A plate, a hole, a position tolerance to A, B and C. Swap B and C in the tolerance indicator and of course nothing changes on the workpiece. Yet in the measurement the hole can be within tolerance in one case and outside in the other. The graphic shows how the datum system is built under ISO 5459:2024 and why the order can decide the conformance decision.
A datum system is not built in one go. ISO 5459:2024 establishes the datums one after the other, in the order of the datum references in the tolerance indicator (A.2.4.1). Each datum inherits only the orientation of the datums before it: the primary datum constrains the orientation of the secondary and the tertiary, the secondary additionally that of the tertiary. Location is not inherited. In the words of the standard: the orientation of the associated features to each other is theoretically exact, but their location to each other is variable (6.3.4).
On the measuring table, the seating plane. It locks one translation and two rotations. In top view it changes nothing, so the graphic leaves it out.
The first workpiece side, nominally perpendicular to A. The association keeps this perpendicularity exactly and fixes the rotation about Z.
The second workpiece side, nominally perpendicular to A and to the secondary datum. Both angles are kept exactly; only the location remains: move until contact.
The tolerance zone is defined in a fixed location relative to the datum system by the theoretically exact dimensions (TEDs). What changes with the order is the position of the workpiece in the datum system. Because the real workpiece surfaces are never exactly perpendicular, the whole system can tilt as soon as the other edge sets the orientation. Whether a drawing chooses the order to suit the function is something we check in the drawing review.
Switch the order, choose the form deviation of the edges and play the sequence. The graphic shows in seven steps how each datum is established: filter [EM], association [GE], the workpiece settling onto the datum, then the same for the second datum, finally the evaluation. The constraints currently in effect are shown on the left.
Each datum is established in two stages: first the filter [EM] from Annex J, then the association [GE] from Table A.2. How both work and how [GE] differs from other criteria is shown in the article Association: how the real line becomes the datum. Here the result is enough: the secondary and the tertiary datum each touch the filtered workpiece at one point.
For the inspection laboratory this means: the order in the tolerance indicator is not a formality. It decides which edge aligns the workpiece, and with it the conformance decision. In CMM programming we build the datum system in exactly this order and set filter and association criterion explicitly instead of relying on the software default. In dimensional inspection we apply the order exactly as it appears in the tolerance indicator.
The position deviation is the diameter of the smallest circle around the nominal position that encloses the hole median line, twice the distance between median line and zone centre. It is compared with the tolerance value Ø0.1.
Everything in this article rests on ISO 5459:2024. For those who want to read it up:
The graphic simplifies on purpose: it shows a section parallel to A, and the workpiece only has deviations in this plane. Datum A acts there only normal to the drawing plane. On the real workpiece A locks three degrees of freedom first, then B and C follow as shown.
