Official Luthiers Forum!

What neck woods most benefit from the CF treatment? For the (few, I'm a
newbie) acoustic and set neck electrics that I have built I used mahogany or
sapele sandwiching maple sandwichin a strip of walnut (5 piece). I am
currently finishing up a tele style with a rock maple bolt on.

The second part of my question might be answered by Mr. Turner as he
seems to fly the CF flag regularly. Would CF ever NOT be used with some of
the harder neck woods for the reason that the combination may stiffen the
neck to the point where it would impede truss rod adjustment?

Thanks, K

   I am sold on this stuff. It is light doesn't add alot of mass and the benifits are more than you can count .
john hall

I don't use it in all necks mainly because it can be a price point issue given that I make some instruments that sell for around $2,000.00 at retail, and we aren't having any major issues with neck problems.   With our price formula, CF adds about $200 to the retail price of an instrument. However, I'm using it more and more, and I could see a time in the future when all of what I build has CF in the necks. I'm working on a fingerboard design where the cost savings in labor may offset the increase in materials cost from using CF.   

Functionally and sonically, I see and hear nothing but good from adding CF to necks.    

Carbon will stiffen any neck up, as its resistance to bending is about a magnitude better than wood. It's available in many sizes as well.
So weak floppy woods benefit the most. It also helps with sound transmission through the neck IMO as it has a higher stiffness to weight ratio than wood.
A typical mahogany neck with 2 pieces 1/8 X 3/8 cf on either side of the truss rod still allows for plenty of adjust if you need it. I've found that with a straight neck string tension pulls in some string relief. I believe Terry Kennedy (sorry if got the person wrong here)posted in another thread he got .009 of relief this way with that size rod. I've never actually measured mine, but this seems about right for me also.
Sorry the questions kind of broad and hard to give a good answer to, but hope this helps some.

_________________
Jim Watts
http://jameswattsguitars.com

OK, this seems like a good place to address a particular CF issue I've heard about (though I've never used CF).



#1-I get the impression that some kinds of CF rods are just a simple
mixture of graphite powder and epoxy, and yes while they are nice and
light they also have no grain orientation and so there is no preferred
orientation to their stiffness.



#2-I've heard that other CF rods have a definite orientation of the
graphite fibers along the length of the rod, much like a fishing rod
(which is actually a tube).  So they are very stiff in
compression, not very stiff in flexure.



What say ye?  Correct me if I'm wrong but I would think you'd want
rod #2, but how do you tell the difference when sourcing these?

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#2- let me add that while they are not as stiff in flexure as they are
in compression, they are still more stiff than an equivalent piece of
most woods.

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I make my own 5 piece necks with Mahogany being the main component surrounding 1/8" layers of hard maple and a contrasting red species. These laminates are quite stiff on their own but I use CF from Los Alamos Composites on all 14 fret necks as a matter of policy. So far, I have felt no need for CF on 12 fret necks...but I'm considering using it there as well.

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

Erik, the rods made from powder are usually called graphite bars. Builders should be using pultruded carbon fiber rods with epoxy adhesive.

On guitars with short-scale, 12 fret necks (1 piece H-M shaft, built up heel, spliced head) I am using 1/8 x 3/8 carbon fiber alone, no other reinforcement. I make the neck and fretboard dead flat, and so far the strings have pulled up the correct relief.

Long scale 14 fret necks get a regular truss rod. So far, no "mixed media"

_________________
https://www.facebook.com/profile.php?id=100008907949110

Eric, powdered carbon rods are what pencil centers are made of.   Nobody uses that for reinforcements. If you've heard that, you're listening to folks who need educations.

The carbon fiber used in epoxy or vinyl ester CF composites is a long chain molecule whose unique property is a very strong resistance to stretching.   There is very little elongation under a tensile load until the stuff simply snaps.   The carbon fiber strands are made from a precursor (it used to be rayon) which is held under tension in an oxygen free atmosphere and heated to the point where everything except the carbon vaporizes away.    

There are many grades of CF and there are also many resins used to stick the stuff together.   The best CF materials have as little resin in the matrix as it takes to simply hold the fibers together.   

Our own James Watts here is a fantastic supplier of CF rods, and I hope he chimes in with more technical information. He's being modest in his above post...

CF fiber rods get their important strength qualities from the tensile strength and lack of stretch of the carbon fibers, as Rick says. In compression, the carbon fiber is doing very little--you are mostly getting the compression resistance of the matrix.

From this it follows that the maximum benefit of CF rods is had when they are as much as possible in tension, which is when they are as much as possible behind the neutral axis of the neck. I.e., as deep as possible without coming out the back of the neck. Rick's method of setting them partially into the fretboard would seem to diminish the effectiveness of their tensile strength. I assume that there is something gained in tradeoff, but I'm not sure what that is. Rick?

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Thanks Rick & Howard for the detailed replies (appeals to the
scientist in me), and for straightening me out on the CF issues.



I can see now how the compressibility of CF is not nearly as high as
the tensile strength....but maybe the compressibility is still higher
than an equivalent piece of (pick your favorite) hardwood..???

Erik, good questions.

In regard to #1 there are plastics that have powders and very very short fibers mixed in them at some low percentage of loading,15-20% kind of thing. These are primarily used in the injection molding industry. For example if a polycarbonate part is boarder line from a structural point of view a loaded polycarbonate material would probably kick in into the acceptable range.
The mechanical properties of this type of material are very far below the mechanical properties of a true carbon fiber composite material.

#2 is a pretty complicated question but you are right in that different processes and fiber orientations give different results Your conclusion that they are good in compression and not flexure stiffness is incorrect however.
Generic carbon fibers have a stiffness or resistance to bending slightly better than steel (some are much better) along the length of the fiber and when put in tension are many times better than steel.
In a nut shell what you want for maximum stiffness are continuous unidirectional fibers. As the fiber moves off the axis of the load it’s ability to resist the load decreases. For a guitar neck you want all the fibers running parallel to the strings.
A lot of pre-made laminates will be what’s known as a quasi-isotropic lay-up. This gives good properties in all axis, but not as stiff as it could be in one axis.. As another example a lay-up made with fabric will about half as stiff in one direction when compared to a unidirectional lay-up, but stiffer in the off axis than the unidirectional lay-up, so it pays to have a tailored solution.
Fishing rods are typically not unidirectional as they’re trying to tailor flex into the rod which they do by fiber orientation and cross sectional area.

I don’t know if you’re a GAL member or not but I answered a similar question in the spring or summer issue of this year and it’s probably clearer there. I hope this makes some sense anyway.

BTW, the product Los Alamos Composites sells is unidirectional lay-up made from pre-preg in order to control the resin content.

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Jim Watts
http://jameswattsguitars.com

I find that CF rods are very, very good in compression as well as tension, and dadoing them up into the fingerboard allows me to use 1/2" tall bars instead of 3/8"...see the cube rule discussion.   Also, what I'm doing is making structural fingerboards that are so strong and stiff that the neck means less and less.   I'm after the ultimate in stability where it counts...at the fret top. With this I also get very predictable relief. Ultimately, it's all about the fingerboard, not the neck.

There is also now an additive that is being used in the resin matrix used to hold CF together; it is made of carbon nanotubes which have tremendous compression strength. This additive apparently greatly improves the compression strength of CF composites.   Exotic stuff... Love to get some! James?

Also, as James quite rightly points out, you can tailor the stiffness of CF in your layup.   One composites engineer described CF parts as being like wood that you can design yourself. This is somewhat akin to how traditional boat builders make structural knees out of trunk to branch areas of trees.   The grain curves in the way the parts are curved. Wooden boat builders used to have major collections of naturally grown knees to use in hull construction.

A few clarifications:

Unidirectional carbon-fiber reinforced epoxy is just as stiff in compression as it is tension, but it is not as strong.  The reason is that, under stress, the fibers undergo out-of-plane buckling that limits the strength of the material to about that of the resin alone.  Unidirectional composites placed in flexure, where the neutral bending axis falls somewhere in the middle of the beam, will have properties in between the pure compressive and tensile properties.  Overall, properties are highest in pure tension. 

Howard is correct that the stiffest neck will result from having the CF beam nearest the player's thumb because that places the carbon in line with the highest tensile stresses inside the neck.

As mentioned, there are many grades of carbon fiber, including both short and continuous fibers.  Carbon fiber is generally a "graphitic" structure that is analogous to a rolled-up newspaper.  However, there are differences in the orientation and defects of the graphitic structure.  In the best case, you have single-crystal graphite "whiskers."  These will generally not be found outside of specialized (and high-cost) aerospace applications.  It is technically a misnomer to refer to CF as "graphite."  As Rick mentions, there are also carbon nanotubes, which can be though of as the smallest unit of a graphite whisker.

Resin matrix stiffness could be enhanced with carbon nanotubes to reduce damping in CF-reinforced beams....  However, organoclay is a much cheaper and more readily available alternative.  In fact, epoxy will bond with untreated montmorillonite, which is dirt cheap because it is literally dirt.

-Ben

Bear in mind that while it is true that a surface of the back of the neck made of CF composites (the subject of my 1978 patent, by the way) does make for the highest stiffness and most efficient use of CF, there are plenty of other reasons to not do this including being locked into very expensive tooling and not having any wiggle room to make custom shaped or widths of necks.   

There is also the matter that stiff enough is stiff enough, and you can over-engineer a neck beyond the beyond if you wish and if you have deep pockets.   

What I'm going for is an engineered fingerboard structure that has in and of itself most of the stiffness and strength needed.   The neck is now little more than something that fits the hand comfortably.   I can vary the width, the taper, the hand feel of the neck at will.   I'm combining what if feel is the best of both worlds...the great stiffness and predictability of CF composites with the adaptability of a custom guitar shop. We can CNC our necks or we can hand carve them or do a combination and still have the advantages of the CF reinforcement.

And then there's just that feel and look thing. A lot of guitar players love wood, and so do I. I feel I'm making the wood better with what I'm doing with the CF. Once again, it's a best of both worlds thing.   

Ben, Thanks for catching my error on the compression concerning stiffness. You are absolutely right about that. I was pretty distracted while I was typing the other post.

One other thing that is eluded to but may not be clear to all, is that the resin matrix and amount of resin in the lay-up reduces the effective stiffness of the composite laminate. While I stated that the fibers are slightly stiffer than steel, I was talking about the fibers only. Once bonded together in a matrix with a controlled amount of resin a unidirectional laminate will be about 2/3 as stiff as the equivalent piece of steel.
Also, laminates that contain excess resin will have a even lower effective modulus of elasticity, or stiffness. Some processes are better at controlling the amount of resin than others.

_________________
Jim Watts
http://jameswattsguitars.com

Stiffness in a structure is a matter of pitting high tensile modulus materials...ones that resist stretching...against materials with high compression strength.   So the most efficient and stiffest neck is one in which the fingerboard surface is optimized for low compression and the back of the neck is optimized for low stretch.

As a practical matter, there are a number of ways to achieve the desired results.   My own direction is in this "structural fingerboard" direction.   With this approach, the back of the neck could almost be balsa and the only issue would be making the peghead-to-fingerboard joint strong and stiff enough.   It's doable...

One interesting approach was the Parker Fly neck...very good engineering on that, though locked into extremely expensive tooling and a very limited number of shapes practical.

Rick, have you checked out the modulus spruce/CF necks, it's the Genesis line I think?
I'm sure you have. I've only seen it, haven't played or heard one.

_________________
Jim Watts
http://jameswattsguitars.com

Yes, James. They're nice basses, and the neck concept works well. Once again, it's kind of a best of both worlds concept.

Hi, Gang,

I didn't mean to imply that ultimate stiffness was desirable.  It was merely a point of information about the bending moment vs. the design of the beam for those uninitiated in the math.  I offer my apologies if it came across differently. 

Rick - I also don't like the look and feel of "plastic" neck components a la Parker and Moses.

I was under the impression that the Genesis design was unique to guitars.  Is that incorrect?  The design had a flat piece of composite sandwiched between the neck and fingerboard with a box-section channel running the length and extending into the headstock.  The neck wood was either cedar or some other soft wood.  I don't know if there was a truss rod or not.

Another interesting design is this one from Kritz:
http://www.kritz.com/engels/inventions/4evernecklayout.html

I'm certain he's not the only one that had this idea, but he's patented it anyway.

-Ben

OK...did some data digging, and this is pretty cool if I have my numbers right (James, please correct me if I'm off here...)



CF rod rupture modulus is ~2700 MPa, about 25X higher than rock maple (109 MPa) and about 14X higher than ebony (190 MPa).



CF rod compressibility is ~1900 MPa, about 35X higher than rock maple (54 MPa) and 21X higher than ebony (90 MPa).



CF rod tensile strength is ~50,000 MPa, about 500X higher than rock
maple (~100 MPa).  I can find no data on tensile strength of ebony.



So even if you're not taking total advantage of the tensile properties
of CF rod, you're still doing pretty well in bending and compression
compared with wood equivalent.



 

Eric, practically speaking you're right...but it is precisely the tensile modulus of CF (or anything else) that is at the heart of bending resistance. It's that surface down to the neutral axis that is in tension which resists by not stretching.

The other thing that hasn't been mentioned yet here is the incredible "memory" that CF composites have as long as the resin is tough and resistant to cold creep.   You can put a CF rod into a bow for years, and it will come back straight when you release the tension. Not so with wood.