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Thread: Position tolerance with parallelism

  1. #1

    Position tolerance with parallelism

    This is a seemingly trivial question that is everyone I have talked to has a differing opinion on.

    For this part, what I care about it the location of the central hole's axis w.r.t the center point of the two pins and the parallelism of the hole's axis w.r.t. the left pin axis. Dimensions and magnitudes of tolerances are made up.

    The "conventional" way shown on the LHS would be to define a plane A that both the pin and the hole are perpendicular to, but I don't really care about the form or orientation of this common plane, so I am would be over tolerancing this part to my manufacturer (which is most likely a plastic molder, so the form and orientation are not trivial asks).

    What I want is to use the pin axis to set the orientation of the tolerance zone of the hole. In this RH case though, there is now no normal feature that makes sense to list as a primary datum for the position tolerance in the RHS, as the primary datum in position tolerance almost always refers to a perpendicular feature that sets an orientation. I am not sure how to describe what I want in an FCF in this case. I am assuming the position tolerance on the RHS is not correct, but my intention with the callout is to let the parallelism control the orientation (2 DoF) and the position tolerance to define the remaining two positions and not orientation (the axial rotation is idle and the axial position is confined to the thickness of the plate, so that's all I need). I would prefer to not use profile, as, without going into too much detail of the application, I truly only care about the axis of the hole, not the surface.

    How would you all go about this? Simply list the pin axis as the primary datum in a position tolerance and have a second FCF refine parallelism? Leave as is? If it makes any difference, this will be inspected solely on a CMM.
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