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Principle:
 In one rotation screw moves forward through a distance equal to the pitch of the screw.
 In a micrometre screw gauge measurements are done actually through a precisely made integrated screw with a pitch of usually 2 threads per millimetre, which means that on completion of one revolution the displacement achieved is 0.5 millimetre.
 Micrometer screw gauges are more precise than vernier callipers
Construction:
 Micrometer screw gauge is a device incorporating a calibrated screw is widely used for precise measurement of components in the manufacturing of mechanical parts.
 The body used to hold the anvil and barrel firmly in their place is called frame, in micrometre screw gauges, thick C shaped frames are used.
 It is the fixed part mounted at one end of frame exactly parallel to the moving spindle which moves towards it. The object whose dimension to be measured is held between the anvil and the spindle.
 The cylindrical part which displaces by rotation of thimble decreasing the distance between itself and anvil until the object being measured become stable between the two of them is called the spindle.
 The stationary part with having linear scale onto it is called the main scale. It covers the screw mechanism of screw gauge.
 Thimble is the part through which measuring screw is rotated, this screwing results in the displacement of spindle and thimble itself.
 Ratchet is a small device which is used to provide a limited applied force.
 Micrometer screw gauge has two scales, one rotating scale which can be found on its rotating cylindrical part it is also called circular scale and the other one can be found on its stationary sleeve which is called main scale or sleeve scale.
 Generally least count on the main scale is 0.5 mm. The circular scale is divided into 50 or 100 equal parts.
Least Count of Micrometer Screw Gauge:
Errors:

Zero Errors of Vernier Calipers:
 When the anvil and spindle of micrometer screw gauge are made to touch each other, then the zero on the main scale should match with a zero on the circular scale.
 However due to wear and tear or manufacturing defect the two zeros usually do not coincide with each other, then the vernier is said to have zero error.
 There are two types of zero errors.

Positive zero error:
 If on bringing the anvil and spindle of micrometer screw gauge together, the zero mark of the circular scale is below the main scale line, then the zero error is said to be positive.
 To find positive zero error, note the division on the circular scale (C.S.R.) coinciding with the line of the main scale line. Then the positive error is equal to the product of the number of division on circular scale matching with main scale line and the least count of the micrometer screw gauge.
 To get correct reading this error is to be subtracted from overall reading.

Negative zero error:
 If on bringing the anvil and spindle of micrometer screw gauge together, the zero mark of the circular scale is above the main scale line, then the zero error is said to be negative.
 To find negative zero error, note the division on the circular scale (C.S.R.) coinciding with the line of the main scale line. Then the negative error is equal to the product of the number of division on circular scale matching with main scale line and the least count of the micrometer screw gauge.
 To get correct reading this error is to be added to overall reading.
Use of Micrometer Screw Gauge:
 The object whose dimensions to be measured is held between the anvil and spindle of micrometer screw gauge with gentle pressure.
 Note down the main scale reading just before zero of circular scale. This is called main scale reading (M.S.R.)
 Note down the number of circular scale division (n) which coincides with main scale line. Then circular scale reading, (C.S.R.) = n X Least Count.
 Add the M.S.R. and the C.S.R. to get reading.
 Subtract the zero error with a proper sign from above reading to get the correct reading.
 Example: Let us consider a micrometer screw gauge with least count 0.01 mm.
 The Main scale reading is 2.5 mm and circular scale reading is 38. Hence the total reading = MSR + CSR x LC = 2.5 +38 x 0.01 = 2.5 + 0.38 = 2.88 mm
Micrometer Screw Gauge to Measure Inside Dimensions:
Micrometer Screw Gauge to Measure Depth:
Example – 1:
 The screw of micrometer screw gauge moves through a distance of 2 mm when it is turned through 4 rotations. Find the pitch of the screw. If the circular scale is divided into 100 equal parts. Find the least count of the micrometer screw gauge.
 Solution:
Distance traveled by screw = 2 mm
No. of rotations given = 4
Pitch of screw = Distance traveled by screw / No. of rotations given = 2mm /4 = 0.5 mm
No. of divisions on circular scale = 100
Least count of the micrometer screw gauge = Pitch of screw / No. of circular scale division = 0.5 mm / 100 = 0.005 mm.
Hence the pitch of the screw is 0.5 mm and the least count of the micrometer screw gauge is 0,005 mm.
Example – 2:
 If the pitch of the micrometer screw gauge screw is 0.1 mm and its circular scale is divided into 100 equal parts. Find the least count of the micrometer screw gauge.
 Solution:
Pitch of screw = 0.1 mm
No. of divisions on circular scale = 100
Least count of the micrometer screw gauge = Pitch of screw / No. of circular scale division = 0.1 mm / 100 = 0.001 mm.
Hence the least count of the micrometer screw gauge is 0,001 mm.
Example – 3:
 If the pitch of the micrometer screw gauge screw is 0.1 mm and its circular scale is divided into 100 equal parts. Find the correct reading for reading as shown. The nozero error.
 Solution:
Pitch of screw = 0.1 cm = 1 mm
No. of divisions on circular scale = 100
Least count of the micrometer screw gauge = Pitch of screw / No. of circular scale division = 1 mm / 100 = 0.01 mm.
Main scale reading = 4.5 mm
Circular scale reading = 28
Shown reading = M.S.R. + C.S.R. x Least Count = 4.5 + 28 x 0.01 = 4.5 + 0.28 = 4.78 mm
Corrected reading = shown reading – zero error with proper sign = 4.78 – 0 = 4.78 mm
Hence the corrected reading is 4.78 mm.
Example – 4:
 When a screw gauge with a least count 0.01 mm is used to measure the diameter of a wire, the reading on the sleeve is found to be 0.5 mm and the reading on the thimble is found to be 27 divisions. What is the correct diameter of the wire if the zero error for the gauge is +0.005 cm?
 Solution:
Least count of the micrometer screw gauge = 0.01 mm.
Main scale reading = 0.5 mm
Circular scale reading = 27
Shown reading = M.S.R. + C.S.R. x Least Count = 0.5 + 27 x 0.01 = 0.5 + 0.27 = 0.77 mm
Zero error = +0.005 cm = 0.05 mm
Corrected reading = shown reading – zero error with proper sign = 0.77 – 0.o5 = 0.728 mm
Hence correct diameter of wire is 0.72 mm.
Example – 5:
 When a screw gauge with a least count 0.01 mm is used to measure the diameter of a rod, the reading on the sleeve is found to be 1.6 cm and the reading on the thimble is found to be 48 divisions. What is the correct diameter of the rod if the zero error for the gauge is – 0.003 cm?
 Solution:
Least count of the micrometer screw gauge = 0.01 mm.
Main scale reading = 1.6 cm = 16 mm
Circular scale reading = 48
Shown reading = M.S.R. + C.S.R. x Least Count = 16 + 48 x 0.01 = 16 + 0.48 = 16.48 mm = 1.648 cm
Zero error = – 0.003 cm
Corrected reading = shown reading – zero error with proper sign = 1.648 + 0.o03 = 1.651 cm
Hence correct diameter of wire is 1.651 cm.
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