Datum

A datum is a fixed reference element on the part: a face, an axis, a point, or a combination of these. Geometric tolerances refer to it with a letter (A, B, C), so that measuring and clamping both start from the same reference.

What it means for the calculation

The choice of datum affects how the part has to be clamped. If the datum sits on a face that is only reachable in a second setup, that costs an extra setup and an extra alignment step. A part with three clear datums in one orientation is often cheaper to make than the same part with datums on opposite sides. The datum also sets the measuring arrangement, and with it the time the inspection room spends on the part.

Where it appears on the drawing

As a letter in a datum feature symbol: a box with a letter, joined by a filled triangle to a face or a dimension line. And in the second to fourth compartments of a feature control frame.

Common mistakes

Choosing a datum on a rough, unmachined surface, which hangs the whole dimensioning scheme on an inaccurate reference. Also specifying three datums that contradict each other, or reversing the datum order A, B, C during measurement, which can reject a part that is actually good.

Is a datum the same as the zero point in the machine?

No, a datum is a drawing requirement; the machine zero is a setting the work preparer aligns to it.

Why does the order A, B, C matter?

It fixes which face makes contact first during alignment; a different order gives a different measurement.

Does each geometric tolerance need a datum?

No, form tolerances such as flatness and circularity only control the shape itself and need none.