Disassembling old cold-welded aluminium barrels

Help!
I have read on many places about how old aluminium barrels get their threads cold-welded by time and humidity, and about the risks involved, if you try to disassemble them by force.
Even S.K. Grimes site advises that the risk is yours, if you send them a lens in such conditions.

Anyway, i just got an american made Goerz Dagor 6.5" f6.8, on an old dial-set. I got it for a very reasonable price, but the lenses are very dirty, from evaporated oil and possibly from a small fungal infection.
In the auction it was clearly stated that the vendor could not remove the optics for cleaning, because the front and rear cells were stuck in place. He optimistically affirmed that "with the right tools" it would be possible to remove the cells and have them cleaned. Unfortunately when i had the lens in my hands, i found that the shutter works well only with fast speeds (now it's stuck open after i tried one of the slow positions... but of course it's nothing to worry about) and that the aluminium barrel of the back cell was badly scratched after an unsuccessful attempt at removing it from the shutter.
Of course the scratch is not a problem in itself, after all the paint has peeled off in part, so a good sansing and a new black paint should restore the original appearance.
What's a problem is that there is evidence that an effort, strong enough to leave a dent, couldn't remove the cell from the shutter.

What to do now?
The most dangerous thing is that the "glasses" can't be removed from the outside, probably the retaining rings are accessible from the inside, and not viceversa. This means that if you apply a lot of force, and the aliminium barrel collapse, the lenses would likely be destroyed, or at least badly damaged.
The lenses are uncoated, so i think it should be possible to safely apply one of those sprays that are used to help remove old bolts.
Do you have any suggestion.
Do the use of a hot airgun be allowed, or the shutter would be too badly affected?
If you have any trick or simply some suggestion to follow, i'd be very grateful.
Fortunately SK Grimes people write on their site that they never broke a single lens, advising at the same time that you take a risk, probably implying that fortune doesn't last forever :-)

cheers

CJ

Cold welding of aluminum is a specific process with specific parameters to make it work. It's not easy to do, and it doesn't happen by accident. What you describe isn't cold welding.

What some aluminum based alloys can do is creep. Threads seem to be rather susceptible to this. If the parts were threaded together excessively tightly, it's possible that the threads from one or both sides will "relax" into each others' contours making for a very tight fit. This in turn often makes it very difficult to disassemble. It takes the right tools, and the right skills. Not something many photographers are likely able to do well.

What to do depends on how much the lens is worth, how much it would cost to get a machine shop (preferably a shop with optics experience such as SKGrimes) to do it, etc.

If it were my lens and I thought it was worth it, I'd send it to a shop like SKGrimes for a CLA and have them give it a try. I would not try to do this myself unless the lens was deemed expendable and I just wanted to play. But that's just me. Clearly, YMMV.

Just a few thoughts off the top of my head -

Use a strap wrench, or a wrench that is specifically for tubes such as these
here at McMaster - http://www.mcmaster.com/#strap-wrenches
If there are scratches on the lens barrel then most likey the previous owner used
slip joint pliers, bad idea, on a tubular shape.

Penetrating oils may work, like Liquid Wrench, spray in a cup and apply
sparingly to the joint with a brush, it may take a few applications ( read days )
in order for it to soak/break thru the corrosion.

Same as above but with a little gentle heat from a blow dryer, not a heat gun ( too hot )
the idea is to get the joint to expand then contract so that the oil can penetrate.
Leaving it out in the sun covered might work as well.

Same idea as heat but with cold, either way , heat or cold
caution is advised because you may crack the glass.

Some pictures of it might help in more suggestions.

"Cold welding of aluminum is a specific process with specific parameters to make it work. It's not easy to do, and it doesn't happen by accident. What you describe isn't cold welding."

Most likely aluminum oxide crystals have grown between the threads, jamming them together, same effect. You could try an ultra-sonic bath the loosen it up.

"Anyway, i just got an american made Goerz Dagor 6.5" f6.8, on an old dial-set."
"The most dangerous thing is that the "glasses" can't be removed from the outside, probably the retaining rings are accessible from the inside, and not viceversa. This means that if you apply a lot of force, and the aliminium barrel collapse, the lenses would likely be destroyed, or at least badly damaged."

It's much more likely that you will break the shutter housing, they are very fragile. You could remove the back side of the shutter and see the inside of the cell, it may be rolled over the glass, not removable.

I totally agree with with Paul's diagnosis. It may not be just aluminium oxides but also other alloy metal oxides - or even nitrides or carbonate salts (from reaction with N2 and CO2). Corrosion of some aluminuim alloys can be quite dramatic and the "tube" can become quite powdery. My Chemistry qualification is more than 40 years ago but I would have thought that a weak acid (acetic?) would supply a lubricant and turn the oxides into more soluable metal salts for a time, that is, until the water evaporates. Trying with organic solvents will just form a barrier on the oxides which will protect it against the dissolving action of weak acids. If you get the threads clear, remove all the powdery layer with a wire brush and then apply light oil before reassembly.

Quote:

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Originally Posted by Paul Fitzgerald

Most likely aluminum oxide crystals have grown between the threads, jamming them together, same effect. You could try an ultra-sonic bath the loosen it up.

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Possible but unlikely in my experience. Aluminum oxide typically forms exceedingly rapidly. One of the keys to making a cold weld actually work is the removal of the aluminum oxide layer in the region to be welded. We found that such removal was fairly easily accomplished with the proper application of a wire brush. Unfortunately, you then have a very limited time (around 30 seconds) to make the weld before the aluminum oxide reforms. In "normal" atmospheric conditions inside an industrial plant, freshly exposed aluminum will "seal itself" in aluminum oxide in a just a minute or two. This research was on electrical grade aluminum, so very pure aluminum and not an alloy. I suspect that alloys of aluminum display similar but different characteristics -- they will probably oxide in similar ways; perhaps somewhat slower. But even if it takes 10x as long, that's still in the range of five minutes.

IOW, after the threads were cut in the lens barrel (on a lathe presumably) they immediately started to oxide. The threads were almost certainly fully sealed before they left the thread cutting station.

Also, I would think it likely that the barrel pieces would be anodized if the process was available (if made after the mid 1930s or so). I don't know if your parts are anodized, but anodizing would clean up and degrease the parts, seal them tightly (by increasing the thickness of the oxide layer), improve surface hardness, and improve adhesion of paints. I have no idea what the effect of anodizing would be on a threaded assembly however. Presumably it would decrease galling, but IDK. Never studied that.

CJ,
The white metal cells on Dagors seem subject to thread corrosion and seizing, as you have experienced. Also the barrels on the dial shutters are often made of thin brass, and subject to distortion if pressure is applied. When distorted, the barrel is even harder to remove.

The key to getting a lens apart without damage is to use tools which grip the lens and the barrel tightly and securely, without distorting the lens cell or barrel. Generic tools are designed for working on pipe and the like and aren't suitable for taking lenses apart.

I grip the lens cell in a six jaw lathe chuck that is padded with rubber or paper. This holds the lens with a secure, even grip. Then I rotate the barrel with a rubber padded ring wrench. You can see what these look like on the Micro-Tools web site.

Penetrating solution is a good idea. Again domestic solutions like WD are designed for automotive use and are too thick to readily penetrate fine lens threads. A very, very thin oil/solvent is needed. You can try the electrical contact cleaner that comes in a small spray can at Radio-Shack or other electronic supply. Don't spray it on. Spray it into a container and then apply it with a Q-tip.

The lathe and ring wrench trick seldom fails. When it does, it is necessary to cut away and replace the barrel.

I hope this is helpful and I wish you luck in getting your Dagor apart. Let us know how it goes.
Cheers,
Photomagica

Good luck. I've tried to get a few frozen ones apart, expendible ones, and no go. Not even a pipe wrench would budge them. They sometimes have a funny characteristic, though, where less grip pressure on the barrel will allow them to unscrew. Probably due to the grip pressure distorting the thing and making it tighter than ever. If I remember corectly, the dial set Compur will come out of it's shell by removing the screws on the back. Remove the iris assembly if attached to the back of the shell. That should give you direct access to the back of the lens. If the lens has a threaded retainer, you may be able to get it apart without unscrewing the barrel. My fear is that the glass is held in by a rolled over edge. Some of that design can be taken apart, at risk of never being able to re-use the rolled edge, thus making it useless. Some will come apart with great care and patience and can be re-assembled. It's a crap shoot, pure and simple. On a positive note, those shutters are easy to repair and are quite reliable.

The corrosion of the aluminum is most likely because it is actually an alloy containing any or all of copper, silicon, manganese, and magnesium. Pure aluminum does passivate (a self-limiting surface oxidation) in seconds, but it is virtually never used for mechanical assemblies because it is much too soft. The alloys machine more readily, but are susceptible to pitting corrosion where the alloy constituents create local electrochemical cells. The corrosion products are bulky and chemically complex, and can do a fine job of locking threads together. Penetrating oil will reduce friction but not dissolve the corrosion products; fortunately, the powdery "oxide" is brittle and if you can ever break things loose, the parts should be salvageable.

I didn't have enough faith on my mechical skills (and even less on the tools i own).
I gave the lens to a local amateur photo repairer. He is not a professional, but (apart from being an expert about stereo photography) he's very experienced and i have been very satisfied by his work in the past.
He was a little dubious, but i have some hope... At least he's got a lathe with a clamp that could be used to securely fit the barrel of the lens, while trying to unscrew the body of the shutter with a padded wrench with a long handle. Of course he will try to penetrate the threads with one of those spray products that are used to help remove oxidized bolts.
I'll report about the outcome in a few days.

What's making me a little more optimistic is that a few lenses with damaged shutters, probably deemed as not economically repairable by Ebay vendors, were considered as a cheap and simple work by my photo mechanic. I still don't know if the vendors were too lazy, or the man is too optimistic!
We'll see...

have fun

CJ