Acoustic Guitar Construction

I bought several thicknesses of CF a couple of years ago with the intention of laminating it into "brace sandwiches", with the CF being perpendicular to the braced surface. I reached a two conclusions.

1. The best place to put CF is on the outside of the sandwich, not in between two wood layers. This makes it easier to attempt modification of the stiffness of the brace once glued up. The CF is so much stiffer than wood that meaningful modification works best by altering its thickness as well as its height, though in theory, height alone should work. Protect your lungs if you choose to sand it though.

2. CF is incredibly stiff. When used this way, it makes a very stiff brace. I questioned whether there is any need for a brace this strong in a standard guitar. Bass, maybe - I don't know much about them. Maybe if you found some extremely thin stock? Or used the cloth like version refereed to below?

If you use it the other way, as a layer parallel to the braced surface (like lattice braced makers typically do), it looks like it would not be possible to tune the top at all. You would need to get it right the first time or suffer the consequences. I have read Greg Smallman does, at times, get it wrong - in which case he simply routs the bad top off and puts on another one. He uses a cloth like form of CF and controls the number of layers that are epoxied to the top of his balsa braces, which are pre-shaped and glued up into the lattice configuration before the CF is applied. (I used the pre-fab 1 inch by 18 inch strips that you can get from several sources.)

CF is interesting stuff. I would not rule it out for a neck.

About necks: Try having someone grasp the head-stock firmly while someone else is playing an acoustic guitar. See if you hear a difference.

As I looked at my reply, I realized there was an assumption I made about the contemplated CF braces that I did not need to make.

My intention was to use them as the main legs of an A-frame that would extend from the upper bout to the lower, much like that found in carved arch tops. Montelone comments, pointedly I think, that it is 10 times harder to move string energy across the grain of the top than with the grain, so I was considering the addition of very light de-coupled extensions to include more of the top. (Montelone says most of his arch tops use an X-brace because of the across the grain difficulty.)

My problem was the effect of gluing down a bridge which would then form a very strong triangle down the middle. I have no empirical measure of that effect because I chickened out and made nothing. I know of no one who has tried such a configuration on a flat top, but I guessed the strength down the middle would have been overwhelming.

Now as I looked at my post, I realized I could use a movable bridge and trapeze and avoid the triangulation. Might get out those old drawings and take another look at where they might go.

One very far out idea would be to turn the grain of the top 90 degrees to translate string energy across the plate, and rely on the extreme strength of the CF reinforced A-Frame (without glued on bridge) to carry the load.

Bob,

Could you tell us more about the CF you laminate to the bottom of your fingerboards? The idea fascinates me.

John wrote: One very far out idea would be to turn the grain of the top 90 degrees to translate string energy across the plate, and rely on the extreme strength of the CF reinforced A-Frame (without glued on bridge) to carry the load.


I'm going to have to think about that, it IS a far-out idea. I like far out ideas.

John, I use a 15 K unidirectional carbon fiber that I vacuum bag to the fingerboard before I epoxy it to the neck. I use Resin Research epoxy and I bag it before the frets are cut, if you do it after the frets are cut, the epoxy can get pulled into the slots - not good! I usually use 21 lbs of vacuum pressure for the laminate, I've noticed that the epoxy is pulled thru the grain on all but ebony which I feel can only help to stabilize the fingerboard. I have also noticed a change in the tap tone after it's been laminated, they ring more on rosewood, even ebony brightens up with the carbon. Now I know that this can be debated if this is a good thing or not, but I've used this method on 65 guitars and nobody is complaining. I have over 40 years experience working with carbon and epoxy and it has it place when used properly. I noticed in Gore's book that he's using carbon in his bridges and I think I'm going to go there too. I've used it on a few bridge plates before but really haven't noticed anything good or bad. I bet if the old timers had epoxy and carbon fiber they would have found a use for it.

Thanks Bob. As I diagram your necks in my mind, they look like they would be very very stable. I know there is discussion here and there about the sonic contribution of the neck, but to me, the primary function is to provide accurate and stable frets that suit playability needs. Although ... there is something to the effect of firmly grasping the head-stock.

I am waiting for Gore's books to arrive from down under.

I need to correct one item on my last post, it should read 21 in. not lbs. Sorry about that!