Norman, Phil, Russell, Pekka, thank you for your thoughts here -- it really makes this interesting and helps greatly in weighing the options.
STEPPER:
Phil I wonder how big a threading stepper and reduction on the spindle I'd need to cut, as a practical maximum let's say an 8 tpi Acme thread on say 1" dia mild steel rod.
Would a 270 oz-in stepper motor handle it with say a 4 to 1 reduction? (The spindle is roller bearing supported, and the leadscrew would be separately driven, so, not a load in this case.)
CHASE THREADING:
I see lathes UK calls the master screw and followers "chase threading". Must have evolved from the hand chaser and Tee rest method.
I'm still kind of attracted to this one because of its wonderful simplicity. I would definitely drive the apron in front, though, and run a pushrod parallel to the leadscrew, unlike any of those I've seen so far.
I would likely put the master screw on its own idler spindle with timing pulley -- close to the pushrod bearing, on the gear side of the headstock -- and run a belt to the spindle. This would reduce the length of the pickup arm and allow a change of simple pulley ratio between spindle and master screw. That last would reduce the number of master screws and follower nuts needed for a good range of threads. Pratt and Whitney did this, I think with master screws of 5, 5.5, 6, 6.5, 7, 8, etc.
It would also utilize the carriage and ways guidance for the cutter, rather than requiring the heavy rod on the rear of the lathe as a guide. The front mounted apron pushrod would no longer need to be of heavy proportions of the old style chase threader attachment. To me, the old style needed beef because they were acting effectively as an auxiliary cylindrical ways, supporting their own toolpost. It might be argued that the rear toolpost was a benefit, but that can also be achived on the conventional carriage -- as is done for an auxiliary parting toolpost these days. I was already planning on that anyway.
CHANGEWHEELS/GEARBOX:
Still a possibility. I think the things I hate most about the Craftsman could maybe be tamed somewhat in a new lathe. The blackened oily gears, heavy floppy three slot banjo, indecipherable gear chart and awkward insert system with conventional bolts could probably be better thought out.
I guess one benefit of plastic gears is no oil needed. Maybe timing belts could replace some of the gearing. Or the gears run for threading only without oil somehow. Simple things like an internal light and custom made gearing wall chart would make sense -- and probably everyone else is smart enough to already have that. What about a quickly removable gear frame, so you could lay most of the train out on top of the workbench while changing gears around?
I could use the Craftsman gears on the new lathe, and with these changes make the operation more tolerable.
Also, feeds could be handled the same way many of us add feeds to a mill -- a DC motor and speed controller -- detach changing feeds from the need for gear changes in threading. That could be done through two leadscrews, or one with a simple dog clutch and engagement lever for the motor (as done on mill tables to allow manual use).
Okay so if I did something like an adjustable DC motor drive for feeds, and a removavble "frame of gears" for threading. The frame could be stored out of the lathe normally, and just popped in when threading was needed. Easy to work on, easy to keep clean, and set already to the last thread cut -- no need to revert to a feed ratio. That doesn't sound too bad....