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  1. #1
    johncollins is offline Aluminum
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    Default Internal thread minor diameter

    I have a one-only job to make a female threaded adapter for a guitar stand. The male thread I have to match is a odd size (near 7/8th's) and is 26tpi. My question is how to determine hole size for single point threading? My idea was to measure major diameter of the male threaded piece and subtract the double depth of 26tpi (.050) extrapolated from the figures found on a Starrett center gage. Is this the way to figure this or is there a better way?

    Thnx - John C

  2. #2
    TAG
    TAG is offline Aluminum
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    You sure it's not metric? As in 22-1.0 mm thread?

  3. #3
    johncollins is offline Aluminum
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    Default Internal thread

    Sorry, I made a typo, the male thread is near "5/8ths" (.607 is the major dia.). I tried a 1.0mm thread gage and it fits fair but not as good as a 26tpi gage to the best of my vision. It measures 15.44mm and I can't find a 16 X 1.0 tap in any of the catalogs I have.

    Am I in the ballpark on figuring the minor dia. hole size for any oddball size female thread with the double depth of the thread pitch subtracted from the major dia. of the male thread I want to fit?

    Thnx - John C

  4. #4
    R.P. Herrmann is offline Plastic
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    Default

    Subtract one thread pitch ( .0384 ) from the major dia. and you get tap drill size.

  5. #5
    gusmadison is offline Aluminum
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    I think you'll find the actual TPI is 27.

    Gus

  6. #6
    walker1937 is offline Aluminum
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    Default

    Quote Originally Posted by johncollins View Post
    I have a one-only job to make a female threaded adapter for a guitar stand. The male thread I have to match is a odd size (near 7/8th's) and is 26tpi. My question is how to determine hole size for single point threading? My idea was to measure major diameter of the male threaded piece and subtract the double depth of 26tpi (.050) extrapolated from the figures found on a Starrett center gage. Is this the way to figure this or is there a better way?

    Thnx - John C
    Take a look here. They show a fine metric thread 16 x 1 with a 15 mm tap drill

    http://www.kramerusa.com/DrillTapChart.htm

  7. #7
    Glenn Wegman is offline Stainless
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  8. #8
    johncollins is offline Aluminum
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    Default Internal thread

    Thanks guys for the info. I bored the hole based on the formula (major dia - one thread advance, .615 - .038 = .575 and 26TPI) and it fit the external thread I had to match perfectly. I think it was originally 5/8" x 26 but I cant imagine a mfg using such an odd size.

    John C

  9. #9
    bob ward is offline Aluminum
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