Measuring conditions

Temperature

The reference temperature for dimensional specifications is 20 °C (ISO 1). It is an international agreement: a size means its value at 20 °C, so parts made and measured in different places can be compared. Materials grow as they warm: gauge-block steel by about 11.5 µm per metre per °C, other steels by about 11–12 µm.

Example: a 100 mm steel part 5 °C above the instrument is about 100 × 5 × 11.5 × 10⁻⁶ = 0.00575 mm, roughly 6 µm, longer. When the part and a steel instrument are at the same temperature, much of this cancels, because both have grown.

• Let parts cool after machining before final measurement.
• Do not hold the part or the instrument in the hand longer than needed; if a micrometer has an insulating plate on its frame, hold it there.
• Keep the instrument and the parts in the same room before measuring.

Cleanliness and burrs

A chip, a film of coolant or a burr between a measuring face and the part adds to an outside reading; on an inside measurement it makes the reading small. Wipe the measuring faces and the part, and deburr the edges you measure across.

Measuring force

Too much force deflects a caliper’s jaws, and an outside reading then usually comes out small. Close with a light, steady force. A micrometer has a measuring-force limiting device (ISO 3611), usually a ratchet stop or friction thimble, so that every reading is taken with about the same force.

Before you read

1. Part and instrument at room temperature, near 20 °C if possible.
2. Measuring faces and part clean, burrs removed.
3. Zero checked.
4. Instrument square to the feature.
5. Light, steady force; ratchet on a micrometer.

In KELIBRON

The lab shows two of these conditions as wrong-use demonstrations: debris on a jaw face, and too much force. It does not simulate temperature: its parts are always at their drawing temperature.