Nomex tops

[DarwinStrings]

I have just been having a E chat with a Guitar builder in the USA. He has built 30 nomex tops and reckons the nomex tops are stiffer than a solid spruce top. I am having trouble seeing that a 3mm thick spruce/nomex/spruce top could be stiffer than a 3mm thick spruce top given that the spruce in both is of equal stiffness to begin with. I understand it can be lighter but how does it get stiffer? Any of you engineer mob have something that might help me ease my confused head?

I suppose I could get some nomex make beams and deflection test but then I would need to sort out the materials as well as some sort of efficient gluing technique to get reasonable results, anyone got any nomex offcuts they would be happy to post to me?

Jim

[jeffhigh]

There are a lot of claims made by the Nomex builders and many of them do not stand up IMHO.
I will however admit that I have not used it so my comments below may be off.
The nomex by itself is not a particularly stiff material, apparently LMI supplies it rolled up.
It is primarily acting as a separating layer between the two skins much the same as the honeycomb layer between door skins.
So a sandwich of wood/nomex/wood will be just slightly less stiff than a solid of the same thickness, but lighter( if only the minimum glue quantity is used)
However most builders are using solid wood under the bridge, soundhole and edges so the mass saved is not going to be quite as significant as you might hope.
I'd like to hear others thoughts on this.

[simso]

They are stronger.

The strength of a material is subjective to its grain density and distance being spanned IMO.

In an area 1 inch by 1 inch then I doubt sp/nom/sp would be stronger than solid spruce, however when you span a distance the strength is dependant on the grain structure itself. If you were to re-inforce that grain structure with another material which supports each section of long grain with other sections of long grain then strength is added. (think of your roof trusses)

However, IMO this defeats the purpose of a top. If we are after strength, why not make the top out of maple or rosewood etc, we want a soft wood for our tops so that way we can get movement of the soundboard, strengthening it with nomex IMO defeats the whole purpose of using a good sounding softwood

[jeffhigh]

There you go Jim, two different opinions.

Where a nomex top may appear to be stiffer is if it is laminated into a dome but compared to a flat solid plate before it is pressed into a dome when bracing.

[ozwood]

Hi Jim,

Strato has built quite a few of them with his students , they all sound pretty good , one of the classical's played by John ferguson at the dinner you attended was a double top , as was the steel string played by Greg at the same dinner , I have some nomex that's been sitting in the shed for a few years now, I have watched a few being built at the AGMS , and started one myself a few years back but abadoned it , it's a dead set kerfuffle..................... I am yet to be convinced that the juice is worth the squeeze.



Two tops thinned to 0.8 mm a centre peice of solid material too reinforce the bridge and sound hole area's , for there to be any advantage the amount of epoxy used needs to be strictly controlled ....too much and you have lost any weight/ mass advantage, too little and the whole thing de-laminates.
The top layer the solid center peice and the nomex is vacume bagged, once this has cured the epoxy is then applied to the other side and the proces is repeated.

I have seen some strange interaction between the nomex and top veneer, so humidity need to be strictly controled also, and with a 0.8mm veneer to work with all care needs to be taken, it's real easy to destroy a top.

So in summary the ones I have heard sound great ...... Bettter than a well constucted solid top, I'm no so sure.

I will probably make one one day ... just for the challenge.

Cheers,

[DarwinStrings]

I must be going nuts, though I replied to this last night, maybe I didn't click submit.

Cheers Jeff, the dome is a good thought and sounds like it could explain it.

Not quite sure what you mean Steve.

Yep I remember that guitar Paul, the steel string and from memory it was falcate braced, I doubt I will ever bother building with it but am really curious about the mechanism that makes it stiffer for the same thickness. It just doesn't sit with my understanding at the moment and I thought I was starting to get a grip on this stuff, another spanner in the works.

Jim

[Trevor Gore]

There's a difference between strength and stiffness.....strength is about the loading when it breaks, stiffness is about how much load it takes to give unit deflection.

For a Nomex laminate to be stiffer than a solid wood top, the Nomex and epoxy that replaces the wood needs to be stiffer than the wood replaced.

To a first approximation, in a bending test it would be fair to assume that the skins are in tension on the outside of the curve, compression on the inside of the curve and the core (Nomex) is only subject to shear stresses. So the question to ask is "Is the Nomex + epoxy (remembering that the the epoxy will also stiffen the wood as it soaks in) stiffer in shear than the wood it replaces?"

The answer to that is that I don't know, because I've not built a double top and done the tests. And whilst it is possible that it could be, it also might not be.

I've heard a good few double tops, some better than others, but none so good as to persuade me that it is the only way to build. As Paul says:

I am yet to be convinced that the juice is worth the squeeze.

[gpj1136]

I think it would be the same principal as an I beam in construction. It gives a great stiffness to weight ratio. I also think you could build two beautiful sounding guitars buy the time you are done goofing around with it.

[DarwinStrings]

Cheers Trevor. Yep GPJ1136 it is not on my list of things to build, for a start I don't want to add vacuum clamping to my already straining at the seems shed.

Jim

[needsmorecowbel]

I would have thought dust/ foreign particles could very easily dent the very thin tops during the vacuuming process. It seems like a process which you'd really have to be 100% switched on to do repeatedly to avoid registration issues, slipping, dints etc. Do you have to replace the vac bags regularly ?

Stu