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measuring a bolt hole circle

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  • measuring a bolt hole circle

    So there is an ASA vacuum flange on a rather large, old vacuum system that I'd like to reproduce a drawing for. I haven't done any measuring yet, since its rather bulky and its squirreled away in a storage room. Anyway, in the past I would estimate the center of the holes and measure the diameter. Then I'd round to a nice number, whether it was a fractional number or just something nice like 1.60 or etc. That works fine for most things, if you don't mind having slightly oversize holes, but what about for an odd number of bolt holes?

    In this case, I can probably take the existing cover (out of polycarbonate) off and just match drill a piece of SS but I was wondering what tricks you guys had to accurately measure a bolt hole circle.

  • #2
    how about something like this?

    Or more likely this


    • #3
      Measure the distance between adjacent holes. Use pin gages or self-turned pins to get an accurate measurement. Call this distance C.

      Then the BCD is given by:

      BCD = C/sin(180/N)

      where N is the number of holes.
      Regards, Marv

      Home Shop Freeware - Tools for People Who Build Things

      Location: LA, CA, USA


      • #4
        Chordal Method.


        I have a set of 8" dial calipers equipped with center probes.

        1. measure chord of holes, usually check all, had many examples of 7, 9, 11 etc, equally spaced, but one offset, to prevent mounting incorrectly, would only assemble one way.

        2. Take the sin of 180/number of holes, then reciprocate it, (the (1/x) key), then multiple by the chordal dimension.

        eg. 7 holes.

        Chordal checks 2.168". (Actual should be 2.1694"), but we are using a guessing stick!

        180/7= 25.7143° Sin = 0.4339, reciprocal = 2.3048

        multiple by chordal = 4.9967" BCD, the actual BCD is 5.00", but you would figure that right?

        Edit. Getting Slow see above, same thing.

        Last edited by LES A W HARRIS; 09-23-2008, 02:21 PM.
        Les H.
        The Impossible Takes Just A Little Bit Longer!


        • #5
          Ah ok - basically just going backwards from the way you'd lay it out if you didn't have a DRO. I should probably memorize that equation or at least print it up on a shop "cheat sheet". I hate having to stop mid project to fire up the computer. Much easier if there is an in-shop refrence!

          Thanks guys!


          • #6
            If I've got digital calipers handy I will use the inside jaws to carefully measure from far side to far side of two opposite (or for odd hole numbers, adjacent) sides. Then, zero the calipers. Now measure one of the holes again with the inside jaws. Calipers now read the center-center distance of the two holes. Negative of course.

            Some obvious limitations but is quick and fairly accurate for most cases.


            • #7
              From Bob Warfield

              From Bob Warfield.


              And another:
              [End edit]

              I think that these will answer all questions:


              [End Edit2]
              Last edited by oldtiffie; 09-24-2008, 09:01 AM.


              • #8
                hmmmm, draw a circle of the desired diameter. Divide 360 degrees by number of holes. Mark off degrees from center with straight lines. Drill where straight lines intersect circle?


                • #9
                  Yep that would work for creating a bolt hole circle, but I wanted to measure an existing bolt hole circle. Its the "desired diameter" that I had to figure out!


                  • #10
                    I have used Pin Gauges of the exact size of the holes. Put the Pin into the chuck then lower into the hole. Mark as center of hole. Do over for center or other holes. This is not dead on but close enough for work I do.


                    • #11
                      Usually for the stuff we have to back engineer at work we normally do BC by placing studs in opposite holes and then zeroing a caliper over the diameter of one of the studs* (or using the average of both as zero) and measuring across them. Of course some of the machines we have to mearsure for drawings have some holes in BCs off set to maintain the relationship between parts. When stuff is say 750mm on the inside diameter and spinning at 2500 RPM and balanced to normally less than a gram total not a bad idea. Thankfully when people design things with offset holes they are usually lazy and will offset a hole in 8 hole BC 5 degrees. But you still have to trig it out a couple of different ways to be sure

                      *One place where stripper bolts really work nice
                      Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.


                      • #12
                        Dale, that would work if the measured distance was in the same plane, say X, but if you have to move Y to set the pin in the next hole as you move X you will then have an angle and you would have to use trig to figure the actual distance from 1 to 2. It's better to measure across the pins on the outside and subtract the diameter of one pin to get the center distance.
                        It's only ink and paper


                        • #13
                          Center Attachment for Dial Calipers.

                          Originally posted by Fasttrack
                          Yep that would work for creating a bolt hole circle, but I wanted to measure an existing bolt hole circle. Its the "desired diameter" that I had to figure out!
                          Here you go went & dug them out.

                          Take a reading.

                          1.0910" avg three holes.

                          Sin 60° = 0.8660, reciprocal = 1.1547 times 1.0910" =1.2598"
                          Metric unit convert times 25.4 = 31.9984mm, 32 mm basic BCD. 0.0016mm difference = .0001". Good enough for me.

                          Les H.
                          The Impossible Takes Just A Little Bit Longer!


                          • #14
                            Other ways.

                            There are "involved" and "difficult" ways and there are "easy" and "easier" ways.

                            Most holes on a pitch circle are clearance holes or are for drilled and tapped holes for bolts that will engage with clearance holes. The pitch circles are usually "standard" as well.

                            Most flanges for pipes etc. are "standard" and can be obtained from tables on the net.

                            So, all in all, most times the sizes and dimensions are neither "super-accurate" or "critical".

                            Most - especially even numbers of holes - are quite easy.

                            The eye is very accurate when used properly with a good rule(r).

                            The distance between centres of 2 holes is exactly the same as between the "left" sides or the "right" sides of those adjacent holes. Just put any "inch" (or 100mm) mark on a rule accurately on or over the left side of the left hole, "eye" the rule into/over "centres" and read off the rule at the left side of the right hole. Repeat three times, add up and divide by three to average it out.

                            So much for the distance between holes.

                            If the number of holes is an even number, the same can be used to measure the pitch circle diameter.

                            If the number of holes is an odd number, use the centre distance between holes and any of the formulae posted previously. There are other ways including but not limited to clamping a bit of sheet metal over the flange and just adjusting dividers until they "fit".

                            It might just surprise you just how accurate and quick some of these older work-shop practices are. I use them first as I learned them in shops. Others seem to approach it as an exercise from a lesson in geometry or trigonometry. It is actually an exercise in applied "solid geometry".

                            So take it easy. The "hi-tech" and "use lots of formulae" approach is often not needed.

                            You should spend a week with or ask a good "Sheet-Metal Worker" or Boiler-Maker and see how they do it.


                            • #15
                              Spray it with a light coat of oil then lay a piece of white paper on top. Remove the paper and take the measurements from the paper in safety and comfort of your shop, it's flange no need for tool room accuracy.
                              Non, je ne regrette rien.