Hi John Ruud
I can't find the article right now, but have read if I use a 47/37 combo, I transform my leadscrew from a 8TPI to a 2,5mm pitch and the error is less than 0.015%, but then I have to buy the 47 and 37 gears.
Then again, if I was to use a 21 gear, my leadscrew would catapult the apron down the bed so fast it would need a airbag at both ends of it's travel, or do I misunderstand anything ?
Mind you I come from a total manual lathe like the one on this pic, but without the gearbox and changewheels