Replace change gears with a stepper?

If you a looking for a project to fool around with, go for it. Its a hobby, Have fun.
If you are thinking of the time saved from changing gears fahgeddaboutit. You will sink way more time in your project than you will ever spend changing gears.
 
A stepper motor drive for the lead screw could work but only if you have a means of knowing the angular position of the spindle. A stepper driven CNC is capable of complex coordinated movements in x, y, and and a if you have an RT 4th axis. Addition of a rotary encoder to the spindle and some means of knowing how many revolutions were made from a "home" position would presumably do the trick.Rotary encoders are capable of decoding thousand of positions per revolution.

The Tormach Slant Bed Lathe is capable of single point threading.
 
As has been said, you would need an encoder on the spindle - I haven't put it to full use yet, but put a pair of Hall-effect sensors* to read tooth position from one of the 70-tooth** gears on my lathe spindle in quadrature (e.g. one lined up with the centre of a tooth, one with an edge) which allows electronics to get 4 x the tooth-count and also the direction of rotation. I was putting one in for the spindle tacho, thought "why not, while I'm in here? Future development..." This will give me 280 positions / rev, I hope that'll be enough if I ever obtain the Tuits...

The stepper / servo should have an encoder, the risk of missed steps from a stepper is either a bad thread or a broken tool-tip... A third sensor on each encoder would help, used as an index mark to get absolute rather than relative position (e.g. the stepper would rotate faster/slower until the angular position matches the spindle BEFORE commencing a threading pass) so they always start with the same angular relationship, as you'd get with a single-tooth clutch.
It would be good to micro-step the leadscrew (intermediate positions between the e.g. 200 steps), it's possible to get e.g. 16 intermediate positions for much smoother motion, at the loss of some torque, by incrementally varying the winding currents (even the Chinese 10-Local-Currency-Unit stepper drivers do this).

There's an open-source "electronic leadscrew" you can probably google up, which uses common logic ICs and vintage-looking thumb-wheel switches, and I'm pretty sure there are Arduino versions, the maths to make one work are pretty simple! It might be worth searching YouTube for "Holbrook CNC", Andy Pugh (a fellow on the Holbrook group) did a full conversion and it may give you a good start on stepper / servo requirements?

EDIT: I had a quick look at Mach, LinuxCNC and how they do it - Mach only uses the "index" pulse once/revolution when threading, so an encoder-based solution would beat that... LinuxCNC allows any count/rev, and uses it properly :)

Hope this helps, rather than confuses!

Dave H. (the other one)

* Allegro ATS667 - tiny, built-in magnet, give a TTL-compatible output. 3 wires: ground, +v supply, output plus one to leave disconnected (the "test" pin)
** Had to hack the tacho board by swapping the timing crystal for one 7/6ths the frequency because of the unusual tooth-count - if it had been a 60-tooth I could have stayed with the original and read frequency from the sensor, read out RPM when set to measure frequency...
 
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Lathes like this have been around for a long time and they do digitally controlled threading. No interest in having one but would love to see what they're all about. I believe they are Hardinge work-alikes.

south_bend_sb1009.jpg
download (2).jpg

Ray
 
You have several choices for existing solutions:
There is a group on Yahoo for Electronic Leadscrew. The product web site is http://autoartisans.com/ELS/
There is a modernized version from Germany: https://www.youtube.com/channel/UCMvLkegD66QvfeOzFVr1lYg, product site is https://www.rocketronics.de/els/?v=3e8d115eb4b3
(this is the one that Stefan Gotteswinter uses on his 9x20 lathe)

On YouTube there are also some videos of a Russian version using Arduino's. Source code is available but the comments are in Russian.

Using LinuxCNC is a bit overkill but it works.

The favourite encoder for this type of work seems to be the AM102/AM103 since the number of pulses/rev is selectable over a considerable range. And it is affordable. For an example of its versatility, https://www.youtube.com/channel/UCF91fwCPdAZfGbnRfu8_dXg/videos
 
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My project to use a stepper to drive the leadscrew is just about complete. The software is complete and I'll finish the cases to hold the electronics this week. I don't intend it for threading. Synchronizing the spindle to the leadscrew appears to be pretty difficult (though not impossible).

My lathe is a Grizzly G4000 (9x20). The system uses an Arduino Nano for the controller, a DM556 stepper controller and a Nema 23 3 N-m stepper. The display shows the spindle RPM, the feed rate and an animation showing the feed direction. The controller measures the spindle speed then calculates the proper stepper speed for the desired the feed rate. I can adjust the feed rate from 0.001"/Turn through 0.010"/Turn, forward and reverse. There is an optical sensor that stops the stepper when the (adjustable) limit is reached. When the stepper stops (almost instantly), it reverses 1/2 turn to release the pressure on the feed lever.

Driving a stepper under load requires proper acceleration profiles. This entailed quite a bit of research. I found a paper by David Austin that described how the acceleration profiles can be calculated in real time. His work was quite good. My adaptation of his equations worked perfectly.

My intent is to publish my work on this forum. I'll include the schematics and software for those of you who are interested.

Glen Zoerner
Spicewood, TX
 
It is interesting to watch CNC lathes thread at low spindle RPM, they will rapid the tool to the start position then pause until the spindle reaches the correct radial position then feed through the thread. They move fast enough that at higher spindle speeds it is not noticeable.

One of the advantages of such a system is that one may produce mating threaded parts of any thread lead you desire, one may easily produce an assembly that is threaded 3/8-16 1/2 TPI, this will cause considerable head scratching for someone working on the equipment in the future.

It is fun however.
 
Also make proprietary parts so they have to buy from you :p
 
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