Datum Hierarchy Question

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  • Datum Hierarchy Question

    Say you have a basic A B C alignment in a feature control frame. And say your secondary datum B is a line. That line serves as rotation, but MUST it also serve as an origin? Or can the print arbitrarily make C be two origins?

    I'm of the belief that if that B line can serve as an origin, then it must.

    Hope that's clear.
    Last edited by Paperback Rocker; 09-30-2019, 11:50 PM.

  • #2
    C would be your x origin. B would be your y origin. A would be your z origin. Is that what you’re asking?
    Darroll
    2018R2

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    • #3
      Yes, I say B would be an origin.I agree with you. The print designer only rotated with the B line, then put the x and y origin on C.

      A datum is something that is measured from, right? Can it only serve as a rotational feature?

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      • #4
        It’s a rotational feature but you’re measuring from it. Sometimes when I am measuring, I will level to a plane then take a measurement. When I’m done, I will recall my DCC alignment. You can rotate to a feature too and measure. But you still have your original datum alignment.
        Darroll
        2018R2

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        • #5
          Originally posted by Paperback Rocker View Post
          Yes, I say B would be an origin.I agree with you. The print designer only rotated with the B line, then put the x and y origin on C.

          A datum is something that is measured from, right? Can it only serve as a rotational feature?
          The print designer isn't correct. B would be rotational and origin in one axis. The print would need to flip Datums B and C for B to be only rotational.

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        • #6
          What bfire said.

          Although I'm not sure it's written like this anywhere I've always followed the Can-May-Must rule.

          If a datum feature CAN constrain a degree of freedom, and it MAY (i.e. that degree of freedom hasn't already been constrained) then it MUST.

          ACB in your case would achieve this.
          Automettech - Automated Metrology Technology

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          • Peter Fuller
            Peter Fuller commented
            Editing a comment
            +1 This is the rule in most cases.

        • #7
          Thanks for confirming my thoughts, guys! I appreciate the input.

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          • #8
            IF it is called out that "C" is origin for X & Y, then "C" is origin for X & Y.

            As long as it is spelled out on the print, the LEVEL & Z for "A", rotate only for "B" and X&Y origin on "C" is allowed.
            sigpic
            Originally posted by AndersI
            I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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            • NinjaBadger
              NinjaBadger commented
              Editing a comment
              Called out how? In words, or simply that the basic dimensions originate from 'C'?

              I guess we need to ask what standard the print is drawn to (but as he's discussing it with the designer we could assume it's a current standard. If it is current then I thought implied datums were a thing of the past.

          • #9
            Originally posted by Matthew D. Hoedeman View Post
            IF it is called out that "C" is origin for X & Y, then "C" is origin for X & Y.

            As long as it is spelled out on the print, the LEVEL & Z for "A", rotate only for "B" and X&Y origin on "C" is allowed.
            Matt is correct. If the print has the XY origin on C then that is where it is.
            Time for the Trolls to leave.

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            • #10
              I always go in FCF order of precedence. How many degrees of freedom can the first datum constrain? Then it shall constrain them. How many degrees of freedom can the second datum constrain that isn't already used by the first? Then it shall constrain them. The rest of the degrees will be held by the third datum. If a third datum does not exist, then the feature in question should constrain the rest of the degrees of freedom.
              It is wholly dependent on the features that are Datums and how they relate to each other. If Datum A is a cylinder with a vector of X, Datum B is a plane in the vector of Y, and Datum C is a plane in the vector of X, then A would level X and origin in Y and Z (4 degrees constrained) Datum B would rotate only Y about X (1 degree constrained) and Datum C would origin in X. (1 Degree constrained.)
              What about A being a plane with B and C being holes located on plane A? Level and origin to Plane A (3 degree constrained) Datum B is origin only in the 2 direction left (2 degrees constrained) and datum C would rotate only, most likely in line with B or rotated basically.

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              • #11
                Hello
                you are correct. If the Line can be a orgin then it will be. A datum will always lock the degrees of freedom it can lock unless its been locked by previous datum.
                Although, there are modifiers you can apply so the line only locks the rotation and not the orgin (looks like this "><")

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                • #12
                  It is legal to make modified DRFs specifying which degrees of freedom are being constrained. It could look something like this: [A,z,u,v][B,w][C,x,y] That being said, how does the part assemble? Which DRF makes more sense?
                  Last edited by RanDawgg; 10-02-2019, 11:25 AM.

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                  • AndersI
                    AndersI commented
                    Editing a comment
                    Do note that you also have to make the current alignment match the modified DRF - explicitly said in the Help.

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