Datum reference order for composite rolerances

[CTengIS]

What is the logical or functional reason for the ASME Y14.5 rule that the datum order and material boundary conditions need to be preserved down all segments of a composite tolerance feature control frame?

 

[3DDave]

From a practical standpoint the lower segments are refinements, and it's not a refinement if one had a different basis that is unrelated to the original basis.

One can always create separate single segments if there is a desired change in basis; composite is a special case where there isn't.

In application the mating part won't have multiple different ways to interact with the given features; it will only have certain primary features to control location and orientation.

 

[CTengIS]

Thank you 3DDave!