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I'm building an OM neck with .200 thk carbon fiber rod reinforcement- the first time I've used carbon fiber.

What kind(s) of glue do you use in your shop for this process?
SM site says epoxy, CA or any wood glue.
Just watched a video where Dan Erlewine used Fish Glue.
TiteBond is appealing due to its reversibility, but I don't know if TB is really a good idea...

This neck will be slightly on the thin side, so I want to reinforce it.

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Measure Twice,

Karl Borum

It might depend on how your carbon rod was manufactured. I had one that hide glue would not stick to. The only option was epoxy for that one. Another manufacturer, I can’t remember which, promised that hide glue and other wood glues would stick to his. You might have to contact the manufacturer or experiment. When I used the one I had with epoxy, it didn’t go well. The epoxy was too thick to escape along the sides of the slot. I ended up making another neck (without carbon).

I use epoxy. Either Smiths or West Systems. I use .125 x .325 bars on either side of the truss rod and run into the headstock. I run them through the thickness sander lightly if the fit is too tight.

Mask off the slots, brush a generous coat of epoxy on the bar, push into the slot, wipe out squeeze out, lay down sone wax paper and clamp with 1/8” dowels and spring clamps. I make the slots slightly deeper than the bar so the dowels self center.

IMG_0495 by Terence Kennedy, on Flickr

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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 use Titebond, but first I clean the CF bars with acetone to remove any residues.

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"Act your age, not your shoe size" - Prince

I use epoxy and do more or less what Terrence does, except I put epoxy in the slots, then the CF. I run tape along the edges of the slots to catch the squeeze out so that it doesn’t soak into the neck surface and prevent the titebond I use to glue on fingerboards from soaking in. I like how running them out the face of the headstock prevents the Gibson break, as I have a fairly prominent volute that would be a pretty significant stress riser without them.

In a fantasy world the slots would be in line with the string path, but in practice they are parallel to the truss rod slot.

What I've seen/done on an acoustic bass guitar neck replacement repair job is as Mr. Bond does. Prior to glue-up, the the CF was cleaned with polar and non-polar solvents to remove any parting agents, etc. (water/Dawn dish soap , then naphtha), mechanically roughened with maroon non-woven abrasive pad, re-cleaned, and then glued with West or Maas structural epoxy. I am not convinced that with the wealth of glue area a CF rod presents, most other common woodworking glues which are reasonably brittle after drying/curing would not work as well for the job.

My limited understanding of the bending loads seen in the neck are that the constant bending load from string tension is likely the sum of the load from individual strings, so would be pretty well aligned with the truss rod slot axis of the neck once those side-to-side forces cancel out (with differences in string tension across a set of strings adding some variability).

Note that the 'bending center' of the neck is probably not too far below the glue line between the fingerboard and the neck, according to the computer models. This means that the load on the rod there is shear, and is mostly taken up by the resin used to bond the CF fibers. In theory the glue line between the rod and the neck material isn't doing much. As the rod bends the tension/compression loads increase, and the fibers start to take up more of it, as does the glue line. Putting the rod deeper into the neck takes better advantage of the high tensile strength of the CF, and the glue used matters more. In either case the rod will limit the displacement under load, of course.

I'll do a test in scrap wood with Fish, titebond and epoxy.
Terrance, when you run rods through the headstock, how did you trim flush at headstock?

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Measure Twice,

Karl Borum

I use System 3 General Purpose adhesive for carbon fiber and most other gluing I do with epoxy. Things like laminated veneers, head plates, fingerboards that I don't want to introduce water to. The viscosity of Sys3, GP is lower than most epoxies. Some epoxy is so thick you just can't squeeze it into a thin glue line.

I cut blind end slots for CF neck reinforcement short of the nut and about mid heel in length. It's my feeling that more slots in neck wood going all the way thru weakens the wood structure. Most of the head breaks I've seen break thru the short grain beneath the neck shaft past the nut so I don't think it is adding head strength in the right place if I run them thru (just me). I used to run the slots all the way thru too.

I cut the CF slots a bit deeper (maybe 1/32) than the CF so once I slather the slot with epoxy and press in the CF it sits a bit below the neck wood and fill any void above with a thin layer of epoxy. I don't like sanding CF because I worry about the fibers going airborne. I treat CF dust like asbestos but maybe I worry too much. After the glue is cured I like to flatten the top of the neck shaft one more time before the FB goes on.

I've been using 1/4 x 1/4 square CF tube (from Diamond-plate) for neck reinforcement because it is lighter and feels just about as stiff as solid 1/8x3/8 CF and is stiffer side to side. I put one on each side of the truss rod parallel with the edges of the neck not parallel with the TR. I like the 1/4x1/4 because it gives a bit more clearance to the back of the neck shaft too.

I never really gave adhesion any consideration as the rod/tube is encapsulated within the neck so didn't think it was going anywhere and there is so much gluing surface. So, I do no CF prep but am listening to you folks in that regard. Learn something every day.

Has anyone ever had CF neck reinforcement bonds break?? Or in other applications?

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Brian R, Wood Mechanic
N8ZED

For identical construction, the 1/8" x 3/8" CF rod oriented vertically in the neck will be 1.69 times the stiffness in bending of the 1/4" x 1/4" CF rod. The 1/8" x 3/8" rod will also be 3/4's the weight of the 1/4" x 1/4" rod.

That is a significant advantage to the 1/8" x 3/8" CF rod - both lighter and stiffer.

I mark the bar during trial insertion, rough cut with a disk cutter on a dremel and fine tune with the belt sander.
I avoid the bandsaw as CF seems to dull blades quickly.

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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 mark and cut with a hacksaw, then glue them in just very slightly proud of the headstock, then sand flush on the edges sander…

Hey Woodie

I should have measured and weighed first but went from memory, bad doggie. Thanks for catching me!

Here's better data.

For clarification the 1/4x1/4 is square tube as mentioned and the rectangular rod I had been using is 1/8x5/16 solid.

I weighed both products in the 4' length they come in.

The 4' sq tube weighed in at 45 g. The solid 1/8x5/16 weighed 49 g.

I stuck both in a vise full length and hung a 100 g balance weight on the end and measured the deflection. The solid rod (vertically) was a bit stiffer and the end of the rod dipped 7/8". The square tube dipped 1". So about a 12% difference. 1/8X3/8 solid would be stiffer yet but heavier (~59 g) too.

While I was at it I stuck the 1/8x5/16 in the vise horizontally (flat) and hung the weight and in that orientation it dipped a full 5". I expected something like that.

That is probably why I "sensed" the sq tube was stiffer in my hands.

I'd take it further and say the square tube probably does more to prevent twist which is also a concern in neck building. All from my engineering arm chair and the seat of my pants.

Of course we haven't tried to answer the question of how stiff is stiff enough or even too stiff for a truss rod to work. ALL good stuff to ponder in the art.

Measure first - write later. I should know that!

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Brian R, Wood Mechanic
N8ZED

That sounds reasonable. The square tube likely saves some weight (25% for 1/16" wall thickness) over a solid rod and does not give up all that much stiffness. I tend to agree that -particularly for guitar scale lengths, CF reinforcement is more an insurance policy than necessity.

Preventing headstock breaks at the volute is the primary reason I use the CF, the extra neck stiffness is a bonus.

Definitely reinforces that area, particularly if using a peg head-adjustable truss rod. Would that Gibson consider that approach, we'd see a lot less neck repair work.