Carbon Fibre rod neck reinforcement?

Kekani said:
I'll have to disagree a little, but not for the reasons you may think.

I leave my cf rod proud of the neck, and route a channel under the fretboard to match. The proud cf rod is the cornerstone of what I call "centerline neck jigs", which is based off my mortise and tenon neck jig, which is mentioned in the out of the box thread (and Luthier's Insights on Shawn's website). Admittedly, this may have been one of those solutions to a problem that didn't exist, but I'm sticking with it.

And yes mikey, add the cf rod. It allowed me to thin out my necks considerably, although recently I've been adjusting the shape a little thicker, but still flat. But that wasn't all. I found it had almost the same effect as a heavier headstock. Good stuff. . .
Click to expand...

That's contrary to a lot of advice in terms of the CF rod doing it's job. I can only quote Al Carruth, who is much more of a science type than I will ever be:

'Carbon fiber itself has a very high young's modulus. Since it's mostly the tension and compression of the material near the surface that resists bending a rod can be very stiff. However, the epoxy matrix that sticks the fibers together has a much lower Young's modulus, and also has a low shear modulus. A CF rod inlaid under the fret board is pretty much riding on the 'center of moment' of the neck, as was shown by Dave Hurd. At that point there is very little tension or compression; most of the load is shear. Thus most of it is taken up by the epoxy matrix rather than the CF itself. Of course, as the neck flexes up some of the load is taken up by the fibers, and at some point they will take enough of it to keep the neck from flexing further, but that's generally 'way more of a bend than we want to see. Putting the CF deeper in the neck puts more of it into tension, and helps it to resist bending better.'

I've also read of makers stating that they flex for near 0.4 mm's before the fibres kick in and stop the flex. I guess this is the 'epoxy matrix' that Al refers to in the quote above. Of course 0.4 mm of flex is pretty much the maximum relief that one would want to see even on a classical guitar. Even then that usually only applies to the wound bass strings. For the plain strings I (and others) either dial in no relief or dial in a mere fraction to prevent back buzz.
I have no personal experience of using CF rods in necks. I do have experience of veneering the back of necks. It's amazing how much stiffness a mere 0.6 mm veneer can add to a neck just by veneering it. I suspect that it's the position of the veneer (back of the neck) that makes it much more effective.
Veneering necks was traditional in lute making where often the core material of the neck was just made of Spruce and baroque lutes can exert a lot of force on the neck, just by sheer number of strings.
 
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Michael Smith said:
Just want to point out to those new at this all carbon fiber rods and not created equal. You can get fairly inexpensive rod from the kite supply place Good Winds but it is not nearly as stiff as the rod LMI sells at the same dimension. I have been using the LMI stuff but it aint cheap If anyone knows of a deal on the better stuff I would appreciate posting it.
Click to expand...

I watched a youtube video about building necks at Mya Moe. Gordon shows a neck with the channel routed for the rod, which looks quite short, maybe 5 or 6 inches and is placed from the heel end, leaving 3 or 4 inches at the nut end without.
I've adopted this idea and that way I get 3 uke rods from 1 guitar rod, thus making them cheaper.
https://www.youtube.com/watch?v=eZ_7aE0SQzM
 
quite short, maybe 5 or 6 inches and is placed from the heel end"
Click to expand...

I saw that and it always seemed to me to be the least effective location to put reinforcement. It "seems" that if you were using a short stiffener, it would be better placed close to the nut end where the neck is much thinner/weaker. :confused:
 
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Michael N. said:
That's contrary to a lot of advice in terms of the CF rod doing it's job. I can only quote Al Carruth, who is much more of a science type than I will ever be:

'Carbon fiber itself has a very high young's modulus. Since it's mostly the tension and compression of the material near the surface that resists bending a rod can be very stiff. However, the epoxy matrix that sticks the fibers together has a much lower Young's modulus, and also has a low shear modulus. A CF rod inlaid under the fret board is pretty much riding on the 'center of moment' of the neck, as was shown by Dave Hurd. At that point there is very little tension or compression; most of the load is shear. Thus most of it is taken up by the epoxy matrix rather than the CF itself. Of course, as the neck flexes up some of the load is taken up by the fibers, and at some point they will take enough of it to keep the neck from flexing further, but that's generally 'way more of a bend than we want to see. Putting the CF deeper in the neck puts more of it into tension, and helps it to resist bending better.'

I've also read of makers stating that they flex for near 0.4 mm's before the fibres kick in and stop the flex. I guess this is the 'epoxy matrix' that Al refers to in the quote above. Of course 0.4 mm of flex is pretty much the maximum relief that one would want to see even on a classical guitar. Even then that usually only applies to the wound bass strings. For the plain strings I (and others) either dial in no relief or dial in a mere fraction to prevent back buzz.
I have no personal experience of using CF rods in necks. I do have experience of veneering the back of necks. It's amazing how much stiffness a mere 0.6 mm veneer can add to a neck just by veneering it. I suspect that it's the position of the veneer (back of the neck) that makes it much more effective.
Veneering necks was traditional in lute making where often the core material of the neck was just made of Spruce and baroque lutes can exert a lot of force on the neck, just by sheer number of strings.
Click to expand...
There was recent discussion on the OLF where Alan commented on cf. He goes on to mention too much stiffness causing the truss to work too hard, and even posed the question of necessity.

As stated, I disagreed with the first statement, but not for the obvious reasons.
I went to a deeper cf (3/16") from the one I started with (StewMac 1/4") in alignment with what Alan describes. I just leave about 1/32" out for reasons explained. Again, a problem solver for one that didn't initially exist.

On such a short scale as an ukulele, with nylon strings, not a whole lot of pull involved. So why install cf? Fear. I've seen too many twisted necks, and probably not because of pull from tension, but that's just a guess. Installing cf gives me one less variable, so I guess I could say one reason is to prevent twisting, rather than add stiffness (although that would naturally be the primary).

Not unlike laminating the backs of necks, just a different path in the same direction. BTW, I like laminated necks, and I'm not sure I'd install cf for stiffness if I did one. Note: I'd install it anyway because my process dictates it.
 
I really like Aarons method of protruding the CF 1/32 to align the fingerboard- yep another jig to make...

Luckily, i now have Jake MacLaurie to help !!!! 12575743_10153514541184825_1195592754_n.jpg

I have found the goodwinds CF is strong enough but id certainly be interested in a stronger cf if i could buy it-
 
mikeyb2 said:
I watched a youtube video about building necks at Mya Moe. Gordon shows a neck with the channel routed for the rod, which looks quite short, maybe 5 or 6 inches and is placed from the heel end, leaving 3 or 4 inches at the nut end without.
I've adopted this idea and that way I get 3 uke rods from 1 guitar rod, thus making them cheaper.
https://www.youtube.com/watch?v=eZ_7aE0SQzM
Click to expand...

I'm not saying my way is the best way but here is how I install a CF rod. I see only benefits by extending the rod as fas as I can. Not only does it make the neck stiffer but it also add some strength at the break angle.
BTW, buy from www.goodwinds.com. Choose the 48" lengths and they'll only cost you a few bucks each after you cut them to your length.
 

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Veneering necks was traditional in lute making where often the core material of the neck was just made of Spruce and baroque lutes can exert a lot of force on the neck, just by sheer number of strings.
Click to expand...

I have also seen Neopolitan mandolins with veneer over a spruce neck. I thought it was just a cheap method of manufacture but obviously there are other benefits
Miguel
 
Moore Bettah Ukuleles said:
I'm not saying my way is the best way but here is how I install a CF rod. I see only benefits by extending the rod as fas as I can. Not only does it make the neck stiffer but it also add some strength at the break angle.
BTW, buy from www.goodwinds.com. Choose the 48" lengths and they'll only cost you a few bucks each after you cut them to your length.
Click to expand...
Exactly how I used to do it, until I started installing them shallow. Funny, I was JUST thinking about doing it like this again, and I'll still be able to install it proud.

And yes, the extra break angle strength would be the reason.
 
Moore Bettah Ukuleles said:
I'm not saying my way is the best way but here is how I install a CF rod. I see only benefits by extending the rod as fas as I can. Not only does it make the neck stiffer but it also add some strength at the break angle.
BTW, buy from www.goodwinds.com. Choose the 48" lengths and they'll only cost you a few bucks each after you cut them to your length.
Click to expand...

Do you recommend a cross section size? .250 x .200 would work with a 1/4' router bit? Glue in with epoxy?
 
Beau Hannam Ukuleles said:
Aaron- doe the slot in the bottom of your fingerboards (to fit the cf) go all the way from end to end or do you leave both ends un slotted??
Click to expand...

I just run it all the way through. Binding covers the soundhole end, and the other end os butted against the headstock veneer- remember, the nut resides IN the fretboard.

I guess I could cut it short, but now there's vwry little sqeezout over the body.
 
dkame said:
Do you recommend a cross section size? .250 x .200 would work with a 1/4' router bit? Glue in with epoxy?
Click to expand...

Exactly what I do except I install the CF rod tall so 1/4" bit will be too wide. I found a couple of table saw blades stacked together to be a perfect fit for the .200" width.
 
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Kekani said:
The one thing I didn't like about the SMD cf rod was the specific .200" bit.
Click to expand...

I make a cut with my table saw blade then add a shim piece of wood against the table saw fence and make another cut. The shim is equal to final width you want less the blade width. I find this method faster than pulling out the dado blades. I usually am only doing one or two necks at a time.
 
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