Micrometer

The micrometer is another end - to - end measuring device used for more accurate measurements.  In this, the auxiliary scale is made on the thimble, which rotates about the main scale as shown in Figure. The thimble rotates about the main scale unlike the linear motion the vernier scale, thereby improving the measurement accuracy.  

         In this case, the smallest division on the main scale is 0.5 mm. The thimble is divided into 50 divisions. Thus,

         Least count=0.5/50=0.01 mm

The reading=10.50+16×0.01=10.66 mm

Features of micrometer

               

The constructional features of an outside micrometer are shown in Figure. It is provided with a fixed anvil and a rotating spindle, between which the part to be measured will be placed.  The spindle and the anvil are connected by means of a U-shaped frame.  The gap indicates the maximum measurement that can be made using the micrometer, the spindle can be rotated by means of the thimble till it touches the part surface.  The ratchet at the end ensures that only the spindle does not apply too much pressure on the part.

Micrometer principle

A number of variations are available in micrometers to suit the measuring function being contemplated The following are a few of the varieties: 

• Inside micrometers (Figure) for measuring the inside diameters.  

• Depth micrometers (Figure) for measuring the depth of slots and blind holes.  

• Screw - thread micrometers (Figure) for measuring the screw thread parameters discussed later.  

• V-anvil micrometer (Figure) that can be used for measuring the diameter of objects that have odd number of symmetrical or evenly spaced features, for example, for the diameter of a 3 - fluted end mill.

Inside micrometer

Micrometer to measure depth

Screw thread micrometer

V-anvil micrometer