Constructing a Circle from Half an Octagon

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  • Constructing a Circle from Half an Octagon

    I have a part has an half octagonal shaped cutout on an outside surface. So my part has 4 surfaces of the octagon and the rest of the octagon is open. I need to calculate the Maximum Inscribed circle for this octagon-half. I have measured each of the 4 sides with the vision system on my CMM. I then created a constructed circle. I typed in each feature in like this: "feature_name.hits[1..feature_name.numhits]" and selected the Maximum Inscribed circle option. This then creates a circle that only touches the two outermost points that is far too large to fit inside the octagon. How should I go about measuring this feature. As a note I also get an error message saying that I have measured less that 180 degrees of the circle. This is potentially true depending on how large the circle is and where it touches the octagon. Is there a way to force PC-DMIS to touch 3 parts on the sides of the octagon?

    I appreciate any insight! Let me know if my description isn't clear enough.

  • #2
    Inscribed/circumscribed really only work if you have more than 180 deg. of arc to work with.

    Circumscribed, for example, the farthest distance between any two points in your data set becomes your diameter (I hope I explained that clearly) if you have less than 180 deg.

    Have you tried construct circle/3 tangent lines?

    Would mirroring the four sides to create a total of eight work, maybe?
    Last edited by TheyCallMeWoody; 05-18-2015, 03:47 PM.

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    • #3
      Maybe this

      Create auto lines for the sides of the octagon and the outside surface of the part (here should be 6 lines). Construct intersect point between two neighboring lines (there should be 5 points). Construct mid point between two neighboring intersect points (there should be 4 points). Construct a circle from 4 mid points. This should be the inscribed circle created from 4 side of the octagon.

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      • #4
        Originally posted by Nano Vujkovic View Post
        Maybe this

        Create auto lines for the sides of the octagon and the outside surface of the part (here should be 6 lines). Construct intersect point between two neighboring lines (there should be 5 points). Construct mid point between two neighboring intersect points (there should be 4 points). Construct a circle from 4 mid points. This should be the inscribed circle created from 4 side of the octagon.
        This.

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        • #5
          OR, do a local alignment on the octagon, as in rotate to it, so one edge is square, then do a 4-hit auto-circle that hits the centers of the flats.
          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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          • #6
            Originally posted by Nano Vujkovic View Post
            Maybe this

            Create auto lines for the sides of the octagon and the outside surface of the part (here should be 6 lines). Construct intersect point between two neighboring lines (there should be 5 points). Construct mid point between two neighboring intersect points (there should be 4 points). Construct a circle from 4 mid points. This should be the inscribed circle created from 4 side of the octagon.
            This is really smart! Thank you very much for the help!

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            • #7
              Originally posted by XlarXlar View Post
              This is really smart! Thank you very much for the help!
              Thanks, I'm glad i could help. If you have any other questions don't hesitate to ask.

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