# MMB Calculation - Special Case

• 02-05-2021, 12:22 PM
chainecedric
MMB Calculation - Special Case
[COLOR=#000000][FONT=Tahoma]Hi Everyone,[/FONT][/COLOR]

[COLOR=#000000][FONT=Tahoma]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.[/FONT][/COLOR]

[U]First Question:[/U]
[COLOR=#000000][FONT=Tahoma]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?[/FONT][/COLOR]

[U]Second Question:[/U]
[COLOR=#000000][FONT=Tahoma]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:[/FONT][/COLOR]

[COLOR=#000000][FONT=Tahoma]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?[/FONT][/COLOR]

[COLOR=#000000][FONT=Tahoma]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?[/FONT][/COLOR]

• 02-05-2021, 02:54 PM
Kelly Bramble
Where did this image and question come from ?
• 02-05-2021, 03:15 PM
chainecedric
The image is just a quick sketch I made with powerpoint.

To explain where the question come from, we need to think about a tooling that would align on DRF A|B|Dm (like in case 4). This tooling would look a bit like what we can see on the top left section of the attached picture in this reply; Assuming that the simulator for datum D MMB would be 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 left section of the picture, the tooling would not be able to fit on this part.

I was under the assumption that the MMB should 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!
• 02-05-2021, 05:17 PM
Kelly Bramble
[QUOTE=chainecedric;17099][COLOR=#000000][FONT=Tahoma]Hi Everyone,[/FONT][/COLOR]

[COLOR=#000000][FONT=Tahoma]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.[/FONT][/COLOR]

[U]First Question:[/U]
[COLOR=#000000][FONT=Tahoma]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?[/FONT][/COLOR]

[U]Second Question:[/U]
[COLOR=#000000][FONT=Tahoma]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:[/FONT][/COLOR]

[COLOR=#000000][FONT=Tahoma]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?[/FONT][/COLOR]

[COLOR=#000000][FONT=Tahoma]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?[/FONT][/COLOR]

The graphics shown look like a training class example as it is unlikely one would debate the concept as a variable for a design application.

I’m going to give short answers – that’s it as I think this is training.

“First question” Datum B is simulated at RFS – nothing special here.

“Second question” Nothing.. The datum D feature is oriented and located twice (silly requirement) and then tightened up with an orientation to Datum A.

[quote]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?[/quote]

Who knows – without fit, form and function defined relative to the target mating parts or assembly changing tolerances or DRF is academic.
• 02-08-2021, 04:47 PM
chainecedric

I agree with you that the examples look like a training class, but I assure you they are not.

I made this example to support my question as I'm trying to better understand how MMB should be calculated; But I now realize that the example is mostly theoretical and that it's hard to understand why I'm asking theses questions..

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

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!
• 02-08-2021, 05:51 PM
Kelly Bramble
Datums defined @ MMB should be tooling simulated at virtual condition.