The precision of a micrometer is determined by the thread pitch of the spindle.

The spindles of most imperial micrometers are threaded at 40 threads / inch. The thread pitch is therefore 1/40 of an inch, which is 0.025 inches.

The spindles of metric micrometers are threaded at a pitch of 0.5mm.

Each time the thimble is turned and the spindle makes a complete revolution, the distance between the measuring faces of the micrometer (the anvil and the spindle) increases or decreases by the length of the thread pitch.

The circular movement of the thimble is directly related to the linear movement of the spindle.

Turning the ratchet speeder increases the speed at which the thimble and spindle rotate, making it easier to take quick and accurate measurements.

The distance between the measuring faces is then shown on the scales of the micrometer.