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Machining Question - threading

gunplumber

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I've thread thousands of barrels. But I'm not a machinist. I'm totally self-taught. And so I occasionally watch YouTube videos to try to become better at machining stuff.

I watched some videos by Blondihacks and think I can improve my game. What I found using an indicator that .0030 on the compound is only .0026 on the indicator, and I suspect the error will increase with deeper cuts. So, how deep should my cut be?

I am confused with some of the terminology.

Lets take 9/16-24

"Nominal" is .0625", and my understanding is that "Nominal" is what it technically perfect, but we actually need to cut smaller or the nut (flash hider) will be way too tight. In other words, we don't want 100% thread engagement.

I usually turn the barrel to .560, or .0025 smaller than nominal. Then make three passes of about .001" each on the compound, then a cleanup pass.

What I did in practice was try different cut depths until I found where on the compound dial was "just right" and a blend of removing all the dykem, and not having to chase it with a die for being too tight. And that was .003

But now I want to be better. So I look up

Minor diameter .5124. and thread height of .0226"

.0562
-.5124
----------
.0496
/2
--------
0.0248 thread height, not the .0226 on the chart. Which confuses me. Why the .0022" difference?

Which I think means if I move the compound in .025, I'll have the right thread height.

But that assumes I'm starting with the Major Diameter, and not the Nominal Diameter.

If the major diameter is smaller than the nominal, than the same advance of the compound will leave a shallower thread.

So what are these dimensions for "Optimal" 2A threads.
1. What diameter start
2. How far to advance compound
3. Optimal finished major diameter ( can't measure minor that accurately, as the root is narrower than my calipers)

I understand that starting .002" smaller than Nominal will require .001" deeper cut to have the same thread height.

My brain hurts. I just want to figure this shit out because I think I can improve tolerances and consistency.

IMG_0249.JPG
 

tac-40

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If the pic is your setup, then you also have to take into consideration that the movement of the crossfeed is at an angle, not perpendicular to the axis. Your Mike is measuring the distance traveled towards the axis and does not take into account the angular movement, which is measured by your dial. If you measure that angular travel, it should be closer to actual movement.
 

OMR_RDTandE

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Well, I don’t have my copy of “Machinery’s Handbook” readily available, but there’s a method in there for measuring threads by utilizing a thread measuring wire set. Amazon sells both of these items. I also do remember there are different “fit” tolerances for threads. It’s all in the book.
 

STG_58_guy

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Mark, you don't quite understand what pitch diameter is yet. You are close. It would help you to look at the diagrams in Machinery's Handbook. I'll send you my copy if you want. PM me.

The pitch diameter is what determines how tight threads mesh. The pitch diameter is an imaginary cylinder that passes through the thread form so that exactly half the cylinder passes through the thread material, and half passes through the air between the threads. On an external thread, a larger pitch diameter will have a tighter fit. On an internal thread, a smaller pitch diameter will have a tighter fit. Think of increasing the pitch diameter as swelling the threads on an external thread.

The major and minor diameters only effect how much surface contact there is between mating threads, and mating threads only contact on one side of the valley.

Notice how both the major and minor diameter are "truncated" in this diagram. You could increase the major diameter of this thread by leaving a "sharper" peak.

1700475838060.png


You could also make the minor diameter smaller by having a sharper valley, or root. But sharp notches raise stresses significantly, so good threads always have a radius at the root. And the peaks are always truncated so they won't interfere at the rounded root.

The important number is the pitch diameter. That's what really controls the strength and fit. Both the minor and major diameters follow the pitch diameter, not the other way round.

All these values have standard tolerances for every ANSI thread size and thread class.

For timed threads, the timing depends on both the starting point, or phase, and the pitch diameter. A loser fit (smaller pitch diameter on an external thread) will rotate farther before tightening under prevailing torque.

If you have 3 correctly sized, precise wires you can measure pitch diameter very accurately on an external thread. It takes a little practice and a good micrometer. Google "three wire method". Internal threads are a lot harder to measure.

A good way to learn about this stuff is to go to work for a couple guys who were the fastener experts at Case IH on your first job out of school. Guys like that are gone now.
 
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gunplumber

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I have a machinist handbook, 24th edition. And I have google. I have the diagrams (which can't even agree if 24 tpi is NEF or NF). Access to information is not a problem. I don't know how to apply them.

I have the compound at 30 degrees, not 29-1/2, because I alternate left and right hand thread several times a day and I'm not going to keep switching. There is some debate how important being 1/2 degree off is , anyway, with modern cutters.

I think StG guy, that you partially got the question, in clarifying a term - that cutting the major and minor create the pitch diameter, and pitch diameter creates the tightness. But the questions remain - I was looking for specific numbers. "Do this, this far".

As a test, I did one yesterday starting at exactly 0625, and advance .027. The thread was too tight and I had to chase it with a die.
But was it too tight in the starting diameter or the depth of the cut or both?
 

Impala_Guy

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Not a machinist either, but as an ME and constant tinkerer I've done stuff myself in the shop when I had to get it done, and threaded my share of shanks and holes. When using taps and dies you obviously don't have to worry, but if you have a series of thread sizes you are continuously lathe cutting, it may be helpful to get a set of plug and ring gauges for that thread. And of course, you can have a set made to whatever spec you want, and you can have a set of MAX / MIN plug gauges made. Not sure how helpful this would be for muzzle threads, but just spitballing here.
 

jpa61

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As a test, I did one yesterday starting at exactly 0625, and advance .027. The thread was too tight and I had to chase it with a die.
But was it too tight in the starting diameter or the depth of the cut or both?
if the advance of .027" was with the compound @ 30degrees ? if so the advance should be .0312" because of the angle of in-feed, also good practice is to take a spring pass on finale depth (no in-feed)
 

gunplumber

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if the advance of .027" was with the compound @ 30degrees ? if so the advance should be .0312" because of the angle of in-feed, also good practice is to take a spring pass on finale depth (no in-feed)

Thank you! Which explains why .300 has always seemed to work. And yes, I do a final pass with no feed - I call it a cleanup pass, but if it's called a "spring" pass, I'll use that.

Side question - is there any advantage to smaller cuts?

I've been making three passes of .010" (appx) followed by the spring pass. But Blondiehacks suggests even smaller passes at the end. So I experimented with .010, .010, .005, .005, then the spring pass. Didn't note any difference other than the spring pass may have had less material removed (from very little, to almost none).
 

gunplumber

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So with my 30 degree compound, that leaves the other two angles 75 degrees . . . I vaguely remember using Pythagoras for a block wall, but that requires a right triangle. How do I convert the "in toward the work" feed to the depth at 30 degree diagonal?
 

jpa61

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Thank you! Which explains why .300 has always seemed to work. And yes, I do a final pass with no feed - I call it a cleanup pass, but if it's called a "spring" pass, I'll use that.

Side question - is there any advantage to smaller cuts? yes, less tool pressure, as you move in more & more of the tool is in contact

I've been making three passes of .010" (appx) followed by the spring pass. But Blondiehacks suggests even smaller passes at the end. So I experimented with .010, .010, .005, .005, then the spring pass. Didn't note any difference other than the spring pass may have had less material removed (from very little, to almost none).
 

jpa61

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So with my 30 degree compound, that leaves the other two angles 75 degrees . . . I vaguely remember using Pythagoras for a block wall, but that requires a right triangle. How do I convert the "in toward the work" feed to the depth at 30 degree diagonal?

you can trig it out, but I am sure there are charts out there, here is one I found with a quick google, https://kb.wisc.edu/engr/teamlab/page.php?id=81932
 

MK ULTRA

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Don’t worry about the angle and Pythagoras. I was just telling you that’s why you’re getting the wrong numbers. When you use the thread wires you just go by the Diameter over the three wires, 2 on bottom& 1 on top. Use 1 inch mics & get that diameter and subtract what it should be and sneak up on it with your compound. You don’t need that travel indicator coming off the tailstock and hanging in the air. it needs to be on your cross feed. Your crossfeed starts from zero every Pass. So TI puts tool in same place each cut. Quality dept will use either an expensive thread gauge to ck threads or by using the 3 wires method. The wires are very accurate.
Here’s a good short video on threading.

This guy is really good I watched a lot of his his videos tubalcain. This video is #183 on measuring threads using thread wires
#182 is cutting threads

Mark these 3 vids should really help you improve your threading. With these can get to thread fit as close as you want.
 
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MK ULTRA

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I have a machinist handbook, 24th edition. And I have google. I have the diagrams (which can't even agree if 24 tpi is NEF or NF). Access to information is not a problem. I don't know how to apply them.

I have the compound at 30 degrees, not 29-1/2, because I alternate left and right hand thread several times a day and I'm not going to keep switching. There is some debate how important being 1/2 degree off is , anyway, with modern cutters.

I think StG guy, that you partially got the question, in clarifying a term - that cutting the major and minor create the pitch diameter, and pitch diameter creates the tightness. But the questions remain - I was looking for specific numbers. "Do this, this far".

As a test, I did one yesterday starting at exactly 0625, and advance .027. The thread was too tight and I had to chase it with a die.
But was it too tight in the starting diameter or the depth of the cut or both?
Mark when you’re cutting left hand threads you don’t have to change the compound angle. just cut from the left to the right which the same chuck rotation. Same angle. Or turn your cutter upside down and reverse the spindle direction and cut from right to left. Same angle. you’re still cutting the thread at 29 1/2°.
If you use the pitch diameter number which is figured by going straight in. Then thread that much on the diameter on the compound and you won’t be too deep. Then use the good wires after that. it’ll get fast when you get use to them. you cut a lot of the same threads.
Anyway that’s all I have. That’s how threads are cut on manual machines in industry every day using thread wires. Unless they have expensive go no/go gauges. Look at those videos I put links to.
 
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Jeff in Pa

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While threading is a precision operation, it really isn't that hard.

Use a "threading gage" to make sure your threading tool is square to the part.

Verify the threads per inch is set correctly.

Coat part with bluing or magic marker

Set compound at 29-1/2* and the dial to zero.

Set cross slide to zero with part turning.

Do a "scratch pass" and then verify correct threads per inch with either the 60* center gage or leaf thread gage.

Once verified , thread away starting with .010 , then .005 and ended up with smaller amounts and I usually do at least three spring passes.
*all cuts are done with the compound until you get within about .005, I then turn in the cross slide .001 at a time as I sneak up on "perfect threads"

Threads can be easily checked as you approach finished size with a thread micrometer
( I have this exact mic and it works great)

Pitch diameters are listed in the machinery handbook or can be calculated on the internet.


~~~~~~~~~~~~~~~

Any questions please ask. I do this for a living and may have other looked something in my description
Jeff
 
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ratas calientes

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You guys are great! I never did learn how to cut threads on a lathe. The few times I threaded a barrel . . . I turned down the barrel to the OD that I wanted. While keeping the barrel in the lathe, I used a thread die to cut the threads. Kept the die normal to the barrel by referencing off of the the tail stock, with the live centered removed, to start thread. Turned the barrel by hand. Probably not the best way, but way easy for mounting a muzzle thingy one or two times.
 

tdb59

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While threading is a precision operation, it really isn't that hard.

Use a "threading gage" to make sure your threading tool is square to the part.

Verify the threads per inch is set correctly.

Coat part with bluing or magic marker

Set compound at 29-1/2* and the dial to zero.

Set cross slide to zero with part turning.

Do a "scratch pass" and then verify correct threads per inch with either the 60* center gage or leaf thread gage.

Once verified , thread away starting with .010 , then .005 and ended up with smaller amounts and I usually do at least three spring passes.
*all cuts are done with the compound until you get within about .005, I then turn in the cross slide .001 at a time as I sneak up on "perfect threads"

Threads can be easily checked as you approach finished size with a thread micrometer
( I have this exact mic and it works great)

Pitch diameters are listed in the machinery handbook or can be calculated on the internet.


~~~~~~~~~~~~~~~

Any questions please ask. I do this for a living and may have other looked something in my description
Jeff
^ This is how I was taught by Ron Walton- a bench rest shooter and Curmudgeon Korea Marine.

The only thing added was a very gentle dress of the "peaks " with a fine double-cut file, or a block with emory cloth to eliminate drag and burrs.
Factory internal threads are usually blunted at the top.
 

gunplumber

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You guys are great! I never did learn how to cut threads on a lathe. The few times I threaded a barrel . . . I turned down the barrel to the OD that I wanted. While keeping the barrel in the lathe, I used a thread die to cut the threads. Kept the die normal to the barrel by referencing off of the the tail stock, with the live centered removed, to start thread. Turned the barrel by hand. Probably not the best way, but way easy for mounting a muzzle thingy one or two times.

Did it that way for years, because I was afraid of screwing up with the single-point. Finally, I just decided, I was going to learn single-point threading THAT DAY! I had a hundred cut off barrel sections and I kept doing it, and running back to youtube, until it was something I was happy with. Which is where I came up with my settings. And why now I want to progress beyond that to understand the numbers behind what I came up with.

Next probably is going to be metric, and I have the (27?) tooth change gear, but I just don't have enough demand for it to be necessary.

Oh, and then how to align/time with an already threaded part to recut existing threads.
 

FNG3

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Try this to recut existing threads. Set your threading half nut like you were cutting the threads (engaged, locked to lead screw). Move your cross slide, compound, or tool post to line up the tool with the existing threads. Use good light and magnifiers to get a good fit between tool and threaded piece. Back off a few thousandths. Use dykem or sharpie and sneak up on it. Hope this helps
 
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