Acoustic Guitar Construction

What style guitar?

Generally the only braces that get any contour are "X" and tone/bass bar. The other thin braces including the bridge plate simply are not rigid/strong enough to bend the sound board. So in effect to put a contour on them only serves to distort the mating glue surface. When it comes time to glue on the bridge you'll be happy that you did not create any in consistant surface in that area as well.

Note that Martin to this day only has a very slight contour on their bracing 52' even then they like to call their guitars "flat tops"

As a mater of fact in our construction process http://www.kennethmichaelguitars.com/topbraceprep.html
we use the bridge plate as the anchor for all the souund-board bracing and it is indeed cauled and clamped very flat.

Sorry I didn't say what style I am building. It is a J 185, Mahogany back and sides with Sitka Spruce top. The top will be arched on a 25' radius. It would seem that a flat bridge plate would be in direct conflict with the arch created by the X braces that it will lie between. Am I over thinking this??

crain 54 wrote:Sorry I didn't say what style I am building. It is a J 185, Mahogany back and sides with Sitka Spruce top. The top will be arched on a 25' radius. It would seem that a flat bridge plate would be in direct conflict with the arch created by the X braces that it will lie between. Am I over thinking this??

Maybe not over thinking it ... but a brace's stiffness is proportional to the cube of its height. The height of the X brace is probably 4 time higher than the bridge plate so 4*4*4 = 64. So by a first or almost a second order of approximation the flat bridge plate can be ignored.

Also keep in mind that you will end up gluing a radiused bridge on the top. The bridge also acts as a brace and is also a lot stiffer than the bridge plate.

In short you do not need to radius the bridge plate as it will bend and will not have enough strength to really conflict with the arch created by the X brace.

Ditto John

I'll stress this one more time -- regarding the bridge plate and the bridge for that matter, what is paramount is getting perfect glue joint after all that is where the concentrated string tension is applied to the sound board and also where the most movement occurs (rolling/rocking forward and back) constantly.

When it comes time to glue on the bridge fine fitting is also required:

http://www.kennethmichaelguitars.com/bridgeprep.html

And yes, keeping things simple (in guitar construction) often works the best -- especially for the first few builds.

Hey Crain,

Glad you brought up the issue as to whether we should radius the bridge plate; I initially wondered about that myself.

Think I'm gonna have to agree with Ken, Tony, and John on this one.

New to this myself (luthiery), but most definitely not new to woodworking and the principles involved.
Like most, whenever I decided to get into this, I purchased a bunch of books and read, read, read.
Granted, the different authors can certainly have differing methods of construction, but I still haven't come across anything that instructs to "pre-radius" the bridge plate.

This is not to say that nobody "pre-radiuses" the bridge plate; probably a gazillion different ways to do this stuff.
But it is my belief that a bridge plate with a "ground-in" radius is weaker than a plate with consistent thickness across the width.

Thinking about it, there are two ways to achieve a radius:
1. Take a piece of flexible material fairly consistent in thickness and bend it to desired shape. This is what is done to soundboard, back, and sides in "flattop" guitars.
2. Take a thicker piece of inflexible material and shape it to desired shape via sanding. Now you have a piece that is thicker in middle and thinner on outer portions. This is what is done to x-braces, and the strongest/stiffest portion of the braces is in the center of the guitar.

Why would one want to weaken an already-thin piece that's designed to enhance support in what's supposed to be the stiffest part of the guitar?


Rob

The bridge plate will curve to the proper radius when you glue on the x-brace. I know because for all of my guitars, I had to radius the underside of the bridge before it will mate to the soundboard. If the bridge plate kept the soundboard flat in that area, I would not have had to radius my bridges since they glue directly over the plate.
No further questions your honor.
The defense rests.







LAWYERED!!!!

There are a number of luthiers that leave their bridges flat and just clamp the crap out of them to mate them to the bridge plates when gluing. One, who is pretty well-known, says he has never had a problem doing it that way.

Personally, I radius the bridge, and I think there are sufficient reasons to do so.