Position Callout on a Rotated Flange Pattern with Only Two Datums Referenced
A machinist produces a circular flange with six Ø8.0-8.4 holes equally spaced on a Ø120 bolt circle. The drawing shows a position callout: |⌖|Ø0.4(M)|A|B| where A is the flange face and B is the Ø50 pilot bore. No third datum is referenced. The inspector fixtured the part on A and centered it on B, then measured each hole with a CMM and reported each hole's X/Y deviation from its nominal (unrotated) angular location. Four holes show radial deviation of 0.10 mm and tangential deviation of 0.30 mm. The inspector rejects the lot because tangential deviation exceeds the Ø0.4 diameter/2 = 0.20 mm limit. The shop supervisor argues the part should pass because the pattern can rotate about B.
1. Evaluate the inspector's rejection. Is the position of the pattern properly constrained by |A|B| alone, and how should the six-hole pattern be evaluated?
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The inspector's method is wrong. With only A (primary) and B (secondary) referenced, the datum reference frame fixes translation in X and Y and orients the Z axis, but does not lock rotation about B. The six-hole pattern's angular clocking is therefore unconstrained - the pattern may rotate about B to best-fit the as-produced holes.
Correct evaluation: perform a best-fit rotation (pattern rotation) about datum axis B, then check each hole's position relative to the rotated pattern. The 0.30 mm tangential deviation may be largely absorbed by rotation. Only the residual radial + tangential deviation after rotation must fit within Ø0.4.
If the designer intended to lock clocking (e.g., to a keyway or a mating feature), a third datum (a hole, slot, or feature of size) must be added. Absent that, the supervisor is correct that clocking is free.
2. The design intent is that the six holes must align with six mating studs on a fixed mating part that is itself rotationally located by a dowel. What change to the drawing is required, and what happens to the acceptance decision?
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Because the mating part's stud pattern is rotationally fixed by a dowel, the flange pattern must also be rotationally located. The drawing must add a third datum feature - e.g., one of the holes or a separate dowel hole - as datum C, giving |⌖|Ø0.4(M)|A|B|C|. With C referenced, clocking is constrained and the pattern may not rotate.
Once C is added, the inspector's original per-hole check becomes meaningful in principle, but it must still be done correctly: the position tolerance zone is a Ø0.4 cylinder oriented and located per the DRF, and the measured axis must lie within it. A 0.30 mm tangential deviation from nominal does exceed the 0.20 mm radial limit of the zone in that direction, so under the corrected callout the holes would fail. The lesson: the drawing as issued was under-constrained relative to design intent, and the acceptance decision changes with the datum scheme.