MMC Bonus Calculation with New GD&T 2025

[[Iv...]]

Please correct me if I’m wrong, and apologies if this has already been discussed.

When using the New GD&T, it appears the MMC bonus is being calculated based on the “suggestion” evaluation used in GD&T, which may not align with the evaluation selected for the diameter size.

For example, I generated a quick output where I left the diameter and true position at their default evaluations: LSQ for the diameter and “suggestion” for true position.

Based on the above, I calculated the MMC tolerance to be 0.2078. However, the report shows 0.1892.

Using CALYPSO’s calculation, the implied diameter would be 3.1592.

Listed Tolerance-Base Tolerance=Bonus

Bonus+Lower Spec=Actual Size

0.1892-0.03=0.1592

0.1592+3.0=3.1592

After reviewing the diameter evaluation and making an educated guess about what the “suggestion” evaluation was using internally, I applied an Outer Tangential evaluation. This resulted in the 3.1592 size that I was hoping for.

It seems the MMC bonus may be tied to the evaluation method used internally by the GD&T characteristic rather than the reported size evaluation itself.

I guess if you make perfect form holes/bosses, you've got nothing to worry about 😁

 

[[Ke...]]

Yes, definitely. If you are evaluating to an ASME print then Position should use Outer Tangential evaluation. Reported size should also use Outer Tangential, unless you are doing a capability study, then LSQ would be appropriate.

[[Ri...]]

 

 

 

You can always change the evaluation parameters inside of the GD&T characteristic. In addition, always ensure you double check the tolerances for the MMC itself. If you copy/paste characteristics and change the feature via typing the MMC values will be incorrect.