Threading question

[racesnake]

Is there a formula for determining the thread depth when boring for a particular thread. I want to make a muzzle brake with a 5/16X24 thread and need to know what size bit to use.
Dave

 

[X Ring Accuracy]

Not sure if this answers your question. But size would mean 60 degree cutting bit

 

[Bob3700]

It sounds like you are new to the nomenclature of threading.

Are you cutting inside or outside threads?

The tools you use in a lathe are different for each operation.

For the outside threads on the bbl. you will need a 60 degree threading tool. You can consult a machinists manual for the amount you move the compound in order to get the correct thread depth.

For the inside threads in the muzzle break itself, you will need a boring bar with a 60 degree threading insert. Same procedure for determining the depth of threads on the break.

You need to get a couple of "How to" books on lathe operation in order to become more familiar with these operations.

HTHs

Bob

 

[racesnake]

Guess I need to clarify my question. I want to know if there is a method to determine how much i need to under bore for a given size and tpi tap.

 

[Artpro]

WOW! 5/16 diameter thread seem woefully small for any muzzle brake, except maybe for 17 caliber. However try the following formula for selecting the nominal tap drill size.

Nominal screw diameter 5/16 = .3125 inch, minus the screw pitch 1/24= .0417
Answer .2708" Letter drill "I" is closest at .272'

If the nominal screw diameter is 5/8 = .625 inch, minus the screw pitch 1/24= .0417
Answer .5833", and there isn't any standard drill size even close.

So break out the boring bar

 

[racesnake]

Durn! I misspoke in my original post. should have been 5/8" rather than 5/16" Sorry.

 

[racesnake]

Thanks, fellows, for the help. Much appreciated.
Dave

 

[jscandale]

Since it is a 24 TPI, the tips of the thread are .042" apart. Half of that would be .021". So, the minor diameter of the hole would be: .625"- 2(.021"/ TAN 30 degrees). Or, .625"-.058"x2, which is about .509" for the minor diameter.

JS

 

[DOGCAPPER]

Be easier to look at the tap/drill selection chart.........LT

 

[Gundoktr]

Since it is a 24 TPI, the tips of the thread are .042" apart. Half of that would be .021". So, the minor diameter of the hole would be: .625"- 2(.021"/ TAN 30 degrees). Or, .625"-.058"x2, which is about .509" for the minor diameter.

JS

I think your math is a little skewed! You would have a helluva time even getting a tap to start in that small a hole.

 

[jscandale]

Since it is a 24 TPI, the tips of the thread are .042" apart. Half of that would be .021". So, the minor diameter of the hole would be: .625"- 2(.021"/ TAN 30 degrees). Or, .625"-.058"x2, which is about .509" for the minor diameter.

JS

I think your math is a little skewed! You would have a helluva time even getting a tap to start in that small a hole.

Could be, I just did it my head early one morning. The moral of the story is to find the adjacent side of the 30 degree thread (half of a standard 60 degree thread), multiply that value times 2 and subtract it from the major diameter.
I'll redo the math later and correct it. I'll use a calculator this time!

JS

 

[jscandale]

Since it is a 24 TPI, the tips of the thread are .042" apart. Half of that would be .021". So, the minor diameter of the hole would be: .625"- 2(.021"/ TAN 30 degrees). Or, .625"-.058"x2, which is about .509" for the minor diameter.

JS

I think your math is a little skewed! You would have a helluva time even getting a tap to start in that small a hole.

Thanks for pointing out the mistake, I recalculated with a calculator and came up with .552" which would give 100% of thread engagement which isn't very practical for any application. I think if I were to bore and tap this kind of hole, I would drill ream to 9/16 (.562) since it is a standard drill size and then use a small inside threading bar.
Math: .625" - 2 [.5(1/24)/TAN 30°] = .552"

 

[butchlambert]

It is nice to be able to figure it yourself. But I either use a tap drill chart or my Machinest handbook.
Butch

 

[kendog]

Try this.
Thread O.D. - .975/pitch = I.D.

.625" - .0406" (or .041") = .584"

From Machinists Bedside Reader Vol. 1, P. 19

I have used this formula many times over the years and always had good results. It will get you close enough for most of what needs to be done in the gun business.

 

[skinnj1]

Equation you are looking:

single thread depth =.54127xpitch= .54127x(1/24)=.02255

Major Dia-2xsingle thread depth=minor diameter

minor diameter= .625-(.02255)x2=.5799

Short Answer, most use 37/6 drill bit........

 

[butchlambert]

What size drill?
Butch

 

[kendog]

What size drill?
Butch

37/6ths. Dont you have one??

 

[butchlambert]

You guys doing all these mental excersizes is good for your brain I guess, but I need to make chips. Do any of you have more knowledge than the Machinest Handbook, I thought not.
Butch

 

[kendog]

I keep my simple formula on a sticky note on the wall above my lathe. Used it just yesterday to fab a temp barrel fitting nut for an AR-15 (1.250-18 TPI). Takes less than a minute to calc it out, and I dont get my nice clean Machinery's Handbook all greasy.
Just for fun I looked up the 1-1/4-18 and 5/8-24 threads and the min and max internal minor diameters are 1.190-1.203, and .580-.590 respectively. My formula provided 1.196 and .584. How close is that?

Anybody working to 5 or 6 places to the right of the decimal is in a different world than I am.

Thread Diameter - (.975/pitch) = hole size.

All this made me thirsty, must find beer.

 

[skinnj1]

Butch- for your information the equation listed was out of an OLD machinest handbook.I agree, Drill and Tap charts are quickest but they will not give you the answer for non-standard threads. Based upon the original posters question I would ASSUME he doesn't have a copy of the handbook regardless of era. Question asked, Question answered.Why the statement on who has more knowledge than the handbook? Wise people know what they do/don't know and they know when to ask (original poster) for help. I just happened to have the answer handy,as I had to do a similar calculation for a 1.5"x28TPI hole I was threading the other day, doubt you will find that one on a drill chart.
Have a nice day.