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DIY Carbon Fiber reinforcement?
- Thread starter BishopJP
- Start date
I very greatly disagree with this it's perfectly possible to hand wet layup carbon with excellent strength, far better than glass. It's regularly done. Done it myself on my race boats. There are indeed big advantages to vacuum bagging, but they are all about better resin to fibre ratios. Wet that layup out properly and there aren't going to be air bubbles. Consolidate it mechanically with clamps or whatever and you can achieve a pretty decent resin fibre ratio.
For the OP, some thoughts. Firstly, completely separate those fibres. The 90 axis fibres won't just do nothing, they'll be a damn nuisance when trying to consolidate the layup.
Secondly, do you know about parcel tape. The brown mylar parcel/packing tape is a lifesaver because epoxy doesn’t stick to it, so use it to cover everything, and especially moulds and jigs.
Thirdly, there's a lot to be said for making rods off the job rather than laminating fibres in place, even though it ought to be a better job. I recently made a centreblock mandolin with carbon reinforcement laminated in place, so down the headstock, along the neck, curving round the centreblock etc. It was a spectacular pain in the neck to do. Don't know why, I'm used to the stuff, but it really was. Maybe it was because everything is so much smaller than a racing dinghy.
I'm 100% in agreement with the point made earlier about under the fretboard being the wrong place for carbon, but I did it that way anyway because whilst a polished carbon skunk stripe on the back of the neck would look really neat, I reckoned it was going to be a nightmare keeping the maple each side clean until I got some finish on it.
Totally missed this thread until now, but I used some rods made from woven CF sheet offcuts in my first neck build. I agree with what others said, only half the fiber is running longitudinally in mine so only half as stiff as it would be if everything was longitudinal. Even with only half the possible strength for the volume, the rods are much stiffer than the neck wood was (not to mention the long term stability), so still a benefit.
In future builds I will fill the neck channel with wet tows instead.
One thing I haven't yet given much thought to is where I would put the CF channels in the neck if I was going to lay directly into the channel(s). I agree with ideas of putting the CF close to the back of the neck, but several comments here refer to putting it under the truss rod in the center of the neck. That really only deals with bow, and if you are already using a truss rod then I don't really see how that is helping much. To me, the benefit of adding a reinforcing rods, in addition to the truss rod, is that they can provide some rigidity off the longitudinal axis, for example to help mitigate twist.
If anyone cares to add some more thoughts on the positioning aspects I would love to hear them.
In future builds I will fill the neck channel with wet tows instead.
One thing I haven't yet given much thought to is where I would put the CF channels in the neck if I was going to lay directly into the channel(s). I agree with ideas of putting the CF close to the back of the neck, but several comments here refer to putting it under the truss rod in the center of the neck. That really only deals with bow, and if you are already using a truss rod then I don't really see how that is helping much. To me, the benefit of adding a reinforcing rods, in addition to the truss rod, is that they can provide some rigidity off the longitudinal axis, for example to help mitigate twist.
If anyone cares to add some more thoughts on the positioning aspects I would love to hear them.
I place the carbon fiber TOW "backstrap" down under the truss rod, about 1/8" from the back surface of the neck, centered in the neck. It runs from the heel, under the truss rod anchor, to about the 5th fret. Its purpose is to stiffen up and prevent stretching of the wood on the back of the neck, from the heel to the 5th.
This does two things: It prevents the inboard half of the neck from bending much, so it won't develop a 12th fret kink (aka ski jump). Also, under string load, almost all of the bending of the neck happens in the outboard half. The relief curve forms between the 5th fret and the nut. Under load, the fingerboard stays nearly flat from the heel to the 5th, then it curves up. This is the ideal shape for a relief curve.
The truss rod goes most of the length of the neck. But because the inboard half of the neck is held more rigid by the TOW backstrap, tightening the truss rod pulls that curve out at the outboard end down flatter. It doesn't change the shape of the inboard half much. The truss rod and the backstrap work together to adjust the shape of the outboard end of the neck. That's what we want them to do: to finely adjust the amount of relief curve, based on the balance between the string load, and how much the neck bends from the string load. Adding the backstrap to the truss rod does a better job of adjusting the shape of the relief curve.
A related note: I use Double-Acting Single-Rod truss rods, which are set down into a curved trough filled with epoxy. The downward curvature of the trough is called the "droop", and the amount of droop determines the mechanical ratio of the truss rod; how much it pushes up in the middle when you tighten the rod. Related to what I described above, when I rout the trough in the neck, I don't make the curve symmetrical inboard-to-outboard. The curve is mostly flat out to the 5th, then curves up more as it nears the anchor. This pushes the deepest part of the curve farther outboard. So, when the truss rod is tightened, the point where it applies the highest pressure is farther out, like the 4th to 3rd. That's deliberate.
To answer your question, I primarily build 4-string necks. I fight twist by careful cutting and selection of the wood lamination strips, to get the rings and the grain symmetrical and opposing. Good general neck building practice. I put my carbon fiber backstrap right down the center.
If I were building super wide necks, then I might add an additional backstrap on either side, spaced outboard. Same basic design: rout a trough down deep, to within 1/8" of the final back shape, heel to the 5th. Lay in epoxy and a strand or two of TOW, and a maple filler strip above it. That would help to fight against twisting of the inboard half of the neck. Hopefully it wouldn't be needed, if I've done the wood strip selection correctly. But the outboard backstraps would be some extra protection. I wouldn't run them the full length of the neck; just heel to 5th. The outboard end needs to be able to flex into the relief curve.
This does two things: It prevents the inboard half of the neck from bending much, so it won't develop a 12th fret kink (aka ski jump). Also, under string load, almost all of the bending of the neck happens in the outboard half. The relief curve forms between the 5th fret and the nut. Under load, the fingerboard stays nearly flat from the heel to the 5th, then it curves up. This is the ideal shape for a relief curve.
The truss rod goes most of the length of the neck. But because the inboard half of the neck is held more rigid by the TOW backstrap, tightening the truss rod pulls that curve out at the outboard end down flatter. It doesn't change the shape of the inboard half much. The truss rod and the backstrap work together to adjust the shape of the outboard end of the neck. That's what we want them to do: to finely adjust the amount of relief curve, based on the balance between the string load, and how much the neck bends from the string load. Adding the backstrap to the truss rod does a better job of adjusting the shape of the relief curve.
A related note: I use Double-Acting Single-Rod truss rods, which are set down into a curved trough filled with epoxy. The downward curvature of the trough is called the "droop", and the amount of droop determines the mechanical ratio of the truss rod; how much it pushes up in the middle when you tighten the rod. Related to what I described above, when I rout the trough in the neck, I don't make the curve symmetrical inboard-to-outboard. The curve is mostly flat out to the 5th, then curves up more as it nears the anchor. This pushes the deepest part of the curve farther outboard. So, when the truss rod is tightened, the point where it applies the highest pressure is farther out, like the 4th to 3rd. That's deliberate.
To answer your question, I primarily build 4-string necks. I fight twist by careful cutting and selection of the wood lamination strips, to get the rings and the grain symmetrical and opposing. Good general neck building practice. I put my carbon fiber backstrap right down the center.
If I were building super wide necks, then I might add an additional backstrap on either side, spaced outboard. Same basic design: rout a trough down deep, to within 1/8" of the final back shape, heel to the 5th. Lay in epoxy and a strand or two of TOW, and a maple filler strip above it. That would help to fight against twisting of the inboard half of the neck. Hopefully it wouldn't be needed, if I've done the wood strip selection correctly. But the outboard backstraps would be some extra protection. I wouldn't run them the full length of the neck; just heel to 5th. The outboard end needs to be able to flex into the relief curve.
Thanks for the detailed explanation Bruce. Engineering the strength to the right parts of the neck makes much more sense to me when adding reinforcement down the centerline.
Now you have me thinking: what if I put a shorter (ex: 460 mm guitar length) DA truss rod with a headstock adjust. That would put the end of the rod around the 13th fret and the peak force between the 5th and 6th fret. Hmmm...
Now you have me thinking: what if I put a shorter (ex: 460 mm guitar length) DA truss rod with a headstock adjust. That would put the end of the rod around the 13th fret and the peak force between the 5th and 6th fret. Hmmm...
I've thought about that over and over but I'm too chicken to commit and try it on a real neck. I'd be nervous that you would have zero control over the shape beyond the truss rod. Although if you make that part of the neck stiff enough, and build in the right shape to start with, that's probably a good thing.
If you try it out let us know!
Yes, that would work, but you would definitely want to add a stout backstrap to stiffen up the heel to 5th zone. Otherwise, you could end up with a nasty 12th fret kink. Remember, that zone around the 12th fret is the most highly stressed part of the neck. When the string load pulls forward on the nut area of the neck, it becomes a long lever handle. The high tension load on the wood fiber on the back of the neck causes it to stretch over time, or to break.
This thread has turned into a wealth of information; thanks to everyone who has shared their knowledge and experience!
What started this line of thought/questioning is my recently started project (not yet posted, here) which will be a 5-string 35" scale. Here is the neck...
wenge and sapele, grain oriented (mostly) like so \\\\//// except for the center strip which is not-so-straight-grained. My thought, after the previous discussion, is to try the CF backstrap instead of the home-made CF rod idea. When I post the project, I'll link it back here for any who may be interested in how it turns out.
What started this line of thought/questioning is my recently started project (not yet posted, here) which will be a 5-string 35" scale. Here is the neck...
wenge and sapele, grain oriented (mostly) like so \\\\//// except for the center strip which is not-so-straight-grained. My thought, after the previous discussion, is to try the CF backstrap instead of the home-made CF rod idea. When I post the project, I'll link it back here for any who may be interested in how it turns out.
