Cutting Metric Threads on an Imperial Lathe

[MozamPete]

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.

 

[NortonDommi]

Now there's a thought for thought! I have a metric lathe and cut Imperial threads at least 50% of the time so I must try that, DRO should be good for accuracy. Off the top of my head I can't think of any reason it wouldn't work other than there is usually a bit of leadscrew rotation before engagement. Think it will be a case of suck it and see. I'd run out and try it now except I have some work sitting set-up.
Definitely going to have a look at that though.

 

[jcp]

Have you seen this....

 

[Latinrascalrg1]

I dont know if this would be a "Real Issue" or just an issue in my head but I would think you may have a small problem with the play in the lead screw if you just use a carriage stop on the "Reverse" movement to set your starting position. I have only done a few projects that included single point threading and one thing that was an absolute must to do when setting up for the next cutting pass was to move the carriage past the starting point a few turns before reversing the carriage direction to take up the slop out of the lead screw/carriage before moving the cutting bit into position. If you skip this step (unless your machine has Absolutely zero slop in the carriage movement) it would be a safe bet to say that your treading project will not come out the way you would like it to.

 

[Bob Korves]

As I remember, the Tom Lipton video jcp posted above in post #3 explains how to cut metric threads on an imperial lathe by disengaging the half nuts and then immediately stopping the spindle. The tool is then withdrawn, the gearing (edit: motor!) is reversed, the spindle is restarted, and the half nuts are re-engaged when the previous position on the threading dial is reached. I have used that system multiple times with total success. The important thing is to not let the threading dial get beyond its current revolution. If it does, the cutter will be in the wrong place for the next cut. If you just practice it with dry runs for a number of times before committing it to metal, it is easy and reliable. It will not necessarily work in reverse for doing imperial threads on a metric lathe, threading dial usage is more complicated on metric lathes.

 

[benmychree]

Many lathes cannot be stopped "immediately" I will continue to reverse and back up; with a drum switch on the motor, it is not at all difficult to do.

 

[Bob Korves]

Many lathes cannot be stopped "immediately" I will continue to reverse and back up; with a drum switch on the motor, it is not at all difficult to do.

John, I have not done that when threading due to worries about the timing/accuracy of the reversal, along with worries about it damaging the motor. Please enlighten me further. It certainly seems to work for you, what else do we need to know?

 

[MozamPete]

More thinking about it and I’ve realized why what I originally proposing wouldn’t work. Although you would be reengaging onto the same place on the leadscrew, any revolution while disengaged will mean your out to synch with the spindle. And with the strange gear ratio you have between the spindle and the leadscrew to cut a metric thread on a imperial leadscrew they would only very rearly be back in the same synch as the previous cut.

The technique shown by Tom Lipton is an improvement over keeping them constantly engaged, but to me it still seems hard on the motor doing all that starting/stopping/reversing/stopping/etc

 

[Cadillac]

I also do the same as John. Quite simple and no different than stopping and starting the lathe as usual. Run your tool to the relief stopping the lathe. I back the crossslide out .100 then run the motor in reverse to the start. All along keeping thread lever engaged. Once at the start of thread move cross slide in that .100 then I use my compound and infeed for the thread. Hit forward on the motor and go.
Since learning threading away from chuck that my go to way unless I can’t. I would think a instant stop is harder than a stop and reverse.

 

[mickri]

One thing that I found on my belt driven craftsman lathe is that reversing the motor introduces error due to belt slip. Maybe my belts aren't tight enough. I don't know. What works for me is to reverse the rotation of the lead screw. I keep my half nuts engaged. It's a no brainer solution. Shutting off the motor is no harder then disengaging the half nuts. Why add complication when you don't have to.