Measuring an unknown thread pitch.

Discussion in 'Horological Tools' started by flynwill, Mar 2, 2011.

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  1. flynwill

    flynwill Registered User
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    Feb 1, 2007
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    I had cause recently to want to know the thread pitch on the stem of an Elgin wristwatch I was working on as I was unable to find a crown or stem extension to fit. I tried several different things before hitting upon this idea:

    A bit of thin soft metal is drilled about .1 mm smaller than the thread. (I used a scrap of copper because that's what I had handy, brass, aluminum or even plastic would probably work as well). Then with the stem held in the headstock of my lathe I carefully turn it while pressing the copper sheet, and the steel threads will self-tap into the soft metal. Once it's started I setup a dial indicator with it's tip as close as possible to the hole (see photo), and carefully measure 10 turns of the headstock. If I wanted to be even more clever I would have taken the time to make a hollow tip for my dial indicator. That would distribute the pressure evenly and make it a bit easier to get an accurate reading.

    View attachment 4971


    In this particular case the 10 turns took 0.083" --> 0.211 mm/turn which is a lot finer than the standard 0.25 mm for a 1mm screw. That also works out to 120 threads/inch, which given this is a American made watch should not be too surprising.
     
  2. Dave B

    Dave B Banned

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    #2 Dave B, Mar 2, 2011
    Last edited: Mar 2, 2011
    My copy of American Machinists Handbook, 7th Ed., in Table 16, on page 190, lists the standard Elgin thread sizes. The only one at 120 tpi is tap dia=.0428" or 1.07 mm and a drill size of .0328" or .82mm.
     
  3. flynwill

    flynwill Registered User
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    Feb 1, 2007
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    That's probably it. This stem measures 1.02mm as it major diameter. Thanks.
     

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