Re: metals and finishes

Cutting sheet metal in the right shape for bells.

Oct 28th, 2006,

How does one find out the math behind cutting sheet-metal so that you get the desired bell-shape?

And as to making the metal into a bell, what are the advantages/disadvantages to the two main methods:

-the first method, the cheaper of the two, is to precision-cut the metal with a laser, then fold the metal and fuse the seam. This is what Yamaha does when make, say, their marching mellophone, which is the top horn in its class right now.

-the second method, the more expensive of the two, is to generally cut a shape, then heat it and hammer it out on a mandrel until it's stretched out to the right size and shape. This is how all the very best instruments are made.

Now, I'm not a physicist, but years ago I did take a bit of metallurgy when studying engineering, and my understanding of the difference between these two processes is that the first process is how cheap band instruments are made, and the second process is how the top pro instruments are made.

Correct me if I'm wrong, but what the hammering and stretching seems to do is put tension in the metal, whereas just shaping the metal and fusing the seam does not. Having no tension in the metal does produce a pretty good horn, but having tension in the metal obviously does something extra.

I'm guessing that the difference is like the difference between a taut string and a slack string, that the taut string is livelier, and therefore produces that "high-end" sound, whereas the slack string (or metal) does not.

So, I'm guessing that hammered, shaped metal is livelier and more responsive than metal that has simply been shaped and fused.

With both methods, I still don't know how you measure the metal prior to its being either fused or heated and whacked with a hammer and stretched on a mandrel.

Is there a good resource out there for this info?

Also, is there a way to measure the tension that has been hammered into sheet metal?


Reply #1 - Oct 28th, 2006,

Several points I would like to chime in on from an engineer's perspective.

I suspect there are some mathematics to describe the bell shape (I remember a calculus prof describing the formulation of the force field funnel leading into a black hole--looks just like a mello bell). However, mathematics to cut out the sheet metal that will give the bell shape with just one seam is unlikely. We cannot take a flat sheet and wrap it into a 3D curved structure without significant distortion of metal by hammering, etc. The less expensive method you describe must certainly be composed of multiple pieces, each fused to the next. Picture the flat map of the world on your high school classroom wall, the one that is very cut up (Mercator projection?). I picture the bell to be formed of many trapezoidal pieces, except that the nonparallel sides of the trapezoids are parabolic curves instead of straight lines. The greater the curvature, the larger the bell flares. The sum of the lengths of the narrow end of the trapezoids equals the circumference of the bell tube at the joint; the sum of the large ends equals the circumference of the bell termination.

The more expensive method you describe is more craft than math. Lots of hammering to stretch out the metal. This is where engineering and metallurgy come in. The bell metal is not under tension. Guitar strings are--they are being pulled at both ends. The worked metal of the bell is strain hardened. Picture bending a wire back and forth many times--it gets warm and eventually breaks. This strain hardens to the point of brittle fracture. Some metals are more malleable than others so they can be worked on to a greater degree before embrittlement. But strain hardening occurs, which alters the elasticity of the the molecular structure. I propose that it is this strain hardening that affects the timbre of the sound emitted, making it brighter. As for measuring out the piece of flat sheet metal for this process, picture a jig composed of two wheels of different diameter, connected by an axle. The small wheel is the diameter of the tube at the joint. The big wheel is something less than the diameter of the finished bell to all for the stretching (that's where the craft is). Then roll the jig over a big flat sheet, maintaining a uniform angular velocity for the axle, tracing the wheel paths, and voila, you have a pattern to cut out, seam together and commence the hammering process.


Reply #2 - Oct 28th, 2006,

Strain-hardening- that sounds about right. And I think this is what the expensive horns have that the cheap horns don't. I don't think it just affects brightness- I think it affects overall resonance and response, too. Plus, I'm pretty sure it somehow smoothes out performance-over-range in the mathematical sense, blending the horn's characteristics throughout its range.


Reply #3 - Oct 28th, 2006,

But wouldn't annealing take the strain hardening back out of the metal?


Reply #4 - Oct 28th, 2006,

It can, but there must be a balance between warming and whacking. As the whacking follows the warming, its effects are left upon the metal more than the warming.


Reply #5 - Oct 28th, 2006,

Annealing and deep freezing (Cryogenic freezing) can remove stress captured in the metal. Annealing will also change the hardness whereas I don’t believe the freezing will affect hardness.


Reply #6 - Oct 28th, 2006,

A site for Cryogenic freezing. I haven’t tried it. Personally, I can’t afford it.

http://www.thebrassbow.com/brassbw1.htm#Cryogenic


Reply #7 - Oct 28th, 2006,

I'm not convinced that C. freezing really does anything permanent.


Reply #9 - Oct 29th,

A good example of what the hammering and strain hardening can do is the Jamaican steel drum--originally an ordinary oil drum. The hammering thins and hardens various zones on the end to yield real musical tones, bright and resonant.


Reply #10 - Oct 30th, 2006,


I was trying to say that some French Horn bells are made from 2 or more pieces. The large bell area is normally one piece fused to the skinny area that is formed from one wedge of brass. Otherwise, the brass would get too thin to be useful.


Reply #11 - Nov 1st, 2006,

I have a C.G.Schuster Jun. made rotary valve alto Saxhorn which has a very visible continuous straight line fusion,(?) of the bell material which extends from the edge of the bell flare to the bell curve, where the line, of course, follows the curve of the bell at that area. This indicates to me that the bell was NOT hammered, as hammering would have forced the fusion, (?) line to divert from a straight line. How was my bell laid out?

In my opinion, it was copied from another bell that had been hammered over a mandrel, then slit, straightened and the resulting sheet used as a pattern for subsequently manufactured instruments. Opinions?


Reply #12 - Nov 1st, 2006,

That horn may have been spun. That is a process where the straighter funnel bell is placed on a mandrel and "spun" while the craftsman uses a burnisher to form the bell to the mandrel.


Reply #13 - May 8th, 2008,

I was going to mention spinning. This is how most French Horn bell flares are made. A disc shaped piece of brass (think cymbal) is spun on a lathe and the horn maker shapes metal to the mandrel as it spins. The flare is then attached to the bell tail (usually a single seamed piece) after the top half of the screw ring is placed on the tail. The bell is then cut, and the bottom half of the ring is placed on the flare. I know Andy Taylor makes his trumpet bells this way, but he learned to make brass instruments at Paxmans (British Fr. Horn company). High-end makers like Engelbert Schmid offer a choice between several bell flares in different metals and throat sizes, as well as spun or hand-hammered and with or without a garland.


Reply #15 - Nov 20th, 2009,

Many older bells were created from two pieces of metal, with tooth-like edges folded across each other, then soldered into place. The resulting cone was then hammered into shape. As metallurgy got better and we got better math to boot, we progressed to one-piece hand-hammered and then to spun, which can be a step back in my opinion, being a noticeable difference from a hand-hammered bell.

The hand-hammered bells sound different due to being thinner (another tidbit from Engelbert Schmid) less than work hardening, especially as they are annealed afterwards to make sure the bell vibrates evenly-unannealed brass that is work hardened does not vibrate fully, and it's kind of like taping a towel to your horn-it muffles the tone.

I made a few horns in college and am currently designing and building my own French horn now-these are all things I must worry about.

Brazing a patch in, I need to understand that part of the process. I wonder if that is the same thing as silver soldering.

Reply #5 - Oct 3rd, 2005,



It is the same if we look apart from those who want to knock on the soldered place with a hammer to harden the soldering. That would not be necessary here.



It is important to remove all remainders of tin solder, as the slightest amount of tin will reduce the stability of the silver soldering. It really takes very little tin to do that.



BTW, I am not too happy with that guarding rim on that tuba either. in more than 90% of all situations they protect very little, in some of the remaining situations they even make things worse!




Reply #6 - Oct 3rd, 2005,

Are you talking about the original rod that was there or the new guard he is making?


Reply #7 - Oct 3rd, 2005,

I took a course in brazing at the General Welding Works school in Toronto, Ontario, in 1987. They're really one and the same thing, but brazing goes beyond just sticking pieces of metal together. The only real difference I can see is soldering is sticking pieces of metal together, brazing is working on surfaces.



Most of the guys I went to school with were learning brazing to become boilermakers.

The thing that always bothers me about patches is that you can make them look flawless on the outside, but what about the inside?

Especially in the middle of a bow?



What is an English Wheel? I haven't heard that term before, except as it applied to my old Ford Cortina.




Reply #10 - Oct 3rd, 2005,







This is my version. I had a local machinist make it for me. It has a variety of configurations that can be used.




Reply #16 - Dec 11th, 2007,

No surprise, they do the best they can. Then copper plate the horn and sand down the finish to get a perfect exterior finish.



Quote from Dan O.

"Completely eliminate the distortions with a heavy layer of copper that could be sanded smooth, leveling the surface and allowing for uniform polishing prior to assembly and silver plating"



http://forums.chisham.com/viewtopic.php?t=9824#google_vignette