Q.No.4 Write a note on the Screw Gauge

A screw gauge is the instrument used to measure small lengths accurately up to one-hundredth part of a millimeter. It is also called a micrometer screw gauge. Its accuracy is greater than that of a vernier caliper.

Construction

It consists of a U-shaped metal stud at one end. A hollow cylinder (or sleeve) has a millimeter scale over it along a line called the index line, parallel to its axis. The hollow circular scale acts as a nut. A thimble has a threaded spindle inside it.

Circular Scale

The thimble has 100 divisions around one end. It is the circular scale of the screw gauge. As the thimble completes one rotation, 100 divisions pass the index line, and the thimble moves 1 mm along the main scale. Thus, each division of the circular scale crossing the index line moves the thimble through 1/100 mm or 0.01 mm on the main scale.

Pitch

As the thimble completes one rotation, the spindle moves 1 mm along the index line. The distance between consecutive threads on the spindle is 1 mm; this distance is called the pitch of the screw gauge.

Least Count

The least count of a screw gauge can be found as follows:
Least count = pitch of screw gauge / number of divisions on the circular scale
= 1 mm / 100 = 0.01 mm = 0.001 cm
The least count of the screw gauge is 0.01 mm or 0.001 cm.

Zero Error of Screw Gauge

If we close the gap between the spindle and the stud of the screw gauge by rotating the ratchet in the clockwise direction, and if the zero of the circular scale coincides with the index line, then the zero error will be zero. If the zero of the circular scale does not coincide with the index line, then there is a zero error in the screw gauge.

Positive Zero Error

If the zero of the circular scale is behind (below) the index line, it will measure slightly more than the actual thickness, and this is called positive zero error.

Zero Correction: In this case, multiply the number of divisions of the circular scale that have not crossed the index line by the least count of the screw gauge to find the zero error. This error will be subtracted from the observed measurement.

Negative Zero Error

If the zero of the circular scale has crossed (above) the index line, it will show slightly less than the actual thickness, and this is called negative zero error.

Zero Correction: In this case, multiply the number of divisions of the circular scale that have crossed the index line by the least count of the screw gauge to find the zero error. This error will be added to the observed measurement.

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