True Position and MMB


I have a supplier using laser scanning (Roma Arm) and Polyworks to measure parts. Apparently, Polyworks is able to determine MMB shift and is calculating the position of a hole to be zero even though the actual X, Y measurements deviate from the basic dimensions.

How do I calculate MMB shift as it pertains to position tolerance? MMC I understand, but I'm having a hard time understanding how to calculate shift when I have MMB modifiers. I had always been told it was extra bonus tolerance a la MMC, but as I read the actual ASME Y14.5, that thinking is incorrect. Unfortunately, the standard is muddy in how to calculate MMB. At least for me it's muddy.

Unfortunately, my CMM software (MCOSMOS) does not calculate datum reference modifiers as MMB which means I need to figure out a workaround. I can't even find anything about MMB in their help documents.


Forum Moderator
See the linked description of the difference between MMB and MMC. In many cases, hard gages made to check features at MMC were probably checking at MBB. The same may be true for your software.
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