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?To nut or not to nut?

 
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  #15  
Old 09-13-2010, 09:03 PM
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Re: ?To nut or not to nut?

Quote:
Originally Posted by Trickymissfit View Post
But with the nut you actually stretch the barrel thread and thus greatly increase the thread form contact area.
gary
What's the difference between stretching the barrel threads against the recoil lug (against the shoulder at the end of the threaded tenon) without the nut, or stretching the threads with the nut? I don't see any. The threaded shank of the tenon is going to be under tensile force either way - no?
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  #16  
Old 09-13-2010, 10:20 PM
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Re: ?To nut or not to nut?

Quote:
Originally Posted by Trickymissfit View Post
first of all when deal with threads that are larger than an inch in diameter the best you can ever hope for is roughly a 60% thread. Some folks do say they can get 65%, but I've never seen anything close to that. (read the Machinest Handbook, or better yet order a copy of thread form statistics from the Bereau of Standards from the Government. This 60% is a Class 1 ground thread. You cannot get the proper helix angle turning a thread in a lathe. A thread that's around 1/2" is best at around 70% to 75%, but the actual contact of the thread form (male to female) will be less. It just a fact of life.

Secondly, should you manage to get that 95% thread contact; all I can say is good luck putting it together. And if by chance you do get it together; then good luck ever getting it apart

do this: Cut a 1.05 thread to match the Savage nut. Now take a magic marker and coat the threads on the male part. Screw it together, and then take it off by screw the nut all the way accross the turned thread. Look at it under a strong light and a magnifying glass. Better yet, if you have access to a shadowgraph. Then cut a section of the nut, and clamp it to the thread form. Results make you wonder how the threads even went together.

not being critical, but I made a living doing this for almost 40 years, and it's just the nature of the beast.
gary
No problem for me to get a better fit than 60%.

If you are referring to nuts and bolts you are pretty close. But we are talking about lathe cut
threads and on firearms that have to deal with tremendous forces and the tenon size has
to be designed to handel this with a 2 to 1 safety factor.

I also made my living dealing with metallurgical connections of all types including bolt strengths
and thread fits and types of threads.

Gun work requires the very best fit possible and if a smith can't do better than 60% then he
is called a plumber (No disrespect to the plumbers but they use taps and dies not a lathe to
cut threads).

If you are happy with 60% thread contact then that is your choice but If I build a firearm it will
have a much better contact area than that or I won't let it out of my shop.

I'm sure there are other smiths on this site that can vouch for 90 to 95% but may not want to
participate in this discussion and some are capable of better threads than that.

The only reason you can't get 100% engagement is that the major and minor diameters
should have a slight radius to prevent galling.

So we are back to the original question To Nut or not to nut. I chose to not use the nut
because it adds one more problem that is not nessary unless you want to switch barrels
often and even Savage will tell you that the barrel nut is for ease of assembly not to switch
barrels.

J E CUSTOM
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  #17  
Old 09-14-2010, 11:37 AM
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Re: ?To nut or not to nut?

Quote:
Originally Posted by phorwath View Post
What's the difference between stretching the barrel threads against the recoil lug (against the shoulder at the end of the threaded tenon) without the nut, or stretching the threads with the nut? I don't see any. The threaded shank of the tenon is going to be under tensile force either way - no?
mostly shear force. When you seat the barrel via a shoulder against the recoil lug, reciever, etc, you will use the female threads of the reciever as a "jam nut". Odds are very slim that you will ever get the full length of the female thread to ever make complete contact (each single vee form of the thread, and not area contact). Where as the thread screwed into the reciever and then pulled forward two or three thousandths will force the male threadform into the female thread form. When seating on a shoulder you are sorta stuck with the way it comes out (the solid shoulder controlls everything in that setup). Now if you take the Savage barrel or a Remington style barrel and thread it into the reciever, but stopping short a couple turns. Mount a .0005" dial indicator (I prefer an Interapid by the way) ontop the reciever, and somekind of a small smooth block of metal clamped to the barrel. Now try to move the barrel in and out without turning it. You should see a little end play (.001" / .0025") the amount is not super critical as long as it's not excessive. You could make .004" end play work. What you are now seeing is about 50% clearence and 50% lead error in the thread. Lead error will always be in there no matter how the thread is cut (there is another form of error that I will address later). Lead error comes from the device used to cut the threads, but there are some ways to actually fight this. It's mostly caused by wear in the lead screw and half nut (nothing can be made perfect, and the first time you use the half nut you induce wear in it). Now if you thread the barrel up against a once fired case from that chamber (you could also use a go-nogo gauge), and set the indicator back up again. Then tighten the barrel nut. When you seated the barrel against the case shoulders you took out all the endplay in the threads, but now when you tighten the nut you should see the barrel move forward slightly (.0005" to .001"). You've now stretched the thread making it as strait as it can possibly get; plus you have forced the forward face of the thread into the rear face of the female thread for near perfect contact (well maybe 75% at best if the lathe has any wear in the slide or spindle bearings)

Stretching threads is nothing new, but really wasn't thought about much till about forty years ago. Now adays it's a commonly done thing when assembelling machinery that has bearing mounted at both ends of a screw (mostly ball screws).

Now back to cutting threads one more time. Most of the barrels used here are cut from 400 series stainless steel, but still a few big bore hunting rifles will use C/M steel in various grades (lets hope it's a pretreat form). The Savage uses a 20 pitch thread, while a Remington is an 18 pitch if I remember right. The difference between the two means little here as they are very close to each other in size (it would have been nicer if Remington had used a 1.06-16 thread!). If you take a piece of barrel steel and chuck it up in the lathe to cut a thread, and turn the O.D. to the size needed. Now setup a dial indicator on the O.D. of the piece of steel your threading and the set another up on the slide to see if the slide is doing any shift to the side when it comes under tool pressure. When you make the second pass you may see the idicator jump ever so slightly (lets hope it's less than .0005"). That's known as lag in the spindle bearings, and also is a nature of the beast. The higher the load the more flex. But you actually can releive some of this by using high speed steel to cut the threads (Rex 95, Vasco Supreme, Gorham Cobalt). Still .0003" is about as good as it gets in a lathe, and that figure will double normally. (spindle flex will change the thread form) Now clamp a parrell bar to the slide, and try to pull it sidways in both directions in the area your cutting. That's wear and clearence in the gibs, and will change the thread form 90 degrees to spindle flex. It's also why you cut the threads in a series of passes instead of two passes. Cut a thread about three inches long, and then measure the thread over three wires every half inch. Now do the samething 90 degrees to where you made the last set of measurements. You just saw the lead error in the lathe. If you could examine the thread under a microscope you'd see little bumps and valleys in it as well (there is a way to measure this, but it's not important here). But in the end it just proves that you cannot turn a perfect thread in a hand lathe (even a precision finish lathe)

gary
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  #18  
Old 09-14-2010, 11:56 AM
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Re: ?To nut or not to nut?

Quote:
Originally Posted by J E Custom View Post
No problem for me to get a better fit than 60%.

If you are referring to nuts and bolts you are pretty close. But we are talking about lathe cut
threads and on firearms that have to deal with tremendous forces and the tenon size has
to be designed to handel this with a 2 to 1 safety factor.

I also made my living dealing with metallurgical connections of all types including bolt strengths
and thread fits and types of threads.

Gun work requires the very best fit possible and if a smith can't do better than 60% then he
is called a plumber (No disrespect to the plumbers but they use taps and dies not a lathe to
cut threads).

If you are happy with 60% thread contact then that is your choice but If I build a firearm it will
have a much better contact area than that or I won't let it out of my shop.

I'm sure there are other smiths on this site that can vouch for 90 to 95% but may not want to
participate in this discussion and some are capable of better threads than that.

The only reason you can't get 100% engagement is that the major and minor diameters
should have a slight radius to prevent galling.

So we are back to the original question To Nut or not to nut. I chose to not use the nut
because it adds one more problem that is not nessary unless you want to switch barrels
often and even Savage will tell you that the barrel nut is for ease of assembly not to switch
barrels.

J E CUSTOM
actually nuts and bolts are often much looser than 60%. And in most cases the threads are rolled not cut. But if you take an aircraft quality thread (often ground) you'd be lucky to see 65% contact. Take a 1.06-16 male master, and a 1.06-16 female thread master. Screw them together. You'll still see some endplay, and these threads are ground. Guys have been trying to get the movement out of the barrel thread for eons, and there are only two methods that work, and one can be dangerous if done even slightly wrong. (there is a third way that is very tricky todo, and know of nobody that's had the balls to try it yet)

this thread (now there's a pun) has evolved into a good Applied Mechanics (forces and vectors) lesson or Physics 102 question, but better yet how to deal with the beast
gary
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  #19  
Old 09-14-2010, 12:25 PM
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Re: ?To nut or not to nut?

I just picked up on this thread. It appears to me that Gary and JE may be talking about 2 different interpretations of 100% thread fit. Sounds like Gary is explaining the difficulty of achieving 100% thread engagement on both pitch faces simultaneously. I believe JE is talking about the percentage of engagement on the pitch faces that are under load, (maybe better described as the thrust faces). If this is the case, It's one of those rare instances when I agree with both sides of a discussion. At any rate, a very instructive sidelight to the original question; to nut or not to nut?

My opinion from studying both the Savage and the Remington, is that I very much prefer the Remington makeup. I do like the long Savage tenon, but have no preference between the 16 tpi Remington and the 20 tpi Savage thread. These are just my opinions though, not backed by any science.

Good thread.

Tom
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  #20  
Old 09-14-2010, 02:22 PM
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Re: ?To nut or not to nut?

Quote:
Originally Posted by specweldtom View Post
I just picked up on this thread. It appears to me that Gary and JE may be talking about 2 different interpretations of 100% thread fit. Sounds like Gary is explaining the difficulty of achieving 100% thread engagement on both pitch faces simultaneously. I believe JE is talking about the percentage of engagement on the pitch faces that are under load, (maybe better described as the thrust faces). If this is the case, It's one of those rare instances when I agree with both sides of a discussion. At any rate, a very instructive sidelight to the original question; to nut or not to nut?

My opinion from studying both the Savage and the Remington, is that I very much prefer the Remington makeup. I do like the long Savage tenon, but have no preference between the 16 tpi Remington and the 20 tpi Savage thread. These are just my opinions though, not backed by any science.

Good thread.

Tom
in theory a 20 pitch thread will hold it's torque value better than a 16 pitch, but you can't prove it by me! All I'm saying is that a stretched thread is strait and ridgid. The percentage of thread contact becomes a moote point with contact achived via a stretched thread. With a barrel shoulder you put the stress at the shoulder and maybe the closest three to four threads to the shoulder (depending on the lead error stacking up). Then when you have ignition, you have two main vectors of force acting against the bolt and the barrel itself. This is then followed by a like reaction going in the opposite directions. This allows the threaded portion of the barrel to move as pressure builds up. But a barrel in close to full contact will not allow the threaded portion to react nearly as much as the barrel with the floating thread
gary
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  #21  
Old 09-14-2010, 03:06 PM
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Re: ?To nut or not to nut?

There is another good reason not to use the nut because you are not stuck using that tiny ass dam breach......

Loose the nut and put a real barrel on that thing. You will be much happier in the end.

bobby
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