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Shop made rotary broach for the Rivett

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  • DR
    replied
    I think I've mentioned this before, but a repeat doesn't hurt.

    If all you want to do is a couple broached holes (non-production) all you need is the rotary broach cutter itself.

    Grind the cutter from an old end mill with the typical 2 degree relief. Be sure to use an end mill with a center hole on the shank end.

    Setup your part in the lathe chuck or collet with the correct size pilot hole with lead in chamfer.

    Offset the lathe tailstock so a live center in the tailstock pushing on the cutter makes the one degree angle of cutter with respect to the lathe's axis. Use the tailstock feed wheel to push the broach into the spinning work.

    You'll have to jury rig a way to extract the broach cutter from the hole with lathe turned off. Sometimes easy, other times the broach cutter is tight in the hole.

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  • mattthemuppet
    replied
    I did try a larger drill size, although it wasn't quite as large as the diameter of the broach. I did see a lot of flexing though, I'm pretty sure that's the problem. I don't think making a dedicated QCTP holder would be worth it either, it's a pretty small flexible lathe, nothing like Max' Rivett (?).

    I'll have to put a tailstock mounted one on my project list, though they look a fair bit more complicated than this.

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  • mars-red
    replied
    Rare win for the ol' eccentric tool post, lol.

    Sent from my BLN-L24 using Tapatalk

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  • enginuity
    replied
    I'd try going to a larger drill size yet if you still want to give it a go. Try one almost the same size across the corners and see if the wobble does its thing. This would reduce the load on the broach. If it sorta works here you know something else is up.

    That does look like an awful amount of hang out from the tool post. Max's rotary broach is held in his eccentric tool post and there isn't nearly as much hang out. I think you are right - the tool post is probably flexing.

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  • mattthemuppet
    replied
    recut the front of the broach to give it more relief, hardened it with the MAP torch, set the holder to 1-2deg off axis and fed it into a piece of alu scrap with a clearance hole



    the broach rotates, but all it does is push the material into the hole in a ring. It won't cut the corners. I tried a clearance drill of te size across the flats and one larger than that, both with a lead in chamfer, with no success.

    there is some degree of flex in the tool and tool post though, I wonder if that is having an affect?

    Either way, this is a wash for me, but I'll just reuse that holder as a live center, recut the 1/2in shank to MT1, grind a nice piece of tool steel into a 60deg point and I'm done

    still, a fun exercise

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  • mattthemuppet
    replied
    finally got off my butt to try making a new holder, unfortunately with no more success



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  • mattthemuppet
    replied
    Originally posted by mars-red View Post
    Truly a noble profession. Good luck, I hope you find your shot at tenure!
    thanks! I love my job, just wish it was a little easier to find one

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  • mars-red
    replied
    Originally posted by mattthemuppet View Post
    visiting assistant professor of biology Hoping to become a tenure-track assistant professor somewhere in the US so we can stay here, fingers crossed!
    Truly a noble profession. Good luck, I hope you find your shot at tenure!

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  • mattthemuppet
    replied
    Originally posted by mars-red View Post
    Wait, are your a teacher??
    visiting assistant professor of biology Hoping to become a tenure-track assistant professor somewhere in the US so we can stay here, fingers crossed!

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  • mars-red
    replied
    Originally posted by mattthemuppet View Post
    thanks Max! In the 2nd version the broach rotated (or was supposed to) in the broach holder, with ~1 1/4in of bore/broach contact and the 1/4in BB at the end. I was using grease on the BB and 20W oil on the side of the broach holder bore - probably too thick for this but it spun easily by hand. I'll use ISO32 next time as that's much thinner.

    I need to check that the broach isn't bottoming out of the bore of the broach holder first, then I'll recut the face of the broach to give it more front relief to see if that makes a difference. No harm in fiddling with it while I'm waiting for the bearings to arrive. Most likely at the weekend as I'm behind in my grading and I have a job to apply to this week
    Wait, are your a teacher??

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  • mattthemuppet
    replied
    thanks Max! In the 2nd version the broach rotated (or was supposed to) in the broach holder, with ~1 1/4in of bore/broach contact and the 1/4in BB at the end. I was using grease on the BB and 20W oil on the side of the broach holder bore - probably too thick for this but it spun easily by hand. I'll use ISO32 next time as that's much thinner.

    I need to check that the broach isn't bottoming out of the bore of the broach holder first, then I'll recut the face of the broach to give it more front relief to see if that makes a difference. No harm in fiddling with it while I'm waiting for the bearings to arrive. Most likely at the weekend as I'm behind in my grading and I have a job to apply to this week

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  • mars-red
    replied
    Originally posted by softtail View Post
    I wonder about the possibility of using an Allen key with ground relief for the cutter. Seems doable.
    I've heard of people doing exactly that for softer metals, and some Allen keys are hard enough that I think you could get away with a use or two in steel. If it's good high carbon steel, you could even re harden it and probably get as much use out of it as the O1 or W1 broaches I've made.

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  • softtail
    replied
    I wonder about the possibility of using an Allen key with ground relief for the cutter. Seems doable.

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  • mars-red
    replied
    No worries, Matt, just wanted to offer in case it would help.

    Let us know how it goes. As far as the fit, I don't think you'd want any looser. The looser the fit, the greater the angle of the rotating body will be relative to the centerline of the bearings, and that means less effective bearing surface. I think if anything you want as little clearance as you can get while still being "free", to maintain the most bearing surface. You may want to have a bearing surface that runs the entire length, rather than 1/2" on each end.

    From some of your commentary, it sounds like you are applying oil, which you definitely will want for this application. I really think if you had this same close sliding fit for the entire length of the tool body, and loaded it up with oil, you'd notice quite a difference. Just a hunch though. For what it's worth, I use light spindle oil (3-in-1, in my case) in my broaching tool.

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  • mattthemuppet
    replied
    Originally posted by mars-red View Post
    That's a downer! Have you tried tweaking the alignment? What are the dimensions of the broach body that is supposed to turn inside of the bushings, and how much bushing length do you have? Most of the force should be taken by the thrust bearing (ball, in both of our cases), but there will be some lateral force and if you didn't have enough bearing surface I could see that maybe contributing to binding problems. There could be a lot more lateral force involved than I realized too, with mine the entire sleeve of the body of the tool that fits into my toolpost acts as a lateral bearing against the inner broach holder, maybe 2.5"-3" in length or so. If I had designed mine with smaller bearing sleeves on either end maybe I'd be having the same problem. I wonder if a full bearing surface along the entire length of your broach body would help.

    Regarding the dish at the end of my broach (would that be called the "rake" in the case? I'm not sure, but seems like the relief would be the angle behind the cutting edge of the broach), I formed that just using a drill, so the angle is whatever the tip angle of the drill was. As I recall, I used one of my screw machine drills and I believe that set of mine is 135 degree.

    If you want to correspond by e-mail or even set up a Google Hangout or Skype session to try and troubleshoot this, send me a PM and I'll do my best to help.
    thanks Max, that's super kind! I'm happy to go through it here though so that others might benefit from it, as Softail suggested. I learn more from my mistakes than my successes

    I tried from ~2deg (the back rake or relief of the cutter) to 0deg without any difference. The only time the cutter moved was when it got jammed in solid. I tried different speeds to, from a few 100rpm to 20rpm. Even with moderate pressure the cutter locked up solid.

    Just thinking this through, I'll have to double check that the cutter isn't bottoming out in the holder in anyway. It shouldn't be, but it wouldn't take a moment to check.

    I'll have to take some pics to show the broach holder (I'm actually rather proud of it). The toolholder that was I repurposing has two 1/2in ID x 1/2in oilite bushings, with the flange of one acting as a thrust bearing. This works fine for my live drill chuck, but I was pretty dubious about it working with the broach holder. The broach holder has a 1/2in diameter by ~1 1/2in hole, with the 60deg countersunk hole for the 1/4in BB at the bottom and the broach is 1/2in x ~2in. The broach is a very close fit in the broach holder, to the extent that it requires a tug to get it out if there's any oil in there. I'm thinking that having such close tolerances might be the problem - any off axis thrust (or even slightly off center hex on the broach) might be enough to jam it solid.

    I've ordered a couple of 1/2 x 3/4 x 5/16 cartridge bearings for little money (only had 12mm ID bearings in my spares box, grr) and the small OD should mean that making a new holder with 1 or 2 cartridge bearings and a BB thrust bearing shouldn't take too long. It'll give me a nice little project to get one with after classes end in a couple of weeks!

    thanks for the heads up on the front relief, I'll try and recut mine to give a bit more.

    To be honest, even though this hasn't worked yet I'm super pleased with how the machining has gone - the broach holder was bang on tolerance and the hex broach came out really well on my DP/mill using my hex ER25 collet holder (love that thing).

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