
HERE'S
THE EASIEST TAP DRILL FORMULA THAT YOU WILL FIND. Most of us have a tap drill chart for normal taps,
such as a 1/420 which calls for a #7 drill for 70% thread. But how about a
1/440 tap? Most
of us have a tap drill chart for normal taps, such as a 1/420 which calls for a
#7 drill for 70% thread. But how about a 1/440
tap ? We
will start with a 1/420 as it is a common tap.
HERE'S THE
FORMULA Tap
Drill Size = Major Diameter of the tap less the Pitch. In
this case Pitch is the number of threads divided into 1.00" 1.00
divided by 20 = .050" thus the pitch for this tap is .050" So
.250" less .050" = .200" A
#7 drill is .201" ON
TO MORE A
1/440 tap drill size .250"
less pitch = hole size 1.00"
divided by 40 = .025" .250"
less .025" = .225" So
we want a drill .225" the closest drill is a #1 drill .228" This
formula works with all Straight Taps METRIC
TAPS Metric
Taps are even easier. The
Metric Thread System uses Pitch instead of number of threads per inch. ON
TO THE FORMULA We
will start with a 10mm tap with a 1.0mm pitch 10
mm less 1mm = 9 mm so you need a 9 mm drill which is .3543" or the closest
US drill which is a T drill .358" MORE
EXAMPLES 12.0
MM tap with a 1.75 mm pitch 12.0
mm less 1.75 mm = 10.25 mm which is about .401" the closest drill is a Y
drill ,404" 
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