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Posts: 1708
Joined: Jan. 29 2012
From: Seattle, Washington, USA
Neck Reinforcement
Does everybody use neck reinforcement? I didn't used to, but after a guitar came back with a warped neck and I fixed it with carbon fiber reinforcing material under the fingerboard, I decided I better be safe and use it from now on. It adds a bit of weight, though.
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I suppose if the neck is thin/flat it's great idea to use carbon fiber, I thought they used only ebony strips in the middle(which doesn't look too good)
Seem to be making necks on the thinner side at the mo. Also, with this in mind, I've taken to splicing a strip of rosewood down the middle for a bit of added strength. I turn one side of the neck up the other way too just in case of warping. Anders is right about storage but I just have a thing about this way of making for now. I also like the extra feature in the neck. In an ideal world there would be no movement of wood and if you can buy and store then that is fine. Sometimes I just dont have this luxury and have to use off the shelve. I'm never going to do again what one maker did and that was to embed a piece of steel tube in there. The arguement was that the routed material v the steel, was a negligable increase in weight - dont believe it - there is. I like the hidden carbon fibre in Ethan's design - nice and light too I'll bet.
remember that the further away from the strings, the better a reinforcment works. There´s little effect in a 6mm deep reinforcement.
Visually I dont like a neck where the reinforcement can be seen on the back of the neck only. If it goes all the way through the head/neck/heel. It can be very elegant. Good idea to use rosewood and not ebony. Rosewood is a lot more stable.
ethan, did you use a carbon fiber rod or tube? a tube of the same diameter as a rod would obviously weigh less (all things considered equal) but i am not sure though if a tube would be as strong as a rod. a good engineering question? given how inexpensive carbon fiber rods are it seems like something that would have a lot of benefits. i like the hidden rod look (be it ebony or carbon fiber).
question, would a carbon fiber insert reduce the likelihood of future neck angle issues?
I use carbon fiber rods on my classical guitars (higher action) and when I am making a flamenco with a thinner than usual neck. I have never found them to be needed on my regular flamencos. Just out of curiosity, why does your reinforcement stop ~ an inch from the headplate? I like to run mine all the way and even under the headplate a little to reinforce the weakest area. Pretty guitar btw.
Keith, I used a rectangular cross-section bar, 1/4" wide and 3/8" deep. I found a place (dragonplate.com) that sells all kinds of tubes--square and round--and beams, such as I-beams, though. But I figure that I might as well fill up all the space I open up.
Andy, I wasn't worried about strength at the head/neck joint as I've never had trouble there. The guitar that had the problem had bowed in the middle of the neck. Also, this carbon-fiber composite is made with carbon fibers (of course) in epoxy resin and I don't know if it sticks to Titebond glue that I use--I suspect not. So I think it might actually weaken the head/neck joint if I made the slot for it go all the way into the head. (Also, I was not completely decided about doing it until I was ready to glue on the fingerboard, so I made a router template that I could screw to the neck around the first and eleventh fret positions--you can see the screw holes in the photo.)
Hi, Anders. Actually, I just shoved the reinforcement into the slot and saw that it hit the bottom and was flush on top, and didn't glue it in except by gluing the fingerboard over it, with Titebond. (Something in the back of my mind told me not to make it so I would have to route it out if there was ever some reason I wanted to remove it.)
Theoretically the stiffness of the Neck increase with the modulus of elasticity of the reinforcement part. When you use a wood like Hardwood it gives you a better strength for the neck: Cedro/Mahogany E = 10 000 to 12 000 N/mm² Hardwood (Rosewood) E = 15 000 to 18 000 N/mm² Stiffness of the neck blank: I = 60 x 22³/12 = 53240 mm4 E x I = 532.400.000 Reinforced I = 8 x 22³/12 = 7098 mm4 (46141 mm4 rest of the Cedro) ExI = 127.764.000 +461.410.000 = 589.174.000 f = 1,11 The reinforcement with wood can give you approximately 10 % more Stiffness in the neck! But when you split the Neck blank and glue them like this, you increase the stiffness too, because the modulus of elasticity depends of the grain orientation (orthotropic behavior of wood)!
But I decide it very freely with the wood that I use, without any calculation! The basic rule is, use only the best wood and store as long as possible in your workshop before use! Handle the wood like a baby and the neck will be perfect!
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For your interest! https://www.dropbox.com/sh/d2t4sb8u4hqf814/mslQ914Cgv I have calculated three different Neck Systems. One plain System, one with wooden reinforcement and one with a GFK Root. The surprising result: The System 2 (Wooden reinforcement) is 13% stiffer than without reinforcement! The System 3 (GFK reinforcement) is 6% stiffer than without reinforcement! In the System 3 the parallel axes theorem gives less stiffness than in System 2 because the reinforcement is very close to the stiffness center! DropBox Link
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In the System 3 the parallel axes theorem gives less stiffness than in System 2 because the reinforcement is very close to the stiffness center!
Exactly. Nicely explained. My vocabulary does not have such fine words. I think wood reinforcement like you did it is a good approach. It has 2 issues though: Esthetically some dont like it and the schrinking of the hardwood will be very different from that of the cedar and theoretically, that is not the best because it means less stability and more stress in the neck. On some necks with a ebony centerpiece, you can clearly feel that the ebony has schrunk (spell?)
when you split the Neck blank and glue them like this, you increase the stiffness
There is a man named Dr. Thoma in Austria who builds the possibly finest wooden houses out there. ( Me was once thinking of distributing his builds, but could not heave financially.) Especially in terms of dimensional / directional stability and noiseless structure. They are also built exclusively of wood / without any metal. http://www.thoma.at
Anyway, among the special treatments that he employs there is that grain positioning nearly like in your example. I think to remember that he layers boards with the grain diametrically opposed.
Ruphus
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I think to remember that he layers boards with the grain diametrically opposed.
Thats a standard cabinet maker rule for making solid wood table tops with wide planks. If not, the top will warp. Alternatively you can glue thinner sticks together and hope it wont warp
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