Cutting left hand square threads in 416 stainless

[Wolfer]

Sometime back I bought a 1912 Norwegian Krag made in 1923. Caliber 6.5x55. Originally set up as a sniper rifle with aperture sights. Sight is not there but like many others I mounted a scope to the sight block.
However I can’t get the rifle to shoot with any degree of accuracy. Roughly 10” groups at 75 yds.
I’ve ordered a blank from Green mtn in 416 stainless. I have an old lathe( 1894 Porter machine works) but I’m not a machinist by any stretch of the imagination. I am however pretty handy and have threaded and contoured barrels before.

I’ve never cut square threads
I’ve never cut left hand threads
I’ve never worked 416 stainless
Any thoughts or helpful hints would be appreciated.
Thanks, Woody

[bgmkithaca]

Brownell's has square thread cutting bits. The Norwegian Krag has left hand 1-12 threads. To cut the threads I set up the tool post on the back side of the piece
being threaded and run the lathe forward. Keep your speed down- use back gear if you have it to run slow. It is a good idea to use a piece of scrap to check the setup to be sure you have it right. Good luck with your project.

[deltaenterprizes]

Can you grind a HSS lathe bit?
What is the pitch of the thread?
Can your lathe run in reverse?
The width of the tool is the width of the pitch and the depth of the threads is the same as the width of the pitch.
The relief angles on the bit will be different than a right hand bit.
Practice on some scrap metal a few times before doing the barrel!

Above thread says 12 pitch so the tool width needs to be .083” wide and the depth is.083”.
Your compound needs to be set at 90 degrees and advance with the cross slide.
The tool will wear as you’re cutting so if you get to the depth and it won’t thread advance the compound.001” at a pass and it will open the threads to the right width.

[garandsrus]

Some lathes will unscrew the chuck if run backwards and enough pressure is put on the part.

[country gent]

Left hand threads are cut feeding from the shoulder to the end, opposite the travel of a right hand thread.
I cut square threads with the compound set to 0* ( parallel to he part) you need thread tool ground to the correct form there are gauges for this. Set up with cutter square to barrel and cut to depth + ,005 then if it dosnt fit adjust threads .001-.002 by feeding the compound out to widen groove and narrow thread. I grind my tools on a surface grinder and grind them a little narrow so I can make the fit just right.

Stainless Steel is tougher than cold toll run slower speeds use oil and work easy. Anchor lube is very good in stainless. Its a paste and adheres well the draw back is it makes it hard to see the cutter once applied for the cut. Tap Magic xtra thick works well but clean the machine well after as I ave had it discolor metals.

One thing to do when you grind your threading tool grind 2 or 3. Then ( here even cold roll will work) turn a practice tenon and thread it to fit receiver, maybe even do 2 or 3 until your comfortable doing it. Its also handy if you have a live center with a extended point this gives more room to stop the tool at the end of the pass.

You want a good solid set up grind the tool with the thread angle plus 5-7* on the cutting side the tool is feeding at the thread angle so you need a little more to get good cutting relief. Keep the stub a short as possible and supported with a center. keep tooling as short as possible to maintain the rigidity. A good thread relief is a plus to feed and start the tool in. A machinists handbook will give you the dimensions you need to work with.

[Red River Rick]

..................Above thread says 12 pitch so the tool width needs to be .083” wide and the depth is.083”.

Delta:

You need to check double check your info, what your stating is incorrect. If the tool is ground 0.083" wide, you'd have absolutely no thread. The carriage advances 0.083" every revolution, there would be no thread left. The width of the tool needs to be HALF of the pitch. And the depth definitely is not 0.083" in depth.

Here's a pic of a square thread for a Remington RB that I just cut, 1.05" x 12 TPI. The threading tool used is ground to 0.042" in width.

[Wolfer]

I do grind my own bits. I thread at the tail stock using a live center and steady rest. The babot bushings at my head stock are not solid enough to cut good threads.
My lathe was made between 1894 and 1898 and was run from overhead belt. Someone put an electric motor and a model A transmission on it. I have three forward and one reverse.
I plan on running in first gear with the lead screw in reverse. The whole affair has too much play to disengage the lead screw between passes. I hit the kill switch and reverse the tranny. This method has worked pretty well for me given what I have to work with and my level of inexperience.
Thanks for all the tips so far. Woody

[Wolfer]

Once the barrel is off my plan was to grind a bit that fit the existing threads???

[Wolfer]

Since the barrel is RH twist and LH threads will this pose a problem. In theory firing would tend to unscrew the barrel. Does it actually happen? Should I use blue loctight?
I’ll probably get it pretty tight and see what happens.

[country gent]

If the lathe doesnt have a half nut once you engage the feed you cant disengage until done. You have to use the motor reverse to move forward and back. Fine tune where the tool is by hand pulling on the belt.
The half nut dial is engaged to the lead screw and allows the half nit to be engaged at the same point every time. We had several ol machines inthe first shop that we had to thread in this way, it was trickier and had to really pay attention to what was going on.
I will stat again to get a machinist hand book and get the numbers from there. Buy one borrow one or hit the local library. Yhe library might evn copy the page with the charts for small fee.
Look to you tube videos also. Abom79, Kieth Rucker, Joe Pie seem to be pretty good.

[Wolfer]

There is no half-nut dial. The half-nut will only engage when the lead screw is in one position. However by using normal methods threads were horrible to non existent. Hence I do not disengage.
I have Machine shop practice and lots of you tube. I don’t see much with a 130 yr old lathe though.

[DougGuy]

You could also ask if anyone would thread it for you, other members may have more modern tooling and we have a great community here the resources (and experience) are amazing.

[M-Tecs]

Delta:

You need to check double check your info, what your stating is incorrect. If the tool is ground 0.083" wide, you'd have absolutely no thread. The carriage advances 0.083" every revolution, there would be no thread left. The width of the tool needs to be HALF of the pitch. And the depth definitely is not 0.083" in depth.

Here's a pic of a square thread for a Remington RB that I just cut, 1.05" x 12 TPI. The threading tool used is ground to 0.042" in width.

That is correct. For the OP some good info here on post #6. https://www.practicalmachinist.com/v...-thread-94695/

Same for depth. Every firearm barrel thread that I have seen that uses square or acme thread types is much shallower than full form or stub form.

I also set the compound so I can use it for the depth of cut same as I do with any other threads be it 6o degree, 55 degree or acme. In this case the compound would be set to go straight in (perpendicular) to the part.

[John Taylor]

Once the barrel is off my plan was to grind a bit that fit the existing threads???

That's the way I do it

[deltaenterprizes]

Once the barrel is off my plan was to grind a bit that fit the existing threads???

That is a good plan!

[deltaenterprizes]

Delta:

You need to check double check your info, what your stating is incorrect. If the tool is ground 0.083" wide, you'd have absolutely no thread. The carriage advances 0.083" every revolution, there would be no thread left. The width of the tool needs to be HALF of the pitch. And the depth definitely is not 0.083" in depth.

Here's a pic of a square thread for a Remington RB that I just cut, 1.05" x 12 TPI. The threading tool used is ground to 0.042" in width.

I divided 12 into 1 and got .083 that’s the way I was taught to get the pitch.
Thanks for correcting my error!
I will have to Google it to get the correct formula !
I found the formula!
I should have divided the pitch by 2 to get the groove width!
The last time I cut a square thread was in 1999!

[Deadeye Bly]

I threaded a Norwegian Krag barrel last year. I had not cut a left hand square thread for quite a few years so I did a practice piece to refresh my memory and skills. It was a good thing I did. Cut from the headstock out with the compound set parallel to the barrel. That way you can adjust the width of the thread if needed. The major and minor diameters are critical on square threads to prevent loosness. Pay close attention to these. They are not forgiving like V threads to center the barrel. Do as many practice pieces as you feel you need to to get it right. The job is for yourself and you are not on the clock. Be patient and you will end up with a good job.

[Moleman-]

Contrats on keeping an old lathe running. Oldest piece of equipment I have is an E.E. Garvin mill. They dropped the E.E. from the name in early 1890 so it's from the 1880's to early 1890. Still works. Someone put a transmission from an older 1930 lathe on it and a 1/2hp pump motor from something. Even the flat belt driven power feed still works on it. I had to cut a new 2 start 7/8"-3tpi nut for the X axis. That was the most involved square thread I've done and it turned out first try. I dykem the threads and while still wet, roll the front of a new or squared off cutting bit across the threads to transfer the dykem onto the bit. That will give you a little bit of a gideline of what the angle of the cutter should look like. Keep the backrake of the cutter down like you'd use on a cutoff blade so it doesn't grab or chatter since the cut is sort of wide and it sounds like you've got plenty of backlash. I don't think you'll have any issues with the 416SS. I use ridged dark cutting oil.

[M-Tecs]

Once the barrel is off my plan was to grind a bit that fit the existing threads???

Normally hand grinding threading tools requires a thread gauge or at a minimum something to check the angles. With square threads the sides are parallel so using a mic or the old threads is fine. Just out of curiosity I did a quick search. It's appears that the only square thread gauge available is a custom order.

[GONRA]

GONRA warns 55 degree Whitworth threads can sneak up on ya!
Be careful.... They eyeball just like Good Old 60 degrees....
>>> GET THREAD GAUGES. <<<