ISO 1 reference temperature (20 °C)
Dimensions and geometric tolerances on a drawing apply at a standard reference temperature of 20 degrees C, not at shop temperature.
Symptom: a part measures out of tolerance right after cutting but passes later. Likely cause: it is warm, and steel grows with temperature. A 100 mm steel length that is 5 degrees C above 20 degrees C is about 6 micrometres longer (typical steel expansion of 11.6 millionths per degree C, as in the NPL good practice guide; the exact value depends on the material). First check: let the part and the gauge settle to the same temperature before measuring, or correct the reading to 20 degrees C. Do not enter a tool offset from a warm part; if it is still warm, wait and measure again first.
General shop guidance in our own words. The drawing and the standard it cites decide; verify on a sample part.
Related standards
Tell the advisor the feature, the tolerance on the drawing and the material. It answers with machining choices, brand-neutral.
Definition written in our own words and checked against eprintspublications.npl.co.uk. Symbol and term names follow ISO 1; we do not reproduce standard text. See the full index.
Shop note reviewed by our tooling specialist (carbide cutting-tool production and tool measurement background) on 12 October 2026.