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Thread: Metric Threadcutting gears for antique South Bend lathe

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    bobneumann's Avatar
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    Metric Threadcutting gears for Imperial Lathe

    I figured out a way to make perfectly acceptable (Something above 99.5% accurate) metric threads on an old South Bend lathe, just by making a series of 5 different stud gears. The idea should work for almost any imperial lathe. The required gears may vary some from lathe to lathe, the attached Excel spreadsheet can help you figure out your own.

    See the video for details.





    The Excel spreadsheet is designed to simply display the resulting metric thread for every possible combination of stud gear size and gearbox setting. You simply visually find which stud gear and gearbox combination will produce the closest approximation to your desired metric thread. In my case I was able to make a set of 5 replacement stud gears that could produce all the common metric bolts, from the tiny .35M up to the large 5M, all with above 99.5% accuracy to the desired thread. (Most with above 99.9% accuracy)



    The only variables for your lathe are, "what is your "native" stud gear size?", and "what exact tpi settings are available on your QC gearbox?" Enter those in the spreadsheet and it will display the rest for you.
    stud gear metric settings.xls

    Here is the resulting chart for my "single tumbler" South Bend lathe:

    Metric Threadcutting gears for antique South Bend lathe-2019-01-30-08_19_45-stud-gear-metric-settings.xls-%5Bcompatibility-mode%5D-excel.png
    Last edited by bobneumann; 01-30-2019 at 06:54 AM. Reason: More information.

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    Daturat100r's Tools
    You didn't try the 21 tooth gear trick for metric cutting on imperial lathe?

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    Quote Originally Posted by Daturat100r View Post
    You didn't try the 21 tooth gear trick for metric cutting on imperial lathe?
    I'm not familiar with that... By my chart, for this particular lathe, 21 teeth doesn't hit very near anything.

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    Daturat100r's Tools
    It a dodge detailed in L C Masons awesome lathe work book "using the small lathe" will dig out my copy and pick out relevant info,he says it way more eloquently than I can

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    doing a little googling, it looks like that trick applies only to mini lathes with a 16 tpi leadscrew. My lathe (South Bend Heavy 10) has a quickchange gearbox, and I believe, though I'm not certain, an 8 tpi leadscrew. If you open the attached spreadsheet you'll see that, at least for my lathe, a 21 tooth stud gear would turn a perfectly fine 2.0 and 1.0 thread, but that's about it.

    Bob

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    Metric Threadcutting gears for antique South Bend lathe-c4cd25c57855ee3b0a73271b8b1ff0cc8442233b009d08069c1eae388b688ce5.jpg

    Nice videos - I really like your homemade indexer.
    Last edited by Wmrra13; 02-28-2019 at 12:32 PM.

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    Daturat100r's Tools
    Sorry clean forgot to look up info on 21 tooth dodge surely if u have a 8tpi the dodge will work as it does on my myford which has an 8tpi leadscrew,maybe yours is a 6 tpi

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    Daturat100r's Tools
    Stolen from myford site with 8tpi leadscrew the errors of cutting with 21 tooth gearwheel dodge are exceedingly small
    Metric Threadcutting gears for antique South Bend lathe-myford-screwcutting.jpg


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