Some CNC Lathes have only one leadscrew that serves all carriage-moving purposes. For screw cutting, a half nut is engaged to be driven by the leadscrew’s thread; and for general power feed, a key engages with a keyway cut into the leadscrew to drive a pinion along a rack that is mounted along the lathe bed.
The leadscrew will be manufactured to either imperial or metric standards and will require a conversion ratio to be introduced to create thread forms from a different family. To accurately convert from one thread form to the other requires a 127-tooth gear, or on lathes not large enough to mount one, an approximation may be used. Multiples of 3 and 7 giving a ratio of 63:1 can be used to cut fairly loose threads. This conversion ratio is often built into the quick change gearboxes.