Iterative Alignment Questions

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  • Iterative Alignment Questions

    I have a question about PC-DMIS’s preferred method for performing an iterative alignment using manually measured features WITHOUT allowing a DCC re-measure the features.

    The part is a simple tube, with a welded flange on either end, and multiple bends between the two. There is no CAD available (it shouldn’t make a raaatsass bit of difference).

    I've got three manually measured features:
    Two circles, each projected onto its own flange plane, and one intersection of two cylinders. Two of these features do not lie on the “common” plane.
    This gives me three points with which to project to a common plane using print nominals, one of which already lies on the theoretical nominal plane.

    Problem: What is the proper method of projecting these constructed features to a common plane for leveling ?

    I used to do this sort of every day many years ago, like it was no big deal (which it's not), and I could use as many of these features to create an iterative alignment as I wanted to.

    The basic for an iterative alignment is to create the "common" datum plane, projected from the features….repeatedly until you get nearly a perfect “Z” (in this case) nominal returned for each feature. After that, a simple iterative rotation and all would be well.

    Iterative alignments using PC-DMIS with CAD is easy.

    PC-DMIS can certainly do what I want, but how ?
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  • #2
    Any ideas please ?
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    • #3
      So...........

      PC-DMIS isn't capable of such a thing ?
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      • #4
        I think Construct/Offset plane is what you are looking for.
        --Brian

        "The best way to predict the future is invent it. This includes your very next action."

        Support: Hexagon Metrology Support Center
        Training: Hexagon Metrology University

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        • #5
          Originally posted by Brian Gudauskas View Post
          I think Construct/Offset plane is what you are looking for.
          Thanks Brian. That's exactly what I needed. I've never used that PC-DMIS function before.
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          • #6
            .......although it's no longer an iterative alignment..............
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            • #7
              Can be, but doesn't sound like it needs to be. I've seen a lot of iterative alignments programmed simply because they were easy, but in reality not necessary.

              Either way will work, enjoy.

              Originally posted by Robert Masters View Post
              .......although it's no longer an iterative alignment..............
              --Brian

              "The best way to predict the future is invent it. This includes your very next action."

              Support: Hexagon Metrology Support Center
              Training: Hexagon Metrology University

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              • #8
                Well that's the plane - now what features control the other degrees of freedom?

                I only use iterative for a "complete package" alignment to lock down all 6 degrees of freedom.
                _ _ _
                It's all pattern recognition.

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                • #9
                  Iterative "in general" is used to perform an alignment where the features in the alignment are not perpendicular to each other. Because Robert is using constructions based on 3D features that can create perpedicular features, and will measure the same each time they are iterated, he can do a normal 321 from constructions (99% sure based on his description). However, there are many other uses for iterative that are totally valid, just maybe not absolutely necessary. Doesn't matter if it works, and it's accurate.
                  --Brian

                  "The best way to predict the future is invent it. This includes your very next action."

                  Support: Hexagon Metrology Support Center
                  Training: Hexagon Metrology University

                  Comment


                  • #10
                    [QUOTE=Robert Masters;246087]I have a question about PC-DMIS’s preferred method for performing an iterative alignment using manually measured features WITHOUT allowing a DCC re-measure the features.
                    (I've never done this before. All iterative alignments I've used were to pick up the part with the manual measurements, then use DCC to bring that geometry to the "best fit" of where it "should" be.)

                    The part is a simple tube, with a welded flange on either end, and multiple bends between the two. There is no CAD available (it shouldn’t make a raaatsass bit of difference).

                    I've got three manually measured features:
                    Two circles, each projected onto its own flange plane, and one intersection of two cylinders. Two of these features do not lie on the “common” plane.
                    This gives me three points with which to project to a common plane using print nominals, one of which already lies on the theoretical nominal plane.
                    (Intersection of two cylinders is an ellipse, unless they are perfectly planar...is this really what you want to use?)

                    Problem: What is the proper method of projecting these constructed features to a common plane for leveling ?

                    I used to do this sort of every day many years ago, like it was no big deal (which it's not), and I could use as many of these features to create an iterative alignment as I wanted to.

                    The basic for an iterative alignment is to create the "common" datum plane, projected from the features….repeatedly until you get nearly a perfect “Z” (in this case) nominal returned for each feature. After that, a simple iterative rotation and all would be well.

                    Iterative alignments using PC-DMIS with CAD is easy.
                    (I've always created a generic plane with the Z value [or other significant axis] specified, then project those features on that plane. Is this what you're trying to accomplish?)

                    PC-DMIS can certainly do what I want, but how ?QUOTE]

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                    • #11
                      Originally posted by FutureMan View Post
                      I've always created a generic plane with the Z value [or other significant axis] specified, then project those features on that plane. Is this what you're trying to accomplish?
                      From what I take of this, wouldn't this generic plane have to have some relevance to the true part coordinate system to work, or am I reading it wrong ?

                      I say this because unless this generic plane is relevant to the part coordinate system, and the correct working plane relative to the feature axes you want to project, then projecting points onto just any old plane wouldn't accomplish anything.
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