Structural Repairs Using Carbon Fiber TOW Pins

[Lo-E]

…could it be applied to more shallow necks like the classic Gibson Les Paul break? I wonder if just embedding TOW under those splines would make it a lot stronger?

I’ve contemplated this a fair amount. I have an LP junior that has been waiting for years for me to spline it.

The thing about a classic Les Paul type headstock repair is that, unlike Bruce’s pins, the reinforcement really wants to go parallel to the grain rather than across it.

The splines provide much more glue surface area than just gluing the break, which is why they’re so effective, but in order to maximize that gluing area it’s good to make the splines fairly deep. Strands of CF tow could definitely increase the strength of such a joint, but they’d be most effective as close to the back of the neck as possible - which is not where they’d be if they were buried under deep splines; they’d be closer to the face of the neck where they’d be mostly ineffective.

I’ve considered possibly doing a two-step splining operation: installing typical deep splines for a nice, strong glue joint and then routing again, this time much shallower and, perhaps, longer so I can install TOW under shallower splines, placing the strands much closer to the back of the neck. I would end up with a sort of spline sandwich with the CF sitting on top of the initial deeper splines and under the later, shallower splines.

Is this a great idea? I don’t know, but it makes a lot of sense in my head.

 

[Bruce Johnson]

I’ve contemplated this a fair amount. I have an LP junior that has been waiting for years for me to spline it.

The thing about a classic Les Paul type headstock repair is that, unlike Bruce’s pins, the reinforcement really wants to go parallel to the grain rather than across it.

The splines provide much more glue surface area than just gluing the break, which is why they’re so effective, but in order to maximize that gluing area it’s good to make the splines fairly deep. Strands of CF tow could definitely increase the strength of such a joint, but they’d be most effective as close to the back of the neck as possible - which is not where they’d be if they were buried under deep splines; they’d be closer to the face of the neck where they’d be mostly ineffective.

I’ve considered possibly doing a two-step splining operation: installing typical deep splines for a nice, strong glue joint and then routing again, this time much shallower and, perhaps, longer so I can install TOW under shallower splines, placing the strands much closer to the back of the neck. I would end up with a sort of spline sandwich with the CF sitting on top of the initial deeper splines and under the later, shallower splines.

Is this a great idea? I don’t know, but it makes a lot of sense in my head.

Hey Ethan;

You've got the right idea. I'll add a few thoughts:

Doing a spline-style repair, the splines themselves are acting as beams. They are intended to strengthen the repair area by taking bending loads, both up and down (looking from the side). They connect to the remaining neck wood through the shear loading of the glue joints on the sides of the splines.

To make the spline joint strong requires a couple of things:

• The spline needs to be as long and as deep as you can make it, to get as much surface area on the sides as possible.
• The spline needs to be a nice tight sliding fit into its slot, very small clearance. To maximize the strength of the glue joints. This is where epoxy helps, as compared to Titebond or hide glue. Epoxy is much stronger when filling gaps.
• The spline itself needs to be strong and rigid in bending.

One way to do that is to make the spline as a TOW sandwich. It's a rectangular strip of wood with a strand or two of CF TOW epoxied onto its top surface, and another strand or two epoxied on its bottom surface. That makes a very rigid beam, in bending, up and down.

Figure out the depth that the spline needs to be, so the bottom TOW ends up about 1/16" above the final bottom surface of the neck. and glue a strip of decorative wood onto the bottom of the spline.

The plan is to build up this TOW sandwich spline separately on the bench, with epoxy and clamps. Then sand the sides flat, to the right width, and saw it to length. Trim it to fit well into the slot in the neck. Epoxy it in, then round off the decorative wood strip to match the shape. If you did your engineering right, you won't file through into the CF TOW.

 

[Lo-E]

The plan is to build up this TOW sandwich spline separately on the bench, with epoxy and clamps.

I had considered that as well. I also thought about using maple for the hidden part of the sandwich and only using mahogany where it’s exposed. Maple, or even oak, would make a much stronger spline.
Good thoughts all around, Bruce.

My particular Gibson is a problem child because the grain of the mahogany makes a big swirl right at the headstock transition and broke almost perpendicular to the neck* - that means that the splines aren’t just reinforcing the glue-up; they ARE the glue-up. They will be doing literally all of the work. The good thing about being too busy to take the project on yet is that I have a lot of time to think about it.

*That piece of mahogany should never have been used for a neck. The fantasy that all vintage instruments are great or that certain companies’ quality control was entirely good or entirely bad during certain eras is just that - a fantasy. This is a 1960 Gibson Les Paul Jr. A holy grail guitar for some, and it has a neck that should have been firewood. …but this is way off topic.

 

[walterw]

I'm going to drill several small holes up through the cracked area, perpendicular to the crack. pour them full of epoxy, and push in thin wooden dowels with a loop of Carbon Fiber TOW

did you try to glue and clamp the cracks first to close them up beforehand? like with thinned-out wood glue, something strong enough to at least hold it closed while you proceeded? or were they just not going to move?

either way super-cool, especially for showing us how easy and straightforward the C-F ribbon is to use this way

 

[Bruce Johnson]

did you try to glue and clamp the cracks first to close them up beforehand? like with thinned-out wood glue, something strong enough to at least hold it closed while you proceeded? or were they just not going to move?

either way super-cool, especially for showing us how easy and straightforward the C-F ribbon is to use this way

Hey Walter;

At the headstock end, the cracks were open, with light visible through them. Both the horizontal crack in the volute and the separation of the fingerboard. As I was getting ready to pour in the epoxy for the pins, I set it up with vertical clamps. But I kept the clamps loose while I poured the epoxy in the holes. I let it sink in for maybe 5 minutes, so some of the epoxy would seep out into the gaps, from the inside around the holes. Then I tightened the clamps and pushed in the pins and CF. When it was cured, the gaps were closed up with epoxy leaking out. So, I think it's securely glued and reinforced.

The crack at the heel didn't have any visible gap, and may have had some glue pushed into it before. So, I just added the four pins without any clamping.

 

[SomewhatHarmles]

Hey Bruce,

Thanks for sharing this write up. It actually reminds me a lot of the glass fiber bonded post/core we do in dentistry.
I've got a case where a lady's bridge is loose, and we may have to use some bonded glass fiber to buy some time.

I also appreciate the thoughts from all the engineers regarding why stuff works.

 

[Bruce Johnson]

Hey Bruce,

Thanks for sharing this write up. It actually reminds me a lot of the glass fiber bonded post/core we do in dentistry.
I've got a case where a lady's bridge is loose, and we may have to use some bonded glass fiber to buy some time.

I also appreciate the thoughts from all the engineers regarding why stuff works.

Yeah, there's a lot of carbon fiber and glass fiber work in the Medical Repair industry. Making replacement parts for the human body. It's a simple way to reinforce odd-shaped custom forms.

Mechanical engineering in the medical field is probably fascinating work. I never went there, but I thought about it.

 

[Gilmourisgod]

I got hooked reading about the disastrous use of carbon fiber, known to be strong in tension, but not compression, on the Oceangate Titanic sub. A horrifying story of Death By Ego and bad engineering, but also fascinating from a forensic materials standpoint. Luckily, the average bass is unlikely to be subjected to 5000 PSI. I have yet to try the TOW backstrap method, just the pultruded CF bars, which so far seem to work admirably. It’s an amazing material when used in tension.