I am often asked how to set up a part when a datum feature is unstable. On this drawing datum feature A has a flatness tolerance of 0.2. If the feature is produced with a concave surface, the part will contact the simulated datum on the high points and establish a datum plane. However, if the surface is convex or bowed it will “rock” when brought in contact with the datum simulator. As a result, there will be an infinite number of possible datum planes that could be used.
The Y14.5 standard states “the part may be adjusted to an optimum position, if necessary, to simulate the datum”.
So, the question becomes—“What is an optimum position?” For clarification we can go to the ASME Y14.5.1 standard. In Section 1.4.6 the standard describes the Candidate datum set. “The set of all candidate datums that can be established from a datum feature.”
Bottom line, there may be an infinite number of possible datum planes that are in the candidate datum set. The inspector is allowed/required to rock the part in an attempt to find a datum plane within the set that will bring the part features into their stated tolerances. This figure shows a couple of possible datum planes that could be used. However keep in mind the simultaneous requirements rule. Let’s say that all of the features are located with position and profile relative to datum features A, B and C. If the part is unstable or “wobbles” in the set-up, the inspector may rock the part on the datum features to bring all features into the required position and profile tolerances but they must all meet their tolerances simultaneously.