Transcript PRINCIPLES OF MECHANICAL MEASURING INSTRUMENTS

PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
• For increasing the accuracy of direct
observation methods, following mechanical
means of magnification are adopted:
1. Lever method
2. Vernier method
3. Screw & Screw nut method
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS (Cont..)
1. Lever Method:
In fig.2.1 is shown the principle of simple
magnifying lever (consisting of multiplying lever & a
scale).
The lever is supported on a knife edge support.
When a distance ‘l’ is measured by the indicator the
magnification ratio is given by:
a/b = Reading on the scale/bsinα= a. α/bsinα
Where α= angle of displacement corresponding to
the measured distance l.
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS (Cont..)
1. Lever Method (cont..):
It may be observed that the magnification ratio
increases by increasing the displacement angle α.
Thus for equal distance l there will be unequal
displacement angle α, so that scales will have
unequal divisions.
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS (Cont..)
1. Lever Method (cont..):
To eliminate this, the angular measurement range
of the system must be made relatively small. In this
method the magnification is practically limited to
10:1 because of the greater friction involved on
pivots & bearing of the system for greater
magnification
The friction can be reduced by means of specially
designed pivots & bearings & making use of knife
edges
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
2. Vernier Method Principle:
• In this method, a Vernier is used
• It is an additional scale which is used in place of a
pointer or indication line on the moveable
member & it enables the main fixed scale to be
read to a smaller value
• Refer fig 2.2. if the graduation of the main scale
are in cm & each is divided in 10mm, the readings
taken by such a scale will be accurate within 1mm
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
2. Vernier Method Principle:
• If a sliding Vernier is provided , whose scale length equals
9mm but is divided into 10 equal divisions, the difference b/w
the scale division of the vernier & that of the main scale will
be (1.0 -9/10)=0.1mm
• If the vernier scale is set so that its zero coincides with the
zero line of the main scale, the first line on the vernier scale
will be 0.1mm shorter than the corresponding line on the
main scale . Therefore scale value of 0.1mm can be achieved
by using a vernier
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
2. Vernier Method Principle:
• When the scale is to be read, the reading of the main scale is
first taken upto the zero mark of the vernier then the reading
of the vernier scale graduation that coincide with the division
on the main scale is noted, which gives the fraction of the
main scale graduation according to the scale value of the
vernier
• Vernier principle can be applied for increasing the accuracy of
angular measurements
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
3. Screw & Screw-nut Method:
• Refer Fig 3.3. The Screw & nut consists of the following:
Fixed Frame: It has two contact members or anvils. One
anvil is fixed & the other is moveable
Moveable Anvil: It is provided with the threaded part &
can be advanced by means of barrel nut
Barrel nut: The Periphery of the barrel nut is graduated &
its reading can be taken opposite to a fixed pointer
Scale: The number of complete terms of the nut is
indicated by means of a scale engraved on the plain
part
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
3. Screw & Screw-nut Method:
• The scale should read zero when the anvils (fixe
& moveable) are in contact. The readings of the
scale give the distance b/w the end faces of the
anvils
• If p is the pitch of the screw thread & N is the
total number of equal divisions on the barrel nut
then the corresponding axial shift ‘a’ per
movement of one division on the barrel nut is
given as the scale value of the instrument by;
a = p/N
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PRINCIPLES OF MECHANICAL
MEASURING INSTRUMENTS
3. Screw & Screw-nut Method:
• Normally when the pitch , p=0.5mm the barrel will be
divided to 50 divisions. In that case, each division will
read 0.5/50=0.01mm. In this case the main scale must
be graduated in 0.5 mm, which is equal to the pitch of
the screw thread
• When the pitch p=1mm, the barrel will be divided to
100 divisions such that each division reads 0.01mm.
The scale can be divided in millimeters
• When a vernier is employed to read the barrel division,
the accuracy can be increased 10 times so that it is
possible to obtain a reading to the nearest 0.001mm
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