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An interesting way to make a multi tooth form cutter

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  • #16
    IIRC John Stevenson showed that the difference between the two button method and the true involute form only amounted to tenths in the gear sizes we're working with.

    There are also several strategies for developing the cutting clearance. If you're making a tool with two hardened steel buttons the right diameter and the right spacing, you can provide the clearance by slanting the face where the buttons are installed. That compromises the circular form slightly but allows resharpening without losing any button diameter. The more theoretically correct tapered button installed on the flat means that it must be ground very carefully to the correct diameter and can't be resharpened without becoming a smaller circle.

    In one of his last iterations of making a fly-cutting tool with the gear form, he used a tapered end mill as used in the mold building industry. A washer with the target inside diameter was pushed onto the tapered end mill to get the right profile on his fly cutter with necessary clearance. A clever solution.
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill


    • #17
      Originally posted by Ringo View Post
      I had to draw this with my morning coffee, I can see it, but, not the way clickspring did it.
      he never relocated the hold down bolt, how he did that?
      I understand my picture, but it moves the hold down bolt each facet
      how he maintain the bolt to lathe center?

      The hold-down bolt is off-center on the arbor, by the amount needed to produce the larger radius for the relief.

      So it was producing the larger radius already, and all he needed to do was change which part of the tool that radius was produced on by rotating the tool.
      Last edited by J Tiers; 11-20-2020, 10:25 AM.
      CNC machines only go through the motions


      • #18
        Ringo, this might help. I did up this sketch to clarify it in my own mind for the future as much as hopefully help you.

        The center pivot bolt does not need to move. The cutter blank rotates around it and we only need indexing pin holes for each. What is needed though is to cut deeply enough that the lobe we cut away covers an angle that is at least the proper angle for the number of teeth. In this case 4 so we need to cut deep enough to cover at least 90°. Otherwise we leave a little land of the original dimeter. Not really a big deal as long as we cut it away later. But still untidy.

        With the 4 positions cut we get the shape in the first of the smaller examples. Then it builds from there.

        I drew myself into a corner with this one as you can see where the indexing holes end up partially left. Needless to say this would complicate the last cut. But I wasn't about to go back and re-do the drawing to offset the indexing holes. This just shows why it's a good reason though to do this up ahead of time on paper or CAD and avoid such a pitfall.

        But provided it would let us mill it this way we could still use the pins to locate the lobes and the front cuts and end up with a nice disc cutter. Just need a suitable slitting saw style arbor to use with the cutters that can be made in this manner....

        Click image for larger version

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        Chilliwack BC, Canada


        • #19
          Our old friend Sir John Stevenson wrote about this method here and on the old rec.crafts.metalworking usenet group years ago.

          The RCM article was sort of archived here:

          A bit of diligent searching on this forum should turn up his posts as well.

          Edit: OK, found it. Lots of related discussion in the thread, it's worth reading through. If you are in a hurry, the good part with photos starts at post #58

          Last edited by alanganes; 11-20-2020, 06:31 PM.


          • #20
            BCR, thanks for a better drawing,

            I looked at post #58 and thats on page 4 and the whole page is good pictures.
            shows how he offset the ellipse by taking a flat spot on the mandrel shank, and it offsets itself in the chuck jaws. cool


            • #21
              I remember seeing something like this mentioned in Ivan Law's book, but it was only a few sentences and then he got into the Eureka mechanism as a tool to do the same job. The first time I read it (last year) I kinda "got it" and then I tried to read it again... fuzzle... I decided that I actually have to *study* the book, because it's heavy reading. Might be the best 12 bucks I ever spent though, just for brain exercise.
              25 miles north of Buffalo NY, USA