2. The general principle of comparator is to indicate the
differences in size between the standard and the work
being measured by means of some pointer on a scale
with sufficient magnification
It thus does not measure the actual dimension but
indicates how much it differs from the basic
dimension
3. All comparators consist of three basic features
1)A sensing device
• which faithfully senses the input signal
2) A Magnifying or amplifying system
• to increase the signal to suitable magnitude.
Mechanical, Optical, Pneumatic,hydraulic and electronic
methods are used for this purpose.
3) A display system (usually a scale and pointer)
Which utilizes the amplified signal to provide a suitable
readout.
4. Need for a comparator
A comparator is used in mass production to inspect the
components to close tolerances with high degree
of precision and speed. Ex: Piston
• Use of line standards such as vernier and micrometer
calipers require considerable skill
• Many dimensions can be checked in a very short time
Comparators can be classified as
1. Mechanical 2. Optical 3. Electrical &Electronic
4. Pneumatic
5. Fluid displacement comparators
6. Mechanical optical comparator
7. Electro-mechanical comparator
8. Multi-check comparator.
5. Design Consideration
1.Ability to record variation of at least 0.0025 mm
2.Linear recording scale
3.Variance free (Instrument readings should not vary
when repeated measurements of given quantity are
made
4.Scale indicators clear and free from oscillations
5.Free from backlash, unnecessary friction &clearances
inertia should be low.
6.Measuring pressure should be low and constant
7.Supporting frame to be rigid and withstand reasonable
ill usage without permanent damage
8.Capable of measuring wide range.
6. Basic operation
1. The comparator is first adjusted to zero on its dial
with a gauge block in position
2. The work piece to be checked is then placed in
position
3. The comparator gives the difference in dimension
in relation to the gauge block.
7. Uses of comparator
1. Laboratory Standards:
to set the working or inspection gauges
2.Working Gauges
To prevent the work spoilage and to maintain required
tolerances
3. Final Inspection Gauges
4. Receiving Inspection Gauges
5. For checking newly purchased gauges
8. Essential characteristics of a comparator
1. Robust design and construction
2. Linear characteristics of scale
3. High magnification
4. Quick in Results
5. Versatility
6. Minimum Wear of contact Point
7. Free from Oscillations
8. Free from backlash
9. Quick insertion of work piece
10. Adjustable table
11. Compensation from temperature effects
12. Means to prevent damage
9. Working principle of a Mechanical Comparator
A mechanical comparator employees mechanical
means for magnifying the small movement of the
stylus
the magnification of the small stylus movement is
obtained by means of levers, gear trains, rack and
pinion or a combination
The usual magnification obtained by these
comparators ranges from about 250 to 1000
10. Types of Mechanical Comparators
1. Dial indicator (Dial Gauge)
2. Read Type
3. Johansson Mikrokator
4. Sigma Comparator
12. • The simplest type of mechanical comparator
• It consists of a base with a rigid column rising from
its rear
• An arm mounted on this column and it carries a dial
gauge at its outer end
• The indicator is set at zero by the use of slip gauges
• The part to be checked is placed below the plunger
• The linear movement of the plunger is magnified by
means of mechanical means to a sizable rotation of
the pointer
13. This type is generally used for inspection of small
precision machined parts
The dial gauge with various attachments are commonly
used
This comparator is ideal for the checking of components
with a tolerance of + 0.005 mm
16. Working of the comparator
A very thin metal strip at the centre carries a light
pointer made up of glass .
One end of the strip is connected to the adjustable
cantilever strip and the other end is to the spring
elbow, in turn connected to the plunger.
The slight movements of the plunger will make the
bell crank lever to rotate.
This rotation will create tension in the strip and causes
the strip to rotate thereby the strip start to untwist
resulting in the movement of the point
17. The spring ensures that the plunger returns when the
contact is removed
The length of the cantilever can be varied to adjust the
magnification
The mechanical magnification is obtained by
“Abramson Movement”
19. The gauging head is usually a sensitive, high quality,
dial indicator mounted on a base supported by a sturdy
column.
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, and floating block B, which carries
the gauging spindle and is connected horizontally to the
fixed block by reeds C
20. Vertical reed is attached to each block with upper
ends joined together. These vertical reeds are shown
in the figure by letter D. Beyond this joint extends a
pointer or target.
A linear motion of the spindle moves the free block
vertically causing the vertical reed on the floating
block to slide past the vertical reed on the fixed
block.
How- ever, as these vertical reeds are joined at the
upper end, instead of slipping, the movement causes
both reeds swing through an arc and as the target is
merely an extension of the vertical reeds, it swings
through a much wider arc.
21. The amount of target swing is proportional to the
distance the floating block has moved but of course
very much magnified.
The scale may be calibrated by means of gauge block
(slip gauges) to indicate any deviation from an initial
setting.
Sensitivities of the order of 0.25 micron per scale
division.
The mechanical amplification is usually less than 100,
but it is multiplied by the optical lens system. It is
available in amplifications ranging from x 500 to x
1000.
23. 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 which are connected
by thin flexible strips alternately at right angle 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 of Y shape and having
effective length I is attached.
24. Advantages of mechanical comparators
1.Cheaper than all the other type of comparators
2.Does not require any external source of power or
air supply
3.These comparators use a linear scale that can be
easily understood.
4.Usually these comparators are robust and compact
but are very easy to handle
5.These are small in size and can are portable from one
place to other very easily without much difficulty
25. Disadvantages
1.Contains more number of moving parts so there
develops friction which in turn reduces the accuracy
2.Slackness in the moving parts reduces accuracy very
drastically
3.These have more inertia so the instrument is prone to
vibrational effects
4. Limited range of the instrument is another drawback
as the pointer moves over a fixed scale
5. Parallax error may also arise when proper scale is
used
27. In mechanical 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 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 due to reduction of moving members and
better wear resistance qualities.
Optical magnification is also free from friction, bending,
wear etc.
28. • The whole system could be explained
diagrammatically by Fig. below, which gives very
simple arrangement and explains the principle of
above comparator.
In this system,
• Mechanical amplification= l2/l1
• and Optical amplification = l4/l3 * 2.
• It is multiplied by 2, because if mirror is tilted by an
angle δθ , then image will be tilted by 2 * δθ .
• Thus overall magnification of this
system=2*l2/l1 * l4/l3.
30. • The optical system of this instrument involves double
reflection of light and thus gives higher degree of
magnification.
• A lamp sends light rays to green filter, which filters
all but green light, which is less fatiguing to the eye.
The green light then passes to a condenser
which via an index mark projects it on to a movable
mirror M1, whence it is reflected to another fixed
mirror M2, and then back again to the first moveable
mirror.
• The second 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 the eye-piece.
31. The projected image of index line on the graticule can
be adjusted by means of screw in order to set the
zero.
When correctly adjusted, the image of the index line is
seen against that of the graticule scale.
The special end of the contact plunger rests against
the outer end of the first movable mirror so that any
vertical movement of the plunger will tilt the mirror.
The extreme sensitivity of this instrument necessitates
special precautions in its operation to avoid
temperature effects.
33. This is the commercial measuring instrument. This
utilises a plunger titled mirror, objective lens, prism
and observing eye-piece to provide a high degree of
magnification.
The mirror is mounted on a knife-edge and it can be
tilted about the fulcrum by any linear vertical
movement of the contact plunger.
A beam of light passes through a graticule suitably
engraved with a linear scale and the movement of
mirror causes this scale to move up or down past a
translucent screen inside the observing hood of the
instrument.
34. The eye placed near the eye-piece views the image of a
small scale engraved on glass after reflection from the
plunger actuated mirror and the prism in the plan view
in Fig
In the focal plane of the eye-piece, a fine reference
line (Index) is provided and 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.
35. 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 * Eye-piece magnification, where f is the
focal length of the lens and d is the distance between
the knife-edge and the plunger.
36. 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
37. Disadvantages
1. As the instrument has high magnification, heat from
the lamp, transformer etc. may cause the setting to
drift.
2. An electrical supply is necessary.
3. The apparatus is usually large and expensive.
4. When the scale is projected on a screen, then it is
essential to use the instrument to a dark room in
order to take the readings easily.
5. The instruments in which the scale is viewed
through the eyepiece of a microscope are not
convenient for continuous use.
41. charactrestics
(a) Very high amplifications are possible.
(b) As no physical contact is made either with the
setting gauge or the part being measured, there is no
loss of accuracy because of gauge wear.
(c) Internal dimensions can be readily measured not
only with respect to tolerance boundaries but also
geometric form.
(d) It is independent of operator skill.
(e) High pressure air gauging can be done with
cleansing of the parts which helps to eliminate errors
due to dirt and foreign matter.
42. f) Gauging pressures can be kept sufficiently low to
prevent part deflection.
g) Dimensional variations throughout the length of shaft
or cylinder bore can be explored
h) Not only it measures the actual size, but it can also be
used to salvage oversized pieces
i)The total life cost of the gauging heads in much less.
(j) It is accurate, flexible, reliable, universal and speedy
device for inspecting parts in mass production.
(k) It is best suited for checking multiple dimensions
and conditions on a part simultaneously in least
possible time.
43. Advantages of Pneumatic Comparators:
i. The gauging member does not come into contact with
the part to be measured and hence practically no wear
takes place on the gauging member.
ii. It has usually very small number of moving parts and
in some cases none. Thus the accuracy is more due to
less friction and less inertia.
iii. Measuring pressure is very small and the jet of air
helps in cleaning the dust, if any, from the part to be
measured.
iv. It is possible to have very high magnification.
v. The indicating instrument can be remote from the
measuring unit.
44. vi. It is very suitable device for measuring diameter of
holes where the diameter is small compared with the
length.
vii. It is probably the best method for determining the
ovality and taperness of the circular bores.
45. Disadvantages:
i. It requires elaborate auxiliary equipment such as
accurate pressure regulator.
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.
