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I realise it's not the answer yo are looking for, but can you get a metric leadscrew?
20 and 25 teeth, would give a reasonable cut, although you would need to relax the tolerances slightly, and would not be able to use a long nut as it would bind somewhat. Good enough for most applications, and certainly as good as any modern lathe. Just remember that you would need to use a cutting tool that has the metric angles, as there are so many imperial angles that define screw threads.
The S.B. Model C does not have a gearbox for the leadscrew (the A and B do). It has no power feed either, wich can be a bit of a pain!I think it's the sudden start in reverse that makes the chuck unwind, although some machines have a grub screw to prevent that from happening. Also, it may not be a great idea to run lathes with plain metal bearings in reverse. I've noticed it sometimes seems to disturb the "set" of the soft metal in the bearing. BTW, there is another trick for timing the engagements of the half-nuts on the leadscrew. I have a buzzer on my lathe that tells me when to throw the lever that engages them on to the leadscrew..Never saw a "hendy" though.
Thanks Maffsolo. I was trying to work out if the metric precision on the Atlas was better or worse than my method, but I keep messing up the calculation!Anyway, according to my calculations, using my single additional gear I can cut a one millimeter pitch thread with an error of less than 0.0005 mm per pitch. That's less than 0.5 mm per meter, which is probably good enough for most of the things I do, except manufacturing leadscrews of course.