Composite Position - Best Fit

[[Mi...]]

Hello,

I am new to CMM's/GD&T and have a question that is eluding me. I have a print that points to a single feature with a Composite Position callout. The top frame being .004" tolerance to A|B|C| and the bottom frame being .002" tolerance to A|B. A is the bottom plane, B is a center cylinder and C is a dowel hole located 20.5 degrees offset. This feature is part of a larger radius of bolt holes at 2.5591" in diameter.

My understanding is that the top frame controls the location and the bottom frame controls the orientation. However, when I look at the way this is called out, C is what is controlling the orientation or rotation of the bolt hole pattern. My question is, should I report best fit for both frames and use the called out Datums and tolerances for each? There is a diametric position callout for the individual holes in this radius at .010" from A|B|C.

I haven't taken any advanced GD&T classes so I may be completely misunderstanding Composite Positions, but would a Composite be used for the group of individual holes instead of just 1? (For example (6X) M5X0.8 Thru at .010" to A|B|C) I feel like that group which creates the 2.5591" diameter would benefit more from the composite then a single thru hole that happens to fall into this diameter.

[[Da...]]

In a composite positional tolerance call-out, the upper frame controls orientation AND location, and all other frames refine ONLY the orientation.

Since the upper frame has a fully qualified datum system A|B|C, there is no need for a best-fit, there simply isn’t anything to best-fit for the first frame. You just need to best-fit all translational degrees of freedom of the whole pattern of holes to A and B for the second frame.

Of course, it should say something like „24X“ on the print for the pattern or whatever number of holes there are.

But if I understood you correctly, there is a composite call-out for the pattern and also individual ones for each hole?

[[Mi...]]

Thank you for the reply!

This callout is for a single feature which is then Datum E. A second dowel hole is located off of this hole, but is not part of the composite. After looking at it some more, is the composite just to tighten the location more without the Datum C? The other holes are not called out for a pattern, they simply are located off of A|B|C.