MMB Calculation - Special Case 2

[chain868]

Hi Everyone,

I searched a lot, but wasn't able to find a satisfying answer to 2 specific question regarding Maximum Material Boundary calculation. Maybe I'm missing something simple that someone would be able to point out.

First Question:
Let's say I have a case like the one in the attached picture. I think we can all agree about the various MMB values datum d would take for the first 3 DRF case shown. But what would happen in case 4? (If we were to remove the modifier from datum B) Does it matter? Would the value of datum d MMB be different from case 3?

Second Question:
Let's say we stick with case 3 for this one (DRF A|Bm|Dm). What would happen if the requirements on datum d would change to what is shown? Which position requirement should be used to calculate the MMB of datum D in this case:

Should we use the 0.5 requirement because the datums preceding datum d in case 3 are A|Bm and we absolutely need to calculate the MMB with a requirement that use the exact same alignment?

Or should we use the 0.4 requirement because it is more restrictive, even if the alignment is not exactly the same as the datum preceding datum d in case 3?

Thanks for your help!

 

[Belanger]

For now I'll tackle Question 1... the datum shift amount (and the MMB value) would be the same in Case 3 and Case 4, but with Case 4 that amount would only contribute in a rotational direction (the part pivots around datum B, which is RMB).
In Case 3 the datum shift around D might allow it to slop around in many directions (rotation and translation of the part), but that depends on how much shift exists around datum B.

In the 2009 standard, pgs. 60-61 (or in 2018, pgs. 74-75) mention that the MMB is determined from the collective effects of the datum feature's MMC together with any applicable geometric tolerances from the higher-precedence datum(s). So both cases would yield an MMB of 15.6. The MMB modifier on datum B (or lack thereof) just tells us how to handle any looseness around that datum, but it still counts as a higher-precedence datum even if the modifier doesn't precisely match.

 

[chain868]

Thanks for the quick answer!

It makes sense that in case 4, datum B would only allow rotation. But there is still something about this case that feel wrong; let me explain:

If we think about a tooling that would align on DRF A|B|Dm (like in case 4), it would look a bit like what we can see on the top left section of the attached picture; with a simulator for datum D with a MMB Ø of 15.6, perfectly located from self-centering datum B.

Then, we can imagine a real part with some deviation as shown on the bottom left section of the picture. (And assuming all the other features being perfect)

Such a part would pass all control as shown on the top right section of the image. The datum D is deviated away from datum B by 0.4; but still pass the position control of Ø0.5m|A|Bm|Cm because of the MMC and MMB bonus.

But as shown on the bottom right section of the picture, the tooling would not be able to fit on this part.


I was under the assumption that the MMB should be allow all possible variation of the datum, relative to it's preceding datums in the DRF; and in cases like this one, it doesn't work.

What am I missing?

Thank again!

 

[3DDave]

I'm only half following this but it looks like a case where a datum feature is defined in one DRF that is very different from how it is used in another one. There is a reason the Y14.5 standard has skipped around this question, dealing with it only once and getting the wrong answer because they expect that addition and subtraction is all that is required.

In this case it means that whoever would use that specification would be rejecting parts for the case you demonstrated. One DRF says the size variation of the feature can relax the constraint, the other says the size cannot. It's a contradictory use of the same feature so it will reject parts as described.

The gage makers wanted the tertiary feature of the gage to be assumed to be a slot, even though this can be an explicitly defined requirement. The rest of the committee, instead of using the explicit requirement, created the "datum translation symbol".

 

[Belanger]

Those are good points, and I'll have to reassess those numbers based on the sample data you've given.

One key statement that I might question is: "I was under the assumption that the MMB should be allow all possible variation of the datum, relative to its preceding datums in the DRF."
I would say that the MMB modifier on D in Case 4 doesn't permit all the variation of that datum feature relative to the higher-precedence datum B. There is datum shift from datum B felt when measuring datum feature D. Yet that shift doesn't carry over to Case 4, because B is now referenced RMB.
This is kind of what 3DDave is saying; datum precedence is maintained but the modifiers are jacked around. Somewhat confusing, but yours makes for a good academic discussion.

 

[Burunduk]

Such a part would pass all control as shown on the top right section of the image. The datum D is deviated away from datum B by 0.4; but still pass the position control of Ø0.5m|A|Bm|Cm because of the MMC and MMB bonus.

A minor correction to this: the part would pass the position control because of the additional variation allowed by the MMB modifier, but not because of a bonus tolerance from the MMC modifier on the tolerance. That's because with the diameter produced at 15.1 the feature is AT MMC - no bonus available.

But as shown on the bottom right section of the picture, the tooling would not be able to fit on this part.


I was under the assumption that the MMB should be allow all possible variation of the datum, relative to it's preceding datums in the DRF; and in cases like this one, it doesn't work.

Since controlling a feature with reference to a datum feature modified at MMB permits additional variation and is less restrictive than control with reference to the same datum feature at RMB, it only makes sense that the tooling for |A|B|D(M)| will not allow all possible variations of datum feature D relative to |A|B(M)|...|.
You could control datum feature D relative to a more restrictive DRF than the higher precedence datums of the FCF where it is referenced; for example if datum feature D is referenced tertiary at |A|B(M)|D(M)|, there would be no issues if you controlled its position with reference to |A|B|C(M)|. Functionally though, it usually makes sense to use the same datum reference order and modifiers both at the qualification of the datum feature and at the control that references it.

 

[chain868]

First, thank you all for the answers!

It all make sense and agree the details you pointed out. But still, my initial interrogation remains.

I realized that the example I have used is mostly theoretical and it is hard to answer such questions.

So let me try rephrase the exact same interrogation in a simpler and more practical example;

In this new example, what should be the MMB of datum D highlighted in yellow?
or
How much float would that (M) modifier give to datum D?
or
What would a tooling that control the 4 hole pattern look like?

Thanks again!

 

[AndrewTT]

ASME Y14.5-2009 4.11.6.3 - Clarifying Applicable MMB
In cases where the boundary is not clear, or another boundary is desired, the value of the boundary SHALL be stated, enclosed in brackets, following the applicable datum feature reference and any modifier in the feature control frame.

I think it is fair to say that the MMB value of Dm, in the latest example part provided, is unclear. Therefore, the designer of the part (who should know which value of MMB results in proper function of the part, and if doesn't know should really get to work on finding that out...) has to clarify the actual value in the FCF.

For the latest example part given, relying on the reader of the print to calculate the proper MMB value is asking for trouble. So the question is not, "in this instance, how does one calculate the theoretical value of the MMB?". The question is, "in this instance, should the designer clarify the MMB value?". I would say yes, clarify it.