How to read a micrometer

Parts

Frame, anvil (fixed) and spindle (moving) with the two measuring faces, sleeve (barrel) with the millimetre scale and the half-millimetre lines, thimble with 50 divisions, ratchet stop or friction thimble for a constant measuring force, and a lock to hold the spindle. Outside micrometers come in 25 mm ranges: 0–25, 25–50, 50–75 mm and so on.

Reading

1. Read the last whole millimetre visible on the sleeve.
2. If a half-millimetre line is visible beyond it, add 0.5 mm.
3. Read the thimble division on the sleeve’s reference line and add division × 0.01 mm.

Example: 7 mm visible, no half line, thimble on 28 → 7.28 mm. Example: 7 mm and the half line visible, thimble on 28 → 7.5 + 0.28 = 7.78 mm. The most common mistake is missing or misjudging the half-millimetre line: when the thimble reads near 0, check whether the half line has just appeared.

Vernier micrometers add a vernier on the sleeve and read 0.001 mm; digital micrometers display 0.001 mm. In school physics a further digit is often estimated (see the physics guide).

Zero check

Clean the faces with a soft paper, close them with the ratchet (0–25 mm) or on the setting standard supplied with larger sizes (25–50 mm and up), and check that the thimble reads 0 on the reference line. A small offset is corrected by turning the sleeve with the spanner supplied; follow the maker’s instructions.

Measuring

Clean the part and the faces, hold it by the insulated part of the frame (hand heat expands the frame), bring the spindle close by the thimble, then turn the ratchet until it slips a few times. Keep the axis square to the surface: on a shaft, rock gently to find the true diameter. Measure at several positions and, for roundness, in two directions. Unlike a caliper, a micrometer follows the Abbe principle: the scale is in line with the measured length.

Accuracy and choice

A typical 0–25 mm micrometer has a maximum permissible error of a few micrometres (typically ±2–4 µm depending on the standard and grade; check the specification), roughly 5–10 times better than a 150 mm caliper. That is why tolerances of a few hundredths are measured with a micrometer (see choosing instruments). Standards: ISO 3611 internationally; in China GB/T 1216 (product) and JJG 21 (verification).

In KELIBRON

The lab has calipers only at present; micrometers are planned. The basics carry over: clean, check zero, square to the part, constant light force, read to the scale interval — practise them on the calipers below.