Question about compound datums?

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  • Question about compound datums?

    I have a datum system on a drawing I have questions about.

    I can’t post the drawing so I have made a simple example part with the information that is on the drawing.

    The only flat planer surfaces on the part are the bottom that they have marked datum A, and the end marked datum D.

    I am using a model and will align to the model with an iterative alignment using 4 points on datum A for level, 2 points, one in the area of datum B and one in the area of datum C to rotate to, and a point on datum D for origin.

    I’m confused about why they are doing it this way. They show an area 1” in from each end with a phantom line offset from the part for the B and C datum’s. (Wouldn’t points on both surfaces offset to create a plane and call it “Datum B” be the same thing?

    But anyway, I can’t deviate from the drawing.

    It’s my understanding that a constructed feature set is not good for a datum surface. So my plan is to create a plane with offset points on one plane for B, and one plane for C.
    Then I construct a plane with points from plane B and plane C and call it plane B-C.

    Will that work? As I said I’m unsure why they are doing it this way. But I have to deal with it.

    My other question is about creating the B-C datum in exact measure. It won’t let me name a datum B-C; only BC. Is there a way to get the dash in there to keep from having drama?

    I’m using PC-DMIS CAD++ 2017 R1 and programming to a solid model.COMPOUND DATUM 1.jpg
    2015.0 SP1 CAD++, Catia Translator
    B&S Xcel, Renishaw PH10MQ

  • #2
    It looks to me like they want the diameters at the ends to be datums B and C, although I'm not sure they are called out correctly on the drawing to represent that... I would construct a cylinder from 2 circles at depths of ~.05 and ~1.00 and call those datums B and C. Then construct another cylinder from all 4 circles and call this datum BC.

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    • DmisDave
      DmisDave commented
      Editing a comment
      There are no diameters on that part. The areas at the ends of the part are non planar surfaces.

  • #3
    Points on both surfaces can't create a plane as the 1" area from each end is not a flat surface. Each end has an angled surface included. You could treat a set of all of the points you measure on the B and C areas as Datum "B-C".

    D might even be redundant, if the angled section of B and C is along the length of the part, then technically B-C constrains translation and orientation.
    "This is my word... and as such is beyond contestation."

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    • #4
      You can add the dash between B and C when you're creating the dimension in Xact measure. Once you've selected B from the drop down then type C.
      Kevin

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      • #5
        Originally posted by DmisDave View Post
        My other question is about creating the B-C datum in exact measure. It won’t let me name a datum B-C; only BC. Is there a way to get the dash in there to keep from having drama?
        You can't name a single datum "B-C". The "-" signals to PC-DMIS to calculate a compound datum from datums "B" and "C". This kind of construct is only allowed in some special places in PC-DMIS (at least historically). I know it works in position and runout, I haven't tried to use it on form...

        AndersI
        SW support - Hexagon Metrology Nordic AB

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