Creating a dimension with MMB

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  • Creating a dimension with MMB

    Hi there,

    I need help. I've received a ASME drawing in inches from a client (i usually work on ISO and with mm) and they've demanded that I measure a position with the MMC condition in the hole and for each datums (B and C).
    The thing is that i don't know what values i have to put in the datum MMC size and why the third datum (C) appears like a plane in the repport when actually is a hole?

    What i need to know is what values do i have to put in the datum MMC size and why the third datum have to be a plane for pc-dmis?


    Attached ishttps://www.pcdmisforum.com/image/gif;base64,R0lGODlhAQABAPABAP///wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw== the dimension requirement.

    Thanks in advance.

    MMB position.jpg

  • #2
    To properly compute the MMB size of datum B, there should be a geometric tolerance (probably orientation) controlling datum feature B with respect to primary datum A (at RMB). You must add this dimension in PC-DMIS before the one in question. If there is no tolerance, PC-DMIS will effectively assume an orientation tolerance of 0 at MMC as this represents a mating situation and is the most conservative assumption.

    To properly compute the MMB size of datum C, there should be a geometric tolerance (probably position) controlling datum feature C with respect to primary datum A (at RMB) and secondary datum B (at MMB). You must add this dimension in PC-DMIS before the one in question. If there is no tolerance, PC-DMIS will effectively assume a position tolerance of 0 at MMC for the same reasons as above.

    All you have to enter for the limits of size are those that are seen on the blueprint for each datum feature. MMB size is a combination of feature size limits and geometric tolerances as described above.

    Since your tertiary datum is referenced at MMB, it must be a feature of size and thus cannot be a plane. There is nothing requiring the tertiary datum to be a plane in PC-DMIS software.
    Last edited by JacobCheverie; 02-19-2021, 09:24 AM.

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    • #3
      The (M) for the feature would be .5136 +0.0003/-0.0002, for instance. What features are A B and C. I assume A is a plane, and B and C are holes? You need to know the size and tolerances of B and C in order to apply correct MMB to those features.

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      • #4
        Thanks for your help. bfire85 you're correct. A is a plane and B and C are holes. Attached is the datum B and C information.

        b and c position.jpg
        That's what pc-dmis ask me for.

        pc-dmis dimension.jpg

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        • António Gualdino
          António Gualdino commented
          Editing a comment
          in this case what values exactly do i have to put in? Sorry, i'm from Portugal (english is not my primary language) and i really want to know if i've understood correctly.

      • #5
        Originally posted by António Gualdino View Post
        Thanks for your help. bfire85 you're correct. A is a plane and B and C are holes. Attached is the datum B and C information.

        b and c position.jpg
        That's what pc-dmis ask me for.

        pc-dmis dimension.jpg
        Do not put anything in for the <size>. Go into the other tab, depending on what version you are using it is either Nominals or Advanced...enter the tolerances in there.

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        • #6
          Thanks bfire85 . That really helped me a lot.

          Comment


          • #7

            Just one last question. It's normal that the feature appearence of Datum C to be the one bellow (like it was a flat feature)?


            dimension.jpg

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            • #8
              Originally posted by António Gualdino View Post
              Just one last question. It's normal that the feature appearence of Datum C to be the one bellow (like it was a flat feature)?


              dimension.jpg
              No. Are you sure CYL_C is truly a cylinder? Can you post the code from Cyl_C.

              Note: When naming dimensions, only use numbers, letters, and/or underscores. Spaces, hyphens, and special characters typically don't work well for PC-DMIS.

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              • António Gualdino
                António Gualdino commented
                Editing a comment
                The code is here:

                CYL_C =FEAT/CONTACT/CYLINDER/DEFAULT,CARTESIAN,IN,LEAST_SQR
                THEO/<2.4034,0,0>,<0,0,1>,0.283,0.3
                ACTL/<2.4034,0,0>,<0,0,1>,0.283,0.3
                TARG/<2.4034,0,0>,<0,0,1>
                START ANG=0,END ANG=360
                ANGLE VEC=<1,0,0>
                DIRECTION=CCW
                SHOW FEATURE PARAMETERS=NO
                SHOW CONTACT PARAMETERS=NO

            • #9
              If you want to use the MMB for the datums, you will need to put in the size. The tolerances are added in the Advanced or Nominals tab, as bfire85 mentioned(it's good to tell us what version of PC-DMIS you are using as things change from version to version.)

              I see the tertiary datum look like C does when it's not facing the same direction as the primary and secondary datums, like a side hole.
              PC-DMIS 2016.0 SP8

              Jeff

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              • #10
                Hi Jeff,

                Thank you for your help. Below i show you where are the 3 datums and the relation between them. Notice that A is the plan.

                P.S.: My PC-DMIS versions is 2017 R1.

                part_image.jpg

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                • JacobCheverie
                  JacobCheverie commented
                  Editing a comment
                  If I am not mistaken, PC-DMIS will show a planar zone in the graphic because the tertiary datum only constrains rotation about the axis of datum B whereas datum B constrains translation in two mutually orthogonal directions.

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