I can envision the cone center planes being in orientations that are efficient in constraining the movement about C whereas the actual datum feature A is not. I think datum B would provide the missing clocking but not sure if there is one definitive datum ref frame since datum B is skewed to cone axis. I was thinking that the cone shaped datum A would have two center planes the way the std shows it for a cylindrical feature. In actual use neither the datum A or B features physically constrain the part that mounts to C datum, but if I understand your point, that's another matter entirely. I just automatically default to using primary datum for that. I never thought of it that way before, picking a secondary datum that has a physical orientation more effective in removing the remaining DOF. by using as primary the small surface now replaces the two rotations and one translation that was most capable of constraining and now leaves with the remaining two translations and one rotation that it is least capable of arresting due to its limited area and its attitude relative to. "For your two hole pattern using as primary. I think I missed the point of your comment: RE: partial cone surface as datum cjccmc (Mechanical) If the coordinate system is well defined, measurement is repeatable and variation can be mapped to its effect on function then the specification does its job. Inspectors commonly compare profiles of fabricated surfaces to modeled surfaced data. If the conic section does functionally stop two rotations and three translations is it capable of repeat ably stopping translation along the conic axis? So have you captured the features that orient and locate the part functionally? Error in form, measurement, and error due to limited circumferential sampling may make locating the apex extremely difficult. as the cone angle decreases tending more cylindrical the apex becomes increasingly difficult to determine. You said that the conic section constrains all but rotation (about the conic axis). given that the tolerance stack accumulation will be complicated by the non-functional substitution. It is always preferable that the datum feature selection and precedence replicate how orientations and locations are determined in subsequent assembly but it is also permissible to depart from that for the sake of measurement stability. then you can determine whether the features on the inside of the conic section ultimately orient and locate the cone or vis-versa. If you know how it assembles and what its function is. Pembroke Pines, FL, USA RE: partial cone surface as datum cjccmc (Mechanical) Maybe you save a bit by sending out undimensioned drawings, or maybe even no drawings, but I'd predict you'll lose much more money just talking on the phone, never mind the costs that rack up when you have to go to court. I.e., if MBD was intended to save money, I'm failing to see how it does so. It just sounds _guaranteed_ to produce conflict between producer and inspector, because they will be trying to compare/contrast an infinity of potential actual dimensions from any one feature to every other feature. In light of that, I'm thinking that you are, in effect, making the tolerances tighter than they appear to be, because the stated tolerance applies universally to any arbitrarily selected or conjectured dimension from a particular feature to _any_ other feature. I'm even more perplexed by the prospect of someday having to prepare an Inspection Report for an article that's (not)dimensioned in that way.įairly often, I've run into parts that would pass inspection if a dimension goes from to, but the same part would fail if the dimension went from to, without regard to whether the part would fit or work. The MBD concept is new to me, and I'm having a hard time wrapping my head around the idea of applying tolerances to dimensions that aren't there.
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