Datum
Glossary · Tolerance & Measurement
At a glance
- Category /
- Tolerance & Measurement
- Related /
- 2 terms
- Source /
- Written in the shop
A datum is the reference surface, axis or point on a part that all other measurements are taken from.
In practice
Datums are labelled A, B, C on a drawing and establish how the part should be held and measured. Datum A is usually the primary locating surface, B constrains rotation, C fixes the remaining direction.
Why it matters
Measuring from the wrong face can make a good part look bad or a bad part look good. If a drawing has no datums, the shop and the customer are free to measure differently and both be right — which is how disputes start.
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