Metrology and measuring Instruments

1. Metrology and
Quality control
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MMIT/Mechanical
engineering/MQC/Prof.S.S.More

2. COMPARATORS
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• Works on a relative measurement
• Dimensional difference in relation with
basic dimension
• Compare variation with amplification
• No need of operator skill
• Range of instrument is small
• No need of full scale calibration

3. Types of comparator
• Mechanical comparators
• Mechanical – Optical comparators
• Electrical comparators
• Electronic comparators
• Pneumatic Comparators
• Fluid displacement comparators
• Projection comparators
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4. Mechanical
Comparators
• Rack and Pinion
• Cam and gear train
• lever with toothed sector
• Dial indicator
• The Johansson Micrometer
• Reed type mechanical comparator
• Sigma comparator
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5. Mechanical
Comparators
Cam and gear train.
In this case the measuring
spindle acts on a cam which
transmits the motion to the
amplifying gear train
 Rack and Pinion.
In it the measuring spindle
integral with a rack, engages
a pinion which amplifies the
movement of plunger through
a gear train
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6. Mechanical
Comparators
 Lever with toothed sector
 In this case a lever with a toothed sector at its end engages
a pinion in the hub of a crown gear sector which further
meshes with a final pinion to produce indication
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7. Dial indicator
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8. • Dial Indicator
 One of the most commonly used mechanical comparators
 It consists of a robust base whose surface is
perfectly flat and a pillar carrying a bracket in which is
incorporated a spindle and indicator.
 The linear movement of the spindle is magnified by means of a
gear and pinion train into sizable rotation of the pointer on the dial
scale.
 The indicator is set to zero by the use of slip gauges
representing the basic size of the part.
 Used for checking out-of-roundness of a cylindrical component
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9. Johansson
Micrometer
• The simplest and most ingenious method for obtaining the
mechanical magnification
• Works on the principle of a button spinning on a loop of string.
• A twisted thin metal strip carries at the centre of its length a very light
pointer made of thin glass.
• The two halves of the strip from the centre are twisted in opposite
directions so that any pull on the strip will cause the centre to rotate
• One end of the strip is fixed to the adjustable cantilever strip and the
• Other end is anchored to the spring elbow,
• One arm of which is carried on the measuring plunger.
• As the measuring plunger moves either upwards or downwards, the
elbow acts as a bell crank lever and causes twisted strip to change
its length thus making it further twist or untwist
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10. Johansson
Micrometer
• The pointer at the centre of the
twisted strip rotates by an amount
proportional to the change in
length of strip and hence
proportional to the plunger
movement
• dϴ/dl = l/ w2n
• Magnification :- 5000X
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11. Sigma comparator
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• Magnification in the range
of 300 to 5000
• Vertical displacement of
plunger-knife edge(ø)-
rotation of moving block-
driving drum-pointer
• First stage magnification :-
L/a
• Total magnification(L/a*l/r)
• sensitivity in the range of
0.0001

12. Mechanical optical
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 Works on the principle of Optical law
 Small variation in the plunger movement is
magnified: first by mechanical system and then
by optical system
 Mechanical Amplification :- x2/x1
 movement of plunger causes the mirror to
tilt by an angle ø , then image will be tilted by
2ø
 optical amplification :- 2*x4/x3
 overall amplification :- (2*x2*x4)/(x1*x3)

13. Pneumatic (solex
gauge)
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 Changes in length of surface
displacement into changes of
pressure of air
 restriction to the escape of air
will increase the back pressure
 change in dimension is
converted in to the pressure
differential
Used mainly to check
roundness, flatness concentricity

14. Electrical (LVDT)
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 Works on the principle of
Whetstones bridge circuit
 Movement of measuring contact
is converted into electrical signal
 Armature and two coils
 Change is position of armature
will imbalance the bridge and
current will flow through the
galvanometer
 Magnification of order 3000x

15. Optical comparator
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16. Electronic
comparator
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 Application of frequency modulation or radio oscillation
 variation of dimensions is converted into difference in
frequencies and noted the same