• Spindle: movable measuring face.
• Sleeve/barrel: main scale.
• Thimble: rotating scale.
• Ratchet stop: helps apply constant measuring force.

Good practice

• Clean the anvil and spindle faces before use.
• Place the object gently between the anvil and spindle.
• Use the ratchet stop and do not overtighten.
• Lock the spindle while reading to avoid movement.

How the micrometer works

Turning the thimble moves the spindle a small distance. Each full rotation usually moves the spindle by 0.50 mm.

• Sleeve/main scale: shows whole millimetres and half millimetres.
• Thimble/rotating scale: usually has 50 divisions. Each division equals 0.01 mm.

Step-by-step: how to take a reading

1. Read the sleeve scale

Read the last whole millimetre mark visible to the left of the reference line. If the 0.50 mm mark is visible, add 0.50 mm.

Example: 8.00 mm visible.

2. Read the thimble scale

Find the thimble division that lines up exactly with the reference line. Each division equals 0.01 mm.

Example: division 42 aligns = 0.42 mm.

3. Add both readings

Add the sleeve reading and the thimble reading to get the total measurement.

Example: 8.00 mm + 0.42 mm = 8.42 mm.

4. Check for consistency

Repeat the measurement at a different point and take the mean. If readings are not consistent, check for dirt, damage or inconsistent pressure.

5. Record correctly

Record the measurement to the nearest 0.01 mm. Do not estimate between divisions.

Correct: 8.42 mm. Incorrect: 8.425 mm.

Worked examples

• Example 1: sleeve reading 6.50 mm + thimble reading 0.23 mm = 6.73 mm.
• Example 2: sleeve reading 9.00 mm + thimble reading 0.41 mm = 9.41 mm.
• Example 3: sleeve reading 3.50 mm + thimble reading 0.12 mm = 3.62 mm.

Common mistakes

• Overtightening, which can damage the instrument and give an incorrect reading.
• Reading at an angle, causing parallax error.
• Not using the ratchet stop.
• Misreading the half-millimetre mark on the sleeve.
• Estimating between thimble divisions.

Key reminders

• Least count/resolution = 0.01 mm.
• One full thimble turn = 0.50 mm.
• 50 thimble divisions = 0.50 mm.
• Each thimble division = 0.01 mm.
• Take several readings and calculate the mean.
• Handle with care and store properly.