Datum
Also called: Datum reference frame
A datum is the reference surface, axis or plane on a drawing from which geometric tolerances are measured.
A datum is a theoretically perfect reference (plane, axis or point) established from a real feature of the part. On the drawing it appears as a letter in a triangle (A, B, C) attached to a face or axis. Position, orientation and runout tolerances are measured from these datums.
How it works
A set of three datums forms the datum reference frame: the primary (A) locks three degrees of freedom, the secondary (B) two more and the tertiary (C) the last one. Order matters, because it mirrors how the part sits on the inspection fixture and in the real assembly. Choosing datums well means choosing the faces that actually locate the part in the assembly.
Practical example
On a bracket bolted down by its base and pushed against a side wall, the base becomes datum A, the wall datum B and a dowel hole datum C. The mounting holes get a position tolerance relative to A|B|C, exactly as the part is assembled.
Common mistakes
- Choosing a rough or unstable face as a datum, which gives no repeatability in measurement.
- Swapping primary and secondary order relative to the assembly.
- Calling out datums the shop cannot reproduce in fixturing.
FAQ
Does every part need three datums?
No. It depends on how many degrees of freedom the tolerance needs to lock. Flatness uses no datum; a hole position usually uses two or three.
Updated
Related service
Manufacturing documentation
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