Don Young wrote:
"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

Has anyone any idea how/if these threads (in particular the 0.75mm one for
lens filters) might be cut on a Myford ML7 (NOT a Super7) with a gearbox?

Dave (UK)