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Dean said:The part designer is in the best position to decide when the mating envelope should be oriented normal to the surface the feature is cut into instead of the normal orientation constraint from the feature itself.
Dean said:Y14.5 should be improved with regard to supporting the axis of a cone. I find paragraph 7.3(e) Figure 7-3(e) in Y14.5-2018 that make it clear that an axis of a conical feature exists, since it is part of the datum when a conical datum feature is specified as a primary datum feature. It is only sensible then to assert that a conical feature can have a position tolerance applied which would apply to the conical feature's axis. In Y14.5-2018 mandatory appendix I, paragraph I-4 (page 307) speaks of applying a size tolerance to a gage diameter, which doesn't help us with this question. I still wouldn't hesitate to apply position on a conical feature. If you happened to read the public review draft of the next version of Y14.8 "Castings, Forgings, and Molded Parts" you may have noticed the "Full Feature" <FF> modifier, which clarifies that a position or perpendicularity tolerance applies to the axis of the full feature if a size (with +DFT, -DFT,or "Draft adds material" or "Draft reduces material) and associated feature control frame are applied to a drafted feature (a cone or wedge). The tolerance zone for these Full Feature cases will be a cylinder for a conical feature (drafted pin or hole) or two-parallel-planes (drafted slot or wall).
Dean said:For now, I would recommend that either profile of a surface or position (without the <FF> modifier) be used if (M) or (L) is the functional case for drafted features. This means that the evaluation can only sensibly be the resolved geometry method, since the +DFT or -DFT size tolerance only applies to the mold line (either a circular mold line or two parallel line mold line, depending on the type of feature). I don't think the surface method can be used for this case.
Dean said:For MMC or LMC, the functional case is that clearance/space must be available for MMC, or material must be present for LMC. Since the mating feature will not center itself within the considered feature the axis is not relevant (or not relevant enough) for these cases, the surface is the feature element that should be controlled, from a functional point of view.
Dean said:My concern about expanding use of the <FF> modifier to drafted features with tolerances modified at MMC or LMC is that it would require that those boundaries be constrained in translation in the features axial direction (for conical features) and a similar restriction for wedge features. Since the surface boundaries must be tapered to match the theoretically exact draft angle, they become variable in size if they can translate in the axial direction. This make them similar to controlling a conical feature with profile, for which an axial translation constraining datum feature is a must if more than form and coaxiality are to be controlled (to control the "size" of the cone). This consideration does not exist for cylindrical or width features, so I think this is at least an odd difference. I think a bigger issue is that we would have to extend local size to tapered/drafted features as well, since the feature being within its size limits is the other requirement that goes along with a tolerance at MMC or LMC. We have a hard enough time trying to define and work with local size for simple features of size. Extending local size to drafted features isn't something that I think we should do.
Kedu said:I am thinking that the virtual condition boundary (the surface that cannot be violated) is to be calculated from the size dimension only and not from the drafted one. So the surface boundary (VC) is a cylindrical surface and not a conical one (also a width VC boundary is a WIDTH and not a wedge)