Datum feature

How to read it

Berez’s UNC Charlotte notes separate three things: the datum feature, the real, imperfect surface or hole on the part; the datum, the perfect theoretical plane, axis or point derived from it; and the datum simulator, a precision embodiment of the datum feature such as a surface plate. The symbol marks the datum feature. The National Physical Laboratory’s guide notes the letter in the box appears again in the feature control frame of every tolerance related to that datum. Placed in line with a dimension line, the symbol makes the datum feature the feature of size the dimension measures (Cogorno). Cogorno’s guide adds that the letters can be any letter except I, O and Q. ASME Y14.5-2018 (paragraph 6.3.2, quoted by Berez) doesn’t allow the symbol on a centre line, centre plane or axis, and NIST’s seminar notes repeat the rule.

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Primary, secondary and tertiary

The order of the letters in a frame sets which datum comes first. NPL’s guide describes the primary datum as the one that levels the part, normally on a surface or axis, the secondary as the one that fixes its rotation, and the tertiary as the one that completes the coordinate system.

Differences by standard

ISO drawings show the datum triangle filled or open (NPL; Keyence, which describes a white or black triangle). Keyence quotes ISO 5459’s definition of a datum feature as a real, non-ideal feature of the part used to establish a datum.

Look-alikes

A datum target, labelled with a letter and number such as A1, marks points, lines or areas to set a datum from instead of a whole feature (NPL), on parts with uneven or irregular surfaces (Cogorno). The letters in a feature control frame are references back to a datum feature symbol, not datum features themselves.