Construction and Working of Micrometer Screw Gauge

Construction and Working of Micrometer Screw Gauge

It consists of a solid U shaped frame of metal. It has a fixed stud with plane round end. Through the other end passes a fine’ and accurately cut screw “S” having a plane round end. The screw has a cap and it carries on its edge a circulator scale having 50 or 100 divisions. There is another scale graduated in millimeters on the nut of the screw. This is parallel to the axis of the screw and is called main scale. The screw end also carries a ratchet to avoid undue pressure on the object held between the studs.

Pitch of the Screw

It is the distance between the two consecutive threads of the inner screw and is measured by the distance travelled by the circular scale on the main scale during one complete rotation of the circular scale. The least count of the screw gauge is given by:

Least Count = Pitch of the screw/Number of divisions on the circular scale

=1/100 mm

=0.01 mm

0.001 cm

Zero Error

If by bringing the studs in contact by turning the screw the zero of the circular scale coincides with the zero of the linear scale then there is no zero error, otherwise there is a zero error. If the zero of the circular scale is above the main scale there is a positive zero error. If the zero of the circular scale is below the zero of the main scale then there is a negative zero error. To find the zero error notes the number of divisions that coincide. Now multiply it with the least count and then add it to the main scale reading if the error is negative and subtract if the error is positive.