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Thread: Appropiate MMB boundary

  1. #1
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    Appropiate MMB boundary

    The ASME Y14.5 - 2018 says some words about choosing the appropiated MMB boundary: "7.11.6 The appropriate MMB for determining the size of the true geometric counterpart for an external datum feature(s) of size is the smallest MMB that can contain the datum feature(s) of size while respecting the datum feature precedence.

    About this I think I couldn't understand what does it means with datum precedence when choosing which geometric tolerance must be take account on MMB calculation. See the image below:

    SmartSelect_20220614-201455_Samsung_Notes_ac1vhj.png

    Focusing only on datum D from the mid big hole. The FCF says datum D MMB must be located or oriented to datum A (perpendicularity in this case). So when choosing which geometric tolerance must be taken account to calculate MMB must I search for geometric tolerances applied to datum feature D which also refeers datum feature D perpendicular to datum A? Is that what the standard calls "features precedence"?

    Another complementary image just for reference:

    Capture_f135a9.png
    Last edited by ivanginato; 06-15-2022 at 06:50 PM.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    With respect to Datum D the only consideration is large hole refinement |Pos|0.3|A|Dmmc|

    Datum A orients the datum D simulator , perpendicular to be exact.

    The straightness at 0.1mmc is a refinement of the above position control thus is not considered.

    The |pos|0.4mmc|A|Bmmc|Cmmc| is considered as there is a datum A specified within the FCF.

    Thus, Virtual Condition = 7.1 + 0.4 (geometric tolerance referenced A) = 7.5

    Variability specified by Datum B and C are not included within the calculation.

    See figure 7-22 as well as 7-23 (LMC) for an expanded clarification.

    In general, the datum precedence specified associates the related geometric tolerance variability to be included in the virtual condition calculation.

    This is a very technical GD&T question typically outside of classic practical application –why are you asking? What are you doing?
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  3. #3
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    Thank you, Kelly.

    I'm reading a book which cover this topic. I actually already had saw the 7-22 and 7-23 figures which gave me an idea of what is the datum precedence. Just to finish, supposing this FCF: |Pos|0.3|A|B|Dmmc| as the image below:

    Screenshot 2022-06-16 001919.png

    Now datum A and B restraing datum Dmmc. In this case, does the considered tolerance must be the first line, since it has the same restriction as FCF?

    Another question, excluding the MMB from datum D, now must all geometric tolerances applied to datum feature D must be taken account according to 7.11.9?

    7.11.9: "... Where RFS is applicable to the tolerances applied to adatum feature and RMB is applicable to the datum featurereference in a feature control frame, there is no fixed-sizetrue geometric counterpart. The true geometric counterpart shall contract to fit an external datum feature andexpand to fit an internal datum feature. For an externalfeature, there is a maximum external boundary that is thecollective effects of size, form, and any applicablegeometric tolerances relative to any higher-precedencedatums."

    Thank you for you time Kelly!
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  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    Quote Originally Posted by ivanginato View Post
    Thank you, Kelly.

    I'm reading a book which cover this topic. I actually already had saw the 7-22 and 7-23 figures which gave me an idea of what is the datum precedence. Just to finish, supposing this FCF: |Pos|0.3|A|B|Dmmc| as the image below:

    Screenshot 2022-06-16 001919.png

    Now datum A and B restraining datum Dmmc. In this case, does the considered tolerance must be the first line, since it has the same restriction as FCF?

    Another question, excluding the MMB from datum D, now must all geometric tolerances applied to datum feature D must be taken account according to 7.11.9?

    7.11.9: "... Where RFS is applicable to the tolerances applied to a datum feature and RMB is applicable to the datum feature reference in a feature control frame, there is no fixed-size true geometric counterpart. The true geometric counterpart shall contract to fit an external datum feature and expand to fit an internal datum feature. For an external feature, there is a maximum external boundary that is the collective effects of size, form, and any applicable geometric tolerances relative to any higher-precedence datums."

    Thank you for you time Kelly!
    No... consider only the modifier specified with respect to the order of precedence specified.

    Use whichever FCF that includes the datums specified even if only partial.
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