Easily cutting metric lens threads on an inch lathe

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
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On Thu, 19 Feb 2009 16:02:58 GMT, snipped-for-privacy@whidbey.com wrote:

If the leadscrew thread pitch is not a multiple of 0.75mm (eg, an imperial leadscrew), you'll need to keep the halfnuts closed and reverse the lathe back for each pass. It's not a major issue for the odd thread though.
Mark Rand RTFM
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On Thu, 19 Feb 2009 22:09:51 +0000, Mark Rand

Not in this case Mark. I've been cutting the threads and opening the half nuts at the end of each cut. Unless I've just been really lucky. I'm not inserting a 127 tooth gear in the gear train. Basically I'v just changed the drive ratio between the spindle and QC gearbox to get a 36 TPI pitch thread. Which is only .0001 different in lead than .75mm. Eric
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No it's not : 25.4 / 36 = .7055mm, not .75xx as required
--
Boo

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On Fri, 20 Feb 2009 02:51:51 GMT, snipped-for-privacy@whidbey.com wrote:

You're cutting a 33.75tpi thread (36x30/32)
What's your leadscrew pitch?
If it's 8tpi then each thread of the leadscrew is 4.21875 turns of the thread that you are cutting.
if it's 3mm (Chinese lathe with bastard imperial gearbox), then each thread of the leadscrew is 3.986220 turns of the thread that you are cutting.
In either case, you will be losing registration if you undo the halfnuts and wind the carriage back instead of reversing the lathe to move the carriage back.
On a camera lens thread with only a few turns and a very fine thread, you might be getting away with it because you aren't making many passes to cut the thread and the lost registration is small enough that you haven't smeared all of the threads. But I wouldn't bet on it.
One of our posters had an issue with a Myford lathe where the silly sods had fitted a 20tooth idler at the factory (standard on changewheel versions) instead of the 24tooth used with gearbox fitted machines. He couldn't cut a thread to save his life until we got him the right gear (Borrowed off my old man's ML7B)
regards Mark Rand RTFM
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wrote:

If he has an 8TPI leadscrew and a 32T gear on the thread dial, registration will be correct for each full revolution of the dial. This represents 4 inches of travel and equals 4 X 33.75 = 135 threads on the work and 4 X 8 32 threads on the leadscrew. If he has a 16 tooth dial gear then he would need to let the dial go around twice.
Regardless of pitch being cut, leadscrew pitch, or gearing, you can open the halfnuts if you back up the spindle so you can re-engage the halfnuts at the same place AND ON THE SAME REVOLUTION of the dial. If the dial goes around one way you have to move the carriage or spindle until it goes back around the other way. This puts you back exactly where you would be if you kept the halfnuts closed the whole time and the dial never moved.
Don Young
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On Fri, 20 Feb 2009 16:08:57 +0000, Mark Rand

Greetings Mark, I'm not getting lucky. I'm cutting a long thread (.375") and made at least 12 passes because I was sneaking up on the finished size using lenses as the gauges. The machine has an eight TPI leadscrew. I only close the halfnuts at the number 1 position on the threading dial. I also posted the threading info on the optics group and Richard J. Kinch posted the following reply:
" I'm sure someone has already posted this info on usenet ...
Yes, see:
http://www.truetex.com/hf-enco-12x36-pitch.htm
etc., and the generating code which I have posted here."
I haven't checked out his web site yet but apparently he is doing the same thing. BTW, the idler gear tooth count doesn't matter as it is just transfering motion from one gear to another. Eric
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On Sat, 21 Feb 2009 07:04:15 GMT, snipped-for-privacy@whidbey.com wrote:

Typo, I didn't mean one of the idlers, I meant the drive gear after the tumbler reverse. That does make a difference.
Note Don's response re registration.
Mark Rand RTFM
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