A plate, a hole, a position tolerance to A, B and C. Swap B and C in the tolerance indicator and nothing changes on the workpiece. Yet 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 decides 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 tolerance indicator (A.2.4.1). Each datum inherits constraints from the datums before it. Only the orientation is inherited: 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).
The seating plane. It locks one translation and two rotations. In top view it changes nothing, so the graphic leaves it out.
The first side edge. Its orientation in the plane is free; the associated feature fixes the rotation about Z and locks one translation.
The second side edge. Orientation fixed by the implied 90° angle to the secondary datum. Only the location remains: move until contact.
The tolerance zone hangs on the datum system through the theoretically exact dimensions (TEDs) and does not move. What moves is the workpiece with its hole. Because the two edges are never exactly perpendicular, the whole system tilts 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 degrees of freedom run along on the left, the constraints currently in effect below them.
For a single datum from a plane the default has two stages since 2024. First the filter [EM] from Annex J: it removes valleys an ideal mating surface would never reach. Then the association [GE] from Table A.2: the line with the least sum of squared distances under the constraint that it lies outside the material. The standard explicitly notes that [GE] is not the same as [G+], the unconstrained Gaussian line shifted outwards afterwards. How the criteria differ is shown in the article Association: how the real line becomes the datum.
The same criterion applies to the secondary and the tertiary datum, supplemented by the orientation constraints (A.2.4.2). For the tertiary datum the orientation is then fully determined. What remains of the Gaussian criterion is only the translation until the associated feature touches the material: one contact point instead of a best fit.
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 axis, twice the distance between axis 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 in one respect on purpose: it shows the top view, in which datum A acts only normal to the drawing plane. On the real workpiece A locks three degrees of freedom first, then B and C follow as shown.
