Cutting metric threads on an Imperial lathe

When you cut metric threads on a lathe with an inferial leadscrew, the threading dial is relatively useless. The usual solution is to leave the half nuts engaged and use the lathe motor in reverse to drive the carriage back to the starting point. This procedure wastes time (which is probably the reason the threading dial was invented).

However, consider this. If I move the carriage to the right a distance that is BOTH an integer multiple of the pitch of the lead screw AND the pitch of the metric thread being cut, I can reengage the half nuts and still be synchronized with the metric thread being cut.

An example will clarify...

8 tpi lead screw → 1/8 " pitch

1 mm thread being cut → 1 mm pitch

An inch is defined to be 25.4 mm, therefore 10 " = 254 mm

10" = 80 * 1/8 and 254 mm = 254 * 1 mm, therefore a carriage movement of 10" satisfies the requirements stated above. Both 80 and 254 are even numbers so we can divide both of them by two and only move the carriage 5" = 127 mm.

If you have a DRO tracking carriage movement, this may be a useful alternative to leaving the half nuts engaged.

A long time ago, I wrote a program (STICK by name) to calculate the required movement. You can still download it from...

https://web.archive.org/web/20230526.../mklotz/#intro

and run it using DOSbox.

Sample program output...

THREAD CHASING OFFSETS

Type of leadscrew thread (M)etric or [I]mperial?

Leadscrew pitch [8 tpi] ?

Type of thread [M]etric or (I)mperial?

Pitch of thread being cut [1 mm] ?

Move carriage 5.000 in = 127.000 mm to right to recapture thread

I have not done this but thinking it through, another way you could do that is to set a DTI for carriage position on the right. It could even be a hard stop. Engage the lead screw on a same number every time. Am I missing something?