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mwaller
07-15-2010, 06:46 AM
Hello!
What's the best way to shape tone bars and braces to have a "parabolic" crossectional shape? I haven't been able to find pre-shaped bracewood stock, so I assume most builders do this by hand. What is your preferred method, and how do you maintain an even crossectional profile down the length of the brace?
Thanks!
Mika

bryanperk
07-15-2010, 06:57 AM
What I do is I make mark on the side of the wood in the shape and size that I need, then use a belt sander to bring the brace down to that line, which gives me the right shape every time, although there's probably faster and easier methods out there.. :)

Moore Bettah Ukuleles
07-15-2010, 08:32 AM
I too pre-profile my bracing stock on the belt sander, finishing them off by hand. It only takes a couple of minutes for all the bracing.

mwaller
07-15-2010, 10:39 AM
I too pre-profile my bracing stock on the belt sander, finishing them off by hand. It only takes a couple of minutes for all the bracing.

Thanks, guys! Sanding it is, then. I don't have a belt sander, so I'll try to do it with coarse sandpaper on a flat surface...
Mika

Allen
07-15-2010, 11:01 AM
Ridiculously sharp chisel and ibex finger planes do the trick in minutes for me.

erich@muttcrew.net
07-15-2010, 11:18 AM
We also use chisel and finger plane - sometimes a bit of hand sanding with a fine sanding pad to finish. We don't always get 100% uniform shaping, but I never had the feeling it would make a difference in either tone or stability.

Doc_J
07-15-2010, 12:22 PM
Speaking of shaping bracing..... Is shaping along the length of braces really necessary?

I've seen some folks shape them, others don't. Sound is still very good.
Bracing is there there to stiffen the sound board/back and yet allow for a fundamental vibratory shape.
It is the material at the outside of the profile that provides much of the bending resistance. An I-beam shape would be the most effective (low weight to strength ratio). Seems if you have to shape the tops of braces, you might want to use a smaller height brace, and elminate shaping.

Any thoughts or comments. I might be missing something.

erich@muttcrew.net
07-15-2010, 02:27 PM
If shaping were such a waste of time, I wonder why our classical, flamenco and western guitars as well as our Brueko uke all have shaped braces... I think all our instrument plans and books all call for shaping as well. There's got to be something to it, whether tone shaping, increased strength to weight ratio, or even - I would postulate - the aerodynamics of soundwave reflection within the box.

BTW, if the I-beam were simply functioning as a supportive substructure without being attached to any other structural framework but the plate it is supporting, then a T-beam would be called for in terms of strength to weight ratio. So shaping the braces in that direction seems to make sense.

fahrner
07-15-2010, 07:08 PM
Speaking of shaping bracing..... Is shaping along the length of braces really necessary?

I've seen some folks shape them, others don't. Sound is still very good.
Bracing is there there to stiffen the sound board/back and yet allow for a fundamental vibratory shape.
It is the material at the outside of the profile that provides much of the bending resistance. An I-beam shape would be the most effective (low weight to strength ratio). Seems if you have to shape the tops of braces, you might want to use a smaller height brace, and elminate shaping.

Any thoughts or comments. I might be missing something.
As it applies to guitars and ukulele; tests have shown that no strength is sacrificed with the parabola shape as compared to the rectangle shape. The benefit is that the mass of the brace os reduced improving response.
Certainly not the "be all" key to good tone but just one more thing in a total design concept to build light, build strong.

Allen
07-16-2010, 12:01 PM
Shaping the cross section from rectangular to something else is going to reduce brace weight and stiffness. How much you take off the sides will determine both.

By keeping the height of the brace you preserve most but not all of it's stiffness when shaping into a parabolic profile.

You also want to shape a brace to weaken it in specific places. Most notably towards the perimeter, as the closer you get to the perimeter the stiffer the top or back gets due to being attached to the sides. The easiest way to do this is reduce height. Taking the height down has dramatic effects on stiffness. You see this a bit in ukes, but its very obvious in guitars. It's also a way for the builder to shape the tone, projection, presence and response of the instrument. Giving the builder to produce his signature sound. It's where most of the "magic" happens