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Thread: MMB Calculation - Special Case

  1. #1
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    MMB Calculation - Special Case

    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!
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  2. #2
    Administrator Kelly Bramble's Avatar
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    Where did this image and question come from ?
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    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!
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  4. #4
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by chainecedric View Post
    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!
    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.

    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?
    Who knows – without fit, form and function defined relative to the target mating parts or assembly changing tolerances or DRF is academic.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    Thanks for the answer!

    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 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!
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  6. #6
    Administrator Kelly Bramble's Avatar
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    Datums defined @ MMB should be tooling simulated at virtual condition.
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