South Bend 9a Threading Dial keeps drifting? Unable to make threads!
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  1. #1
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    Default South Bend 9a Threading Dial keeps drifting? Unable to make threads!

    I purchased a threading dial off ebay for my South Bend 9a.

    But each time I try to chase the threads the tool does not line up with the previous cut.

    Each time the error gets worse until it comes to the next thread?

    I am at a loss as to what is wrong or I need to do to figure it out.

    My dad and I spent 4 months rebuilding the lathe out of a pile of crap.

    Any help would be appreciated.

    Have been wondering if some of the gears could be the wrong size.

    Thanks

    Roy

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    The gear should be 32 teeth, if it is not that is your problem. If that is not it, the gear or the dial is slipping.

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    A picture or tooth count of each gear between the spindle and gear box would help.

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    Do an experiment but chucking up a shaft that has no value, under cute the first inch near the chuck and set up to cut a thread. and after the tool leaves the cut, shot off the motor, leave the 1/2 nut engaged, retract the cutter and reverse the motor and run it back past the starting point, stop the motor and reverse again and feed forward, move in the tool and see if it lines up. This will tell you if it's the machine or the thread dial. I bet you checked the thread dial gear and key and it's tight right? How does the gear look? You may want to check the collar at the end of the feed screw to be sure the sheer pin or key isn't sheered off...also engage the 1/2 nut when the chuck isn't spinning and crank the saddle to and fro..see if anything moves...good luck

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    Could be as simple as the gear on the thread dial slipping, as has been mentioned. You did not say however if the machine is cutting the correct lead.
    A quick sanity check could be to try cutting 8TPI. Since it's a multiple of the lead screw, you don't need to use the thread dial. First confirm that it is cutting 8TPI, then you can see if you still have the drift problem. At least you will then know if your issue is in the gearing orthe dial.

    Edit: Richard types faster! His suggestion will accomplish the same thing by removing the thread dial from the equation to isolate the problem.

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  10. #6
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    I'll suppose since you went after a TD, that these are the lathe's first threads during your custody.

    HIGHLY likely the TD has nothing at all to do with it and there is a wrong tooth count gear in the left end train

    Great stuff here for digesting

    http://bnordgren.org/files/model-a-gears.pdf

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    Default Threading Dial Reply

    Wow! You guys rock.

    We believe that this is caused by the wrong size "Thread dial" gear.

    I bought a printed replica off ebay for 60$.

    The gear is pretty nasty looking (Like it was made from play dough)and does not fit the feed screw well.

    Small chunks of plastic were being taken off it by feed screw notch even though it is very smooth.

    I returned it and purchased a "Real" one.

    Our test was 20 TPI and thread gauge measures it perfect.

    We confirmed the tooth count on all removable gears.

    I realized that since the TPI is right and that means all the gears are correct.

    The only place where the timing could be altered was in the Thread dial gear.

    Here's hoping the Dial comes fast and works as advertised.

    Will know soon.

    Last lathe was a 1927 18" Lehman rebuilt to amazing tolerances but was to big to move (2200lbs) into a basement.

    So we bought a South Bend 9a junker and rebuildt every inch for the past 5 months, Now making tooling and jigs.

    Thanks you very much for your help!

    Roy

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  13. #8
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    The leadscrew is 8 tpi, the thread-dial gear is 32 teeth, with dial marked 1-4 so each numbered division represents 1" carriage movement. I think this "1 inch per number on the dial" is sort of standard. Although the thread-dial gear is low-stress, a low-quality hot-glue gun rapid proto gear doesn't seem like a good application (obviously it must not slip or skip teeth)--a high-quality thermoset rapid proto gear would work. Cheers


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