I have a model A Monarch lathe, 4tpi leadscrew. I wanted to cut metric threads to make a part for a German auto. My chuck screws on with a right hand thread, so the lathe has no reverse.
I added a 127 tooth transposing gear, and have been able to cut many metric threads successfully since, by doing the following:
1. Made a split set collar with an index mark and installed it on the drive end of the leadscrew.
2. Made a sharp metal pointer that would align with the mark in the above set collar.
3. Made an indexing mark on my backing plate of my chuck.
4. Made a sharp metal pointer that would align with said index mark.
5. Made an adjustable carriage stop for the RIGHT side of the carriage.
To do a metric thread, I then do the following.
A. Place carriage at correct position to begin cutting thread.
B.place carriage stop firmly against right side of carriage and clamp.
C.rotate spindle until mark on thread dial is aligned, (slowly, or by hand) engage half nut.
D. Using a sharpie marker, mark where the pointer for the spindle back plate is on the back plate.
E. Loosen set collar on leadscrew and align with pointer. Mark with sharpie for reference.
F. Make sure gear(s) setting is correct for the thread pitch you are cutting.
G. Make a scratch pass, open half nut when done with scratch pass.
Turn off lathe.
H. Move carriage back against stop.
I. Turn chuck until sharpie mark aligns with pointer.
J.dis engage any lever in gearbox, rotate leadscrew by hand until sharpie mark aligns with pointer,, and thread dial is at correct position, then re engage gearbox.
K. Now, carriage should be against stop, thread dial lined up, and mark on chuck back plate, and mark on leadscrew set collar lined up.
L. Engage half nut and continue thread cutting.
Process is a bit tedious, but absolutely works.
Any questions I would be happy to answer. Unfortunately I haven't figured out how to add photos yet
