Introduction Screw gauge or micrometer screw gauge. It is a measuring instrument used for precision measurement. This is used in the mechanical engineering field. It is used for measuring extremely small dimensions.

Example to measure the diameter of the given lead shot, the diameter of a given wire, the thickness of a given glass plate, sheet metal and etc like measurable.

**Lets we learn what is the theory of screw gauge.**

Parts of the micrometer screw gauge

A = anvil,

B = spindle,

C = sleeve(with main scale),

D = thimble

(with rotating vernier scale),

E = ratchet knob, F = frame, G = lock

Micrometer screw gauge is used for making accurate measurements of short lengths (less than 2.5 cm). It consists of a frame(like in the fig above), a moveable rod inside a sleeve, and a thimble. The sleeve looks like a barrel and a thimble can rotate over the sleeve.

First, the object is placed loosely between the anvil and the rod. Then rotate the ratchet at the end of the micrometer.

The ratchet knob allows the screw to slip in order to prevent too much force from being applied to the anvil and spindle. That no damage to the precision threads, jaws, and the object.

A micrometer screw gauge has a linear scale on the sleeve and a circular scale on the thimble. One division on the linear scale is 1mm long. But we can catch the half of 1mm because it is made two same scales together as the fig above.

The circular scale has 50 divisions, and one complete revolution moves the thimble 0.5 mm along the linear scale.

Movement of the thimble when 50 divisions rotated = 0.5mm

Movement of the thimble when 1 division rotated = ( 0.5mm)/50

Lead or pitch is the linear travel of the thimble or spindle when complete one screw revolution.

the smallest measurement = (lead or pitch value)/(number of divisions on the circular scale)

=( 0.5mm)/50 = 0.01mm

There are two types of micrometer screw gauges. The one in which circular scale has 50 divisions, and one complete revolution moves the thimble 0.5 mm along the linear scale. The other is which circular scale has 100 divisions, and one complete revolution moves the thimble 1.0 mm along the linear scale.

## Zero Error and Zero Correction

When the screw just touches the anvil and the zero mark of the circular scale comes on the reference line, the zero error and the zero correction are nill. See the fig here. We can directly take the reading as the true length of the Physical quantity.

## Positive Zero Error and Negative Zero Correction

When the screw just touches the anvil and the zero mark of the circular scale comes as shown in fig. Here we can see zero marks on the linear scale. Four sections of circular scale are latched backward. Therefore it’s zero error is (0.01mm) * 4=+0.04mm. It’s zero correction is negative because the zero error is positive. To find the true length, This error must be reduced by the reading.

**True Value = Reading – Zero error**

## Negative Zero Error and Positive Zero Correction

When the screw just touches the anvil and the zero mark of the circular scale comes as shown in fig. Here we can’t see the zero marks on the linear scale because it is hidden by the circular scale or we can’t see it 100%. In this case, One section of circular scale is latched forwards or clockwise. Therefore it’s zero error is (0.01mm) * 1=-0.01mm. It’s zero correction is positive because the zero error is negative. To find the true length, This error must be added by the reading. You can understand these two cases well by seeing the video here.

This device is used to measure the thickness of like wires, iron plates, .. etc.

**True Value = Reading + Zero error**

Lets us ready for the next lesson. click here NEXT

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