Gears for cutting metric threads on inferial lathe

[Paul Alciatore]

The conversion gears will work both ways. You just use them in the reverse direction (swap the driver and the driven gears in the chain). 100-127 or 127-100. Same idea for 37-47 or any of the other approximate ratios. The real problems start when a quick change gear box does not have an appropriate setting to use. Then additional gears need to be changed or added to the gear train.

In general the 127 tooth gear is the driven gear (often mounted on the lead screw) and the 100 or some other round number gear is the (a) driver gear when cutting metric threads on a lathe with an English lead screw. To cut English threads on a lathe with a metric lead screw the 127 tooth gear would be a driver gear and the 100 or some other round number gear would be the driven gear.

I mention other possible gears instead of the 100 tooth because, on some lathes, numbers like 50 or 120 tooth often allow more/easier conversion for many metric threads. When I was making and selling a set of conversion gears I made separate 100 and 127 tooth gears which had provision for bolting them together. That way they could be used together as a compound gear or the 127 tooth could be used by itself with gears from the original SB change gear set. This made it possible to cut virtually any metric thread. A quick change gear box with it's severely limited number of settings is a hindrance for this.

How about cutting English threads on a lathe intended for Metric? My old SouthBend 9" is set up for Metric.