Skills · skill
Reading Calipers and a Micrometer
Measuring · Beginner
- Difficulty
- Beginner
- Safety
- None to speak of, beyond dropping the micrometer — which ruins it.
- Done When
- Repeat readings agree within the instrument's resolution, and a known gauge block reads its stated size.
- Tools
- Digital or vernier calipers, 0–25 mm micrometer with a standard, a clean rag, a stable temperature.
- Category
- Measuring
- Summary
- Measuring to a hundredth of a millimetre — the difference between a part that fits and one that almost fits.
Steps
Zero the instrument closed; wipe the jaws.
Calipers: close on the part square, no rocking; read; check with the depth rod and inside jaws where needed.
Micrometer: open past the size, close on the part with the ratchet until it clicks twice; read the sleeve for millimetres and halves, the thimble for hundredths.
Take three readings around the part; they should agree.
Check the micrometer against its standard now and then.
Record to the instrument's resolution and no further.
The Science
A caliper reads directly to 0.02 mm; a micrometer to 0.01 mm or better, by a screw of known pitch: one turn of the thimble is 0.5 mm, and the fifty divisions on it read hundredths. Steel expands about 12 µm per metre per degree — a warm part in a cold hand measures big. The measuring pressure matters, which is why a micrometer has a ratchet: it applies the same force every time.
How It Fails
Reading a vernier one line off. Squeezing a caliper until it flexes. Measuring hot metal straight off the lathe. Trusting the fourth decimal on a calipers display.
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