straightness with Datum

[[Jo...]]

Is it legal to have a straightness FCF with Dia. of a cylinder, plus MMC modifier referenced to a Datum plane ? If so, how do you deal with that ? That would be axial straightness of the cylinder to one plane, so really no indication of relationship.

[[Br...]]

No that does not make sense. A straightness tolerance is a tolerance that controls form of individual features and therefore would be independent of any other features. Y14.5 explicitly states that Form tolerances are not related to datums(ASME Y14.5-2009 para 5.4).

[[Jo...]]

Thank you. We are trying to provide an alternate method to meet design intent. Maybe position with refinement.. Profile maybe.. Don't need the distance from Datum controlled so tightly, as long as it is parallel..

[[Br...]]

It's hard to know how the part is meant to function without talking with the designers. But if you are using Calypso, you can do a position tolerance that evaluates the derived median line like a straightness tolerance does and reference datum A. That might reflect something like that. It's hard to say though because the tolerance applied doesn't make sense according to any current standards. If you could call and ask what they are looking for, that would be your best avenue instead of just guessing.

[[An...]]

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..... and always remember: there is no median line .....
but: there is a __________ m e d i a n_____p o i n t_____c l o u d

[[Br...]]

Except for when there is. Like in the case of straightness, Or in the case of how Calypso evaluates derived median lines. Your issue is how to calculate the center point of each cross-section where the median line comes from. That is a separate problem that isn't well defined in my opinion. But that doesn't mean there is no derived median line. It's the official term for a line that passes through all the cross-sectional center points of a cylindrical feature. So.. its a thing.