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Xact measure confusion

Alex B-shift
First, a little background: I'm using DMIS 2018 R1. The part is basically a sheet metal assembly of 2 layers of tin-plated copper that have been formed, with thin insulation between layers and on the outsides, and has been epoxied and pressed together, before being PEM'ed with M8 PEMS. I have 2 full datum structures on this part, the one on the top surface with a datum A being the flat surface, and datums B and C being 2 thru holes. The D,E,F datums on the lower surface are similar, and the E datum is dimensioned to the B, C datums on the top.



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The odd thing about this part are the position callouts; there is no MMC on the hole dimensions themselves, but there is on BOTH datums, even though the positions for the datums themselves have no MMC callouts. That alone (I think) is improper, and I'm fairly certain that the dimensioning was probably done by a clueless summer intern. But, now
I have to deal with it.

As an example, I'm showing the edit window for the position callout for the "22x" callout in the center of the other pic. All the unPEM'ed holes, whether datums or not, are Ø10mm +.3/-.1. Most of them run around 10.05mm, so my 'bonus tolerance' usually runs about .15 from any datum called out.

First question I have is this: Am I setting it up properly? Up in "Feature Control Frame Editor" window, is it correct that I enter the diameter of the datum at it's MMC value (being 9.9mm)? Or was I supposed to enter something else?

My second question (and the most important) is this: Why isn't the software calculating my position dimension actually using the bonus tolerance from the callout? When I calculate those 22 holes, no bonus tolerance is added, but at the end of the calculation, it does tell you what the bonus from the secondary and tertiary datums are, but it just doesn't use that info in the calculations. Am I missing something in my setup? Is there something in the "advanced" tab that can help me? Or, is this something that only an upgrade to a newer model of DMIS is going to fix?
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  • You shouldn't enter anything in the feature control frame itself for your datum feature MMB size. Ensure the nominal values and tolerances are correct in the advanced tab. Specifying a custom datum feature MMB size is usually not required and, in doing so, you could be overriding the actual MMB size calculated using combined effects of size and any applicable geometric tolerances.



    As said, a material modifier on a datum reference is not the same as a material modifier on a considered feature. (M)/(L) on datum references are MMB/LMB respectively, while (M)/(L) on considered feature references are MMC/LMC respectively. When the material modifier is on the considered feature(s), the size of the position tolerance zone can grow by the measured deviation from MMC/LMC size. When the material modifier is on the datum feature(s), only the MMB/LMB boundary is constraining the applicable degrees of freedom. As the datum feature deviates from MMB/LMB size, the applicable degrees of freedom are only partially constrained and the tolerance zone(s) are now free to float until the measured datum feature makes contact with its MMB/LMB.



    It seems that somewhere along the way, as datum MMC's came into usage, I was wrongly instructed on how to use them. So, just to clarify things, let me ask 1 question for our experts:

    Is having a callout like the one I posted the pic of essentially the same thing as those T.P.s I used to see where the HOLE PATTERN was separately dimensioned, often with MMC?
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  • You shouldn't enter anything in the feature control frame itself for your datum feature MMB size. Ensure the nominal values and tolerances are correct in the advanced tab. Specifying a custom datum feature MMB size is usually not required and, in doing so, you could be overriding the actual MMB size calculated using combined effects of size and any applicable geometric tolerances.



    As said, a material modifier on a datum reference is not the same as a material modifier on a considered feature. (M)/(L) on datum references are MMB/LMB respectively, while (M)/(L) on considered feature references are MMC/LMC respectively. When the material modifier is on the considered feature(s), the size of the position tolerance zone can grow by the measured deviation from MMC/LMC size. When the material modifier is on the datum feature(s), only the MMB/LMB boundary is constraining the applicable degrees of freedom. As the datum feature deviates from MMB/LMB size, the applicable degrees of freedom are only partially constrained and the tolerance zone(s) are now free to float until the measured datum feature makes contact with its MMB/LMB.



    It seems that somewhere along the way, as datum MMC's came into usage, I was wrongly instructed on how to use them. So, just to clarify things, let me ask 1 question for our experts:

    Is having a callout like the one I posted the pic of essentially the same thing as those T.P.s I used to see where the HOLE PATTERN was separately dimensioned, often with MMC?
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  • in reply to Alex B-shift
    I'm not sure what you used to see, perhaps you could share a more precise example? Each hole in the pattern was assigned the same FCF?

    I'd suggest you consult the Y14.5 standard, either 2009 or 2018 edition, to study the difference between MMB/LMB and MMC/LMC. Pre-2009 everything was MMC/LMC, even though the intent was different. Y14.5-2009 introduced the terminology MMB/LMB to emphasize the difference.
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