Micrometer

Micrometer

They enable very accurate measurements to be taken.

Outside micrometers are used to measure:

• Outside diameters

• Thickness of material

• Lengths of parts.

They are available in various sized frames. All sizes,however, have a measuring range limited to the length of

the thread on the spindle.The range is 0 to 25 millimetres.

The principal parts of a micrometer are:

Frame

Anvils

Spindle and Thread

Sleeve or Barrel

Thimble.

A knurled collar or a small lever on the frame can be used to lock the spindle in the barrel. After the anvils have been set against the work being measured, tighten the spindle lock. This prevents any movement of the spindle while you are reading the micrometer scale. Remember to loosen the clamp before attempting to take any further readings.

Principles of a micrometer:

The principle of a micrometer that reads to 0.01 of a millimetre is explained below. Hold a 0‑25 mm outside micrometer by the frame between thumb and first finger of your left hand. Keep the graduations on the sleeve towards you.Loosen the spindle lock. Use the finger and thumb of your right hand on the knurled part of the thimble to screw it anti‑clockwise. This moves the spindle to the right and uncovers the graduations on the sleeve. Look at the gap between the anvils. It is equal to the uncovered 'length of the datum line. Look at the datum line on the sleeve. It is graduated into millimetres and half millimetres, from zero up to 25 mm, and each fifth millimetre is numbered. Turn the thimble until zero is level with the datum line Note the position of the graduation on the sleeve. Turn the thimble one complete turn. The thimble will move along one graduation of the sleeve scale. This is because the pitch of the thread on the spindle is half a millimetre. Two turns of the thimble move the spindle one millimetre. Look at the graduations around the thimble There are 50 graduations and each fifth graduation is numbered Now wipe the face of the anvils with a piece of clean cloth.Screw the thimble inwards towards the frame until the