GlossaryKind of term: Manufacturing termDatum precedence
What is datum precedence?
Datum precedence is the ranking of primary, secondary and tertiary datum in the feature control frame; it decides how the part is aligned for the requirement.
You read the order left to right in the feature control frame. The primary datum locates the part first, the secondary then constrains what the primary leaves free, the tertiary the rest. With three planar datums this corresponds to three, two and one points of contact, the 3-2-1 rule. Position ⌀0.1 A B C therefore means something different from position ⌀0.1 B A C, even with the same letters.
What does the standard say about datum precedence?
ISO 5459 describes how datums and datum systems are derived from real surfaces, ISO 1101 how they appear in the frame. Letters in separate compartments form a datum system with precedence; two letters joined by a hyphen in one compartment, such as A-B, form a single common datum with no precedence between A and B. A cylindrical primary datum removes four degrees of freedom instead of three, so the 3-2-1 count does not apply literally there. ASME Y14.5 uses the same precedence.
How does datum precedence work in practice?
The order often reflects how the part sits in its assembly, and so hints at the setup. If the primary face is the locating face in the final setup, the requirements referenced to that datum are tied directly to the clamping face. Clamp on another face and the error between clamping face and datum enters every dependent requirement. When measuring, the coordinate measuring machine aligns the part in the same order; a different order can reject a good part or pass a deviating one. Whether the intended order needs an extra setup or a dedicated fixture depends on shape, tolerances and quantity.
What is datum precedence often confused with?
Datum precedence is not the alphabet: A need not be primary, the position in the frame decides. Nor is it the machining sequence: which face you machine first is your choice; precedence says how the part is aligned for the requirement.
Frequently asked questions
Does datum A always have to be primary?
No. The letters are names; the position in the frame gives the ranking. In a frame reading B A C, B is primary. Read each frame separately, because frames on the same drawing can use the same datums in a different order.
How many points does a primary datum plane touch?
In the 3-2-1 approach, three, which seats the part stably; a secondary plane two, a tertiary one. Under ISO 5459 the datum is the plane fitted against the real surface from outside the material, not three chosen points, unless the drawing gives datum targets.
May I clamp on a face other than the primary datum?
Yes, a drawing normally prescribes no clamping face. Anything that depends on the datum then carries the error between clamping face and datum. With tight tolerances it is better to finish the dependent features with the primary datum as locating face.