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MACHINISTS GUIDES

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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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Copyright 2001 METALWORKING BOOKS
Last modified: November 29, 2001