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Finding the center of a radius

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  • Finding the center of a radius

    I need to cut numerous openings in electrical panels to accommodate a fancy joystick switch. See pic below.

    I've been detailing the dimensions in CAD, to make it easier to mill out the openings with my mill and DRO.

    Two of the fillet corners appear to be "offset", as evidenced by the LED indicators in two of the corners, with differing chord lengths.

    Anyone have a simple procedure for locating the center of the radius?

    Last edited by jmarkwolf; 10-02-2018, 06:06 PM.

  • #2
    Construct two chords on the arc. Make them as close to perpendicular as the length of the arc will allow. Now construct the perpendicular bisectors of each chord. The bisectors will intersect at the center of the arc.
    Regards, Marv

    Home Shop Freeware - Tools for People Who Build Things
    http://www.myvirtualnetwork.com/mklotz

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    • #3
      That looks like a quarter of an ellipse to me, not an arc. I'd measure the legs and draw an ellipse from them and see if that's correct.

      EDIT: Nope. Or you can do like I often do and bring an image into CAD and draw on top of it after scaling it!



      EDIT

      Here's how, find where the tangent point of the arc off the side is.

      Last edited by gellfex; 10-02-2018, 08:20 PM.

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      • #4
        Radius gauge?

        I recently purchased two sets, inch and metric, on sale but haven't used them yet. This is what they were made for.
        Paul A.

        Make it fit.
        You can't win and there is a penalty for trying!

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        • #5
          Originally posted by gellfex View Post
          That looks like a quarter of an ellipse to me, not an arc. I'd measure the legs and draw an ellipse from them and see if that's correct.

          EDIT: Nope. Or you can do like I often do and bring an image into CAD and draw on top of it after scaling it!


          EDIT

          Here's how, find where the tangent point of the arc off the side is.
          I work like that a lot. Small flat things go on the bed of the scanner. Other things can be figured out by taking pictures with a 6" scale in the shot. a couple key measurements with calipers on the part to piece it all together and it's very surprising how accurate that method can be. Very easy to hit 0.005-0.010" and on some parts, even tighter than that.

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          • #6
            Originally posted by Dan Dubeau View Post
            I work like that a lot. Small flat things go on the bed of the scanner. Other things can be figured out by taking pictures with a 6" scale in the shot. a couple key measurements with calipers on the part to piece it all together and it's very surprising how accurate that method can be. Very easy to hit 0.005-0.010" and on some parts, even tighter than that.
            I have clients that want AL flat plates of various shapes with various wire and tube holding fittings, so I just ask for a photo of their pencil drawing with a ruler in it. I CAD over the sketch, print it out and just spray glue it down to the metal. Easy workflow! And no laborious hole layouts, just center punch on the pointmark. And I do confess there's been times I've imported into CAD photos of stuff I made but didn't document well in order to reproduce them.

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            • #7
              I’d really like to have one of the spindle cameras for this kind of stuff.

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              • #8
                Radius gauges, as Paul has already recommended, will give instant results.

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                • #9
                  I use an optical comparator and blow up the part to full screen and take a photo. Put the photo in CAD so it fills the screen and trace the part.
                  When you scale the part down it also scales any error down as well.

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                  • #10
                    OP here.

                    Thanks for the input everybody.

                    I tried the photo trick with only limited success. My 2D CAD package (Draftsight now that AutoCadLT is so damn expensive) only offers limited importation of pic files, and then I couldn't draw over the image. But I was able to confirm that my first assumptions as to radius were correct.

                    As for radius gauges, forgive me but I don't see how I could use them to find the "center" of the radius. What I'm ultimately after is specifying the "plunge point" for the end mill at the corners of the panel openings.

                    I know I'm close, but will likely have to do some manual "retouching" to get a drop-in fit.
                    Last edited by jmarkwolf; 10-07-2018, 08:03 AM.

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                    • #11
                      Originally posted by jmarkwolf View Post
                      As for radius gauges, forgive me but I don't see how I could use them to find the "center" of the radius.
                      If you know the radius, you can then work backwards laying it out from the the 2 tangent sides if they are truly tangent and it's truly a radius. I'm not completely convinced of the latter since how could the former be true and the legs be unequal?

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                      • #12
                        OP here with an update:

                        I created the the openings for the mirror control joystick on my electrical boxes. Turned out quite nice with a nice slip fit.

                        I ended up using the closest center-cutting end mills I had on-hand (7/8in and 1/2in). I expected that I would need to "post-process" the openings a little bit but the part fit beautifully.

                        Pics below:



                        Last edited by jmarkwolf; 10-16-2018, 05:36 PM.

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