As gunsmiths we sometimes run across a job where there is no published thread data, or, the receiver has had it?s threads recut and it is up to us to figure out what the new dimensions are. Here is a handy formula for figuring threading tool infeed, and, major and minor diameters based on known thread pitch (ASME 60 degree V), and at least 1 (one) known diameter.

The formula: 1 / (TPI) x 0.613 x 1.155 = depth of in-feed. Multiply this by 2. This is either add to, or, subtract from the known diameter, to get major/minor diameters.

The application: You are handed an object with a female thread that measures 0.850 across the minor diameter of the threads. The thread pitch is 12 TPI. What is the thread tool infeed, and, what is the major diameter?

Using the above formula, the first thing is to divide 1 by 12 (TPI) which gives us 0.08333 You multiply this by 0.613 which gives you 0.01083329 You multiply this by 1.155 which gives you 0.059 which is the thread tool infeed. Multiply this 0.059 by 2 and add this to the minor diameter to get the major diameter of 0.968?

Don?t forget to make enough free passes as you advance the thread tool to avoid thread stacking or, undercutting. I usually opt to get it close and then finish it by lapping it in. This formula works pretty slick on those occasional odd jobs. Give it a shot and let me know...

Malm

The "tap drill rule" is taught about the second day of any machinist class: tap drill diameter = major dia. - the pitch. So, if I apply that to your example of a female thread with a "minor diameter" of .850, I would just add the pitch to get the major dia. The pitch of a 12 t.p.i. is .083. So the major dia. would be .933". I would turn a piece to about .935 and start threading and check it with thread wires. I'd guess the thread is a 15/16 X 12.
Show me where I went wrong 'cause I got a different answer but took some shortcuts. Jay

I would say the difference is somewhere in the short cut you took.

Malm

Sorry about cutting that response short, I was pulled away for a moment...

I think the "tap drill rule" you learned on "your second day" of a machinist class is designed to get you in the ball park. Most taps are based on a 75% depth and this may be the reason for the error. While 75% depth may be sufficient for holding a part or a sight, 75% depth just isn't good enough when cutting barrel threads and so the formula I presented is meant as a "guide" for a little more precise fit...

Malm

What is the origin of the constants in your formula? "1 / (TPI) x 0.613 x 1.155 = depth of in-feed" Is the "depth of in-feed" for the crossfeed or the compound set at the appropriate angle? I think you have given us some values from a trig table? Please enlighten me. Thanks, Jay.

Crossfeed at zero, straight in.

Malm

So your figure of in-feed is the base of a 30, 60, 90 degree triange that has a height of pitch/2. I'll have to check the trig tables tomorrow and find the figures 0.613 and 1.155, then I'll be able to rest. Thanks, Jay

G.Malmborg,
I'm curious where you found this equation that you're using. The proper dimension for a 15/16-12 2B class thread would put the major diameter at .9375". A 2B class thread is plenty adequate for barrel threads. That is the standard for i.d. threads on damn near every boeing aircraft in the air.
I would suggest using the Machinery's Handbook for accurate info on threads and other various types of machining.
I've done countless O.D. and I.D. threads, but have never come up with a formula that I would trust. That's why I own the bible of the machining industry. Takes the guessing out of the equation.
By the way, 75% threads are the maximum amount of engagement that you will most likely ever see in precision machining. Most times you can get away with much less, which will significantly reduce the amount of torque that is placed on a tap, for instance. Take a 1/4-28 tapped hole for instance. Recommended call out for drill size is a #3, which is .213. This combination is recommended to give you 75% thread engagement, or max load on tap. If you increase the size of the drill to 7/32, or .218", you will decrease tap load by as much as 15-20%. This seems somewhat insignificant when you're just fooling around with a couple of holes, but when you start tapping 10,000 at a time, your tooling bill will drop dramatically.

I gave up on the formulas when dealing with re-threaded actions. Now I wax the internal threads and make a cast from quick set epoxy then measure that.

It's WAY too common to find threads cut with odd tools or standard V tools that weren't properly located and the resulting thread form is out of kilter a little. By making a positive cast to actually look at matching threads can be cut on the barrel shank.

The formula was given to me by a fellow who taught machine trade at the local community colledge some 21 years ago. I use it now and again on odd jobs and it has worked pretty well. That is why I was passing it along. Now, if you don't like it, it doesn't work for you or you don't know whether it is "trig" based or whatever, then don't use it... Use what works for you.

Malm

I appologize to one and all. I miss quoted this thread depth formula. The actual formula is .613 x pitch = depth of cut. This is a plunge cut with the cross feed set at "0". The depth of cut is multiplied by 1.155 wich represents the amount of feed, using the compound set at 29 degrees. I failed to read the foot note at the bottom of the paper when I passed this along to you. Like I said, I have used this formula (threading off the compound) and it works pretty well for external threads, I've just never explained it as you can tell. Again, I appologize for the miss quote.


Malm

I have the Machinery's Handbook, but it doesn't handle the odd ball stuff, or, not that I can find.

Malm,
Start with page 1629 in the 25 edition. It's very enlightening to your problem. Explains everything you'll ever want to know about internal and external threads. Table 1 on page 1638 would handle the oddball problems associated with undetermined minor and major diameters. It tells you just exactly what the dimensions of the actual thread will be based on it being, for example, 12TPI. Referring back to this book reguarly sure takes the guessing out of the equation.
My only problem is that I don't have a photographic memory, so I find myself looking over the same damn info constantly.

Thanks Matt.