how do you turn thin pieces of rod accuratly??
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  1. #1
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    I am doing a project that will require me to do some accurate machining on a rod that will be about 9in long and .275 at it fatest and possibly down to .150 0r .180 for alot of it. Do you guys have any good ways to turn this down accuratly in a lathe?? I imagin some sort of follower rest but it would have to be small. I'm useing a 13in south bend lathe.

    Thanks
    Grinch

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    You could use a small follower rest (use your stock one, but braze on some more bearing material and then grind it down to the smaller tolerances), or if possible, use the thru hole and do small bits at a time, with the part running in a collet. Alternately, is it possible to make some sort of a die that can be run along the shaft for specific portions? Just an idea...

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    This is a job for a roller box, mainly used on turret lathes/or capstan lathes,in the UK,
    turning something long and thin is difficult for every one,
    You could try this,if you clamped a piece of square steel in the tool post, center drill,and drill (from the lathe chuck)a hole the same as the rod size,clamp the cross slide if possible,to make sure the hole stays on center,
    The turning tool needs to be about 3/8 behind the support hole,enough room for the chips to get away,
    The tricky bit is setting the diameter,by tapping the tool to get the correct size,not moving the cross slide, you need to hit size in one pass,its not possible to take a second cut,
    A more elaborate set up would be to have the support hole attached to the saddle in some way,then you could use the cross slide to set the diameter,

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    Long tapers on a skinny rod. Hm.

    I'd think you need a feed-through lathe where the work is rotated and fed through a rotating bushing. The tool takes a full depth cut giving the rod work its contour without the need for more than convenience support for the overhang. Such lathes were used to make clock shafts, carburetor needles, and other very slender long work.

    I don't know how many parts you need to make but that's the way you should be thinking. Set up a bushing plate in your follow rest and run the tool in and out from a linear cam. Long tapers on skinny work are otherwise a nightmare to make economically.

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    Long tapers on a skinny rod. Hm.
    He does not say tapered?

    John

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    He does not say diameter or step either. This would be an ideal screw machine or (if it is stepped not tapered) centerless grind job. Some of the small, long, "whippy" stuff we would send to screw machine shops that moved the work in z axis instead of the cutting tool. If you must do it in house I would make a split bushing, finish as much length as you can w/o chatter, pull out more, indicate, do it again. Ive done this on cnc lathes where I did not have a follower rest to play with.

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    A Swiss turning machine is the first thing that came to mind when you described your part. Of course that's just not the kind of machine we tend to have in the home shop, but simulating it might be possible on a standard lathe. As Forrest mentioned, making the full depth cut using a rotating bushing is the way it's done on Screw & Swiss machines, but the bushing doesn't need to rotate (and commonly doesn't on Swiss machines) as long as a good stream of oil is constantly feeding the bearing surfaces. The bushing is basically a split collet like 5C, usually but not always with carbide faces. It's a tricky thing to set the correct tension and a little practice goes a long way. This is basically a follower rest with 360 degree bearing surface instead of just two points.
    Note that it's very important that a full-depth cut is made because the bushing does nothing once you've cut even a couple thousandths off the diameter It has to be correct the first time, which can also be tricky and use up some material (though smallshop's idea sounds creative and promising - it worked for him). If you have many parts to make it might be worth it. For a run of 1/2 dozen I'd try to make a follower rest work.
    A "box tool" works similarly, but I haven't used those so I'll defer to the folks with experience.
    If you have the time and inclination, maybe there's a shop nearby with Swiss machines from which you could glean some ideas or even experience. It's an interesting operation. Pose as a reporter investigating an article for "History of Industry Magazine" or something.

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    Though I never have made a part like this, I've seen some of them after they were made, by a machinist friend of mine. I think every proud acomplished machinist you will meet has his little "brag box," usually filled with some of the hardest parts he ever made often where the critical tolerance was blown by the smallest RCH possible to render it useless. Especially if discovered by a novice aspiring to learn more, h an hour or so of listening will lend the listener so much knowledged, and give the master machinist a rare oppurtunity to really share some stories of his true pride and joy. Especially as was my case with a great genenrational and exeperience gap was present afer listeninglong enough and asking intelligent questions chances are you will find your self with a new found mentor and friendship.

    Anyhow one of the machinists I know had such a box, the part looked sort of like this (please note I drew it all from memory, and on Microsoft paint, so pardon the poor quality drawing)

    The whole thing was made out of brass that I believe was then gold plated as it was some sort of tuner for a radio. On top of that those fins I drew also had some weird slots milled crosswise in them as these were tuning forks for some sort of radios. The whole thing was done mind you also before CNC. On top of that he also kept a copy of the print, who ever drew the part had surely spent some time in a machine shop, as he drew it as big as possible and totally out of scale to make it look real easy to make! As drawn that part was about 12" long and 3-4" in diameter. When he started working on them his forman asked him what he was doing on the such a small lathe. Then he put a few completed ones into the forman's hands, he said the forman's eyes buldged out in shock when he realized the acutall dimensions he overlooked on the drawing.

    As to how he did it, he said it was all done on a Hardenge HVLH lathe, I don't remember if he used a follower rest, yet he said the real trick to the part was to turn it out of some rather long stock, and then to do one full quick pass. I think for this part he started with either 3/4" or 5/8" rod in order to turn these from. He said the reasoning for such large bar, was that a real small diameter brass could never support such pressures and load of the cutter.

    Mind you this was just one of several mind blowing parts I saw in his little altoid's tray of smaller parts he'd done.

    Adam

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    Off the wall suggsestion: Would it be possible to use a turntable on a bridgport so that if you used a small diameter endmill you could make many steps and then sand it smooth to shape? Or use that methdod to remove MOST of the material and finish it on a lathe?

    How about make a cavity that is 180 degrees of the part, but full length out of steel...then HARDEN the steel and use grit to grind down to net shape? I know you should not grind on a Lathe you care about ever ever, but it would provide support as it "cut" along the entire length...

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    Here is an exaple of what I have done with the steady rest. The piece is aluminum and is 18 inches long. The base stock is .250" and the major diameter of the threaded section is less than .125"



    Joe

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    thanks for all the responses guy that is why I love this forum.

    I'm not needing to cut tapers just a long rod with a couple different small diameters.

    WJHarston-that looks like a great solution to me do you know what it cost??

    I've seen that thing somewhere before but searched e-bay and couldn't find it.

    Thanks
    Grinch

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    Joe, how do you thread when the follower rest is connected to the cross slide instead of the carriage base? Did you have a relief groove and adjust the follower when backing off for each pass? Thread it in one pass? Hmmm.

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    Slightly OT. I saw a programme on the TV where Roll Royce were having trouble profile grinding mushroom headed valves for engines, the reason being that the valve stem was 18" or so in length. The stem was coming out barrel shaped because of deflection when being ground. They fixed this problem by maching an inverse curve on the template, so the stems ended up true to size.
    Frank

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    The thread was done in multipal passes. You have to lift and adjust the unit at the end of each pass. The thread is a lot coarser for the diameter than normal. I use these rods for mandrels to make some parts for jewelry. The following pictures show the lifting of the steady so the carrage can be moved, closeup of the thread, and the steady in place for the next cut.







    Joe

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    After seeing the Follower Rest I thought it was a good looking product. The AXA size hadn't become available yet, so I made a little version of my own that simply bolts to a double tool holder. Later on I bought the "real one" from their Web site.



    I have more on this project here:

    Mini Follow Rest

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    Frank, Nice looking unit. When I bought mine from Richard I was in the process of designing one for my purposes which would have used small bearings like yours. When I saw his I said why reinvent the wheel and spend time that I didn't have and get on with my project. It worked well for my purposes and I have used it on several other jobs.

    I really like your exploded drawing it almost looks like the real thing taken apart. What drawing program did you use.

    Joe

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    I am so relieved to find this thread. I thought it was just my poor amateur technique (not blaming the Micro Mark lathe).

    I had been trying to make some custom 6x32 tpi bolts for an old Belgian Bulldog revolver (about 1880ish). The original bolt bodies (frame mounted) are full of corrosion pits on the shaft and also someone without the right tools had managed to burr the screw heads at some point in the past.

    The bolts are tiny and under magnification I suspect the threads were cut by hand. Just a tad under a #6 in size about 0.62 long. 3 of them. So I retapped the right side holes to #6x32. That was the easy part.

    Only the bottom .197" has threads. Then the body has a smooth 1/8" shaft with a screw head size of 0.15".

    I am using a steel 3/16" rod. I can usually get things down to about 0.155" before problems begin. When I use a steady rest (or a drill chuck in the tail stock) somewhere around .135" either the rod begins to bow or (worse) it just snaps in two. If I loosen the rest naturally the rod bends.

    I know it can be done and hopefully without simply using just a file and sandpaper (which would probably take longer than I have years left).

    I am using an HSS cutter which is giving a decent finish so I am confident I am okay there, but how to stabilize things so the rod doesn't snap or bow under cutter pressure remains the problem.

    Yes, if I owned a monster press I could maybe swage it through a hole. But I don't have that.

    I am of course also looking forward to the idea of putting threads on a part that tiny.

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  22. #19
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    Where are you at in Virginia? A kind member who I don't recall his name sent me some hands for a 19-2 circa early 60's .357 that I could not find anywhere, everyone was out of stock, and it was a CNC Mill part which I do not have. To keep the Karma going maybe I could help you out, I am almost done with my last job booked on the screw machine. No charge if I have material and tooling in stock and I will even eat the postage.

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    Danke. I sent you my email address. I can send you photos and more precise sizes. Since someone made hundreds of them in the 1800s I know it can be done, but I think what he/she did was spin a .15" pin while holding a file to it at an angle. I say that because the originals use a steep taper to go from .15 down to .125 rather than a step. Then some kind of groove tool to handcut the threads. Ouch. Must be a more modern way..
    Attached Thumbnails Attached Thumbnails 1642515209435.jpg  


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