Does brass absorb and expand over time??

[Dan Daniel]

For the third or fourth time I am dealing with a front-cell focusing lens in a folding 120 camera where the front element is tight because the metal has expanded. Not sure how else to describe it. Disassembling, I think it's typical old grease turned to cement. Finally get the front element out, clean all the threads completely (running a pick inside the threads among other things). And the front element will simply not thread in. Nothing I do will make it go beyond catching slightly at the beginning. Jammed (two Zeiss cameras and one Voigtlander, and maybe another?).

So I take a wire brush to the threads. This last lens, 80mm f/2.8 Jena Tessar uncoated on a 532/16 Super Ikonta, I ended up taking a grinding stone on a Dremel and lightly reducing the diameter of the front element. Then using a file to 'recut' the threads by hand. Jam the lens on, run it back and forth, keep cleaning the threads and now it goes home and will focus fine by hand.

And by tight, I mean the lens element will simply not thread on. Attempting to force it even led to a jam that needed a day in the freezer and a small strap wrench to get it off. This is some serious change in diameter.

Anyone else encounter this? Any ideas on what causes it? People here have seen a lot, but I don't remember hearing mention of this. If this was wood, I'd assume that it had absorbed moisture and expanded. Oxidation inside the threads? Something that I am doing to deform things? Lens elements always measure as round, but maybe it only take a .005mm out of round to make a mess of these fine threads.

 

[AgX]

No, brass does not expand. Except for temperature induced changes, but these are similar to what you experience in all other metals.

But instead you may think of something like rust on iron alloys. Well, there is something similar, but very obvious, seen as green incrustations. But then these need more than just water as in iron alloys.

 

[MattKing]

Did your efforts result in the threads being exposed to heat?

 

[btaylor]

I think it’s likely there is some metal distortion, and when you removed the lens, one or both of the components unloaded from the tension of mating parts, and sprang into whatever distorted shape was a relaxed state.

 

[reddesert]

Brass itself does not expand over time. What is the material it is being threaded into? Is it aluminum? For aluminum, in particular, oxidation produces aluminum oxides that are less dense than aluminum, increasing the volume of the part. Oxidation, in general, can be sped up by reactions at the interface of two different metals. Anyone who has ever dealt with an aluminum seatpost stuck in a steel bike frame (and had to cut it out) will be familiar with that.

Edit to add: Galvanic corrosion: https://en.wikipedia.org/wiki/Galvanic_corrosion , typically deterred by grease, but when the grease breaks down, who knows.

 

[Dan Daniel]

Thanks for the thoughts. Brass 'growing' over time wouldn't happen, of course. Oxidation could expand size, but I clean and wire brush to get to clean brass (then clean out threads with a pick in case any edges were rolled).

In the case of these lenses, it is brass to brass. So aluminum binding to the brass and making a mess isn't very likely.

No heat applied; I would not apply heat to a glass element or its mounting because I don't want to be surprised by assorted expansions cracking things!

I'll lean towards the 'stress relief distortion' theory for now. Rather confusing, and to see it more than once is also confusing.

 

[Sirius Glass]

No brass does not expand over time, but threads can bind over time.

 

[__Brian]

I've had this happen once to a Komura 135/3.5 LTM lens. The brass seemed softer than other lenses, and I believe the tips of the threads became malformed. Too forever to get back together, and filing the ends of the threads was required. I had a spare mount- so put the optics into it before figuring out the real problem.

 

[Digital Wendy]

Brass itself does not expand over time. What is the material it is being threaded into? Is it aluminum? For aluminum, in particular, oxidation produces aluminum oxides that are less dense than aluminum, increasing the volume of the part. Oxidation, in general, can be sped up by reactions at the interface of two different metals. Anyone who has ever dealt with an aluminum seatpost stuck in a steel bike frame (and had to cut it out) will be familiar with that.

Edit to add: Galvanic corrosion: https://en.wikipedia.org/wiki/Galvanic_corrosion , typically deterred by grease, but when the grease breaks down, who knows.

This is very useful - thank you. I have a brass lens that I can not yet remove from (what seems like) an aluminium lens plate.

 

[250swb]

In the scheme of things brass is a very benign metal, so maybe look somewhere else. The surface can oxidise, but so can almost any metal, but brass in cameras is used because it does have a level of self lubrication and self lapping, so a good brass mechanism is usually as very happy mechanism.