What is the micrometer screw gauge?
It is an instrument used to measure diameter of thin wires, thickness of small sheets of glass,plastic etc. It can measure upto 1/10 of mm (or 0.01 mm= 0.001 cm) which is usually called the least count of screw gauge.
Parts of screw gauge
It consists of a U-shaped metal frame F with a fixed stud A.
- The right moveable and R (called Ratchet) is a cylinder type whose other end is called spindle.
- A static linear scale or a main scale (usually in mm) is marked on a cylinder.
- The secound scale (moveable) is called circular scale or a cap scale (with 50 or 100 divisions) which is drawn on a cap called thimble ‘T’ fitted over a cylinder.
- The circular scale is designed in such a way that one complete rotation of the cap moves it horizontally by 1 mm on the main scale if circular scale cap moves it horizontally by 1 mm on the main scale if circular scale division is 100 and 0.5 mm in the main scale if circular division are 50.
Working steps to use of Screw Gauge
- To Find Pitch of Screw
- To Find the Least Count
- To Find Zero Error
The distance moved by linear scale during one complete rotation is called pitch of screw. For a screw gauge of 100 circular division, the pitch is 1 mm.
Least count of screw gauge
A screw gauge of 100 divisions will move the cap scale along the main scale by 1/100 of mm-0.01 mm. This is the minimum value up to which a screw gauge can measure and is known as its least count. Or it is defined as the ratio between the pitch of the screw and the number of division on the circular scale.
Micrometer least count formula
Micrometer least count in mm
L.C= L.C= =0.01 mm
Errors of micrometer
When both ends (stud and spindle) of the screw gauge each other in such a way the zeros of both circular scale and main scale exactly overlap each other, then there is no error. Otherwise, the instruments has an error called zero error. Zero error can be negative or positive.
Negative Zero Error of screw gauge
if the zero of the circular scale above the index line (reference) line, then error is negative. So zero correction will be positive.
Positive Zero Error of screw gauge
If the zero of circular scale is below the reference line or index line then zero error will be positive. So zero correction will be positive.
How to use screw gauge?
Let us find the diameter of Wire:
- (i) Fix the given wire between the stud and spindle by turning the retchet in the clockwise direction.
- (ii) Note the main scale reading (M.S.R) (up to complete division visible on the horizontal line just beyond the circular scale.
- (iii) Note the circular scale division says ‘n’ which is exactly in front of the index line or reference line.
- (iv) Multiply ‘n’ with the least count (L.C) to get fraction of the circular scale reading (C.S.R) to be added into the main scale reading (M.S.R).
- (v) The diameter of the wire will be diameter of wire= (MSR+ C.S.R) + zero correction.
- (vi) Repeat the experiment at least three times by gipping the wire at different places to obtain its average diameter.