spaco wrote:
I haven't had to confront the metric issue just yet, but I can see it
coming if I live long enough.
Every now and then HSM magazine publishes an article on lathe gear
ratios. Might be worth looking there. I vaguely remember such an
article within the last few years where the author developed a set of
tables of threads using standard gears that were so close to metric
equivalents that he suggested using them instead. Not all metric
threads worked out this way, but many did.

I always figured that if I had the need, I'd start out by putting all my
facts into a spreadsheet and go from there.

Why not ask this same question again on this newsgroup as a new post,
so it doesn't get lost in this one?

I've heard the 127 tooth thing, too. It must work. I just went out in
the shop to see what the biggest gear I have is. It is 96 teeth. A
127 tooth gear would be pretty dang big!
You probably don't have a 50-tooth, either. If you use them as a set,
then they don't need to have the same tooth pitch as the rest of the gears.
So, get some nice brass gears in a finer pitch that makes them a nice
size, and make the necessary hub features (bore and two keyways).

But, the real problem is the imperial leadscrew. Using the transposing
gears, you can never disengage the leadscrew, you have to back up the
lathe every threading pass. That means you have to stop the spindle at
the end of the threading pass, or the pull back the cutter very fast. A
royal pain. The proper fix is to go with a metric leadscrew.

Jon