There should be a way to resynchronize the spindle and carriage thread dial. It's going to be some modulo 127 or 100 ratio, but I haven't figured it out yet.
It's going to be a multiple of 127 lead screw rotations.
I have an ELS and I'd like to do this. Have an encoder on the spindle and I can count steps for the lead screw. One of my round tuits that I'd like to figure out this year. I need to just work out the math - hopefully it will become obvious once I plot some stuff.
The difficult part of this is going to be that when you run the carriage back, it depends on how far you run it back. As you wind the carriage back, every time the 1 comes around (or pick your number), the carriage has moved (usually) four inches, and each is a different "instance" of where the thread dial would have lined up. So you'd need a counter on that too, so the ELS would know "WHICH" number one you were on when you drop the half nuts again, so that the software could calculate from there where to go.
As it applies to the original post- That's the key, WHICH number one are you on? If you start metric threading with the thread dial engaged, pick a number that's dead nutz (or exactly repeatable) and leave it there. If you don't disengage the half nuts, the dial will NEVER move a bit.
If you're threading to a shoulder that's challenging to time the stopping of the lathe, you CAN release the half nuts, but here's the caviat- You MUST re-engage them on the SAME NUMBER, and the SAME INSTANCE of the same number. For example, if you release the half nuts, stop the lathe, and the dial advances a quarter turn, you've got to back out of the cut, reverse the lathe, and drop the half nuts on the EXACT same INSTANCE of the exact same mark you had it on before. (It's usually a small amount, quite easy to back it up by hand). If the dial goes all the way around and you drop in, it won't work. In theory, you could let it go around ten times or twenty times, and you could just back it up ten or twenty turns and re-engage. In practice, you wanna get it pretty close so you can keep track. Once you loose that rotation, you have literally thousands of "instances" of the same number 1 on the dial, and it's not really feasible to figure out which one. (I havn't worked it out, it might be as low as 127 times the lead screw pitch, but I suspect it's a bunch more than that as I think you might have to multiply by the gear ratio between the gear chuck and the lead screw as well. I don't care enough to sort that out, as the best case scenario is implausible to work with anyhow). So if you loose your alignment, you're essentially picking up a thread as if you were repairing an existing one, since you will never (with any practicality) find the correct place to re-engage the half nuts.. And I'm not trying to scare you, picking up an existing thread absolutely be done provided you've got the compound installed and are feeding in at 30 degrees, otherwise you'd have to reinstall it at that point, but it's not the end of the world, so long as you know that it got out of time and don't send a wild cut down your shiny new threads. But at that point where you've picked up the threads, you've lost all of your calculated cuts, and you're literally cutting the thread blindly until it fits. You won't be turning out a properly finished thread without a suitable mating thread to test with. PITA, but very doable. So yeah, if there's a mistake, and you collect your thoughts before something goes way wrong, there is a recovery option. Not quick or easy but you can recover.
