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Thread: Calculating tap drill the thread FORMING taps

  1. #1
    mklotz mklotz's Avatar
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    Calculating tap drill the thread FORMING taps

    One way to minimize tap breakage is to use thread forming taps. However, they require a larger tap drill than that used for the equivalent thread cutting tap.

    In my DRILL program, I use the following equation to find the tap drill for thread forming taps...

    TD = MD - 0.0068*DOT/P

    where:

    TD = tap drill size
    MD = major diameter of thread
    DOT = desired depth of thread expressed as percentage
    P = pitch of thread expressed as tpi

    For example, a 1/4-20 thread with 75% DOT yields:

    TD = 0.25 - 0.0068*75/20 = 0.2245"

    Closest conventional drill is 5.7 mm but a #2 is close enough.
    Last edited by mklotz; 09-09-2015 at 11:15 AM.
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    pennswoodsed's Avatar
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    Quote Originally Posted by mklotz View Post
    One way to minimize tap breakage is to use thread forming taps. However, they require a larger tap drill than that used for the equivalent thread cutting tap.

    In my DRILL program, I use the following equation to find the tap drill for thread forming taps...

    TD = MD - 0.0068*DOT/P

    where:

    TD = tap drill size
    MD = major diameter of thread
    DOT = desired depth of thread expressed as percentage
    P = pitch of thread expressed as tpi

    For example, a 1/4-20 thread with 75% DOT yields:

    TD = 0.25 - 0.0068*75/20 = 0.2245"

    Closest conventional drill is 5.7 mm but a #2 is close enough.
    A little confusing, I am easily confused. Took me a moment or 2 to get 5.7mm from math shown.
    Thank,Ed

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    mklotz mklotz's Avatar
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    25.4 (mm/in) is an exact number, not an approximation. In fact, the American inch is defined as 25.4 mm. It's worth memorizing that number.

    Note that the 5.7mm figure does not come directly from the math; it's merely the closest conventional drill size to the size dictated by the formula. It differs by a totally inconsequential ten-thousandth.


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