It's often best to not use datum shifts in PC-DMIS.
2 reasons:
1) PC-DMIS actually deducts the measured position from feature actuals (often reporting perfect measured position), rather than simply adding to dimension bonus. This usually makes the data useless to customers.
2) Except in very limited circumstances, PC-DMIS does not calculate simultaneous evaluation correctly (no CMM software does, without extremely advanced programming).
Now, if you have one of the extremely limited conditions where PC-DMIS doesn't calculate this completely wrong, you can report positions both with, and without the datum shift. Once to tell the customer where the features lie, and once to inform the inspector if that's in, or out of tolerance.
It's often best to not use datum shifts in PC-DMIS.
2 reasons:
1) PC-DMIS actually deducts the measured position from feature actuals (often reporting perfect measured position), rather than simply adding to dimension bonus. This usually makes the data useless to customers.
2) Except in very limited circumstances, PC-DMIS does not calculate simultaneous evaluation correctly (no CMM software does, without extremely advanced programming).
Now, if you have one of the extremely limited conditions where PC-DMIS doesn't calculate this completely wrong, you can report positions both with, and without the datum shift. Once to tell the customer where the features lie, and once to inform the inspector if that's in, or out of tolerance.
Agreed. Probably engineers like datum shift & GD&T gimmicks like this because in theory it gives them more room to play & write out engineering deviation if they need to. But as usual not pay attention that measuring their ideas is a big headache. Same goes to composite FCFs. The machinist doesn't care about those GD&T gimmicks either when creating toolpaths using a model in Mastercam. The features should be in the right location regardless of datum shift or not.
