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  • I too am easily corn fused picturing rotary broaching at work. Wikipedia may help:
    https://en.wikipedia.org/wiki/Broaching_(metalworking)

    The necessary condition is that the center of rotation of what I am calling the "shank" and the center of rotation of the "wobbling" broach intersect
    at the face of the broach. This varies with angle and length of broach. Many designs allow the "shank" to be adjusted relative to the "barrel"
    to accommodate differing length broaches. I settled on fixed 1.5" long broaches and then "dial gauge found" the proper shank
    offset to get the zero intersection. No math or geometry dizziness. Then screwed the two pieces together and welded for good measure.
    Then turned down the frankenstein assemblage into a marginally less crappy looking rounder assembly. Beauty, it ain't.

    As noted before, a mill, lathe, or drill press, etc. may be employed. Rotate the tool, or rotate the toolee, no matter.

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    • Ought to be a youtube video showing a rotary broach in action.
      “I know lots of people who are educated far beyond their intelligence”

      Lewis Grizzard

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      • Originally posted by Dave C View Post
        Ought to be a youtube video showing a rotary broach in action.
        This Old Tony made a rotary broach and video of it working. Two videos actually - the 1st was on the lathe and with the work and broach turning it was not at all clear as to what was happening. So the 2nd one had the broach's shank in the drill press, with the work & cutter stationary. Very clear.

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        • I believe it was AVE that also did a dead simple version too.

          It occurs to me in a sudden flash as I read your last reply that maybe it doesn't need to be all that complicated. If we can live with a slight chamfer at the opening of the broached hole the job could be done with this "loose broach" as seen in the sketch below.

          There is an obvious issue with the sketch since clearly the broach cannot be removed from the retraction cap. I saw that right away but wasn't about to alter the whole "quick" sketch. I think what I'd do is make the ball end "knuckle" a separate and larger piece. And due to the rotational factors the broaches themselves could simply thread into the half ball "knuckle". I would simply assemble the broach and knuckle with the retraction cap fitted to the pair then install the whole works in the offset.

          Did I miss anything on this conceptual sketch?

          What I like about this option is that the broach is centered by the chamfer of the part. So no need for any actual set length. Well, other than the length must be sufficient that the side clearance angles do not go past being parallel with the center axis. So depending on the amount of offset in the bearing body the broaches would have some minimum length. But longer would be OK. And in fact.... .longer would need less of a taper angle with this scheme. Yes? No? With this way of doing it the smaller "waist" at the end which will screw into the half ball can be a set size. And the broach length can be set to any value. Well... within reason. If the broach is too long there won't be enough wobble and actually forming the hole would take a lot longer or may not occur after some value due to spring in the parts. So there is a limit to all this.

          Anyway if this option works it would take away the need for an oddball angle being drilled.

          Click image for larger version

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          Chilliwack BC, Canada

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          • I like this one as well:
            https://www.hobby-machinist.com/thre...9/#post-272748

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            • That's a nice option Mike. I just need three screws to come in from the side instead of that cap like threaded on retractor. That makes the job even easier.
              Chilliwack BC, Canada

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              • Originally posted by MikeWI View Post
                That is nice - it is so simple, cuts to the chase, if you will. As a dedicated KISS-er, I am impressed.

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                • In that simple design with just the single ball as the pivot and rotating bearing he mentioned not using too much relief angle to avoid the broached hole forming a spiral. I wonder if that is related to the drag in the socket due to the spinning? I'm thinking that there may still be some advantage to using a tapered roller as a lower rotational drag socket. I do like the idea of a hard ball and the mild steel with socket formed by a ball end mill though.
                  Chilliwack BC, Canada

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