Gluing carbon fiber to wood with Titebond

[redir]

Wait... That glue doesn't say for wood in the list

My guess is it would be fine.

[LouieAtienza]

I have used polyuretane glue to attach phenolic fretboards to wood with no probkems; it might be a good option if epoxy is not available.

[Jim.S]

Originally Posted by Martijn

I teach first-year mechanical engineering students in statics and strength of materials, so I am pretty aware of the maths behind it.

Cool, do us a favour and show us the maths for that CF inclusion in the manor you intent to use it. I am slowly working my way through flexural rigidity at the moment and having a little trouble adding CF tow into the parallel axis theorem so would love to see someone else do the maths.

Jim

[KevinLPederson]

I've done a few tests with CF and epoxy, and epoxy doesn't necessarily really stick to it all that well. It will stick, but I've been able to pry certain piece's apart. Structurally, its possible it would pop if it weren't "encased" in the channel. Epoxy a piece of spruce to a CF rod and see. When I test for stuff like this, I actually go until it breaks or the piece comes apart. You HAVE to know.

Epoxy gluing the CF rods in, its still a mechanical bond. The epoxy is curing with a chemical bond, but its not chemically bonding with the CF. Its mechanically bonding. It works great to glue the CF rods in.

When the strings are tuned up to tension, they are compressing the neck (not necessarily "pulling a nice even back bow in the neck") and CF rods will resist this, even if they are in the neutral axis. Simply, because they won't compress, like the wood will. If the channel is tight these rods do a good job at this.

Here is a link to how I do my truss rods and graphite rods (this is more about how I do my truss rod, but you can see the CF rods). I do use epoxy to put them in.

http://www.acousticguitarforum.com/f...284337&page=17

[John Arnold]

There is essentially a zone, a line really, called the neutral axis that represents the equilibrium between compression and tension. In this zone nothing you can do will make any difference.

You said it....the neutral axis is a line. Therefore, the only scenario where a carbon rod (even one installed on the neutral axis) won't stiffen the neck is when it has no height.

When the strings are tuned up to tension, they are compressing the neck (not necessarily "pulling a nice even back bow in the neck") ...

The neck is mainly in compression, but since the force is offset from the neutral axis, it introduces a bending moment. That is why necks tend to bow forward (toward the front of the guitar).

[Martijn]

Thank you very much for all the information everyone!

Originally Posted by Jim.S

Cool, do us a favour and show us the maths for that CF inclusion in the manor you intent to use it. I am slowly working my way through flexural rigidity at the moment and having a little trouble adding CF tow into the parallel axis theorem so would love to see someone else do the maths.

Jim

Sure, I will work out an example, give me one or two days. Maybe this already helps a little bit:

Suppose the CF has a 25 times higher Youngs modulus than the wood. So in fact, you add 24 times the strength of wood on the area of the carbon fiber rod.
This is modelled by adding a piece of wood which has 24 times the width of the CF, at the same height.
You have to add this term (twice) in the formula for calculating the location of the neutral axis. (Mind that the location of the neutral axis also changes by adding CF, in my case it shifts about 0.5 mm away from the fingerboard.)
And you add a term to the total inertia.

I did this calculation for a neck with and without CF (two strips of 3.2x8.25 mm), the bending stiffness increased with about 25%. Such an increase could also be obtained by making the neck a bit thicker.

Although, as others also talked about, it is also about creep of the material. The wood is indeed compressed, changing the scale length a little bit. But the main reason for me to use CF is to decrease the deformation perpendicular to the fingerboard.

Originally Posted by KevinLPederson

Here is a link to how I do my truss rods and graphite rods (this is more about how I do my truss rod, but you can see the CF rods). I do use epoxy to put them in.

http://www.acousticguitarforum.com/f...284337&page=17

Your guitars look incredible! I am going to follow that topic, for sure. Your neck design is a little bit different than what I have in mind. I am wondering why you used a different piece of wood under the fingerboard extension, but I might find out the reason after reading the topic.

[arie]

what about the torquing moment?

[Martijn]

The bending moment is calculated by multiplying the string tension by the distance between the strings and the neutral axis of the neck.
The displacement caused by that is inversely proportional with the moment of inertia. In the calculation example I talked about I will show how to calculate de moment of inertia of both a neck with and without CF reinforcement.

[arie]

Originally Posted by Martijn

The bending moment is calculated by multiplying the string tension by the distance between the strings and the neutral axis of the neck.
The displacement caused by that is inversely proportional with the moment of inertia. In the calculation example I talked about I will show how to calculate de moment of inertia of both a neck with and without CF reinforcement.

no not the bending moment, the torque component seen as pitch and roll. the bass strings pull harder then the trebles inducing axial rotation to the neck.

academic of course, nobody really seems to care much about this stuff. most people make something beefy enough and figure they're good to go. still interesting though. then there's the fact the the bridge is not fixed either.

another thing to consider is that some builders will want to stuff a neck full of carbon fiber to make it as stiff as possible then put a truss rod in it to make it adjustable. if you're not careful, the result is that the stiff neck stays put and the trussrod pops the fretboard off when trying to adjust that super-stiff neck

[Jim.S]

Good on you Martijn. Yep I get that you can convert all the materials to the same thing by altering the dimension. As a carpenter/joiner the maths I have done since school has been working out what length to cut the bit of wood I am working with so as you can imagine some of this stuff is a bit daunting for me. I would really appreciate some examples from others to help me make sure I am getting things right (even though the neck is not what I am working). I approached the local university engineering department but their brand of helpfulness was no good to me. They were happy to calculate a situation for me but not to help me learn how to DIY, unless I enroll in the engineering course which seems a bit much when all I need is second moment of area and parallel axis stuff.

Jim

[John Arnold]

no not the bending moment, the torque component seen as pitch and roll.

I call that 'twist'.

the bass strings pull harder then the trebles

If that were a major concern, then the bass side of the neck would tend to develop more relief than the treble side. I have not seen that consistently. If you decide it is a problem, then all you have to do is shift the rod toward the bass side a little. It won't take much.

if you're not careful, the result is that the stiff neck stays put and the trussrod pops the fretboard off when trying to adjust that super-stiff neck

The whole idea is that you don't have to tighten the truss rod much at all....because the neck has little or no bend from string tension. The result is much less variation in neck stiffness from the adjustment. Tightening the truss rod stiffens the neck, and can change the sound.

[Aramgreuter]

Polyservice.nl is a company in holland with a shop near amsterdam and one near rotterdam.
They have all the supplies you need in small quantities as well.
And a good customer service to help you get the right stuff.
And they do ship.

And otherwise there are multiple online shops selling smaller amounts for modelbuilders.

Good luck!

Aram