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. 

[[Iv...]]

Thanks for the feedback. Already considered those things, but good to hear them back!

After thinking more about it makes sense to use the GD&T evaluation for the calculation. Since different evaluations will lead to different sizes and, more importantly, different locations, especially for the true position.

But I guess I'm a little annoyed that the number is not automatically included in the report. Or even better, why not have the characteristics report in the proper evaluation since the standard is known!

Now that it's gone, I somewhat miss the pop-up that Calypso used to give when you applied a material condition and a size had not been applied to the feature. It also gave you a notification if a different evaluation was being used. I'm not the biggest fan of the extra material condition option that is now within the GD&T characteristic. I believe it's overly hidden and presents a high risk of not being updated when there is a change to the size. 

[[Ri...]]

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Please remove this comment. Hahahahaha. 

Calypso used to show the diameter that it calculated, but people complained because it added more lines to their report, so they removed it. 

It would be nice as an option to disable. I prefer choice. 

[[Iv...]]

Haha, I should have phrased that differently. The option would be ideal. Something simple, but this sounds more like PiWeb, so it would probably involve coding 😆