Metallurgy Question
Posted by Nathaniel.
Here’s a trivia question for people who have experience with metal: does brass get work hardened a lot?
Edit: Details and a picture after the jump
Here’s the situation: I was kind of careless with my big camera and left it on the tripod as sort of a room decoration. Unfortunately, I left it in one of Ida’s running paths and she managed to knock it over which rammed the lens straight into the wood floor (it left an impressive dent). Unfortunately, the hood of the lens got dented in fairly well so I can’t screw filters on anymore. I’ve ordered a special lens vise which is specifically made to fix the problem but I’m wondering if the metal is more likely now to bend back into place or if it’s just going to crack.
Anyway, here’s the thing, your brain can really play up flaws until you look at them closely. This dent isn’t huge. It’s the slightly flat spot at about 1 o’clock in this picture.
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on Tuesday, March 25th, 2008 
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March 25th, 2008 at 8:47 pm Using
What’s the wall thickness and diameter of the lens hood?
but anyway, brass is inherently brittle – you have to use special tools and go slow when machining it, and it will often splinter/shatter almost like frozen plastic or something. However, I do seem to remember that brass has a much more flat fatigue-curve than other metals (certainly base 30* stainless), so that if it survived the first bending, it should be safe to bend it back.
The one exception is if the hood is made of stretched brass, in which case I believe it is more likely to fail. I know nothing about stretched brass except that it exists and they used to use it in old blackpowder rifles and it’s not a good idea.
I think you should be safe, though, especially since the vise will allow you to go slowly. Tim, what do you think about heating the ‘joint’ before bending back into place?
March 26th, 2008 at 12:24 am Using
You should be able to bend that dent back to true with the right tool – sounds like your lens-vise is the correct widget for doing just that.
As for brass work-hardening enough in this situation to reach brittle failure, I doubt that will be an issue. I’ve worked with sheet brass (one of the more common alloys) a fair bit, and haven’t seen too many pieces shatter upon repeated bending/flattening cycles.
That being said, I know that rapid working of a sheet brass joint can lead to failure. You know, like how you might break a coat hanger without using shears: bend it back and forth rapidly until the metal fatigue breaks it. Given that this sort of failure is somewhat related to the heat generated by the rapid working, I’d say that you should probably avoid heating your piece.
Okay, I did a little more reading before hitting “submit comment” and here’s what I came up with: the part you’ve got was likely formed/extruded hot using some kind of mold process – much cheaper than machining it from a solid block. If that’s the case, then it’s going to be a high zinc content alloy (around 30-40% Zn), which tends to have favorable work-hardening properties. As in, you can cold-work it without experiencing brittle failure.
Sounds like you’re in fairly good shape, mi amigo. Good luck with the repair!
March 26th, 2008 at 12:07 pm Using
The lens vise is a pretty cool and simple little doohicky. It’s basically just two half-moon shaped steel pieces on a threaded steel rod. As you screw the rod, the two pieces are forced apart and restore the roundness to whatever got dented.
In my mind’s eye though, I kind of built up the dent. I though the hood protruded an inch and a half and it was all folded over. Sometimes it’s very important to actually look at a thing rather than letting your imagination run wild. Given the metal info that I now have, I’m pretty sure the fix is going to be easy.
I just hope that no issues show up with the shutter. Tiny high-precision watch parts would be much trickier to fix.
Thanks for the info on metals.