Fabricating Camera Optical Equipment Lens Adapters

Ed,

How about a lesson in thread chasing?

Kevin Gallimore

Sure. Teach away. I'd be glad to learn. d8-)

My uncle was very good at it. 'Made all his own chasing tools. I tried it on a piece of aluminum barstock, soon after I got my lathe, and I finally got one to come out good -- after three or four tries.

You need to have a good sense of timing. If you're too slow, you wind up with a few closely-spaced, shallow grooves. If you're too fast, you can wind up with a double-lead thread.

Do you understand the basic idea? The tool is a freehand turning tool, which you hold over a tool support much like the one on a wood-turning lathe. The tool looks like a flat chisel on the end, but with four or five teeth, each shaped like a single-point threading tool, only it can be simpler, without any rake.

You start moving the tool along the work, in the direction of the thread you want to cut, and then you plunge the tool into the work while you're moving it lengthwise along the lathe bed. If you get it right, it cuts a single-lead thread and it self-feeds after the first revolution of the work. You'll need to make a few passes to cut the thread to full depth, but after the first pass, the tool follows the previous cut.

It's a bit of an art but people who specialized in it got it right every time.

-- Ed Huntress

Greetings Ed, I've cut thousands of threads of all sorts of different pitches and design. But I have never hand chased a thread. Horoligists apparently hand chase threads sometimes. The smallest thread I have single pointed is 0-80. I tried 00-96 but couldn't make it work. Too much deflection of the screw. Now, after reading your post, I just have to learn how to hand chase threads. I know it will come in handy for tiny threads. Thanks, Eric

I see that there's a lot of info on hand-chasing threads in wood on the Web, and some on the subject in metal. I'll bet that with some of that info and some practice you'd find that it isn't hard.

The good thing is that there is some latitude in the starting speed with which you move the tool, because the second and following teeth tend to follow the start, and they push the first tooth along once they get a grip. So it's not as magical as it may sound. It just takes some practice.

My uncle (a shop teacher) worked with a big astronomy club full of telescope makers back in the '30s, helping them with machining for their hobby, and he got quite good at it. He told me that I should learn because it would come in handy. I can't say it has, but I don't work with optics or clocks.

I thought a lot about the business of making the cutters for extremely fine threads, and it seems to me that's the hard part. If you have the right pitch available on your lathe you might be able to rig a boring-bar-type flycutter, between centers, with the blank tool mounted in the toolholder, and just take tiny nicks with the flycutter to mark the positions for filing in the chasing tool. Then finish it off with a file. It probably requires the very finest of Swiss-pattern jeweler's files for the fine ones, working with magnification.

My uncle told me that the keepers (they must have a name but I don't know what it is) for individual elements in a refractive telescope are typically mounted on a mandrel for machining and threading, because they're very thin and fairly large in diameter relative to the thread length. He also mentioned that a lot of threads on old telescopes were bastards, because the machinist would cut the outside thread and then bend the end of the tool over to cut the matching internal thread. And they worried more about getting a proper fit than about actual thread pitch.

That must have taken a lot of finesse. I got interested in making clock jigs at one time, after I met Dick Moore and read his early books, and my interests went in the direction of precision positioning rather than making threads.

Good luck and give us a report when you get a chance to try it.

-- Ed Huntress

That's all a wonderful bit of lore, but I don't think anything has been hand-chased in optics since H. G. Wells and the Victorian era. I don't see how it could be anything but a degraded version of single-point threading, something to resort to if you didn't have an engine lathe. Single-point threading is especially critical for thin tubes, since the side load is minimized.

It is a lot more free from galling than aluminum (unless you do a good job of anodize after threading. :-)

Interesting. That should work nicely -- as long as the grit does not get loose and rattle against the lenses. :-)

Black paper has more reflection than the threaded and painted surfaces.

Beware of threading to a shoulder, however. With conversion gears such as the 100/127 pair you won't be able to use the half-nuts and threading dial -- unless you can cut full depth in a single pass -- which *might* be possible with the finer threads used for filter mounts and such.

Nice.

If you want some small ground glass for lens testing, and have some spare microscope slides (1"x3" IIRC) you can rub it back and forth on a nice flat whetstone to make the ground surface on one side. The finer the grit the more transparent it tends to be, so experiment for a while with broken fragments to see what works best for you.

Enjoy, DoN.

Hi DoN, one of my concerns about using aluminum was that I was sure that the parts would need to be anodized, which I've read a lot about, but haven't done (any anodizing). I've had to disassemble properly threaded and some nasty cross-threaded binocular parts that weren't anodized, and it's easy to see that anodizing would very likely have prevented most of the problems.

A thick gooey grease or beeswax makes re-assembly much easier, but getting the grease on the optics would be bad. The grease that I've used (sparingly) is a thick synthetic non-silicone lube for automotive disk brake caliper sliding surfaces/parts. I have looked into the special lubricants that optical equipment manufacturers use, but haven't actually tracked any down.

I wouldn't use this lube on anything critical or valuable, since I don't know what it might exude as far as residue creeping about within.

I can't recall ever seeing thread problems with brass parts, which indicates it's superiority for use especially where fine threads are involved.

As I suggested in my original post, I expect to be threading with a hand crank since the majority of the parts only require a very short threaded section, usually adjacent to a shoulder.

Your suggestion for imrovising ground glass is a real gem, it had completely escaped me to consider making some controlled scratches.. doi-ee.

On the contrary, sealed optical assemblies are often gobbed all over inside with grease inside to trap dust on a flypaper principle. Not on the glass, of course.

Bare aluminum-to-aluminum threads work fine if there is no interference. If they bind or gall, you're likely not honoring the thread allowances in your fabrication. Or approximating a metric pitch on an inch lathe versus a mating part that is true metric.

The "special lubricants" are most commonly pure ordinary white petroleum grease, sometimes disguised with the names petrolatum, petroleum jelly, or Vaseline. Wetted with naphtha to make a brushable grease paint. Upgraded to high-temp bearing grease (lithium soap, etc) from the auto parts store if things might get hot.

Once I used my pocket ChapStick when in the field with nothing else handy, but don't tell my customer.

Hi Richard, thanks for the comments and the in-depth detailed webpages.

I've not encountered amounts of internal grease to trap dust, but then my experience is mainly consumer and a few military binoculars/optics, and a few pieces of industrial optical equipment. Encountering unexpected grease could get messy.

The non-anodized aluminum thread issues that I've encountered were manufactured items (pre-and-post WWII to present technology USA, Japan and Germany), some which appear to have been disassembled by previous owners and probably messed up the threads when they tried to reassemble the parts incorrectly.

I haven't started making optical fittings yet, which is why I'm asking questions about how to go about it (the purpose of the thread).

I've not used ChapStick on threads, but I've used pure lanolin (in a plastic tube from Enco) which is a thick paste consistency when cold, but gets thin with heat, to the point that it may run when moderately hot. Lanolin is a fairly good cutting lubricant, too, FWIW.

finally a light came on. I recall that Newnes screw thread tables had a section on old photographic threads.

Royal Photographic Society Screw Thread (Whitworth Form) diameter in inches : threads per inch : core diameter in inches

so... Royal Microscopical Society Screw Thread (Whitworth Form) screw thread for objective pitch 36 threads per inch = 0.02778in approximately (= 0.7056mm) length of thread , 0.125in (= 0.175mm) full diameter 0.7982" max or 20.274mm max. 0.7952" or 20.198mm minimum. effective diameter 0.7804" or 19.822mm. core diameter 0.7626" or 19.370mm.

screw thread for nose piece form pitch as for the objective screw. length of thread not to be less than 0.125" (3.175mm) full diameter 0.8" or 20.320mm minimum effective diameter 0.7822" or 19.868mm core diameter 0.7644" or 19.416mm minimum 0.7674" or 19.492mm maximum.

that all is probably not one ounce of help in the problem at hand but it gets rid of a dark corner in the discussion.

Stealth Pilot (E&OE = errors and omissions excepted)

Amateur telescope makers used the method at least into the '50s. If you haven't tried it, don't knock it.

-- Ed Huntress

Richard, I thought you might be interested in this brief account, from 1972:

To my amazement he mounted the sleeve in a self centering 3 jaw scroll chuck on one of his Boxford lathes, and hand chased a 100 tpi thread on the brass ring's OD. He then removed the sleeve from the chuck, fitted a purpose turned steel collar with a shouldered 4-inch bore, and located the PA circle within it. He then hand chased an internal 100 tpi screw thread, to match that turned on the brass ring. The PA circle was then screwed onto the brass ring, aligned to suit the opposing verniers so that the micrometer would be horizontal at PA 90° / 270°, and a 1/64-inch dia. hole drilled through the thread and a little silver pin tapped into it so the PA circle was locked in place. An amazing job calling for the most consummate skill, performed by a man in his mid 70's with chronic arthritis, perched on a bar stool at his favourite lathe, and all done in less than an hour.

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I have several of the books on telescope making that MIT Press published in the '20s and '30s, and they describe hand-chasing threads for eyepieces and similar devices. One of the Scientific American telescope books, which I bought in the late '70s, (but which was originally published, I think, in the '40s) also talks about it.

If someone here has a copy of Guy Lautard's _The Machinist's Bedside Reader_, he apparently discusses it there, but I don't know if he's talking about original threads or cleaning up old ones. I haven't read it.

Historically, hand-chasing was used in a variety of industries until well after 1900. The "brass-finisher's lathes" and horological lathes then made for precision work had a fairly common type of thread-following attachment, similar to the ones used on early model Unimats, which eventually replaced hand-chasing in production and in much of industry. But skilled lathe hands still hand-chased into the '20s or so, until change-gear lathes had crept into all corners of toolmaking and prototype making, because one might find his set of follower-type thread-chasing attachments didn't include the right pitch for the job at hand. A set of hand thread-chasing tools (sometimes called "comb tools") was far cheaper than a set of followers.

Hand-chasing apparently produces very clean and smooth threads in brass, which is the material that was most used for optical work like this. As I mentioned, skilled amateurs were using the method for decades after it was replaced in industry.

-- Ed Huntress

You can find some info on thread chasing in old books. "The Complete Practical Machinist" by Joshua Rose 1895 has some info. You can get it off the net here:

The following program works really well for viewing said file:

This program will blow the doors off any pdf/acrobat viewer that I've messed around with...

Poke around in the Archive site, there are several more old books in this genre worth getting. On the same Archive page for Complete Practical Machinist try clicking on the "Subject: Machine-Tools" link for starters.

I don't knock it; there are appropriate uses for hobby or historical purposes. I suppose those ATMs just didn't have a proper engine lathe for fine threading. Maybe they spent all their available time skiving leather belts to their steam engines to run whatever they had.

I hand-scraped my worn-out Bridgeport back to new, so its not like I'm opposed to improvising with primitive techniques, when more modern tools are not at hand.

And some of the threads in binoculars (the eyepiece individual focusing ones at least) seem to be rather coarse acme style threads with a thick gray-silver grease in them.

Depending on whether it could be removed with a solvent and without mechanical scrubbing.

Hmm ... I'll have to look into that.

O.K. Let me know if you do -- and where. I would like to clean and re-grease the focusing threads on my 500mm f8 reflex Nikkor (cat lens).

Indeed -- though a lube which is designed for the temperatures sometimes found on brake calipers would probably not give any problems on that line.

It is still *possible* to cross-thread them, but you can tell by feel a lot sooner -- soon enough to prevent significant damage.

O.K. I had forgotten that.

When I need to cut metric threads, I go to my Compact-5/CNC and simply throw the switch into metric mode. :-)

I do have the conversion gear set for my 12x24" Clausing (if a

It took me some tries to get one opaque enough for the purpose. The first tries were too fine and I could see through the glass instead of just seeing the image on the ground glass.

Enjoy, DoN.

About a year ago, I was seriously engrossed in researching and buying binoculars and some of the various other optic stuff, and I was finding some solidified grey residue in the seized eyepiece focus threads and also the center focus parts of some binocular models.

I knew it had been a lubricant when the devices were manufactured 50-70 years ago in Japan or Germany, but it had completely dried out leaving a solid thread-locking mess, very similar to dried grey automotive primer. The only effective solvent for the stuff was lacquer thinner, and not just a little, but putting the parts in a closed container with the threads submerged in thinner for several days. I suspect that this lube was originally something like the old white lead.

One of the resources I found by searching for binocular lubricants and greases was these europa forum archives

One binocular that I bought appeared to have something like the old non-drying Permatex sealer on all of the moving parts, I assume it was put there by someone who was uncertain what type of grease to use after disassembly. What a mess.

From what I could determine, a few of the characteristics for good lubricants are stable consistency over a wide temperature range, not out-gassing? (vapor releasing) and anti-bacterial to combat fungus. FWIW, fungus will actually etch glass, much like squashed bugs on a windshield do.

That aoto caliper lube I referred to may not have any of the characteristics of a proper lube for optical equipment, but I tried some and will find out eventually. The goop works very well for some machine purposes. I use it in several machines with very positive results, such as drill press quill rack and pinion. It's thicker than, but sticky like Karo syrup, so it keeps pulling itself back 'n forth between the 2 mating parts, and doesn't just squeeze out to the sides like many greases do.

Dunno about anti-fungal, but vacuum grease meets the other two criteria and I've used it on optical components where a very thick grease seemed desirable. Permatex makes a clear synthetic grease that has a lighter consistency than vacuum grease and is quite stable, but I don't about its tendency to outgas. It's much less expensive than vacuum grease.

Thanks Ed. I will post results. Hand chased threads have started to really rouse my curiosity. Eric