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  1. #1
    mtbr member
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    Tap drill size calculator

    You probably have a chart/card for this but it's kind of a cool trick:

    Newman Tool's handy hint.For Metric sizes only. Diameter of thread minus the pitch equals the tap drill size.
    eg. M6 x 1
    6 - 1 = 5mm
    and you thought metric was difficult ;-)
    thanks to Jay Steinbuchel of Barksdale Control Products for the following:
    This works for all 75% threads, not just metric.
    For example, the tap drill for 3/8-16 is 5/16.
    The pitch is 1/16". (1 / threads per inch = inches per thread)
    3/8 - 1/16 = 5/16.
    For other sizes that don't work out so nicely, just use the closest drill size.

    This came from here(calculator at bottom of page):

    Tap Drill Sizes

  2. #2
    RCP Fabrication
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    Thats pretty slick

  3. #3
    Plays with tools
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    Now if only I had a metric drill index.

  4. #4
    650b me
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    Today's rainy day project was to figure out fasteners and corresponding tap for attaching badges to head tubes. I went with 18-8 stainless button head socket cap screw, 4-40 thread, 1/4" length. Does that sound about right? Drill size is #43...guess that's a special order bit, eh?

  5. #5
    Randomhead
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    There is a place here in town that stocks number drills, you might have the same. This isn't a particularly big town. 43 seems like a weird size, but since it's the standard drill for 4-40 most industrial places probably stock it.

    I worked with someone that had a formula memorized for SAE tapped holes. I didn't use it enough to remember, so I just google for tap drill chart.

  6. #6
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