
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Constructional details :
The metal ribbon attached to the forked arms passes
around the spindle
This spindle is to rotate in specially designed miniature
ball bearings
The indicating pointer is secured to a boss on the disc
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Operating Principle :
The various movements in the Sigma Comparator are shown in
figure.
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
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Operating Principle :
The cross strip hinge then connected by thin flexible
strips alternately at right angles 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 `Y` shape having the
effective length `L` is attached
If the distance of the hinge from the knife edge be `a`
then the magnification of the first stage is `L/a `
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Operating Principle :
A phosphor-Bronze strip is attached to the two extremities of
the `Y` arm
And is passed around a radius `r` attached to the pointer
spindle
If the length of pointer be `l` then
l/r is the second stage of
magnification
The total magnification is
(L/a)X(l/r)
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Operating Principle :
In order to adjust the magnification ,distance `a` must be
changed by slackening and tightening the two screws
attaching the knife edge to the plunger
In sigma comparator a damping device must be employed to
prevent oscillation of point before coming to rest
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Johanson Microkator
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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:
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.
30
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.
32
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.
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Block diagram of Electrical
Comparator
34
Principle of an Electrical
Comparator
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Whetstone bridge circuit arrangement
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Principle of an Electrical Comparator
The principle of these comparators is to convert the
linear displacement of the measuring stylus into an
electric output
Whetstone bridge circuit is used for this comparator
When the circuit is balance no current is detected
Hence the following condition will exit
R1
R3
R2
R4
Electrical Comparators are extremely sensitive and
are capable of high magnifications up to 30000.
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Principle of an Electrical Comparator
If one of these resistors is varied then imbalance in
the circuit will result in a current flow through the
galvanometer
By arranging the variable resistor a measuring head
mechanism is formed
The galvanometer may be calibrated to read linear
displacement
The whetstone bridge circuit is suitable only for D.C
supply
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Principle of an Electrical Comparator
Although the operating principle is same certain
modification is that the resistors are replaced by
inductors
Two of which are incorporated in the measuring head
When the stylus moves vertically, the circuit will
become imbalance
This is due to the movement of iron armature
between the inductors L1 and L2
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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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PRINCIPLE OF ELECTRONIC COMPARATORS
The transducer converts this movement into an electrical signal which
is then amplified and fed via an oscillator to the demodulator. The
current in D.C. form, then passes to the meter and the probe tip
movement is displayed as a linear measurement. Various measuring
and control units can be incorporated which provide for an extremely
wide range of single or multiple measurements to be made
simultaneously.
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Principle of an optical comparator
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Principle of an optical comparator
In optical comparators 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 simple optical system involving the projection of an
image
The usual arrangement employed is such that the
mechanical system causes a plane reflector to tilt about
an axis
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Principle of an optical comparator
The image of an index is projected on a scale on the inner
surface of a ground glass screen
The whole system could be explained diagrammatically
from fig which gives very simple arrangement
In this system mechanical amplification is x2/x1
Optical amplification is 2( x4/x3)
44
Principle of an optical comparator
It is multiplied by 2 because if mirror is tilted by an angle , then the
image will be tilted by 2x
Thus overall magnification of this system is
2x ( L2/L1) ( L4/L3)
The important point in optical comparators is that mirror used must be
of front reflection type and not of normal back reflection type
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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.
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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.
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Optical Comparator principle
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48
Optical Comparator principle
This is the commercial measuring instrument
This utilizes a plunger tilted mirror, objective lens,prism
and observing eye piece to provide a high degree of
magnification
The mirror is mounted on a knife-edge
It can be tilted about the fulcrum by any linear vertical
movement of the contact plunger
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Optical Comparator principle
A beam of light passes through a graticule suitably
engraved with a linear scale
The movement of mirror causes this scale to move up
(or) 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 scale after reflection from the
plunger actuated mirror
The plan view for the mirror is shown in fig
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Optical Comparator principle
In the focal plane of the eye piece , a fine reference line
( index 0) is provided
The system of lenses is so arranged that the image of the
scale is projected in the same focal plane
Thus with movement of scale the 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 contact plunger
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Optical Comparator principle
The image of the scale and the index line could also
be viewed through a projection system
The overall magnification of the comparator is given
by (2f/d)x eye-piece magnification
Where `f` is the focal length of the lens and `d` is
the distance between the knife-edge and the
plunger
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Principle of pneumatic comparator
The measurement is made by taking reading using
master gauge of known accuracy and comparing with
component reading
It uses a controlled, pressurised jet of air to measure
small dimensional variation in the size of component
The working principle of pneumatic comparator
depends on the flow of air between the gauge head
and the component being checked
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Important characteristics
Very high magnifications are possible
It can be used to measure diameters, length, squareness,
parallelisms, concentricity,taper, and other geometric
conditions.
Internal dimensions can be readily measured , in other
words, while measuring a bore it can reveal complete details
of size, taper, straightness,camber and bell mouth etc.
It is independent of the operator skill
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Important characteristics
High pressure air gauging can be done by cleaning the
parts which helps to eliminate errors due to dirt and
foreign matter.
As there is no physical contact made either with the
setting gauge (or) the part being measured , there is no
loss of accuracy because of gear wear.
Gauging pressures can be kept sufficiently low to prevent
part deflection
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Important characteristics
Dimensional variations throughout the length of the shaft
(or) cylinder bore can be explored for out of roundness,
taperness, concentricity,regularity and other similar
conditions
The total life cost of the gauging heads is much less
It is accurate, flexible, reliable,universal and speedy device
for inspecting parts in mass production
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Important characteristics
It is best suited for checking multiple dimensions and
conditions on a part simultaneously in least possible
time.
It can be easily used for a line measurement of parts
as they are being machined and take corrective
actions.
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Principle of Solex Pneumatic Comparator
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Working principle of Solex pneumatic comparator
This instrument is produced commercially by Solex
air gauges Ltd.
This is generally designed for internal measurement
But with suitable measuring head it can be used for
external gauging also
The arrangement used in Solex gauge is to pass the
high pressure air after filtering through a flow valve
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Working principle of Solex pneumatic comparator
It consists of a tank in which water is filled up to a
certain level and dip tube is immersed into it up to a
depth corresponding to air pressure .
In the Fig it is represented by `H`
Air is sent at higher pressure than required one
Some air will leak out from the dip tube and bubble out
of water
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Working principle of Solex Pneumatic Comparator
And the air moving towards control orifice will be at
desired constant pressure `H`
It is very obvious from Fig that the diameter being
measured at any instant is corresponding to the
portion against two jets
To find the concentricity (roundness of any job at any
section) The work piece may be revolved around the
measuring gauge
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Working principle of Solex pneumatic comparator
Similarly the diameter can be noted down at several
places along the length of bore and thus tapering of
hole is determined
This method is best suited for measuring roundness
and taperness of cylinder bores and gun barrel bores
By having suitable measuring head this can be used for
external gauging also
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Advantages of Pneumatic Comparators
1.
Comparators can be checked with out contact with
finished surfaces
2.
This method needs little skill to operate
3.
No limit to the size of work that can be tested
4.
Provide a simple and direct method of high
amplifications
5.
Dimensional variations can be controlled with close
tolerances
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Comparison of comparators
Comparator
Magnification
Mechanical
X300 to X 5000
Comparators
Electrical
Comparator
Optical
Comparator
Pneumatic
Comparator
X10000 to X
50000
upto
X1000
X9000 to
X 23000
Operating
System
Operation
Complex
Easy
Very robust
And portable
Limited
magnification
Difficult
Few moving
parts
Large
magnification
range
Electric supply
requires
Difficult
Few moving
parts.
Unaffected by
temperature
range
Difficult
Few moving
parts
Large
magnification
Complex
Complex
Simple
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Advantages
Disadvantages
Very low
magnification
Electric supply
requires
Air supply
requires
64
TOOL MAKERS MICROSCOPE
The tool makers microscope is a versatile instrument that
measure by optical means with
no pressure being involved, thus very useful for measurement
on small and delicate parts.
It is designed for:
a) Measurement on parts of complex form e.g. - profile of
external thread, tool,
templates, gauges, etc.
b) Measuring centre to centre distance of holes in any plane.
c) A variety of linear measurements.
d) Accurate angular measurements.
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Tool makers microscope is
shown in fig. The optical
head can be moved up or
down the vertical column and
can be clamped at any height
by means of clamping screw.
The table which is mounted
on the base of the instrument
can be moved in two mutually
perpendicular
horizontal
directions (longitudinal and
lateral) by means of accurate
micrometer screw having
thimble scale and venires.
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A ray of light from a light source fig. b is reflected by a
mirror through 90 It then passes through a transparent
glass plate (on which flat parts may be placed ).
A shadow image of the outline or counter of the
workspaces passes through the objective of the optical
head and is projected by a system of three prisms to a
ground glass screen.
Observations are made through an eyepiece.
Measurements are made by means of cross lines engraved
on the ground glass screen.
The screen can be rotated through 360; the angle of
rotation is read through an auxiliary eyepiece.
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