Tootsall

(Digging out mechanical engineering text again.) Not quite!

To say that the metal is permanently disfigured is to imply that it can never be restored to its original shape/configuration/characteristic. In fact, it may be possible to eliminate evidence of the original bend through a particular form heat treatment called "annealing".... which is what Robert said.

You can bend a metal and have it pop back as long as it is not bent past the point where it's elastic limit has been exceeded. Bending past the elastic limit causes (in the case of brass/copper alloys), slippage of the metal grains past each other. The larger grains of metal start to break up into smaller grains and the metal in this area becomes harder. Repeated slippage (bend a coathanger wire back and forth) results in continued hardening to the point where the wire (or sheet) will snap. If you do this you will also notice that the metal in the area of the bending becomes warm or hot to the touch; this heating is caused by the friction of the metal grains sliding against each other. Heat treatment will restore the properties by allowing the small grains to "grow" back together into larger ones and thus, softening the metal again. Just be sure to cool it slowly or you will build in thermally induced stresses that will harden the material!

Here's a way to remember it:

Bending (either the original bend or the repairwork) = breaking grains = work hardening
Annealing = heating followed by slow cooling = larger grains = softening
Tempering = heating followed by fast cooling = building in stresses = hardening.

So... what happens when you get a really big, ugly dent in the bell of your trumpet? The metal is deformed past the point of it's elastic limit (quite low in brass alloys). It stays in the bent shape until a repairperson comes along and bends the dent back out. Sometimes the original bending followed by the repairman's straightening actually stretches the metal in the area of the bend (if the dent is severe enough... and it doesn't take much). (When you make a sheet of metal longer in any of it's directions, it must also get thinner so as to maintain the original VOLUME). When you try to push that material "back into place", it can leave a small wrinkle behind (it's easier to stretch a sheet of brass than recompress it). All of this bending also causes the metal in the area of the repair to "work harden". This work hardening affects the way the metal will resonate with the vibration in the horn (change the "response"). When that happens, the bell really should be re-heat treated (annealed and possibly retempered) to restore it's original characteristic but note that the "stretch marks" may very well remain!

The amount of heating required for either annealing or tempering can be quite a bit different and is best left to someone who has been trained and has acquired more than "a bit of experience". It (heat treatment) is really quite involved.... you wouldn't just want to pass a propane torch over the bell of your trumpet until it glows nicely.... it has to glow "just rightly"!

A good repairman will know if the bell needs heat treating based on his/her experience... how much is "enough" and how much is "too much"? Different manufacturers (and even different models) have different characteristics due to different alloys, different forming methods, and different heat treatments to the original bell. From what we've read it seems that Monette takes this issue to the extreme with bells so soft that they don't want you using a trumpet stand or the bell may bend under the weight of the horn. Another maker might make exceptionally thin bells that are tempered quite a bit harder so that they "respond" and "ring".

Vive la difference!

Edit: So the "limit of elasticity" or "moment of elasticity" refers more to the breaking of the grains of metal and resultant work hardening. Minor bending past the limit of elasticity can generally be restored provided thinning has not taken place. Severe bending past the limit of elasticity can result in stretching of the metal to the point where thinning of the original sheet occurs and it is difficult to eliminate or minimize the wrinkles to where they can't be seen.

Daff

Exactly.

TopGun

$

musicalmason

I have no doubt that dent enidence can be minimalized by great repair techs, but my local repair tech explained it to me like this, I like the analogy so Im posting it. he said to me "a dent in brass is like pushing your finger into saran wrap streched over a bowl, you can still tighten the wrap but the wripples will always be there" given, he is not a brass specialist (hes actually a luthier for his main job) I just thought it was an interesting way of putting it.

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Tootsall

And a heck of a lot shorter than the way I said it!!!

mike ansberry

Ferrees Tools makes a machine called the Z80 dent machine. It is an expensive tool. I don't own one, but the shop where I learned had one. It is a little hard to describe, but what it has a place to attach various mandrels and an arm that hold a roller. The arm is adjustable to put various amounts of pressure against the mandrel. It does an incredible job on just about any brass dent, including trombone slides. It does mar the finish. I'm planning on buying some time on the machine back home to work on a friend's bass trumpet with a crushed bell.

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"Music is a fire in your belly that has to come out of your mouth, so you'd better put a horn in the way before someone gets hurt" (paraphrase of Bleeding Gums Murphy)

Tootsall

Mike, have you had any experience with those rare earth "supermagnet" balls? The ones where the ball goes into the tubing and you "rub" the opposing magnet along the outside of the tube so that the ball rolls the dent "out"? Comments on their effectiveness?

mike ansberry

Yea, I've got the biggest magnet that MDR sells. I have the big set of steel balls, and some smaller balls that I've had around for a while. It is a strong magnet, and will remove smaller dents well. If the metal is creased or the dent is large, it really takes a while. Overall, I've been disappointed in it. I take it to school and keep my low brass in good shape. But for heavy repair work, I usually take the horn apart and do it the old fashioned way. I have a bari sax apart on the bench now. Thought the MDR would be perfect for it, but alas...

They make it sound like some kind of raging killer magnet. They talk about it drawing metal objects at you in a dangerous fashion. It is pretty strong, and you can hurt yourself with it if you don't know what you are doing. It is easy to get fingers caught under the magnet, and that isn't much fun. But I haven't seen any knives flying across the room yet.

Mike

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"Music is a fire in your belly that has to come out of your mouth, so you'd better put a horn in the way before someone gets hurt" (paraphrase of Bleeding Gums Murphy)

Robert Rowe

Mike.

Had to look in my Ferree's catalog. I have the P80 ... but we call it by it's nickname, the "Rocket Roller".

Definitely a "cool tool" ... (and you're right -- it ain't cheap).

If you ever get a chance, try to meet Gary Ferree. One of the most unforgetable "characters" on the planet. I've had dinner and drinks with him several times. ... talk about "war stories" ... O-M-G !!


Robert Rowe

mike ansberry

Hey, Robert. I mis-spoke. I didn't mean the P80. I have a roller very similar to the rocket roller called a "fast eddie". I like it a lot. What I was talking about was the Z60 dent machine. It is on page 142 of my catalogue, but my catalogue is an old one. 1996. Ferrees just keeps sending me price updates. This machine is really cool and does amazing work. But it is WAY out of my price range. I only repair part time now.

__________________
"Music is a fire in your belly that has to come out of your mouth, so you'd better put a horn in the way before someone gets hurt" (paraphrase of Bleeding Gums Murphy)