Machining/Engineering Question
 

whats is the formula to find the PD (Pitch Diameter) of a 1/4 42 NS2B thread???

I can find out tomorrow or if you have a Machinery Handbook it would be in there. (Don't have mine with me)

If you are just looking for a tap drill size there is a simple *Rule of thumb* formula that works quite well

Major Dia - (1/TPI)=tap drill size about 70% thread
example 3/8-16
.375-(1/16)=.3125 or 5/16 which is the trap drill for a 3/8-16 screw for a 70% tread

I want to know where you got 1/4-42 screw unless you meant 1/4-24 then the tap drill would be a #4 drill.

I'm at work now and looked it up. I can't find a 1/4-42 in my Handbook but here is what I did find

1/4-24 UNS 2B Pitch Dia = .2229-.2277
1/4-40 UNS 2B Pitch Dia= .2338-.2376
1/4-48 UNS 2B Pitch Dia=.2365-.2401

1/4 42 is a size I use at my work and the engineers are having trouble getting it with in torerance. I think that have the drill szie wrong thats why i need a formula. I couldn't get my hands on a hand book.

Okay here is a formula (from the Machinery Handbook) for calculating tap drill size.

For American Unified Thread
Hole Size = Basic Major Diameter - ((1.08253*Per Cent Full Thread)/Number of Threads per Inch)

Per Cent full Thread is expressed in decimal form i.e. 75% = .75


Is this what you were looking for??

Most likely, I'll try it.