GD and T General question

[[Ma...]]

Ok, I realize this is probably a simple one but why can't you call-out TP of Datum C using FCF ABC? I know you can't but how is the part fully constrained if its only called back to AB?

[[DW...]]

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 Can you share a snippet of a drawing or elaborate? Without knowing what type of features A, B, and C are this question is hard/impossible to answer. 

[[Jo...]]

True positions don't have to fully constrain a part.  You can TP to one datum if youd like (You get only perpendicularity, but its legal).  The positions apply with whatever constraints are offered. 

 

Since datums consume constraints as they are available, you need more information than just ABC to help more. 

 

 

Edited December 10

[[Ma...]]

[[Ma...]]

Datum C is just for stopping rotation - it's like using bestfit rotation. Nothing bad here. You still have to have correct distance from B origin.

[[Ma...]]

But wouldn't the proper way to set that up be TP of C to AB and then use Best fit TP Callout, using the feature in the TP callout as a Datum seems wrong.

[[Pa...]]

So this is a funny situation.  It's not wrong but could lead to trouble.

Datum C provides only clocking.  All of the other 5 DOF are constrained.

Datum C remember is NOT the hole itself.  Its DFS will be a best fit pin mutually parallel to B and perpendicular to A.  Whatever size that best fit pin is when constrained for orientation, its CL is the clocking datum.

You could start this hole perfectly on position but if the drill walks and the hole isn't perpendicular to A, I suspect an evaluation of TP with C tertiary would yield different results than the same evaluation with no tertiary.

It is possible to have the C feature machined OOT for TP using C as tertiary because DFS's have perfect form and orientation while your feature will not.

 

[[To...]]

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I agree.