"Wild_Bill" wrote in message
...
I have a Smithy 1220XL, but it's been in a crate for the past couple
of years.

If one of the headstock case Hi/Low selector positions is 1:1, a
gear calculator should werk.. I can't say for sure.

If not you figure out an equivalent leadscrew pitch that is 1:1 to the
spindle with equal-sized gears in the train.

Say the leadscrew pitch is 4 TPI and installing equal-sized change
gears on the banjo makes it turn half as fast as the spindle. You
would calculate threading gear ratios as though it were 8TPI.

On my lathe the quick-change gearbox input shaft turns at 3/7 of the
spindle speed.

jsw