Cutting threads

[twosmokinspades]

I traditionally cut threads in reverse, so way from the head stock. I make a relief cut where I want to start the thread. Because I have a metric lead screw once I make the first cut I lock the carriage in place and never unlock it till I am finished. So, my question is, I have seen threads made in the tenon almost right to the barrel shoulder with no relief cut. How are they doing this? They can't possibly be pulling the tool out at the exact same time. I could see a CNC machine doing this, but most guys I see have manual machines. I ask because I now have a Defiance Elite action and when I want to set the barrel back I'll have to cut off a lot, I think it was around 1.600 cant remember the exact number. With a remington I only had to cut off to my relief cut so I would loose about.0.75 to 1.000 total barrel depending on how the timing worked out.

 

[leelad]

Well there's a few things to remember. The shortest answer is that practice is king. There are guys out there who rebarrel hundreds(or more) rifles a year and that adds up! Also manual machines are still used extensively in industry. I've known a few guys who are "thread guys" who cut threads alllll day long(shoot me now). When a guys like that takes a liking to gunsmithing...

Also, not everyone is using a metric lead screw.

Some ppl/shops DO have CNC.

keep in mind too that if your barrel/action use any kind of a recoil lug, the threads don't have to go quite all the way to the shoulder. This can be further mitigated by chamfering the action threads or even(like some factory actions I've seen) counter boring them Back an eighth or so.

Finally, from a structural integrity standpoint, having an undercut half way along your tennon threads wouldn't actually hurt anything as long as the old/new threads were timed to eachother. It'll look like crap and I wouldn't call it a best option by any means, but it would work just dandy.

 

[guntech]

I set up and use a dial indicator to indicate 0 when I am 10 thou from hitting the shoulder with the threading tool. I use as small a threading insert as possible. I thread at a slow rpm, usually about 60 rpm. I watch the needle on the dial indicator and when it is within 5 thou of 0 I quickly back the tool out and at the same time disengage the feed. I only cut 5 thou depth a couple of times and then 3 thou, eventually 2 thou, then 1, the 1/2 thou finishing cuts...

I can thread very close to the shoulder. Most modern actions have a thread relief cut in them or a recoil lug that accommodates this type of barrel threading...

 

[tiriaq]

Losing a thread or two in the middle of the shank because of a residual clearance groove isn't going to make any functional difference.
An option when setting a barrel back would be to shorten the shank enough that it is gone.
I have cut threads running toward the shoulder, without a relief cut, by pulling the tool back at the end of the pass. I'm cutting in low rpm backgear, so this isn't particularly difficult to do. I also grind the bit so that I can cut as close to the shoulder as possible.

 

[twosmokinspades]

Thanks guys. I have some bar stock and run a few practice runs to see if I can get one technique to work for me

 

[gaff]

I think the answer your looking for is a saddle stop. a lot of the older machines have, saddle stops, that disengages the saddle movement in the same place.

 

[rifleman762]

On my Savage barrels I stop the lathe 1/2 to 1 thread away from the end of the thread and turn the lathe by hand to the very end of the thread. No shoulder and no relief cut. And I thread at about 50 rpm. Some people have a VFD for the spindle motor and can dial it down to a crawl at the end.

 

[laurencen]

having cut likely thousands of threads my machine has a saddle plunger, it will disengage the threading nuts at the same position every time, to add to this when cutting metric where the nuts can not be disengaged I have a micro switch setup, when the carriage gets to position it stops the spindle and engages a brake, its repeatable so cutting to a shoulder the lazy way I use the brake, wind the tool out and reverse the spindle for metric

as for conventional threads to a shoulder I have been successful many times retracting the tool and once in a wile I get unlucky and hit the shoulder, on something I can remake if it screws up not a big deal but a barrel, I would use the brake function

 

[guntech]

Thanks guys. I have some bar stock and run a few practice runs to see if I can get one technique to work for me

Low rpm and a dial indicator are the easiest and cheapest way of doing it... your reaction time quickly adapts to that sweeping needle... just practice disengaging a few times... and soon it will be easy... out with the cross feed and out with the engagement at the same time... as the needle hits 0...

 

[terry_g]

I built a tool that threads into the outboard end of the headstock spindle with screws to stabilize a long shaft or barrel.
I made a crank handle that slides on the tool and locks with a set screw into a hole. I can use it to turn the lathe spindle by hand.
No fear of running the tool into the shoulder.
Make sure the power is locked off!

Terry

 

[laurencen]

thats a interesting way to turn the spindle, you could thread to a shoulder easily and no chance of screwing up

 

[Leeper]

Thanks guys. I have some bar stock and run a few practice runs to see if I can get one technique to work for me

Your only issue is the fact that you have a metric leadscrew so that you cannot disengage the half-nut. Instead, you have to cut, withdraw the tool and stop, reverse, stop, reset the tool, foward, and repeat. This is the same thing those of us who have inch leadscrews have to do when cutting metric threads. If you have a spindle brake on your lathe, it helps. Otherwise you have to let the thing coast to a stop. When you back out, you have to backout far enough to clear the shoulder. All of this is why a relief cut helps and cutting backwards is a less stressful way of doing things.
I used to use a dial gauge as Dennis describes and it is indeed a great method. However after several years of working in a busy machine shop and cutting all sorts of threads, metric and standard, every day, I got fairly comfortable with it. My lathe does not have a brake and, when I cut metric threads, I have to do it just as I described and that is what I do.

 

[trevj]

I traditionally cut threads in reverse, so way from the head stock. I make a relief cut where I want to start the thread. Because I have a metric lead screw once I make the first cut I lock the carriage in place and never unlock it till I am finished. So, my question is, I have seen threads made in the tenon almost right to the barrel shoulder with no relief cut. How are they doing this? They can't possibly be pulling the tool out at the exact same time. I could see a CNC machine doing this, but most guys I see have manual machines. I ask because I now have a Defiance Elite action and when I want to set the barrel back I'll have to cut off a lot, I think it was around 1.600 cant remember the exact number. With a remington I only had to cut off to my relief cut so I would loose about.0.75 to 1.000 total barrel depending on how the timing worked out.

With a Metric lead screw cutting inch threads, a crank is like to be your best bet. A little tedious, but easy to control.

If you get a lathe with inch threads, like as not it will have a back gear for very slow speed rotation of the work.
If you stick a chunk of tape, or make a sharpie mark on the work or on the chuck, it is not that hard to get to the point where you can exit the cut pretty much at the exact same point each time and leave a pretty good looking lead-out of the cut.

I can't dance, to fat to fly, and generally gots no rhythm, but with a little practice, I can get in to the moves to manage that. Typically I start nodding my head in time to the mark going by, four to six threads out from where I am ending, and this gives timings to snap the half nuts open with one hand, while retracting the cutting tool with the other.

There are commercially available (and very expensive) retracting threading tool holders that can make threading a bit easier, there are more than a few sets of plans out there for variations of the theme too, that may be of interest to you.

But mostly, practice. Too many guys tread threading like it's some dark art, and all it really is, is a skill that is learned. If the guys that fear it spent half as much time practicing it, as they do trying to find ways to avoid it, they'd be way better off!

 

[terry_g]

I had a metric Southbend lathe for a lot of years. It came with a thread dial that mounted on a pin on the end of the carriage.
It had four holes for the pin and four gears for the screw. It allowed me to open the half nuts and move the carriage back the same
as an inch machine.

I found a picture of it. Its in the lower left corner.

 

[The_Shmoo]

Sort of tangentially related, I'm bored and I feel like making something, so I might try and turn out a muzzle brake type thing for my new 6.5 creed AR-10 barrel I've got coming. I've never really done internal threading on the lathe before, and i'm not sure any of the threading tools i've got will even fit in a 5/8" hole. I do have a 5/8-24 tap laying around somewhere, so assuming I drill/bore to the proper size for the minor, are tap cut threads sufficient for a muzzle deivce? Obviously not ideal I know, but I'm working with what I got, unless someone can point me towards a good small bore internal threading tool.

 

[terry_g]

I have a couple tool bits ground for internal threads. I just looked for them and I don't know where I put them.
They are very similar to the one in the picture but ground to a 60 degree point.
High Speed Steel is your friend.

 

[quinnbrian]

Great idea , making a wrench to fit the spider gear! I usually, use the chuck key to turn the head..but I like your idea, a lot better...I don’t think I could live with out a rear spider to help center up the bore. I also have a heavy 10..nice old lathe...first lathe I ever bought. Had it for a couple years years.

 

[guntech]

Sort of tangentially related, I'm bored and I feel like making something, so I might try and turn out a muzzle brake type thing for my new 6.5 creed AR-10 barrel I've got coming. I've never really done internal threading on the lathe before, and i'm not sure any of the threading tools i've got will even fit in a 5/8" hole. I do have a 5/8-24 tap laying around somewhere, so assuming I drill/bore to the proper size for the minor, are tap cut threads sufficient for a muzzle deivce? Obviously not ideal I know, but I'm working with what I got, unless someone can point me towards a good small bore internal threading tool.

If you are only doing a few brakes, it much nicer and cheaper tapping internal threads over threading in a lathe...

 

[tiriaq]

If I cut the threads with a tap, I finish turning the brake by mounting it on a trued threaded arbor. That way, the bore and the exterior are coaxial with the threads. And, of course, finishing boring the brake after it is threaded onto the muzzle of the barrel guarantees that the bores of the barrel and brake are exactly lined up.
Cutting internal threads - cut on the far side with the bit inverted. Then you can see what is happening.

 

[The_Shmoo]

Yeah, it's likely a one(or two)-off project, I'm just really bored and haven't made anything in a while. I kind of want to try out a few designs rattling around in my head.