Comparators

Matoshri College of Engineering and Research Center
Nashik
Department of Mechanical Engineering
Subject:- Metrology And Quality Control
Class:-TE Mechanical
Unit: II Comparators, Thread and Gear Metrology, Surface
Roughness Measurement(Session 1)
Semester:-V
2020-21
By
Prof R.S.Patil
(Assistant Professor MCOERC )

Syllabus
 Unit –II Comparators, Thread and Gear Metrology, Surface Roughness
Measurement
 Comparators: Mechanical, Pneumatic, Optical, Electrical (LVDT).
 Measurement of Thread form: Thread form errors, Measurement of
Minor, Major and Effective diameter (Three Wire Method), Flank angle
and Pitch, Floating Carriage Micrometer (Numerical).
 Gear Metrology: Errors in Spur Gear form, Gear tooth Vernier, Constant
chord, Base tangent (Numerical), Gear Rolling Tester. Profile Projector, Tool
maker’s microscope and their applications
 Surface Roughness Measurement: Introduction to Surface texture,
Parameters for measuring surface roughness, Surface roughness
measuring instrument: TalySurf.
Matoshri College of Engineering and
Resarch Center Nashik

CONTENT
 Comparators
 Mechanical
 Pneumatic
 Optical
 Electrical (LVDT).

COMPARATORS
 A comparator is a precision instrument employed to compare the
dimension of a given component with a working standard (usually slip
gauges).
 It thus does not measure the actual dimension but indicate how much it
differs from the basic dimension.
 In mass production identical component parts are produced on a very
large scale.
 Micrometer and Vernier caliper are not more feasible because skill
involve and time required to measure the dimension .
 In such case comparator is more suitable for the measurement.

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
6. Combined Comparator: The combination of any two of the above
types can give the best result.

MECHANICAL COMPARATORS
Some of the comparator which belong to this
class are:
1. Dial indicator
2. Reed type comparator
3. Sigma Comparator
4. Johnson Mikrokrator

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.
 The indicator is set to zero by the use of
slip gauges representing the basic size of
part.

Dial Indicator: (different uses)

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 check the alignment of lathe centers by using suitable accurate
bar between the centers.
5. To check trueness of milling machine arbours and to check the
parallelism of shaper arm with table surface or vice.

Johansson Microkator :
 Principle:
 It works on the principle of a Button spring, spinning on
a loop of string like in the case of Children’s toys.
 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.

Reed Type Mechanical Comparator
 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.
 The floating block B carries the gauging spindle and is connected
horizontally to the fixed block by reeds C.
 A vertical reed is attached to each block with upper ends joined
together, as shown in the figure . Beyond this joint extends a pointer
or target.
 A linear motion of the spindle moves the free block vertically
causing the vertical reed to slide.
 This movement causes the pointer or target to swing through an arc
over the scale proportional to the distance traveled by the spindle
and of course very much magnified. Comparators of this type may
have sensitivities of the order of 0.25 micron per scale division.Matoshri College of Engineering and

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

Mechanical - Optical Comparator(Cont.)
 In mechanical optical comparator small
displacement of the measuring plunger
are amplified first by mechanical system
consisting of pivoted lavers.
 The amplified mechanical movement is
further amplified by a simple optical
system involving the projection of an
image.
 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.

Mechanical - Optical Comparator(Cont.)
• In this system,
Mechanical amplification = x2 / x1
Optical amplification = (x4 / x3 ) * 2
• It is multiplied by 2, because if
mirror is tilted by an angle δθ, then
image will be tilted by
2 x δθ.
• Thus over magnification of this
system = 2 (x2 / x1) *(x4 / x3)
• 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.

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
 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.
 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 .
 When the force is applied on the moving block,
it will give an angular deflection.
 AY-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.

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:
• Range is limited as the pointer moves over a fixed
scale.
• Pointer scale system used can cause parallax error.
• There are number of moving parts which create
problems due to friction, and ultimately the accuracy
is less.
• The instrument may become sensitive to vibration
due to high inertia.

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.

Pneumatic Comparators(cont.)Working:
 As shown in Figure 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

Pneumatic Comparators(cont.)
Metrological Advantages of Pneumatic Comparators
Reasons for Pneumatic Inspection of Small Holes
1. Pneumatic gaging elements can be very small. This permits precision
measurement of smaller inside diameters that is possible by other means of
measurement.
2. Deeper holes can be inspected because depth has little effect on ease of
measurement.
3. Greater accuracy is obtained because rocking and centralizing are unimportant.
4. Nominal gaging force permits thin-walled and yielding materials to be
measured.
5. Absence of metal-to-metal contact protects finely finished surfaces.

Disadvantages of Pneumatic Comparators
i)They require elaborate auxiliary equipment such as accurate pressure
regulation.
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.

Electrical Comparator
Principle of working of Linear variable differential
Transformer(LVDT)
 It work on principle of mutual inductance which state that
when core is inserted in a magnetic field generated by
primary winding then the voltage induced in secondary
winding is proportional to core position

LVDT working
• When core is centralize at the middle
position of two secondary windings
then the voltage introduced in each
secondary coil will be same and out of
phase
• When the plunger move on the surface
to be checked then the core which is
connected to plunger will start to
move from its mean position
• The differential voltage appears across
secondary in series which is further
calibrated in term of linear movement
of the core()plunger

Assignment
Q1) Explain working principle of Mechanical comparator(any one)
Q 2) State different types of comparators, explain sigma comparator in
detail.
Q3) Principle of working of Linear variable differential Transformer
(LVDT)
Any Question
patil.roshaneng@gmail.com

Comparators

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