mixing MMC and LMB
mixing MMC and LMB
(OP)
I'm looking for a little help with determining the appropriate datum feature simulator sizes when the datum modifier does not match the material condition modifier which appears in the FCF of the datum. If you look at Fig 4-16 in ASME Y14.5-2009 I see that the datum feature material boundary for D is 7.3 (option b). This is the maximum feature size (7.1) plus the perp tol (.2). This makes sense to me as the perp tol applies at MMC per the callout for datum D.
What if instead the perp tolerance had applied at LMC and option b still referred to D at MMB? Would the MMB still be 7.3 OR would you need to consider the maximum envelope which the perp tolerance at LMC would allow. I'd think that would be 7.5 (LMC zone would be 6.7 and a 7.1 dia could be line to line with that to give a 3.75 radius * 2 = 7.5).
Can anyone chime in and confirm which line of thinking is correct? The standard appears to assume the datum modifier matches datum feature material modifier. Perhaps it's silly to ever call out the datums and features with mixed modifiers but this is what I have.
Thanks
What if instead the perp tolerance had applied at LMC and option b still referred to D at MMB? Would the MMB still be 7.3 OR would you need to consider the maximum envelope which the perp tolerance at LMC would allow. I'd think that would be 7.5 (LMC zone would be 6.7 and a 7.1 dia could be line to line with that to give a 3.75 radius * 2 = 7.5).
Can anyone chime in and confirm which line of thinking is correct? The standard appears to assume the datum modifier matches datum feature material modifier. Perhaps it's silly to ever call out the datums and features with mixed modifiers but this is what I have.
Thanks





RE: mixing MMC and LMB
But I guess we can still pursue the question in a theoretical sense. If the perpendicularity tolerance is changed to the LMC modifier, but MMB modifier remains on the position callout (option "b"), then it means for the position callout that we must still simulate the datum as if it were at its MMB, which would now be a worst case of 7.1 + 0.2 stated perp tol + 0.2 bonus tol = 7.5 mm (which is what you came up with).
That being said, this doesn't mean we can trot out a fixed-size gage, because that 7.5 is not a constant boundary, but a worst case for only a pin of size 7.1. (The MMB for a pin of diameter 7.0 would be 7.3 mm.)
Let's see if other agree with my rambling here... And, depending on how deep this discussion goes, we might also want to look at Figs. 2-16 and toss in some terminology about the boundary conditions (inner/outer locus). Ugh.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: mixing MMC and LMB
However like you said this might be solely an academic discussion since it might not make sense. I guess I'd like to know if the standard implies that it is flat out wrong to do this, or if it allows for it and it's a bit of an awkward inspection.
RE: mixing MMC and LMB
When an LMC modifier is on an OD like this example, the only constant value is the inner boundary, often called the virtual condition (also see Fig. 2-16). The outer boundary, which is what the MMB concept is seeking for a datum, is not a constant boundary -- notice that Fig. 2-16 has blanks in the last column except for the resultant condition, which is akin to the value of 7.5 in our discussion.
I don't think the standard has a dogmatic stance on the general question you propose; I'd lean toward your last statement of it being an unusual and awkward thing.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: mixing MMC and LMB
John-Paul, I agree with you that the outer boundary (resultant condition) of the pin is not constant. But the maximum material boundary, the volume in which there could be material, is 7.5. My gut says that the simulator should therefore be a fixed size of 7.5. I understand that the simulator could accept a datum feature that isn't to spec, but I don't think that matters. The simulator's purpose is not to verify the tolerances on the datum feature - that would be done separately. The simulator just needs to fit over any datum feature that is in spec, and therefore needs to be sized at 7.5.
What do you guys think?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: mixing MMC and LMB
My hesitation is that I don't know if the intent is that the MMB symbol be interpreted as the worst-case MMB or the actual MMB for each part made.
IOW, although the datum simulator isn't meant to check the datum itself, how can we know if the datum shift that appears is really allowable?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: mixing MMC and LMB
I just read through sections 4.11.5 through 4.11.6.3. It all points to a worst-case MMB, as the collective effects of MMC and geometric tolerances. I didn't find anything that hints at the idea of an actual MMB. This would be very difficult to accomplish physically anyway - the simulator would have to somehow sense the UAME size of the datum feature and then offset out by a calculated amount.
Again, the fixed size 7.5 diameter simulator may not detect an out-of-spec datum feature placed in it. But this can also occur in other contexts. If a part has a datum feature whose size is past its LMC limit, it will still fit on the simulator and allow unrealistic shifts. The part would be flagged as nonconforming in the separate step of verifying that the datum feature conformed to its tolerances.
This leads to the contentious question of what should be reported for a geometric tolerance, when one (or more) of the datum features is out of spec. In other words, is it meaningful to report a value for a Position tolerance if one of the datum features is oversized? Or undersized?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: mixing MMC and LMB
Some other thoughts:
- Although Y14.5 does not specifically show and describe such kind of mix, I would not say it is clearly forbidden.
- Agree with J-P that outer boundary is not a constant value, however I am rather leaning towards Evan's opinion that presence of (M) modifier causes the datum feature simulator D for option b) to be fixed at 7.5 size. Unfortunately standard is not clarifying the definition of MMB in cases like we have here, therefore my opinion is just based on my personal intuition.
- Y14.5-2009 introduced quite powerful and useful possibility of clearly stating what should be the size of datum feature simulator for a geometrical tolerance if its size is not clear. Even if we find an agreement here on size of datum feature simulator D for option b) we will not be sure that other readers of this particular drawing will get into the same conclusion. Therefore we can use a method defined in 4.11.6.3 and give a precise information about a size of simulator.
- Maybe a little bit off topic, but what really intrigues me is a statement at the beginning of 4.11.6.3: "in cases where[...] other boundary is desired". I am wondering if for instance [dia. 7.6] could be specified instead of (M) for datum D in positional FCF. So the value which is greater than any MMB of datum feature D shown in fig. 4-16. This concept IMO would make sense for some functional reasons, and I'd like to know if you see any problems with that.
RE: mixing MMC and LMB
It's crazy how we were discussing that figure on page 61 all this time, yet I didn't bother to read the paragraph that you mentioned, pmarc!
And yes, I suppose that this alternate method allows a datum simulator size that even exceeds the worst-case MMB.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: mixing MMC and LMB
To me, the meaning described in 4.11.6 and 4.11.6.1 is clear and the simulator should be sized at 7.5. But I agree that others may not draw the same conclusion, and explicitly specifying the desired boundary size wouldn't be a bad idea.
I don't see a problem with specifying a boundary size that is larger than any MMB. There could be functional reasons for doing that.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: mixing MMC and LMB
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