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 1/4-20 which calls for a
#7 drill for 70% thread. But how about a 1/4-40 tap?
Most
of us have a tap drill chart for normal taps, such as a 1/4-20 which calls for a
#7 drill for 70% thread. But how about a 1/4-40
tap ?
We
will start with a 1/4-20 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
1/4-40 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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