GlossaryKind of term: Manufacturing termDatum
What is a datum?
A datum is a theoretically exact reference, such as a plane, an axis or a point, derived from a real feature, to which geometric tolerances refer.
It can also be a combination of these; orientation, location and run-out tolerances refer to it, form tolerances take no datum. The real feature it is derived from, such as a face or a bore, is called the datum feature. On the drawing the datum feature carries a capital letter in a frame, connected to a triangle. With the triangle on the surface or on its extension line, the surface is the datum feature; in line with a dimension line, it is the axis or median plane of that feature of size.
What does the standard say about a datum?
ISO 5459 describes how a datum is derived from a datum feature and how datums form a datum system. In a feature control frame they appear in order of precedence: ⌖ ⌀0.1 A B C means A primary, B secondary, C tertiary. The primary datum constrains as many degrees of freedom as its geometry allows, the secondary and tertiary only those still free. A different order gives a different datum system and so a different measurement. ASME Y14.5 follows the same principle but differs in details of derivation and simulation.
How does a datum work in practice?
Once machined, a datum face is the natural place to clamp or align: machining and inspection then share the same starting point. That is why datum features are often made early in the route, though that is a choice, not a rule. In inspection, the coordinate measuring machine or the fixture simulates the datum system; measuring from a convenient edge instead of from the datums checks a different requirement. A datum on a face that cannot be reached in the first setup can change the order of setups.
What is a datum often confused with?
A datum is not the machine's work origin: the datum is a requirement on the drawing, the origin is the position you probe and set per setup, often on a datum feature. Nor is a datum a clamping face: you may like to clamp on it, but the drawing does not prescribe that. And the datum is not the datum feature: the feature is the real, imperfect surface, the datum the ideal reference derived from it.
Frequently asked questions
May I clamp on a datum face?
Yes, and once the face is machined it is often the sensible choice: you then clamp and measure from the same starting point. Clamping on a rough face that later becomes a datum makes the features that depend on it more sensitive to error.
Does the order A, B, C matter?
Yes. The first datum constrains as many degrees of freedom as its geometry allows, the second and third what is still free. Measure in another order and a good part can be rejected or a bad one accepted.