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  • BCRider
    replied
    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.

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  • Bob Engelhardt
    replied
    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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  • BCRider
    replied
    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.

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

    Leave a comment:


  • BCRider
    replied
    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

Name:	Wobble broach tool.jpg
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  • Bob Engelhardt
    replied
    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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  • Dave C
    replied
    Ought to be a youtube video showing a rotary broach in action.

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  • wbc
    replied
    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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  • danlb
    replied
    Originally posted by Dave C View Post
    I've tried to understand how a rotary broach works since first hearing of the things existence. All I've got so far is a headache🤔
    In a nutshell, the work rotates and the broach rotates with it. Start with a hole in the work. The broach is on an eccentric that makes it wobble a few degrees. The wobble changes the edge of the broach that is cutting. It's as if you were slicing away the edges of the hole a sliver at a time.

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  • Dave C
    replied
    I've tried to understand how a rotary broach works since first hearing of the things existence. All I've got so far is a headache🤔

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  • BCRider
    replied
    Thinking more on this if the off center coning axis is reflected into the sleeve's bore by the angle on the arbor and mounting face then yeah, one could spin either work or the tool. Wbc, is that the case?

    Leave a comment:


  • Hopefuldave
    replied
    Originally posted by BCRider View Post
    Nice!

    If I'm reading this right this style of tool has to be rotated against a stationary work piece. It could not be used in a lathe tail stock for example?

    And if I'm thinking correctly that is because the offset angle has to rotate?

    For a version that could be used in a lathe I could make one where the offset and angled bore for the broach would be only in the spindle and the body and shank would be axial and thus be able to remain motionless while the rotating angle wobbled around with the work piece?

    Can be used in either, have it held stationary in the lathe tailstock against rotating workpiece, or vice-versa, as long as there's the ?nutuating? motion so the cutting edge point of contact moves around the circumference of the hole you're broaching.
    I'm thinking that on the lathe it'd be possible to mount the rotary broach on the carriage and apply the offset angle by e.g. rotating its axis on the toolpost, that'd give power feed too? As long as the cutting face remained on centre it would correctly track the pilot hole, I think. I might have to try it!

    Dave H. (the other one)

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  • BCRider
    replied
    Nice!

    If I'm reading this right this style of tool has to be rotated against a stationary work piece. It could not be used in a lathe tail stock for example?

    And if I'm thinking correctly that is because the offset angle has to rotate?

    For a version that could be used in a lathe I could make one where the offset and angled bore for the broach would be only in the spindle and the body and shank would be axial and thus be able to remain motionless while the rotating angle wobbled around with the work piece?


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  • wbc
    replied
    Yep. Could dispense with the two piece barrel-shank arrangement and use a boring head and tilt the workpiece in a mill.
    But I never made or used a broach before, and didn't know what angle best to use, so I took the exploratory route.
    Making another having this experience as a model, could have made as a total of 3 pieces. But I think you have to
    either use a CAD program or be good at geometry to figure out the proper tilt and offset for cutting the shank portion
    as one unit, given a pre-determined length for the broaches so that their cutting ends intersect the point of zero wobble.

    An example of various prior fancier art is better covered here:

    https://www.hobby-machinist.com/thre...roaches.19945/
    Last edited by wbc; 06-20-2020, 11:04 PM.

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  • Cenedd
    replied
    So the barrel ID, OD and spindle were all concentric. The shank was concentric but the end was milled off at an angle so when mated to the barrel the shank is at an angle. Then you turned the final OD down to make it look right. Is that right?
    So if I've got it right you could also have turned a shank from the solid body by using a boring head on the mill for OD boring? Not trying to say it's be easier/better, just trying to get my head around how it works exactly.

    Leave a comment:

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