Cutting Metric Threds

[Clark]

I talked to my brother about this, and he pulled out the manual for his Jet 14x40 gear head lathe. He made the spider with metric threads 15 years ago, but could not remember. The manual says to never open the half nut, and run the lathe backwards.

 

[Robert]

I talked to my brother about this, and he pulled out the manual for his Jet 14x40 gear head lathe. He made the spider with metric threads 15 years ago, but could not remember. The manual says to never open the half nut, and run the lathe backwards.

I have done thousand tilmes the contrary : cutting imperiakl threads on a metric lathe, with occasionally the contrary.

In metric thread cutting, we are used to reverse, as many thread sizes are not fractions of the leadescrew pitch. For example, 0.5 ,75, 1.0,1.5are submultiples for a 3mm leadescrew, but not .8, 1.25 , 2.0 and 2.5, which made the reversing the normal situation.

The first lathe I worked on was a US surplus Le Blond who came in Europe with Gen. Pershing, so then screwciutting metrics led me to learn early to reverse. This lathe had a dial permanently engaged in leadescrew allowing to return to the exact position to re-engage the half-nut, but I never used it.

I also remlember of US Va,n Norman lathe who had a reversing device for the leadescrew and feed, allowing to return while the spindle always running same. This was a wonderful device that I never saw it on any other lathe.

R.G.C
c-g-desugns.blog4ever.com

 

[jdh47]

Yes that is why metric thread dials have a stack of different gears that one must be chosen from to correspond to the particular thread being cut. Metric threading isn't so easy even on a metric lathe.

I do not wish to argue the metric vs imperial virtues but in thread cutting, at least to my mind, imperial wins the argument every time. As an added bonus threading imperial threads puts lots less wear on the lead screw and half nuts.

Joe

 

[Robert]

Yes that is why metric thread dials have a stack of different gears that one must be chosen from to correspond to the particular thread being cut. Metric threading isn't so easy even on a metric lathe.

I do not wish to argue the metric vs imperial virtues but in thread cutting, at least to my mind, imperial wins the argument every time. As an added bonus threading imperial threads puts lots less wear on the lead screw and half nuts.

Joe

Joe,

I would also never enter in the metric/imperial debate.

Having worked 2 years on US engine reconditionning machines, with imperial dials and measuring tools...at a time calculators were non-existant, I can say I am quite fluent in both !!.

Now, on your last paragraph, I think no one have ever seen a lathe worn out by the leadescrew first?? LOL

R.G.C
c-g-designs.blog4ever.com

 

[shortgrass]

Joe,

I would also never enter in the metric/imperial debate.

Having worked 2 years on US engine reconditionning machines, with imperial dials and measuring tools...at a time calculators were non-existant, I can say I am quite fluent in both !!.

Now, on your last paragraph, I think no one have ever seen a lathe worn out by the leadescrew first?? LOL

R.G.C
c-g-designs.blog4ever.com

And the half nut (split nut to old school machinists) is usually made of bronze.

 

[Rustystud]

I own a Grizzly "PRECISION"Tool Room 13 x 40 lathe. They no longer sell my model. However multiple companies sell the exact same model under their names. I requested a new parts list for my lathe model. They list (3) different lead screws. One combination standard/metric, one metric only, one standard only. My machine came with the combination lead screw. It requires not disengaging the halfnut or marking the bed so you can find your halfnut engage point. If I could find an inexpensive version of my lathe I might buy the metric only lead screw and leave it set up to do just metric threads.
Nat Lambeth

 

[carlsbad]

reading this thread I see some confusion. let me try to clear it up:

The thread dial allows you to disengage the half nut and re-engage when you are cutting threads that are a multiple of the threads on the lead screw (the long acme rod that drives the half nut). It allows you can use the half nut and re-engage it (with some guidance on which numbers are acceptable for which threads). Also, with some limits, it works on fractional and odd threads. This is math (arithmetic) and it works for TPI threads but not for metric.

If your lead screw is threaded in TPI, you can use your half nut for TPI threads.

If your lead screw is metric, (I've never used a lathe with metric lead screw), then you can use your half nut for metric threads.

I believe all the lathes belonging to people discussing this thread have a TPI lead screw, even if they are made in China, Europe, or California, so the half nut has to stay engaged for metric threads. One poster suggested a trick to disengage it and re-engage in the same position which will work but probably shouldn't be attempted by anyone having to ask this question. Certainly practice on scrap.

So if you want to cut metric threads on a lathe with a TPI lead screw, you'll need metric gears (NOT a metric lead screw). Metric gears are just a gear ratio that results in about 25.4 revolutions for the spindle per inch of carriage travel, or some multiple there of. Most small lathes use 127/120 x 24 =25.4. They aren't a secret decoder ring, they are just a gear ratio. So metric gears installed doesn't change the fact that you can't use the half nut/threading dial. Use your metric gears per the directions with your lathe, but don't disengage the half nut. If you disengage your half nut, you lose your setup.

it isn't that big a burden to not be able to disengage the half nut. Thread slowly. If you can thread away from the shoulder, that will make it even easier. Use your foot brake. Learn how far your lathe coasts after you turn it off. Practice on scrap.

--Jerry

 

[Rustystud]

Most Chicom lathes come with different gears for non metric and metric threads. They come with a universal lead screw. The gear change(s) are listed both the literature and outside of the lathe. These lead screws require the operator leave the lead screw engaged for the whole threading process. Cumbersome at best. A second way is to mark both your lead screw with a marker and mark the timing of that mark on your bed. Both methods work. Again most Chicom lathes are made for multiple markets most outside the U.S.A. use metric threads so these lathes are available with "metric" lead screws. Changing the gears would take the average guy about an hour. Changing the lead screw will only take about ten minutes. With both the metric gears and leadscrew one does not have to leave the halfnut engaged for the whole process.