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How to cut left hand thread on chinese lathe

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  • How to cut left hand thread on chinese lathe

    How do you cut a left hand thread on a lathe that has power feed in only one direction?

  • #2
    You need to come up with a way to reverse the leadscrew relative to the spindle. Depending on the lathe you have, try and see if anybody else has done a mod like this before.

    Typically, if there is an idler gear in the change gears driving the leadscrew, a reverser can be added. Try looking for mods to the 9x20 lathes. They don't come with a reversible leadscrew, so this is a mod that many make.

    Why doesn't a 9x20 come with this option, when a lowly little 7x?? does have it??

    Andrew

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    • #3
      I just went through this on my lathe as well. What type/size lathe is it? Any pictures of the change gear section? You will need to build a reverse tumbler for your setup, you can take a look at mine Here. It was just a quick setup, I'll be hobbing a gear with a press fit bearing sometime soon.

      Andrew D beat me to it.

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      • #4
        I donno. My little HF mini lathe has a reverse lever on the change gears, and the motor is reversable, with the switch.
        I've got a 1"X 24 TPI small screw-jack on my cofee table that I made with both inside and external left-hand threads.
        I made the screw-jack left hand thread to mess with people picking it up.
        I've also got a right-hand thread #5 Acme thread jack that sits next to it.
        The hardest part was accurately cutting the tool bit.
        No good deed goes unpunished.

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        • #5
          Strictly speaking, a tumbler gear is not necessary. The lead screw will be reversed by adding in another gear (if there is another stud available so that another gear can be put in the change gear train), or by taking two gears out of the train and substituting a larger gear. Doing this will not change the ratio, the ratio is dependent on the driving and driven gears only. The gears in the middle simply transfer motion (that's why they're called idlers ) UNLESS there are two gears mounted on the same shaft - one driven and the other driving the next gear in the train (that's how metric/English pitch translators work).

          Really, all a tumbler reverse does is substitute one gear for a pair, depending on the position of the carrier. I think it's called a tumbler because the carrier typically has a spring loaded pin(tumbler) holding the carrier in position. If you were really dedicated, you could make your own reverser...
          Design to 0.0001", measure to 1/32", cut with an axe, grind to fit

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          • #6
            Just thread it normally then turn the workpiece inside out. That's how I always do it.

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            • #7
              Most 9 x 20's have an idler gear that is "somewhat" removable to do left hand threads but it's a clumsy bit of a lash up. Build the tumbler.
              The shortest distance between two points is a circle of infinite diameter.

              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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              • #8
                GK

                Can you post a link to a youtube video of that? I'd like to see that!

                I suppose we'd find it right next to the square hole drill video??
                Design to 0.0001", measure to 1/32", cut with an axe, grind to fit

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                • #9
                  Originally posted by loose nut
                  Most 9 x 20's have an idler gear that is "somewhat" removable to do left hand threads but it's a clumsy bit of a lash up. Build the tumbler.
                  Agreed - mine requires removal of the gear train to swap in the reverse gear. Definitely a nuisance.

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                  • #10
                    Originally posted by camdigger
                    I suppose we'd find it right next to the square hole drill video??
                    Here's the bit. Just put the holder in your tailstock and away you go.



                    Here the video of the general process.

                    It is pretty neat how they do it.

                    Don't you hate it when the imaginary is a reality.

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                    • #12
                      Originally posted by rancherbill
                      Here's the bit. Just put the holder in your tailstock and away you go.



                      Here the video of the general process.

                      It is pretty neat how they do it.

                      Don't you hate it when the imaginary is a reality.
                      Iv seen them work great is screw machines you can threadroll and rotary brooch stuff bullet fast. both at once if you tool up and have fat bucks or a wire edm.

                      great stuff.

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                      • #13
                        I wound up purchasing a SB with tumbler reverse, but I did consider the import lathes. I have a copy of a internet post by someone named Jack F. that I saved during that process. It is a page of text with some pictures inserted. I'm not sure how good it is as I never built it. From a quick read, I believe he used the gears supplied with the lathe.

                        If you want it, send me a PM and I will e-mail it to you. I don't have the original web address and it may no longer be there anyway.
                        Last edited by Paul Alciatore; 03-24-2009, 02:40 AM.
                        Paul A.

                        Make it fit.
                        You can't win and there is a penalty for trying!

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                        • #14
                          Here's the link that Paul has I think. If you gear train works like the 9x20 lathes that will work or the others posted. But if it is setup like my 10x18 you'll need to build something a little different.

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                          • #15
                            OK I saw the pics. Thats great but. My intent was to be able to cut left hand threads. My lathe has a gear set up very simular to the 9x20 . When selecting what thread pitch I want to cut I have to change gears. The problem is I may have to change one or all the gears to achieve the correct feed. Thus to make a reverse work for a particular pitch I cannot just pick any gear, I must use 2 that have the same amount of teeth to reverse rotation and remain with the same feed.

                            I'll look closely at it this weekend......

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