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Thread: How to cut left hand thread on chinese lathe

  1. #1
    Join Date
    Feb 2005
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    947

    Default 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. #2
    Join Date
    Jan 2009
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    3 hours west of Winnipeg, Manitoba
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    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

  3. #3
    Join Date
    Oct 2008
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    Alberta
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    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.
    Check out my work Here

    Clinton

  4. #4
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    Nov 2008
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    Kingman Arizona
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    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.

  5. #5
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    Jul 2007
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    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...

  6. #6
    Join Date
    Apr 2006
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    Northwest Missouri USA
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    Just thread it normally then turn the workpiece inside out. That's how I always do it.

  7. #7
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    Sep 2006
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    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.

  8. #8
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    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??

  9. #9
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    Mar 2005
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    Oroville, WA
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    Quote 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.

  10. #10
    Join Date
    Sep 2006
    Location
    DeWinton, Alberta, Canada.
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    Quote 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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