Official Luthiers Forum!

Has anyone made a double top guitar with kevlar honeycomb instead of nomex? DuPont's literature says it's lighter and stiffer than nomex, which leads me to believe it would be a better core material for a double top.

Can anyone recommend a supplier?

I've used Kevlar a few times for canoe projects- not honeycomb, and not in guitars- so my experience isn't too relevant.
However, I found Kevlar fabric very difficult to work with-hard to cut, and it turns to 'fuzz' if sanded.

It will be interesting to read the responses here. Thanks for asking the question!

BTW, is there a cheaper source than LMII for nomex honeycomb in the right thickness for guitar tops?

Cheers
John

Wow, I'm surprised to read that Nomex works so well!  (After seeing your post, I Googled the construction technique and found Fritz Mueller's website.)

Kevlar and nomex have a similar molecular structure.  Kevlar can be fairly stiff, but it depends on which type of fiber.  For example, the fiber used in bulletproof vests is completely different from what you'd want to use in a guitar.  IIRC, the stiffer stuff is Kevlar 149.

Compared to graphitic carbon, Kevlar is a high-damping material.  It's used in helicopter blades to dampen resonant modes that would otherwise cause the blades to come apart.  However, it may not be high-damping compared to wood.  If it's truly stiffer than Nomex, and Nomex already works fine, then it could indeed be an improvement!

Kevlar fabric is almost impossible to cut and requires special shears if you're doing your own layup.  If you're buying a pre-made honeycomb, then a fine-toothed bandsaw blade will do.  I'd love to hear how you can thickness a whole sheet of the stuff.  It must tear up planer blades like nobody's business!

-Ben

After posting, I realized I had a question.  Has anybody used the sandwich construction method to allow minor load bearing across the soundhole?  In other words, can the honeycomb be extended across the entire soundhole and reduce the required bracing?  Only the wood would need to be perforated....

-Ben

The physical properties of honeycombs vary in relationship to it's density assuming the same material. Cell size also is a contributor, of course smaller cells increase the density.
But as an example, when comparing Nomex core to Kevlar core (same family) Kevlar will make a stiffer/stronger panel pound for pound. Primarily due to the fiber form, woven vs. paper. The woven fibers are oriented +/- 45 deg in the core material vs the Nomex paper which are random fibers. This difference can be made up however by moving to a denser Nomex core.
Searching through some of my old data sheets show that Nomex core with a density of 3lbs/cuft and a cell size 1/8" will have better mechanical properties than than 2lb cu/ft 1/4" cell Kevlar core for example.

The real kicker here however is that Nomex core is not to hard to come by, where as Kevlar core is a specialty item and may be very hard to come by unless your name is Boeing.

BTW- A stiffer top can be had simply making slightly thicker, at 2-3lbs/cuft there is almost no weight penalty by going a little thicker. The weight in the double tops mostly comes from the skins. 3 lb/cuft - 1/8 cell Nomex core makes a great double top, I also know of other builders using 2 lb/cuft Nomex.

As far as working with it, do not send it through a planer. You can send it through a sander and yes it is stringy, take it to 220 and you'll have fine fuzz that will bond up just fine.

Ben, I'm not sure I understand your question. The honeycomb core material is only valuable as a core material, it must be skinned on both sides to have any structural integrity, so you cant just have open core. Well maybe if it was dense enough an tall enough etc..
but it's really meant to be a core material. I probably misunderstood your question but my interpretation was core with out the skins.

There are a lot of variables here and the combinations seem endless. It's fascinating however.

Just some thoughts. So to actually answer your question: I haven't used it in a double top and have no idea where you could get it unless you wanted to by a lot of it.

_________________
Jim Watts
http://jameswattsguitars.com

Thanks for the clarifications, Jim!

You got my question right.  I understand that the core's primary value is to transfer loads to the bearing "skin," but I was curious if it could provide enough structural benefit unskinned to reduce bracing.  You got right to the point:  if it's dense and thick enough.  I'd wager it's not worth doing.

Regarding density, there's two factors at work here.  One is the overall rigidity of the top, which has a geometric component and provides the fundamental resonant mode.  The second is the dynamic response, which relates to the loss modulus in the core material.  Ideally, most of the sound is generated by the stressed skins, and the core is just going along for the ride.  However, in reality some vibrations will be absorbed by the core.  It would be cool to compare and know for certain, but it sounds like the supply of the Kevlar material is a problem.

-Ben

BTW, Jim, your guitars are stunning!

-Ben

The obvious internet search for the Kevlar yields this supplier:

http://www.plascore.com/honeycomb/para-aramid-honeycomb.asp

Do these folks only fill huge orders?

-Ben

check out aerospace composite products. They have all kinds of composites...kevlar, graphite, fiberglass, etc.

Al

Thank you for a very informative post Jim. Indeed I have not been able to find a single supplier for kevlar honeycomb that did not ask me to set up a meeting with some corporate sales lackey that would no doubt laugh at the tiny quantities I'm after. I have emailed plascore to see if I can get some samples, hopefully that will pan out.

Now that I think of it, some of the folks I went to engineering school with ended up at Lockheed Martin... perhaps they have access to some material and they could score me a sheet or two.

As for nomex, aerospace composite products does have it at a reasonable price. Also aircraftspruce.com seems to be pretty reasonably priced as well, though 1/8" thick is as thin as they go.

Another thing to consider when laying up kevlar is choosing a resin. Standard epoxy resins, polyester resin, bond poorly with kevlar so vinyl esters are preferred. The VEs use MEK based hardeners so if you have an aversion to MEK... beware. Ben is right about the dampening characteristics of kevlar. It's also used extensively in aircraft cowlings to dampen engine noise. Seems like a poor choice for a soundboard layup but hey, what do I know. BTW, Aircraft Spruce sells Nomex HC in small quantities. 

_________________
Jim Womack

"Be thankful we're not getting all the government we're paying for. "
Will Rogers