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3 holes on bolt circle

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  • 3 holes on bolt circle

    I am making an adapter plate to mount my little 4 jaw chuck to the top of my rotary table. I need an accurate measurement of a bolt circle on my chuck . The chucks mounting plate is held on by 3 bolts on a bolt circle that I want to know what it's dimensions are. What is the proper way of measuring it? I did know at one time ,but have forgotten how to do it now.
    Old age or alzshimers is setting in. Thanks in advance. Boot

  • #2

    What lathe do you have?

    If you think you understand what is going on, you haven't been paying attention.


    • #3
      The way I do things like that is use shoulder bolts in the holes and
      measure the spacing (inside and outside of the three places) if they
      are all the same then the angle is 120 deg between the holes.
      So it's a simple bit of trig to get the rest of the data.


      • #4
        Well, speaking strictly as an amature i'd locate the holes on a piece of card, marking them like you'd make a gasket,, then take a compass and draw a circle the fit that bolt pattern.

        (In reality that may not be proper, but it could be made to work.)


        • #5
          Transfer screws - if you just want to mark the adapter plate.

          Point up some screws that will fit in the chuck and then lay the chuck on the plate and whack with a brass hammer.

          If you want actual numbers, try this:

          Last edited by wb2vsj; 02-22-2012, 10:00 PM.


          • #6
            As Lew said, simple trig, measure the distance between holes, a shoulder screw works best, but any screw tightened will work. I usually measure all three and average the result. .866 X distance should be the circle dia. Don't have my calculator handy , but remember certain trig values. This only works for 120 degree spacing though. Bob.


            • #7
              Bob, that (.866) is the height of the entire triangle the "center" will
              be 2/3 of the height for that configuration. The intersection of the
              altitudes or in this case bisectors of the angles is 2/3 of the height.
              This is the radius of the "bolt circle".


              • #8
                For my little Emco Maier rotary table, I did as suggested, sharpen three screws, & malletized the chuck on top of an aluminum plate.


                • #9
                  First time I tried the calculations for a 3 hole, I snatched up a piece of scrap aluminum, grabbed my compass, made the marks, punched, drilled, and nailed it dead on.

                  I've never been able to duplicate that feat. Use transfer screws.


                  • #10
                    I would put the chuck and a reference dial caliper on my flat bed scanner and take a picture of it. Unless the chuck weighs more that my leg.


                    • #11
                      I'd measure each hole, then measure between the holes on the back of the chuck. Average the hole sizes, add that figure to the average measurement between holes, then construct a triangle to match, using those chord dimensions. Calculate the dimensions to lead to a radius, and double that for the BC. 120 degrees at that BC and you're accurate enough given there is clearance on the bolt holes anyway.
                      Scientists have shown that the moon is moving away at a tiny yet measurable distance from the earth every year. If you do the math, you can calculate that 85 million years ago the moon was orbiting the earth at a distance of about 35 feet from the earth's surface. This would explain the death of the dinosaurs. The tallest ones, anyway.


                      • #12

                        These are all for a basic "1"

                        If your bolt/hole centre distance is say 3" then "S" is 3.

                        Bolt pitch circle diameter = D = S x 1.1547 = 3 x 1.1547 = 3.4641


                        • #13
                          3 hole bolt circle

                          Thanks guys. I like the formulas that Old Tiffie presented. The holes as far as I know are 120° apart. I'll do a measurement on the space between them and use D= Sx1.1547 to get my answer if they are equally spaced. It's funny how we can forget these important ways of figuring out stuff.
                          Old age doesn't come easy and it doesn't come alone . Remember that. I thought I had it made when I retired and then I found out what Lymphoma was. Thank God I'm beating it . I feel good too. Thanks Boot.


                          • #14
                            Can't you just set the chuck on your backing plate (presumably located by a feature on the backing plate) and use a transfer punch to mark the holes?