Model Threads 3/16-40 vs 10-40

[Bill Shields]

For tap drilling.. my thumb rule is OD - pitch

Been getting me close enough for 50+ years

[Peter Crisler]

For tap drilling.. my thumb rule is OD - pitch

Been getting me close enough for 50+ years

One of the very first Rules of Thumb that I learned in the earliest days of my apprenticeship, and it works quite well.

In the 26th edition of Machinery's Handbook, on page 1900 there is a formula for figuring tap drill sizes.

Basic Major Diameter = 1.08253 x pitch x % of thread depth.

This will yield a tap drill size a wee bit bigger than the OD - pitch calculation or even what is on most tap drill charts. On page 1890 it states "Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread. Often, a 55 t0 60 per cent thread is satisfactory, although 75 per cent threads are commonly used to provide an extra measure of safety."

A slightly bigger tap drill size, when tapping difficult material with a small tap, could make the difference between a good thread and a broken tap.

Peter Crisler

[Rich_Carlstedt]

Peter, you mentioned some good things that all home shop folks should be aware of.
During the WW II, the Machine Tool Association Leaders got together and agreed ( common war effort)
that all higher strength Steels and Stainless should only be tapped at 55-60 % thread, even though the charts (70 %)
may be different . The charts are just guidelines and as you said, but it will save the loss of a tap particularly with small taps

The other standard to be aware of is materials and thread depth.
The standard to use there is that when using same materials ( hole and screw) , you need thread depth to match diameter
So if you are using a 1/4-20 steel screw in steel, you only need to go 1/4" deep to get maximum strength . any more engagement is a waste of effort.
Conversely , If you are using steel screw in Aluminum or brass, you need more thread depth. Manufacturers have sort of standardized that as either
1-1/2 or 2 times diameter for depth of thread..Along with that , know that a blind hole needs to be drilled one diameter deeper to account for the
front clearance portion of the tap. So in the case of the 1/4" tap, if you wanted 2 times thread depth ( .500") you need to drill to 2 x diam + 1 diam or .75" to achieve that

Rich