True Position with MMC Not Applying Datum Bonus

[[Ka...]]

Hello everyone,
I’m having an issue with my Zeiss Contura G2 when evaluating true position with MMC, and I’m hoping someone can help me understand what I might be missing.

The drawing calls out true position with MMC. In Calypso, I applied MMC to the datums in the true position characteristic. However, when I generate the report, it looks like the CMM is only calculating the MMC bonus from the measured feature itself, and not from the primary or tertiary datums.

I’ve attached screenshots for reference.
Does anyone know why this is happening or what setting I may be overlooking?

Thanks in advance for any help.

 

[[To...]]

MMC on datum features does not give you additional tolerance on the position.  On datum features, MMC is really MMB and only allows for the datum feature to shift in order to help it fit, like jiggling a part onto hard fixture.  

[[Pa...]]

Tom is absolutely correct.  In addition, where the requirement is simultaneous for multiple features, MMB provides datum shift but all features use the same shift (magnitude and direction).  This fits nicely with Tom's "jiggling the fixture" comment.  

Unlike MMC, MMB does not produce a number that can just be "added" to the position tolerance.

[[Cl...]]

Tom. So, when the circled M is next to a datum it can only be MMB? There aren't

two symbols that differentiate the two, but rather where they are placed? Is that correct?

[[Ka...]]

Tom, so you’re saying that in my case the cylinder diameter Ø.120-.001 is a datum feature at MMB and therefore does not provide any additional bonus tolerance. That’s honestly the first time I’m hearing this.

However, if you look at my screenshots, the issue I’m running into is that the CMM is only calculating bonus from the datum feature. It is not applying any bonus from the primary or tertiary datums. At the same time, the CMM is calculating feature MMC, even though you’re saying the datum feature should be treated as MMB.

 

That mismatch is really the core of my confusion.

[[To...]]

Referring to the position window, the Feature uses the feature's tolerance to calculate the max material condition, and the difference from the actual to the mcc is added to the position tolerance.  The Datum Features do not.  They shift and nothing on the report shows you what it did.

 

 

Edited Tuesday at 02:55 PM

[[Ka...]]

Hey Tom,

I did a bit of research on MMB and also spoke with our engineering team—interestingly, they hadn’t even heard of it.

i will have to educate myself and our team a bit better about this .

 

I’m still a bit stuck though. As you can see in picture #3 from my original post, for the true position callout I set the CMM to use MMC on  features, and both datums were also set to MMC.

Shouldn’t the report still show the additional bonus tolerance if everything is set to MMC? Or does the CMM automatically recognize this as MMB, which is why it’s not adding the extra bonus tolerance even though MMC is selected?

 

[[Ze...]]

Calypso has the incorrect terminology for datum modifiers in the dialogue box. It's always been shown as MMC, but the correct term is MMB when applied to a datum feature.

MMC = Maximum Material Condition and is applied to the considered feature of size in the FCF.

MMB = Maximum Material Boundary and is applied to the datum/datums in the FCF. MMB provides no "bonus" to the feature itself. It applies mobility/shift to the datums depending on their size relative to their limits. This is not reported in Calypso. The calculations are not a direct tolerance that can be applied to the feature itself.

[[Pa...]]

MMB was first disambiguated from MMC in ASME Y14.5-2009.  The engineers at your company are not the only ones who haven't quite found time to read it yet.  I've read the entire spec many, many times now, and learn something new every time.  There are newer versions.  This is the document that defines GD&T.  It is the only source (unless you're an ISO shop; I'm less familiar) for this information.  I'm not looking at 2009 now but rather at 2018.  Here are pertinent screenshots, but there are more than 300 pages in the spec rich with more info.  I'd start your research here.