"Don Young" wrote in message
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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
I like your thinking on the gear ratios. Be very careful about opening the
half nuts as the success of that has to do with the pitch of the
leadscrew, the thread dial gear, and the pitch being cut. I do not believe
that your system will reliably re-engage the feed without loosing its
synchronization if you allow the dial to go all the way around.
A metric thread re-synchronizes with an inch leadscrew every 127 turns of
the spindle so a 0.75MM thread synchronizes every 3.75 inches. If your
thread dial goes around to the next starting point when you move the
carriage 3.75 inches you are good to go. About the only one I know will
work is 0.8MM which synchronizes every 4 inches and that is the distance
for a 32T gear on a 8TPI leadscrew. Of course if you are not using a 127
tooth gear your error multiplies for each revolution of the dial also.

I think the best way, if you really need to open the half-nuts, is to just
watch the dial and back up the lathe so it does not go around.

Don Young


I noticed on another group that you indicated your success at opening the
half nuts was due to you actually cutting a 36TPI thread. Because of the
gear change you are actually cutting a 33 3/4 TPI thread. Your thread dial
will work for this if its gear has teeth equal to 4 times the leadscrew
pitch and you always use the same point on the dial. This requires 4 inches
for the gear to go around and will work correctly for threads that are a
multiple of 1/4 TPI. Good work!!

Don Young