Composite GD&T Position Callouts

[Mightee856]

I often use holding fixtures to measure parts and assemblies.  These fixtures have known tooling ball locations which allow me to do a center point best fit alignment and therefore find the part. 

My problem lies with the composite position callout for a pattern of holes.  Without datums defined or used (because I use the holding fixture for alignment), The Feature Relating Tolerance Zone Framework (bottom half of composite callout) position does not calculate correctly and simply matches the PLTZF.

Any advice on a workaround would be appreciated.

[cappleby1]

Hard to visualize without a print can you post the print? black out info for privacy if necessary. A pic of the part in the fixture may be helpful too.

[jvenlet]

The easiest way to dimension a composite position is to use a best fit to modify the alignment.  I'm not sure how to handle this in PolyWorks.

If your position is ABC and your composite is only A, then you can rotate and translate to fit the holes.  If AB, then you can only translate.  It all depends on what degrees of freedom are 'unlocked' by your FRTZ.  Again though, I am unsure how to handle such alignment changes in PolyWorks.  Have you tried defining datums from the part? 

[Jeff]

We also use a lot of fixtures for our parts. We usually define the datum manually and then copy the nominal to the measured. With the fixture you are call all the datums perfect. That is the best we can do because the datums are blocked by the fixture and are unmeasurable. If anyone asks we say the datums are set by the fixture.