Key centring...
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Says the repairman’s best friend.
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A key arm can be carefully be bent right or left using the fingers. The brass does not need to be heated. There is a school of thought that it should be bent slightly further than necessary and then bent back a small amount to account for "metal memory". A word of caution though: bending the key arm can sometimes cause the hinge tube connected to the arm to bind on the hinge rod. I sometimes keep the key arm in a fixed position with parallel jaw pliers close to the hinge tube as I bend the forward part of the arm to avoid key binding. Votaw sells a key cup aligning tool to move sax keys right or left for $45. You can find the same tool of a lower quality and lower price on Amazon and Ebay.
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Metal accumulates damage at the atomic scale - it has no memory. Too many cycles back and forth leads to fatigue failure - the number of cycles depends on the material state, plastic strain amplitude, and the accumulated plastic strain.
Bottom line: Don’t bend it more than you have to.
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You are certainly the expert. In the band instrument repair trade it is a commonly held belief that when brass gets bent or moved as in drawing toneholes there is a "tendency" for the material to try to move back to its original state or position sort of like an elastic material after it has been stretched. We are taught in the trade to slightly "over bend" keys that need to be re-positioned and then bend them back slightly to the desired spot to counteract this tendency. Are you saying there no scientific basis for this "belief"?
So why does a piano that's sharp and is tuned down go up?
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There is elasticity in metal, but that occurs instantly upon the release of the applied force. So yes, you do have to bend past the desired resting place, but you do not have to bend them back.
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I don’t know why that would happen - it doesn’t occur on a guitar. There must be more to the system of the string, soundboard, tuner, etc. I have never heard it happen - but then I don’t tune pianos.
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Metal memory is just an easy made up non-professional term in instrument repairs that refers to how the key/metal moves back after bending. Knowing how much keys can be bent etc. mainly comes from experience, but after decades of bending (or straightening) keys I haven't been able to break a key, even after very severe bends and many small bends. Obviously you need to be careful but they can take more than most think (except those lousy MAZAC keys).
Like you wrote the key/metal instantly springs back, like... a spring, because it is a type of spring in a sense, but it's a bit more than that. If you press a key to bend it, it will take a certain amount of force to get it to a specific position. Then it will automatically spring back some depending on material, thickness, etc. If you bend it to more than where you need it, allow it to spring back, but do nothing in addition, it will then almost always require significantly less force to bend it back more in that direction. The key/metal might not move more by itself (or would it over time...?) but it is definitely in a less stable condition where it would move in one direction much more easily, so much that it could easily get out of adjustment from the force of playing, moving the instrument, etc.
I've never bent a key, let it spring back, then just waited... but I've tested this many times and the difference in force needed in the back direction is very obvious (sometimes). Try the same force in the other direction and it wouldn't do anything. It is this unstable condition that you solve by bending it back, or rather wiggle it to sort of find the most stable and relax condition.
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Well, I'm not going to comment on the metals mythology vs. actual engineering knowledge bit, but what I will say is that bending the key arm to center the cup on the tone hole has some significant risks to it. Yes, it's the way you center it up, but the problem is that there's nothing to grab on to.
If we're talking about a stack key, the tube, which is the obvious thing to grab on to, is weaker than the short, stiff key arm, so it's extremely easy to have the tube bend rather than the key arm. Now you've got a bent tube binding on the rod rather than an off-center key cup. If you yank on the thing while it's installed, you can also shift posts, if it's a non-ribbed construction. Finally, where you push on the cup is important - it's not hard to bend the thin sheet metal of the cup while trying to bend the thick material of the key arm. You can easily end up with a potato chip shaped cup.
Key in bending things is to secure BOTH sides of the bend. This is tricky if you have a short stiff key arm. This is the reason tools like the Feree's key benders exist.
I've done this kind of thing many many times over the years, but I also know how to secure both sides of a bend and how to check afterwards to make sure I haven't created new problems while fixing an old one. Go ahead and try it but go slow and check everything frequently.
If we're talking about a stack key, the tube, which is the obvious thing to grab on to, is weaker than the short, stiff key arm, so it's extremely easy to have the tube bend rather than the key arm. Now you've got a bent tube binding on the rod rather than an off-center key cup. If you yank on the thing while it's installed, you can also shift posts, if it's a non-ribbed construction. Finally, where you push on the cup is important - it's not hard to bend the thin sheet metal of the cup while trying to bend the thick material of the key arm. You can easily end up with a potato chip shaped cup.
Key in bending things is to secure BOTH sides of the bend. This is tricky if you have a short stiff key arm. This is the reason tools like the Feree's key benders exist.
I've done this kind of thing many many times over the years, but I also know how to secure both sides of a bend and how to check afterwards to make sure I haven't created new problems while fixing an old one. Go ahead and try it but go slow and check everything frequently.
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Thanks for sharing your experiences and observations, Nitai. The difference in backforce is the Bauschinger Effect.
Sometimes bending is the only way but it should never be a casual thing because of the accumulation of bending stress as already explained. Moreover, a key may not have to be perfectly centered on the tone hole - it depends on where the seating ring falls between the tone booster and the edge of the pad. In fact, I doubt if a sax exists with every pad perfectly centered. Make sure this is not just a visual thing that bothers you. If in fact the cup is too far off, the key arm can be bent but you would use non-marring tools and think it through before doing anything.
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Forgive me for being Capt. Obvious, but....just wanna state....
(I think most respondents are assuming the keycup is empty, and gonna take a new pad)...
IF the OP's intent was to bend the key to align the keycup but LEAVE the existing pad....that will create a leaky pad. Again, stating the obvious but I don't know the OP's level of experience with repair. If retaining the pad after re-aligning cup with hole (not a good idea to keep the pad, but) then in the very least the pad is gonna need to be refloated....
(I think most respondents are assuming the keycup is empty, and gonna take a new pad)...
IF the OP's intent was to bend the key to align the keycup but LEAVE the existing pad....that will create a leaky pad. Again, stating the obvious but I don't know the OP's level of experience with repair. If retaining the pad after re-aligning cup with hole (not a good idea to keep the pad, but) then in the very least the pad is gonna need to be refloated....
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