Threading coarser than my lead screw

Probably a bit of a noob question but I'm having trouble finding the answers on the internet. My problem is that I have a lathe with a 12tpi lead screw. I need to cut 6 TPI Acme threads. I have all the gears I would need to set it up to cut that way I'm just not sure if overdriving the lead screw 2 to 1 would generally be considered too much.

That is not a problem, at my place of employment we turn stuff with a 1 in 24 pitch down to 1 in 1, its neat watching the chuck turn at 1 rpm and the handle wheel on the carriage almost spinning that when we do 1 in 24.

Possibly OK

South Bend 80 years ago decided it was dumb to keep selling lathes that were expected to live with 3 and 4 to 1

(8 and 6 pitch screw and 2 TPI)

So the highest after that was 2 to 1

Beefsupreme said:

Probably a bit of a noob question but I'm having trouble finding the answers on the internet. My problem is that I have a lathe with a 12tpi lead screw. I need to cut 6 TPI Acme threads. I have all the gears I would need to set it up to cut that way I'm just not sure if overdriving the lead screw 2 to 1 would generally be considered too much.

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Uhh.. AFAIK, all you need to do is expect to make more passes, lighter touch each go than when cutting finer threads.

A lathe "genteel" enough to ship with a 12 TPI LS would probably have to do that on 6 TPI Acme form, regardless, so...

Beefsupreme said:

Probably a bit of a noob question but I'm having trouble finding the answers on the internet. My problem is that I have a lathe with a 12tpi lead screw. I need to cut 6 TPI Acme threads. I have all the gears I would need to set it up to cut that way I'm just not sure if overdriving the lead screw 2 to 1 would generally be considered too much.

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Frequently done. Some 30 years ago I was making very long, multiple start threads. I've made this attachment that allowed me to multiply the leadscrw pitch from 2:1 to 2000:1 in 12 steps. So the coarsest thread was about one turn in 170". For this operation the headstock was driven by the leadscrew (via a stepper motor attached with a timing belt to the leadscrew and the lathe motor disengaged). The "tool" was a carbide milling cutter in a motorized high speed spindle mounted on the toolpost. Very light cuts...

Wow, that looks like one heck of a gear box. Did it allow different ratios by rotating to different output gears on the outside edge?

billzweig said:

Frequently done. Some 30 years ago I was making very long, multiple start threads. I've made this attachment that allowed me to multiply the leadscrw pitch from 2:1 to 2000:1 in 12 steps. So the coarsest thread was about one turn in 170". For this operation the headstock was driven by the leadscrew (via a stepper motor attached with a timing belt to the leadscrew and the lathe motor disengaged). The "tool" was a carbide milling cutter in a motorized high speed spindle mounted on the toolpost. Very light cuts...

View attachment 215157

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EPAIII said:

Wow, that looks like one heck of a gear box. Did it allow different ratios by rotating to different output gears on the outside edge?

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Yes, by rotating the whole and engaging each of the 12 gears with the gear on the spider.

I AM impressed.

My old French Sculfort lathe has a knob that can be turned to "Coarse feeds and pitches", it switches the drive from spindle>feed to feed>spindle, so that the headstock can be put even into neutral and the feed box is still driven.
You have to use it a bit to grasp what that means, but it means that when cutting very coarse pitches the drive enters the feed box and from there to the spindle gearing, so that the lighter load is always at the end of the gear reduction.
With this setup I can cut a pitch of 1 turn in around 7 or 8", obviously way beyond what would be possible if the power entered from the spindle. This may be more common than I would think, as it's the only lathe I've ever run that's like that.
With the spindle in neutral and locked it is possible to cut keys using the boring bar as a shaper bar by using the leads screw.

To do this with a small lathes what you'd want to do is input the drive torque at the output end of the feed box, but only when driving high pitches. The torque would then pass back up through the gearing of the headstock from the feed box instead of the other way around.

Pratt and Whitney (and I assume others) does exactly the same thing discussed above on their thread milling machines. For long leads the power is sent from the lead screw drive to the spindle and for shorter leads the power is reversed as in a conventional lathe- from the work spindle to the carriage drive. Just pointing out that its not that uncommon.

Yeah Dan, I know some hobby machinists add a small variable speed gearbox to the tailstock end of the feed rod via a dog clutch, they disengage the lathe's feed box and use this to do feeds instead.

But if the dog clutch were also made available to the lead screw then very highly compounded threads (Very coarse pitches)could be done on most any lathe without stalling the main drive motor. The "output" at the left end of the threads box would be driven very slowly and with mechanical advantage so that the power going to the head stock would work without forcing anything, in fact the main motor might be left unconnected.
If one were designing a "lathe" for extreme ratios for whatever reason that's probably how it would be done, the main drive motor could be clutched to either end of the feed/thread ratios, that is, for fine feedings and threads by the common method but for helices either at the output side of the feeds box or at the other end of the lead screw.

Or, so it seems to me. In fact, use a ball "Lead" screw and eliminate the feed rod entirely. Control the ratio by rotary encoders just like a CNC lathe does whether feeding or threading.

You mentioned a mill that shifted power to the side where most needed, have you seen any lathes setup like my Sculfort?

Personally never seen this on a lathe at all so yours might be unique. I was just pointing it out as it's very common on thread mills, as they are essentially a special purpose lathe with a milling head on the carriage rather than a turning tool and the work spindle turns much slower than a conventional lathe. They have a few other unique features as well but . . ..

I would wonder if the lathe can handle that level of cutting force. A 6tpi Acme is a chunk to cut even a nibble at a pass, with a small lathe.