Datum Features

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  • Datum Features

    Good Afternoon

    I am going over a part that has a B-C in a feature control frame for a true position. I have not seen this before. Does this mean both datums must be used? They are 2 seperated ID's one is B and C respectively.
    Since I gave up hope I feel much better !!!

  • #2
    No clue what your part looks like..... Think of a Shaft with a bearing surface on both ends. 1 is B and 1 is C. So in effect you have a center point from a Ø on each end. This would make line/ DATUM structure B-C. This may not apply to your part without seeing the print.


    • #3
      That means "Datum -B- THRU Datum -C-".

      This type of callout can only be used if the two datums are coaxial to one another.

      Design intent means to program it such that it would simulate chucking into both of those IDS at the same time and then spinning the part on that created axis and then checking applicable gdt back to that axis.

      What you should do:

      If using Legacy Dimensions:
      -Probe -B- as a cylinder
      -Probe -C- as a cylinder
      -Create start point and end point in each of the cylinders (4 points total)
      -Construct a 3D axis through those 4 points
      -Use the axis in your alignment (leveling as well as origin for two of the three axees)
      -Recall that alignment and then measure applicable GDT back to it

      If using Xact or GeoTol Dimensions:
      -Probe -B- as a cylinder
      -Probe -C- as a cylinder
      -Define -B- and -C- as datums using the DATDEF command
      -Create the feature control frame as shown on your print. PC DMIS will allow you to make the "|B-C|" as opposed to "|B|C|" (which is very different)

      Hope this helped. If you need more assistance check back & am sure someone will hop in
      Last edited by DAN_M; 05-26-2022, 02:15 PM.


      • JacobCheverie
        JacobCheverie commented
        Editing a comment
        For coaxial datum features B and C, I don't believe |B|C| is even valid - Datum feature C would not constrain any DoF.

    • #4
      JacobCheverie response to 3.1....

      Agreed! |B|C| would be invalid in this situation


      • Schlag
        Schlag commented
        Editing a comment
        So in theory here...... If this was a turned shaft and ALL we are talking about was the " coaxial " portion here of different Ø's back to B-C..... We don't really need to lock in the last DOF ? Would it be even needed. YOu could be " linear " from the end of the part to where the Ø's start and stop ( they may not be critical ?

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