Screwcutting Gear Ratio Calculator Threading Screw Cutting Lathe Gear Selector

Interesting, but wouldn't work with my Smithy, which has a gearbox between the change gears and the lead screw (said gearbox having a fast/ slow lever that changes the ratio by 2:1).

I've just got a chart of all possible combinations, with most of the repeats winnowed out.

The gearing isn't that difficult to figure out. If the spindle and leadscrew turn at the same speed, the lathe will copy the leadscrew's pitch. For other pitches the cut-thread-to-leadscrew ratio equals the gear ratio.

jsw

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.

Maybe you could send a copy of the chart to the metalworking dropbox:

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