Position Tolerance with MMC and MMB

Hello Everyone

I'm curious about the way PC-Dmis calculates TP with a CF that includes MMC and MMB (Maximum Material) both on the secondary and tertiary datums.

I made an "Excel calculator" to try to compare the PC-Dmis results, but I have a few questions, hopefully you can help me answer them.

I'm attaching a screenshot from the calculator and the TP specification:

TP + Bonus - I added the TP Nominal tolerance plus the Bonus tolerance of the Feature only. Should I also sum the Bonus tolerances of the 2° and 3° Datums?

Thank you for the support.

JacobCheverie over 1 year ago

You don't add the "bonus" gained from the datum feature's departure from its MMB. Instead, you gain "datum shift", allowing the part to move (via translation and/or rotation) through the degrees of freedom that are partially unconstrained. Calculating the shift is an optimization problem in (up to) 3 dimensions and is not a simple calculation. The rules for computing the MMB of the datum features can be found in Y14.5. I'd personally use the Geometric Tolerance command. You likely won't be able to obtain the same results in Excel unless you remove the material modifiers in PC-DMIS.
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dwade over 1 year ago

Picture in your mind a hard gage with MMB gage pins for the datum features and envision how that will shift and rotate within the limits of those gage pins
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