Hi All,
I was originally on JP's side on this, but I can see CH's reasoning as well. The problem is that the standard gives a mixed message with regards to what types of dimensions can be used in conjunction with Profile tolerances. I don't think we can say that one is right and the other is wrong - there is evidence to support both opinions.
Here are some thoughts on CH's latest figure with the Profile tolerance on the cylindrical surface with a directly toleranced diameter. Is this compliant with the standard? On the one hand, the true profile is not fully defined by basic dimensions which violates Paragraph 8.2. On the other hand, there are figures showing a Profile tolerance on a conical surface with a directly toleranced diameter. Applying this same idea to a cylinder is not a big leap. To me, the standard contradicts itself here. I would say that if you polled the Y14.5 subcommittee on whether or not CH's cylinder application is legal, the result would not be unanimous.
My personal opinion is that the true profile should always be fully defined using basic dimensions (this includes basic linear dimensions, basic angles, basic diameters and radii, etc.). This is the only path to a rigorous definition of the tolerance zone. I do not support the use of directly toleranced dimensions in conjunction with Profile tolerances, even though the standard still includes figures showing this. One reason is that these examples are non-rigorous and require certain assumptions to be made. The second reason is that these examples muddy the waters as to how a Profile tolerance zone works (resulting in many discussions like the one in this thread). In the examples involving Profile on a conical surface with a directly toleranced diameter, the form of the tolerance zone is theoretically exact but the size of the tolerance zone is not. This makes the Profile tolerance zone behave like a form tolerance or a Total Runout tolerance, but only for features that can have a directly toleranced dimension. There are no rules describing exactly how this works, only examples.
So I wish that the Y14.5 would make it clear that Profile tolerances must work in the way that J-P describes, but I acknowledge that the standard allows them to work in the way that CH describes. Y14.5 is trying to move forward with more sophisticated and rigorous Profile techniques, while not abandoning past practices involving simple geometry with directly toleranced dimensions. This is still a work in progress.
Evan Janeshewski
Axymetrix Quality Engineering Inc.