Ballscrew conversion

Just spent yet another evening trying to figure out how to shoehorn
cheap ballscrews into an old lathe that wasn't designed for them, and
hence has a fairly small channel in the carriage that won't really
accomodate even a 5/8" ball nut (1" sq + return channels) without
milling out more material than I'd like.

So I got to thinking and remembered an alternative I might have heard
suggested somewhere: What if the screw were fixed to the cross slide
and the nut were turned instead? It could be captured between ball
bearings and needle thrust bearings in a housing that would bolt onto
the back of the carriage, so cutting forces would actually put the
screw in tension. Power transmission would be via toothed timing
belt, with a sprocket bored out and bolted to a flange that screws
onto the ballnut threads.

Problem though is I don't see a reasonable way to get a 2nd ballnut in
there to permit preload, unless I make the whole assembly really long.
Anyone able to estimate what kind of lash I would likely see on the
5/8 or 3/4" .200 lead rolled components from McMaster? I've also
heard rumors about replacing the balls with slightly larger ones to
make them tighter...

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
30 Griswold Street, Cambridge, MA 02138
http://www.mdc.net/~stratton
(617) 492-3358 home/shop