We have been in discussion over how to evaluate position tolerance when you have mmc on the feature measured and the secondary datum which also has mmc. For instance, picture a square plate, the base is datum -A-, a 50mm+/-.5 hole in the center of the plate, Datum -B- and a slot on the edge for orientation, datum -C-. Four holes, 4mm +/-.25 dia equally spaced around -B- at 77.5mm from center in each direction. The top feature control frame requirement is Position .5dia @mmc to A, Bmmc, C. The interpretation is that we add up the .5, plus mmc for the measured hole, plus the mmc for datum -B-, which could add up to as much as 2.00mm position tolerance. On our Zeiss Prismo using umess-ux by selecting a reference datum, the software will add this up. But I contend we cannot arbitarily do this. We can only evaluate the effect of mmc on -B- by using the paper gage method to determine how much mobility we have from -B- to accept/reject for any position above the .5 plus mmc on the measured hole. Any thoughts on this matter would be greatly appreciated. Also, is there any way for the software to determine this correctly, I have been asked to pursue this issue.

The lower composite requirement is position .17mm dia to -A-, this is accomplished by performing a 2d best fit, by rotating and shifting to determine if the hole pattern is true within itself.
Our problem is the top with mmc on the measured featured, Secondary datum and I have seen mmc on the Tertiary datum, which can really get sticky.
Myrle L. Harrison
CMM Quality Engineer
Lockheed Martin Missiles and Fire Control
Camden Operations
Camden, Ar
(870) 574-0200 ex 4633