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I'd like to start using carbon fiber as reinforcement in necks and I'd like to use it in tandem with a truss rod. Is anyone willing to tell me where you route the channels in reference to the truss rod channel & what dimensions cf you use?

I looked at Los Alamos and it looks like I could use two rods on either side of the truss rod, 1/8 thick by 3/8ths deep. Is that the way to go? If not, what's customary? If so, how should the channels be routed? It seems like there's a danger: If the space between the truss rod channel and the cf channel is less than 1/8th, might the router not tear it out? And if the space is great, isn't the getting really close to the outer dimensions of the neck?

Any advice would be appreciated! Thanks.

Flori.

Cut the CF channels first, epoxy in the CF rods, then route the truss rod channel. You could have 1/32" of wood left between the CF and the truss rod and it would hold together (it's epoxied to the CF!).

I believe people are spacing them out a lot more than that, though. If you leave a 1/8" or even 1/4" gap between the truss rod and the CF rods you'll still have lots of neck underneath. The CF rods don't 'push back' against the wood like the truss rod does, so as long as there's enough wood under them that you don't 'carve through' you should be fine.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I don’t think there is any stead fast rule but I’m sure others will chime in here.
I would leave at minimum an 1/8” of wood between the truss rod and the CF. The best thing to do is draw a cross section of the neck profile that you use and make sure the CF doesn’t get too close to the outside edge.

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Rich S

"The inconvenience of poor quality will linger long after the thrill of a bargain has been forgotten"

I like the 1/8" X 3/8" stuff, spaced about 1/8" from the truss rod on either side.

I use a table saw to do the truss rod and CF channels, its fast and safe.

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Jordan Aceto
Ithaca, NY

I also use 1/8" x 3/8" and space the channels 1/8" on either side of the truss rod. (With a 1/4" truss rod slot) I like running them across the headstock joint to strengthen it. When I did some V shaped necks I drew a profile to scale and there was enough room. If you use super glue to anchor be sure to shellac the bottom of the channel. It can run through the pores and come out the neck side and affect staining. I use a router table but the table saw is a great idea. Below is a little jig I use to get the spacing equal on the router table. It would work on a table saw too.
TJK

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I space mine 1/8" from the rod slot as well, but I wouldn't go much more without checking carefully. Using my 1st fret profile gauge for a 1 11/16" nut, I saw there'd only be about 5/64" of wood left between the cf & the carved exterior at the first. (I thought about switching to 3/16" x 1/4" CF, for this reason & to keep the stiffness "high" in the neck, but 1/8" x 3/8"CF has more than twice the beam stiffness for the same weight.)

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Dave
Milton, ON

Using a table saw to cut the CF slots, you can safely cut the slots quite close to the truss rod, if it is a concern. 1/8" from the TR works for most necks i do, but i have cut them much closer, on thin necks.

Who remembers the famous Somogyi botch up that led to his ultra cool back of neck purflings? (he used an unfamiliar table saw blade and sanded into the slot during neck shaping)

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Jordan Aceto
Ithaca, NY

Same router table method and dimensions as Terry...but I love the little spacer indicator! I'll be sure to make one for next time...thanks Terry!

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JJ
Napa, CA
http://www.DonohueGuitars.com

Thanks for the replies, all of you!

I haven't priced CF lately but if you wanted to save a little money, instead of using a rectangular piece in those slots, you could use a square piece and fill with wood. That would have roughly the same strength as the rectangular piece.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

You'd need to use 1/4" square and the 1/8" by 3/8" would still be 12.5% stiffer (and 3/4 the price at LAC).

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Bob, do you have your calculations handy? I've been meaning to calculate the difference in stiffness between a solid piece and one like I described and have been a little too lazy to dig out my old engineering texts or search on line.

Actually there's a couple configs I've been meaning to check - one with CF just in the bottom of the slot with the rest of the slot filled with wood; the second being a rectangle with CF at the edges and wood in the center vs. solid CF.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Actually, I booched my calculations! I squared something I should have cubed.

The normal stiffness relationships for solid beams: width is linear, depth is cubic

So 2x the width is 2x the stiffness, 2x the depth is 8x the stiffness

So we'll take a 1/8th denominator out and for the 1/8 by 3/8 beam we get 1 * 3^3 = 27 of our units of stiffness

A square beam would be such that w * h^3 = w^4 = 27 units so w = 27^(1/4) 8ths = 2.27 8ths = just over 9/32"
A 1/4" beam would actually be only 16 stiffness units (so only about 2/3 as strong as the 1/8 by 3/8)

I would assume that a CF sandwich would compare favorably assuming the wood in the middle was glued on perfectly, was very strong in shear, etc etc but that's a lot of questions considering that it'll be a lot of extra work for a small weight savings. I'm not an engineer, though, so once we get into composite beams I'm down to design principles that work rather than being able to put numbers on things anymore. I'm not totally sold on wedging one piece down in there working very well for the above reasons.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I know that Mark Swanson uses 1/4" tubing and puts half of it into the fret board. This also allows him to put fiber optic side dots in his boards.
Here's where I get my carbon fiber.
http://dragonplate.com/ecart/categories.asp?cID=20

That's the thing, I can't remember the composite beam formulas either and why your 12% number seemed high. I just figured one could find 1/8" square CF. Put in the bottom of a 1/8 x 3/8 channel, it should be just barely not quite as strong as the 1/8 x 3/8 solid CF. If the price difference is small, it wouldn't be worth it.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I also use the router table method for TR and CF channels - but I rout mine against a fence while the neck blank is still square.

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The member formerly known as erikbojerik....

Has anyone considered a CF square or rectangular tube to house the truss rod and then be epoxied into the neck?

It may help with how the TR pushes against the inside of the neck and would reduce the # of cuts into the neck.

But, on second thought, will the tube restrict the movement of the TR, make it harder to adjust the neck?

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Steve Walden
Aspiring Builder,
Bartlesville, OK

If you are going with tubing it seems to me the round would be better then square.gives you more area clear of the side of the neck. I would put the rod inside of the CF. I think with 2 rods it might help to prevent twist as opposed to one down the center but I couldn't say for sure.