Official Luthiers Forum!
http://www-.luthiersforum.com/forum/

Best brace shape for top?
http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=15066		 Page 1 of 1
Author:  j.Brown [ Thu Dec 20, 2007 7:54 am ]
Post subject: 
Another thread on the top radiused vs. flat got me thinking about this.
For glueing up top braces that will follow the same radius as the top, what do some of you do and why?
If the brace is shaped the same as the top in the sanding dish and glued up, it really isn't stressed a lot after the glue up and is fairly idle with no strings on.
If those same braces are not shaped, but bent into place when glued up in the go bar deck, it would seem that this would almost have a pre-loading effect where the braces would be trying to always pull the top down into the body or at least to the flat state that they were in prior to glue-up. Does this hold true? Any opinions on which is better?

-j.
Author:  Michael Dale Payne [ Thu Dec 20, 2007 8:08 am ]
Post subject: 
Using arched braceses during glue up the top is stressed the braces are not. Using flat braces both are stressed

Stressed meaning forced into a shape other than its natural form. Keep in mind that the top is forming a dome. The top is forced to shape in a spherical shape not just a cylinder shape. Arched braces actually get stressed some when removed from clamping by the natural spring back the top is imparting on them.

using flat bracing the braces in an domed gluing form, I would assume that as soon as you take the top and braces out of the clamping the braces will want to flatten out a bit despite the fact that they are glued to the top. There by trying to compress the grain structure of the top.

Now all this is just using what seems to me to be natural mechanical logic so it is possible I am missing something.
Author:  muthrs [ Thu Dec 20, 2007 9:21 am ]
Post subject: 
Torres used unarched fan braces on classical guitars while the harmonic braces were arched. Since fan braces run fairly parallel to the top grain, it is important not to arch them as they will telegraph very strongly throught the top in short time. Actually when Larivee was developing his bracing pattern he discovered this and decided to go with cross braces in the belly of the guitar.

Since there is downward pressure in front of the bridge it would make sense to me to arch these braces. However, since there is upward pressure behind the bridge, it may make sense to leave the tonebars unarched and stress them to conteract the string tension. In this way it may be possible to use lighter bracing in the belly of the guitar. I use a double X pattern and have always arched all of my braces. However since learning about Torres' techniques, I am considering leaving the lower X brace unarched and making it even lighter than I do now. On my daughter's parlor guitar I used an X/fan hybrid and discovered the telegraphing problem, although it sounds great. So I will be revisting this approach and hope to have a OO for the Miami show with my new RXF (Radial X Fan) bracing scheme.
Author:  muthrs [ Thu Dec 20, 2007 2:33 pm ]
Post subject: 
I'm bumping this thread because I think it is and interesting point and would like to hear what others think about it. I don't recall seeing this discussed before, although I might have missed it.

Also I don't know if this will work or not, but I changed the thread title to something a little more descriptive.
Author:  KenH [ Thu Dec 20, 2007 3:14 pm ]
Post subject: 

All of my braces are arched with the exception of the transverse brace just above the sound hole. I am getting excellent results with what I am doing, so for now I wont make any changes.


Of course I am making steel string guitars. Classicals and flamenco's may be a horse of another color.
Author:  David Collins [ Thu Dec 20, 2007 4:07 pm ]
Post subject: 
There are a number different approaches. Obviously shaping the brace to
the same radius as the top is common, but it would be incorrect to
assume that the top and braces are not stressed in this method. Any time
you force a flat plate of wood in to a compound radius, it will be stressed
and want to flatten out. Gluing up with this method will often give you
and end radius somewhat flatter than the radius it was shaped and glued
at, with both the braces and top stressed.

Some builders, again more classical than steel string, may radius the
braces and glue them down to a relatively flat top. Now the braces are
trying to pull the flat plate in to a radius, rather than the plate trying to
pull the braces in the above example flat. Or is there a difference.
As they say, it's all relative.

My preferred method is to bake the tops dry (low heat, high circulation,
very dry), brace them in to a shallow radius - say 60' - with shallow
radiused braces. Then as the top expands it is forcing both the top and
the braces in to a radius, quite differently from the above examples. It
takes a bit of experimenting with your own tools and procedures to
achieve consistency, but it's stability is incredible, and I've personally
found some elements of tone and projection with this method that I have
not been able to achieve otherwise. This is exactly how piano builders
have been bracing their tops for at least the last century or so. They do
this to achieve the desired crown for tonal reasons, more so than
structural.
Author:  Arnt Rian [ Thu Dec 20, 2007 8:18 pm ]
Post subject: 
[QUOTE=David Collins]
My preferred method is to bake the tops dry (low heat, high circulation,
very dry), brace them in to a shallow radius - say 60' - with shallow
radiused braces. Then as the top expands it is forcing both the top and
the braces in to a radius, quite differently from the above examples. It
takes a bit of experimenting with your own tools and procedures to
achieve consistency, but it's stability is incredible, and I've personally
found some elements of tone and projection with this method that I have
not been able to achieve otherwise. [/QUOTE]

That is interesting. Could you describe some of the "elements of tone and projection" that you are able to produce by doing it this way? Do you have a target radius ( or 'crown'?) that want to see the top return to under 'normal' humidity conditions; did you just experiment until you found a procedure that gave you the desired results, or did you develop the method closely to hwo piano makers do it?

Sorry for all the questions, but this is the first time I have heard about this method.
Author:  muthrs [ Fri Dec 21, 2007 12:27 am ]
Post subject: 
David wrote:

Some builders, again more classical than steel string, may radius the
braces and glue them down to a relatively flat top. Now the braces are
trying to pull the flat plate in to a radius, rather than the plate trying to
pull the braces in the above example flat. Or is there a difference.
As they say, it's all relative.


Dave, in the two examples you gave they do seem like two methods of construction to give the same end result. However, in the method of Torres, both the braces and the top are trying to flatten out against the pull of the strings. I think we need to keep in mind the stress the strings are applying to the top both in front of and behind the bridge. I think Torres was able to achieve the delicate balance between the pull of the strings and the stresses built into the top and braces. Romamillos described it as a "floating" plate. Of coarse this is what we all strive to achieve, but I think perhaps a lighter system can be developed by using unarched braces behind the bridge. I am quite happy with my current method of construction, but I remain open to new possibilities and by my very nature like to experiment.

I think your method is also interesting. I can see why it is so stable , since as the top gets too dry it just tries to flatten out into the shape in which it was originally constructed which has even less stress. It seems this method of construction must put some shear stress on the glue line between the braces and the top, but I'm sure it is not enough to be of any consequence.
Author:  JackBarton [ Fri Dec 21, 2007 2:03 pm ]
Post subject: 
Going back to Benedetto... He calls it "spring". He describes it as, leaving an
empty tapered space about 4" at each end of the braces. He says it produces
a much more livelier sound. But it shouldn't be done because after about 5
years the tension and "spring" will be lost, resulting in a duller, smaller
sound. This works for violins and cellos because it is routine to replace their
bass bars every so often.

This is all in relation to archtops of course. I have absolutely no idea how
things work in the flat top world.
Page 1 of 1 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/