Oh my, all this metric versus imperial versus 8 million gear combinations and which ones will fit on the banjo, yada, yada. So much pain.
And now, for the real black art secret: it's all so unnecessary and so last millenium (he said with tongue somewhat in cheek). After all, the computer can do for lathes what it has done for the phonograph.
CNC, of course, makes all of this easy. But CNC is such a religious issue. Many don't want to deal with CNC and all of it's learning curve and cost. But there is a waypoint between the mechanical and the full CNC that is pretty nice. It's called an "Electronic Lead Screw". Here is a YouTube video:
[ame]http://www.youtube.com/watch?v=RRaVIBhLgF0[/ame]
So what you do is mechanically use a stepper motor to drive your leadscrew. It's not hard to lash one on there. You attach a little sensor to the spindle, which enables the little computer (it's all in a box, you don't need a separate PC, just mount the little box right on your lathe next to you VFD) to create an infinitely variable simulated gear train. Now you can dial in any thread or feedrate you might want.
If you are more ambitious, the ELS can control both axes and do tapers of any kind, but now you really are starting to get more down the road to CNC. I just love the idea of an infinitely variable electronic gear box that is cheap and cheerful to put in place on almost any lathe.
Here is the Yahoo Group for the thing:
http://tech.groups.yahoo.com/group/E-LeadScrew/
There are professional "manual" lathes that have this feature. A Hardinge clone is pictured on the site. John Danmeyer manufactures the little kit and frequents the HSM boards. He may even be lurking here.
Worth a look see. Short of CNC I can't imagine a more versatile answer to this vexing problem.
Cheers,
BW