Cutting square threads?

I have a 1903A3 action (and a rolling block that could use a new barrel) that has been drilled and tapped for scope bases and it waiting for a barrel. I have been thinking about .35 Whelan for the 03 action and have checked out some appropriate countoured blanks that would need to be threaded. I am thinking that cutting square threads is a bit different than V shaped threads and am guessing that the carriage does not get disengaged at the end of each cut. Is this right? I have no brake on my lathe and wonder about the time it takes for the machine to coast down to a dead stop. Will it stop before the cutter crashes into the shoulder? Anyway, I've never cut square threads before and wonder how it is done. Thanks for any comments.

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Same. Disengage half nuts, carriage stops.

If needed, withdraw tool in the process of opening half nuts, though this is unneeded if a thread relief is provided.

John Oder

Hhmmm, I may haave a problem with my machine or maybe not. When I engage the halfnut, it is rare that I am right on the mark - it is either to one side or the other. This has not been a problem so far as everything seems to thread together okay and I have always assumed that the V profile was a bit more forgiving or that it was just the way single pointing on the lathe worked. The square thread seems like it would need to be dno (dead nuts on) though.

Your thread chaseing dial needs to be adjusted. Usally the face can just be rotated so it lines up with the marks, it just got off a little bit. There is no forging with the half nuts, they either ingage or not. But square threads are hard to cut. There is a lot of information needed to cut square threads, but I usally make a tool that is a little narrow and cut the treads and then use the proper size tool to finish. There is very little room for mistakes in square threads and that makes them hard to machine. That is also the reason that almost everyone uses acme threads these days. Square threads are better at transfering motion than acme threads but not enough to warent the agravation.

Depending on the lathe and the number of TPI, when you close the half nut can be more or less critical.

The thread dial is simply a mechanical way of syncronizing the angle of rotation on the chuck with the axial position of carriage so you can re engage the half nuts to thread.
For most threads there are many more positions on the thread dial other than the standard 4 ticks that will work, but if you chose a single mark and stick with it, it will always work.

If the pitch your cutting is the same as the lead screw, anywhere you engage works, if you are cutting a very course thread, you may be able to engage on one mark only.

Ran into a situation where I was cutting 6 TPI, double lead. turned out that if I engaged on even numbers it cut one lead and if I engaged on the odd numbers it picked up the other.
saved a lot of trouble making the part.

I turn my compound parallel to the ways. I grind my tool a little narrow and then I can dial in a couple thousandths and slim the thread as necessary.
Butch

My thread dial has no adjustable "face". The outside is just one casting and there was no hash mark on it when I got (it came with the lathe - a 1948 9" Southbend) so I added my own mark which may be off from where it needed to be. It's placement was just at 12 o'clock which seemed to make sense. I like your idea of putting the compound parallel with the ways Butch. I thinkit would work and you could cut until the receiver fit.

Butch Lambert said:

I turn my compound parallel to the ways. I grind my tool a little narrow and then I can dial in a couple thousandths and slim the thread as necessary.
Butch

Click to expand...

This is a GREAT way if not THE BEST way to work threads.

You can cut down on tool interface, and even just cut one side at a tiime on a course thread by using a nose radius a couple of thosandths smaller. This is one of the CNC things that is shared with manual.

Great way to pick up a thread for repair.

A lot of my work on a CNC now is threads. Little that is standard. Ijust go by pitch, pitch dia., and thread form. Thread form meaning angles and radii.

Without doing what Butch does, repairing parts that have to have their pitch diameter changed by only a couple of thou and still have perfect thread form would be near impossible. Certainly not cost effective.

I was never a 29 1/2 degree person.

Sorry I got OT.

Gazz,
If youv'e never cut threads.. I'd says get yourself a copy of 'how to run a lathe' ( lindsey books) and practice before you work on a barrel.. When your confident make a 'trial' piece and and check for fit before you cut the 'good' piece.. That way you can tweek your setup if need be first..
While not gun related here's a pic of two of a left hand acme thread that I made for a Craig Donges vise project on my tired old heavy 10 southbend...
As for the compound setting I try to angle it 1/2 of the pitch angle.. so on a 29° acme the compound is set at a 14 1/2°... Hope this helps..
Stay safe
Calvin