Sorry for the long subject line folks,
Anyway, it seems that camera lens threads are all, or mostly all,
75mm pitch. Cutting metric threads on an inch lathe is of course a
hassle. 34 TPI would be close enough but my gearbox doesn't have a 34
TPI option. Looking at the change gears that came with my lathe (which
has a quick change gearbox), I figured that there must be some
combination of two gears and the quick change gearbox that would get
me close enough to the inch equivalent of .75mm. It turns out that I
have a 30 tooth gear and a 32 tooth gear and that the ratio between
these gears, times 36 TPI, which my lathe does have, results in a
pitch only .0001 different than .75mm, calculated to 5 places. My
lathe has two ratio settings for both the headstock gearbox and the
threading gearbox. The headstock gearbox output is either 1:1 or 2:1
in relation to the spindle. The threading gearbox input is either 1:2
or 2:1. So, in order to get the .75mm pitch I set the headstock
gearbox to the 2:1 ratio and the quick change (threading) gearbox to
36 TPI. The change gears are mounted so that the headstock gear is the
32 tooth gear and the quick change gearbox gear is the 30 tooth gear.
The ratio between the 32 and 30 tooth gears is 1:1.06666666667. The
pitch of 36 TPI is .02778 and .75 mm is .02952. 1.066666667 times
02778 is .02962, which is only .00010 more than .02952(.75mm). I'm
pretty certain that all lathes with quick change gearboxes will have
even ratios of spindle revs to quick change gearbox input revs, so as
long as a 32 tooth gear drives a 30 tooth gear (or any set of gears
with a 1:1.0666666667 ratio), through an idler or not, you should be
able to cut a .75mm pitch thread and still release the halfnuts after
each pass. I'm sure someone has already posted this info on usenet but
I needed this today and thought it might be useful to someone else.
Especially considering the ubiquity of asian lathes.
Cheers,
Eric