Screw-gauge
The screw has a known pitch such as 0.5 mm. Pitch of the screw is
the distance moved by the spindle per revolution. Hence in this case, for one
revolution of the screw the spindle moves forward or backward 0.5 mm. This
movement of the spindle is shown on an engraved linear millimeter scale on the
sleeve. On the thimble there is a circular scale which is divided into 50 or
100 equal parts.
When the anvil and spindle end are brought in contact, the edge of
the circular scale should be at the zero of the sleeve (linear scale) and the
zero of the circular scale should be opposite to the datum line of the sleeve.
If the zero is not coinciding with the datum line, there will be a positive or
negative zero error as shown in figure below.
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