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  • drill size?

    I've got a 9/32-32 tap, need to drill a hole the right size. I'm thinking its between a 1/4 and 5/16, so that would be between about .235 and .260+/-? Maybe a K or L drill bit? Any suggestions for figuring a drill size for an oddball thread?

  • #2
    Tap Drill Size

    9/32-32, about .2473"- .2548", letter "D" .246" will probably cut a tad large and be on the low, .250" dia in the midrange, "F" .257" a bit oversize!

    Cheers,
    Les H.
    The Impossible Takes Just A Little Bit Longer!

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    • #3
      Subtracting the pitch from the nominal major diameter will give you approximately 75% thread depth, so in this case pitch is .03125 and your major diameter is .28125, so you end up with .250 for the tap drill.
      Stuart de Haro

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      • #4
        Originally posted by hornluv
        Subtracting the pitch from the nominal major diameter will give you approximately 75% thread depth, so in this case pitch is .03125 and your major diameter is .28125, so you end up with .250 for the tap drill.
        I get the same answer.
        Paul A.
        SE Texas

        Make it fit.
        You can't win and there is a penalty for trying!

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        • #5
          0.2549" for a 65% thread

          0.2508" for a 75% thread

          0.2467" for an 85% thread

          A quarter inch drill would probably be fine.
          ----------
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          • #6
            Yep. That "system" for metric tap drill works just fine for any
            dimensioning system.
            "Basic dimension minus pitch"
            ...lew...

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            • #7
              Cool, and I already have a 1/4" drill bit to use. Thanks for the help guys.

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              • #8
                Does that rule (nominal dia minus pitch) result in 75% for BOTH metric and std threads?
                Lynn (Huntsville, AL)

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                • #9
                  On a drill chart. find the proper size size tap drill dia for a 32 pitch thread. IE. (10-32- tap is a #21 (.157). <187- .157 = .020.
                  Take your major Dia of your thread and minus that Dia. (-.020)
                  This will work in all instances. All pitches and all Diameters. 1/2-13 or 5/16-24 and plus to infinity.
                  DOT is the same.
                  I'm just an old shop fart.

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                  • #10
                    Originally posted by lynnl
                    Does that rule (nominal dia minus pitch) result in 75% for BOTH metric and std threads?
                    Works for any 60 deg thread.

                    Whitworth and acme you have to look up. :-)
                    ...lew...

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