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Thread: Has anyone done threading on a lathe with the tool upside down ?

  1. #1
    Boolit Master Jedman's Avatar
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    Post Has anyone done threading on a lathe with the tool upside down ?

    I seen a video where a guy was threading the shank of rifle barrels in a different way.
    Using a 60 degree tool bit mounted upside down in the tool holder and the compound set 30 degrees to feed from the barrels shoulder towards the breech end and running the rotation in reverse.
    Threading this way you don't need to watch closely to disengage the feed when you get to the shoulder. You start at the shoulder dail in your depth of cut and start threading away from the shoulder and you can relax and let the tool run off the end of the shank before disengaging the feed. Looked like it worked well in the video and I can see the advantages of using this method.
    I haven't tried it yet but sometime when I get the time to play I plan on trying it on a scrap piece and get the feel for it.

    What do you all think ?

    Jedman

  2. #2
    Boolit Buddy
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    It's a pretty common practice when threading to a shoulder , the only caveat is if you have a threaded spindle you need to lock the chuck on someway so it doesn't unscrew and come loose or fall off .

  3. #3
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    Bent Ramrod's Avatar
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    I threaded a Ballard receiver casting that way, using an internal thread tool upside down, on the opposite side from normal on the workpiece, and the lathe running backwards.

    In the absence of X-ray vision and the reflexes of a cat, itís the only way to go. Much easier to start at the shoulder than risk a crash stopping at one.

  4. #4
    Boolit Master
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    Yep, best way to thread from a shoulder to open space, much lower stress level than cutting toward a shoulder.

  5. #5
    Boolit Grand Master

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    Yes done it that way for years now. Great for doing compensators and silencers.

  6. #6
    Boolit Grand Master

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    I did it whenever possible at work it gave several advantages over up right tools.
    1) you could run faster I had a live center with and extended point so had extra room there also. Cutting closer to the correct sfpm gives a much better finish.
    2) coolant worked n to the cutting edge better and flow was more even around the tool. In this way of threading the relief angles are up forming a funnel for coolant to the cutting edge. Coolant also does a better job pushing chips away.
    3) gravity works to remove chips along with coolant they drop off the tool instead of building up and being pulled thru the cut again.
    4) coolant floods can be used easier since you don't need to see start and stop points.

    You do need to grind the tool accordingly for this but that's not a big deal. Tooling may need to be modified to get to center line of part as it needs to be a little higher. Cutting the thread relief and feeding into it and then cutting away from the shoulder makes threading much easier.

  7. #7
    Boolit Master
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    I'll have to try that - I always leave the half nut engaged and stop with VFD. Makes it easy to always get tool back in exact spot. Also can clean up threads by using the compound to slowly adjust tool point to exact center of thread with half nut engaged.

  8. #8
    Boolit Master Idz's Avatar
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    On a small lathe there is so much backlash in the gears and usually no slow motor speed that threading is difficult. My solution is to turn the chuck by hand to get slow speed and control at the shoulder. When a pass is done I back the bit way out and rotate in reverse to handle the backlash problem. Some folks get fancy and put a hand crank on the spindle.

  9. #9
    Boolit Master bikerbeans's Avatar
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    Quote Originally Posted by Jedman View Post
    I seen a video where a guy was threading the shank of rifle barrels in a different way.
    Using a 60 degree tool bit mounted upside down in the tool holder and the compound set 30 degrees to feed from the barrels shoulder towards the breech end and running the rotation in reverse.
    Threading this way you don't need to watch closely to disengage the feed when you get to the shoulder. You start at the shoulder dail in your depth of cut and start threading away from the shoulder and you can relax and let the tool run off the end of the shank before disengaging the feed. Looked like it worked well in the video and I can see the advantages of using this method.
    I haven't tried it yet but sometime when I get the time to play I plan on trying it on a scrap piece and get the feel for it.

    What do you all think ?

    Jedman
    Time for me to dream up another project?

    BB

  10. #10
    Boolit Master
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    You would have to be sure to get all the backlash out before the following cut,or it would all disappear.......I personally see the system as being unworkable,or at best ,very slow ,threading away from a shoulder,unless you include a large rebate........which then means conventional threading is just as easy.

  11. #11
    Boolit Master
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    Quote Originally Posted by john.k View Post
    You would have to be sure to get all the backlash out before the following cut,or it would all disappear.......I personally see the system as being unworkable,or at best ,very slow ,threading away from a shoulder,unless you include a large rebate........which then means conventional threading is just as easy.
    Your fears are unfounded. The rebate needs be no larger than normal and backlash will not be a problem. It works the same as threading normally,backlash works the same in any direction.

  12. #12
    Boolit Buddy
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    I thread to the shoulder in the conventional way. I've never crashed the shoulder. I use a 2" travel dial indicator mounted on a minimag that stuck to the ways and riding the carriage. I set my tool at the shoulder where I want to stop the thread, and watch the dial indicator wile threading. Very easy to back the cross feed and pull the half nuts at the same time. You have a very nice thread ending right at the same place each time and no relief cuts.

  13. #13
    Boolit Master uscra112's Avatar
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    Quote Originally Posted by B R Shooter View Post
    I thread to the shoulder in the conventional way. I've never crashed the shoulder. I use a 2" travel dial indicator mounted on a minimag that stuck to the ways and riding the carriage. I set my tool at the shoulder where I want to stop the thread, and watch the dial indicator wile threading. Very easy to back the cross feed and pull the half nuts at the same time. You have a very nice thread ending right at the same place each time and no relief cuts.
    Exactly what I do. My barrel-work lathe is an old South Bend 10" Heavy with a threaded nose. But it also runs at 30 RPM or even less, so I have no trouble threading up to a shoulder.

    However I've seen the upside-down tool trick used countless times on CNC lathes.
    flectere si nequeo superos, acheronta movebo

  14. #14
    Boolit Master
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    The chuck would unscrew on my old lathe if I tried to thread backwards, but it's a nice idea for people whose equipment will support it. I don't think my lathe will run fast enough that the typical barrel thread will cause a crash unless I'm just not paying attention. Just make sure you have a decent relief cut.
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  15. #15
    Boolit Bub
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    I make the initial cut with the cross slide. All other cuts are make using the compound slide set at 30 degrees. This causes the tool to cut on one side only, making a much cleaner cut. I don't believe running the threading backwards with an upside down tool would allow this. If you are paying attention to your work, there is no need to run a threading operation backwards.

    However you cut threads, if it works for you, keep doing it. This is just how I do things.

  16. #16
    Boolit Master Cap'n Morgan's Avatar
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    I used to do all internal threading upside-down - a trick my machinist dad showed me. That way you were using the same moves as when cutting external threads. He also taught me always to keep the cross slide handle at the three o'clock position and using the compound rest for depth - otherwise, you might end up accidentally pushing the tool into the work piece instead pulling it out. These days I do all threading on CNC mills (or bribe the lathe-guy)
    Cap'n Morgan

  17. #17
    Boolit Master
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    I guess I was spoiled by the lathe I learned to thread with.
    It was an ancient 1923 model Pratt & Whitney tool room lathe.
    That old lathe had a smaller threading cross slide within the normal cross slide.
    To use the threading cross slide you locked the power feed cross slide and unlocked the threading slide. The smaller internal cross slide had a cam slot instead of a lead screw. The cam slot had a very high helix angle and would retract the tool much faster than turning the normal cross feed hand wheel. When you cranked the handle back it was normally 1/2 turn or 180 degrees and the cam slot pulled the tool back about 1" to give plenty of shoulder clearance. Since I spent days and weeks sometimes doing production threads I got used to paying attention and pulling the tool out at the right time.
    That lathe was built long before there was carbide tooling so it had plenty of low rpm spindle speeds to pick. Though it was a 13" lathe the top RPM was only 1800. It had a 10" screw on chuck that was put on arm strong tight with a bar in the jaws. Even so I would never run it backward if I could avoid it and I do not like single point threading when I cannot see what the cutting edges are doing.
    Last edited by EDG; 03-18-2019 at 01:25 PM.
    EDG

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