10 Replies Latest reply on May 2, 2018 10:55 AM by Tran Van Quang

    Effect of datum precedence

    Tran Van Quang

      Hello every one,

      I’m sorry for posting this question again.

      I don’t undertsand about the precedence of reference datum in this drawing.

      I asked my colleague and he just say it obeys principle of MMC.

      I wonder without  regarding to MMC, is there any diffrence if I change the order of reference datum  in tolerance frame.(X,A,C to A,C,X)

       

      Snap3.jpg

      Snap4.jpg

      Snap5.jpg

        • Re: Effect of datum precedence
          Brian Brazeau

          My Answer would be no since this is a 3D profile tolerance and therefore applies to the entire surface with respect to the 3 datums that completely lock down the part in 3D space. Having said that I don't see where the datums are defined in this drawing. Usually they look something like this when given off a planar face or an axis.

            • Re: Effect of datum precedence
              Tran Van Quang

              @Brian Brazeau, thank you. The datum are define in sections in the lower part of that picture. I read ASME Y14.5 M-1994, it seem have difference for order of datum, I just only don't understand how to apply that difference into manufacturing. For example, in your picture can I change order of A and B datum without affect of the

              quality of product?

              Snap4.jpg

              Snap5.jpg

                • Re: Effect of datum precedence
                  Brian Brazeau

                  Do you mean this as definition of Datum A for example? If so, I'm not sure that the datums ABC properly set up your part.

                  In the example above the the geometric symbol is True Position not 3d Profile. It is also a lathed part which allows setting up datums different than prismatic part.

                    • Re: Effect of datum precedence
                      Tran Van Quang

                      Snap6.jpg

                      Those are defined as A, B, C datum. I am concerned about frame reference. In the original picture, with the profile tolerance the datum precedence is X,A, C  and for position tolerance is A,C, X. I wonder the frame reference which are setup in those cases are same or different.

                        • Re: Effect of datum precedence
                          Brian Brazeau

                          I suppose they may be different in this case, but I'm not sure the true position is even used correctly. Wouldn't the true position be more useful attached to one of the 2 diameters associated with the slot/plane that datum B is attached to? 3mm is just a linear dimension. Is this a case where you are trying to define the location of the entire slot? If so (see below)

                          Slotted Features:

                          Another common way true position can be called out is with slotted features. If you have a slot in your part that must always be located correctly, you can use true position to ensure that each of the planes that make up the slot are always located in the correct position. Symmetry can also be used in this case – but only if the slots have a referenced datum plane that they are symmetrical about (and measuring symmetry is very difficult!).

                          taken from.   True Position | GD&T Basics