LinuxCNC as ELS for lathe?

I'm not an expert by any stretch in LCNC, but I can think of a couple ways of using LCNC for this.  But a dedicated ELS is likely simpler, assuming you can borrow someone else's design and microcontroller program.

And creating a user interface for LCNC will be a challenge if you've never done a GUI before.

Considering the only difference between doing an ELS and full CNC control in Linuxcnc is one stepmotor. What's the point of only doing it half way?

Perhaps someone simply wants a manual lathe and not a CNC lathe, but they want the ability to cut many different threads or feed-rates.

I think of them as enhancing a manual lathe rather than being a half-a$$ CNC lathe.  Like a power down-feed on a mill quill - not necessary, but makes a huge difference in user experience.

Just because you add cnc capability doesn't mean it has to loose the ability to be used manually.

I certainly would want to keep manual capabilities on a lathe, if I were ever to convert one.

No eBay APP ID and/or Cert ID defined in Kunena configurationBasically, the ELS requires a way to set the TPI / mm pitch.
You can certainly do this with LinuxCNC, and the fact that LinuxCNC has all the drivers might be a good enough reason.

I have always fancied doing an ELS with BCD switches (eg ) and a metric/imperial switch. That way you could run the Pi without a display and run a non-gui non-g-code HAL-only config.

I would probably write a custom component for the job, rather than glue together all the required existing modules, but it certainly could be done with standard HAL modules.

I think three digits is probably enough for most purposes? 1 to 999 TPI and 0.01 to 9.99 mm pitch.
Then you would need 4 IO pins per digit + units. so 13 inputs. Unfortunately the 7i96 only has 11 inputs, so this scheme would maybe have to use a scale of 0.01 to 3.99 mm and 1 to 399 TPI.
(though you could use the Pi built-in GPIO for the inputs instead of / as well as the 7i96.