Question regarding interrupted threads

Does anyone know how to time and cut an interrupted thread (ACME or buttress) with manual lathes or mills?

I would like to start collecting the fixtures and tooling for cutting interrupted threads for a few gun-related projects. I would like to make bolts/receivers with interrupted threads for locking lugs. I would also like to make some firearms with quick-detach barrels.

If the fixtures and tooling need to be custom made, then so be it. If there is something that is commercially available, then even better.

Thanks,

Kevin

mill the profiles of the 2 lobes, and thread as per usual.

I would thread first, then remove the sections I didn't want. As far as timing goes, I think you would have to set up the barrel tight, scribe witness lines and mill to the lines.

For making a receiver/bolt, maybe add the bolt handle after timing the parts.

Interrupted threads

Both suggestions sound fine. I was wondering how to cut and time the threads on a production level, not just on a small lots.

There were thousands of machineguns made during WWI and WWII that used interrupted threads for the barrels, and I doubt that each of them were individually timed to their respective receivers. There must have been a way to mass produce them.

Thanks,

Kevin

Timing

As the 1903 springfield book is back in view...

Springfield barrels had the thread milled in a fixture which had a master thread to allow timing.

The milling allowed both threading in a single pass and the cutting of a horrible square thread with stress concentrators at the corners (it was all done a long time before Griffiths and his calculations of stress concentrations inside tight corners). V form threads with nice radiused roots are far stronger. (Rope threads are the most resistant to fatigue - no sharp corners at all, which is why they get used in percussion rock drill rods - but I don't think you'd want to go there...)

I have a photo somewhere of a several foot length of coiled square wire that used to be the square thread in a drop hammer for standard penetration testing down boreholes. That one didn't hurt anyone when it stripped, but it happened in Angola where skilled labour is ultra expensive and machine shops are very very few and very very far between, and spare parts have to wait for one of the 2 flights a week from Johannesburg (3 hour on a 747) and pay (extortion) import duty to a bunch of commies...

Number 4 Lee Enfields were made with the breech face of the barrel tight against an inner collar, and, the reinforce tight against the front of the receiver ring. They were meant to tighten to about 10 o'clock by hand, then crush to bring the sights upright. naturally that didn't always happen, so, in later production, peel off shims were used to get to the 10 o'clock position.

The little browning .22 semi auto rifle my mother used to have had a lock nut to take the wobble out of the interrupted thread barrel, when the sights were upright. As it was blowback, headspace was set by the bolt face resting on the back of the barrel, there was no problem with slight variation in barrel position.

Lindsay publications did a little reprint on the manufacture of Ross rifles. You might like to check that out, as the Ross (and the P14 / M17 Enfield and Lee Enfields) all had helical locking seats - effectively interrupted thread.

For your order of work, cutting the thread after the slot milling is going to produce fewer burrs and tears, and the ones you do make are going to be in an easier position to clean off. You can also bevel where the threads will come out into the slots, to minimise the chance of any little gouges there, and to ease the tool into the cut on the other side

If you thread first and then cut slots, you'll probably have a great weekend of work with a needle file, cleaning up the ends of the threads.

Keep us posted!

Interrupted threads

Thanks a lot for your post. I will have to look more at the books.

For a standard magnum rifle (7mm Rem. Mag. or .300 Win. Mag.), I am planning on using a bolt with an interrupted 0.875" buttress thread. An interrupted 1.250" buttress thread will be used on the .50 BMG rifle that I am working on.

Kevin

Threads

Check out the old Model 12 Winchester shotgun as it appears to be a threaded barrel connection to the receiver but is really an interrupted series of thread like grooves. The design worked for a long time.

JRW

Thread the barrels and receivers first. Then make a set of gages, one to thread into the receiver and one to thread onto the barrel. Thread the barrel gage into the receiver gage and mark both to show where to mill away clearance. Then thread the gages to the barrel and receiver to mark them for milling. That way there is no loss of timing between operators or interrupted operations.

Further thoughts on my previous suggestion

My recommendations above could result in each receiver-barrel group being different from all others. If you desire to have barrels interchangeable between receivers then all the receivers need to be timed the same. One way to accomplish that would be to use a fixture to insure every receiver is located the same in relation to the threading tool. For example, as the threading tool touches the receiver the top dead center of the receiver should be straight up( or down or ?-the same as all others). By having a threading tool with a shoulder added to the shank, you could use that to set your receiver index. (If the thread depth will never be more than X inches, put the shoulder at X+1/4 inch from the tip of the tool. Use a setting block of X+1/2 inch length. Engage the half nuts and advance the threading tool until the shoulder is X+1/2 inch from the face of the receiver. While maintaining that distance with the setting block, index the top of the receiver to the desired location. Then cut the threads and all receivers will be indexed the same. The important part is that the receiver is indexed to the tool while the half nuts are engaged and all backlash is eliminated. I would suggest that this will require an indexable threading tool to insure repeatability.)

Then the barrels can be indexed to the receivers and can be interchanged to all receivers. Before indexing the barrels the threads will need to be deburred and set. Cratex is good for deburring and polishing. To set the threads the barrel should be torqued into a receiver blank several times. I would use blank "receiver" that has polished threads and shoulder that is harder than an actual receiver. Torque the barrels into the receiver 3-4 times using at least twice the torque desired for installation after the threads are interrupted. After setting the threads, use a gage as recommended in my previous post to index the barrels to the standardized receivers.

If you did not understand any of this post I will be glad to explain further. If you use any of it, I would like to know how it works for you.

This may help...

http://www.practicalmachinist.com/vb/general-archive/interupted-buttress-threads-88482/

I posted a similar question over five years ago concerning interupted buttress threads on the breech of an artillery piece(155mm howitzer).

Some good insight from RKI's here.