# Cutting Metric Threads on an Imperial Lathe



## MozamPete (Jan 17, 2019)

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.


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## NortonDommi (Jan 17, 2019)

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.


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## jcp (Jan 17, 2019)

Have you seen this....


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## Latinrascalrg1 (Jan 17, 2019)

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.


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## Bob Korves (Jan 17, 2019)

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.


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## benmychree (Jan 17, 2019)

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.


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## Bob Korves (Jan 17, 2019)

benmychree said:


> 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?


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## MozamPete (Jan 17, 2019)

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


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## Cadillac (Jan 17, 2019)

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.


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## mickri (Jan 17, 2019)

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.


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## RJSakowski (Jan 17, 2019)

MozamPete said:


> 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?
> ...


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.


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## benmychree (Jan 17, 2019)

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.


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## pontiac428 (Jan 17, 2019)

mickri said:


> 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.


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## mickri (Jan 17, 2019)

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.


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## pontiac428 (Jan 17, 2019)

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.


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## Bob Korves (Jan 17, 2019)

pontiac428 said:


> 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


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