Cutting Metric Threads on an Imperial Lathe

[RJSakowski]

When cutting metric threads on a lathe with an imperial lead screw you are always told you can't disengage the half nuts and you have to reverse the lathe leadscrew to move the carriage/tool back to the start for the next pass.

I was thinking (that can be dangerous), what if you set a carriage stop to the right of the carriage just before the start of the thread so that you were always bringing the carriage back to the same start position, and always reengaged the half nuts on the exact same position on the thread dial?
My lead screw is 8TPI and my thread dial has 8 indication marks, so if always engaged on the same number (say 1), and the carriage was always in the same position due to the stop I had set, wouldn't I be ensuring I was always re-enggaged onto the same position on the leadscrew and could do away with all this reversing the lathe hassle?

I'm away from home for work at the moment or I would of just gone out to the garage and tried it to see if it worked.

Releasing the half nuts and reengaging on any identical thread dial position won't work.

It works for inch threads on an inch lathe because the thread dial is timed to the lead screw by the number of teeth on the thread dial. For instance, if you have an 8 tpi lead screw and the the thread dial gear has 32 teeth, the dial will make one complete revolution in four inches of carriage travel. If you are cutting an integral number of threads per inch, and you move exactly 4 inches to bring you to the same position on the thread dial, you are still engaging in the same position relative to the prior pass.

A metric thread doesn't have this nice integral relationship with the lead screw. Moving 4 inches is 101.6mm. You have to reengage at exactly the same spot on the lead screw. This is why Tom Lipton's method works. He disengages the half nuts and immediately shuts the lathe off. The spindle coasts to a stop. He then reverses the spindle and reengages when hits the same dial position. He continues in reverse until he is past the start of his thread, shuts the lathe off, and advances the cutter for the next pass.

Another approach would be to start at the shoulder and thread towards the tailstock. The lead screw direction would be reversed and you would need to either flip the cutter upside down or thread from the back side of the lathe. Joe Pieczynski has a good You Tube video on this technique.

 

[benmychree]

Yes, Bob it works for me, and I have been doing it with that motor for a good 40 years now. It was a centrifugal pump motor, 3450 rpm and I had it rewound for 2 speed, 1750 and 3450 rpm with a 2 speed drum switch with fwd and reverse. I installed another drum switch ahead of the high/low switch that I am going to connect to a rod extending to the tail end of the machine that can be reached from the far end easier than the one on top of the headstock.

 

[pontiac428]

One thing that I found on my belt driven craftsman lathe is that reversing the motor introduces error due to belt slip.

I don't see how that would be true, since the leadscrew is driven off of the spindle. Belt can slip all it wants, the relationship of the thread start to the chuck is unaffected by drive slippage unless you're skipping change gear teeth.

 

[mickri]

Pontiac I agree with you but for some reason if I reverse the motor the threads don't seem to come out as nice as when I reverse the lead screw. I don't know why. I thought it might have something to do with belt slippage. I haven't cut very many metric threads. Only a couple. Reversing the motor is a easier. I'll do some more practice threads.

 

[pontiac428]

mickri, how much back lash do you have in your leadscrew/half nut assembly? That can be a pretty large gap on an Atlas. Another possible contribution to not being able to catch the thread on return on Atlas lathes could be spindle play or even loose gibs. If everything is adjusted tight, it should work out for you. Cadillac's post describes how it should work.

Another comment above that had me with one hooked eyebrow was about the threading indicator. No matter what kind of thread you are cutting, even if you completely made it up, you will always have ONE point per rotation on the threading indicator dial that is in sync with your work. That won't change for a simple, single-position threading operation.

 

[Bob Korves]

if you completely made it up, you will always have ONE point per rotation on the threading indicator dial that is in sync with your work.

Not "ONE point per rotation on the threading indicator dial", but rather THE SAME point on THE SAME REVOLUTION, if you are doing a metric thread on an imperial lathe. There is no repeating match up when mixing them at 254:100