Double Top Guitars

[Jim.S]

Good on you for taking the time to answer Tim, it looks like I will have to research more to satisfy my curiosity.

Originally Posted by Tim McKnight

Nomex by itself is not stiff at all. The Doubletop gains its stiffness after the Nomex is glued to the inner and outer skins. Nomex is not stiffer than spruce if comparing the two different materials. The spruce is stiffer than the Nomex when they are compared before they are joined.

Yep I had assumed that was the case.

Originally Posted by Tim McKnight

Imagine if you had two playing cards, you held one between both hands, you are able to flex them easily, right? The playing cards would be an example of the inner and outer spruce skins. Now imagine that you separated the two playing cards by a certain distance, say .060" and glued a perpendicular member (the Nomex) between the two cards. You have created an I-Beam shape with the three separate pieces. The I-Beam shape is what gives the three "joined" pieces it's unique stiffness.

Yep, Got that too, using I beams is a way to reduce weight and maintain stiffness but they do not gain stiffness over a same sized solid beam of the same material. Like a 4" X 8" steel I beam is a similar stiffness (perhaps a bit less) to a 4" x 8" solid steel beam but much lighter.

Originally Posted by Tim McKnight

I have glued Nomex to only one piece of spruce and it is extremely weak, floppy and not stiff at all. Its not any stiffer than the single piece of spruce. After the second pieces of spruce is glued to the other side of the Nomex, that is when the sandwich gains is stiffness.

That I can easily understand in my head and as well as from experience with building custom doors with skins and the paper honeycomb.

Originally Posted by Tim McKnight

As the thickness of the Nomex increases so does the stiffness of the completed sandwich based on the cube root rule.

Yep I understand that bit

Originally Posted by Tim McKnight

Traditionally we adjust the stiffness of the top plate by graduating it. One is unable to do this on a Double Top because a .030" piece of spruce does not have enough material to graduate. Add to that if one dings the top, during construction there is no way to sand that ding out. Its a mind boggling and tedious process to say the least.

It does sound like a very time consuming and exacting process good on you for taking up the challenge and not being able to shift the stiffness once it is all laminated up means you need to be sure of your sizes first and quite experienced by the sound of it.

I am not having any trouble with the fact that a nomex top is stiff what I am having trouble with is, like the steel I beam example is that I can't see how say a 3mm thick spruce/nomex/spruce top becomes stiffer than a 3mm thick piece of solid spruce provided that the spruce used in both cases has the same properties to begin with. I can easily see a weight reduction but not a stiffness increase. Having said that though when someone of your obvious experience and knowledge says that of those two 3mm examples the spruce/nomex/spruce is stiffer then I take notice, so now I need to convince myself that my understanding of I beams or lamination is off the mark or totally wrong.

Can you see what my head is doing? with the 3mm double top compared to the 3mm solid spruce all your are doing is removing the middle of the beam and replacing it with a lighter medium. It is difficult for me to see how it increases stiffness when the stiffness is in the outer skins.

Originally Posted by Tim McKnight

Logically it sounds like it should remain the same stiffness but it does not because you are replacing the "guts" with a material that is MUCH stiffer & lighter than the material which was removed.

This quote confuses me even more, it seems you are saying the nomex is "MUCH stiffer and lighter" than the bit of spruce that was removed

Jim

[Tim McKnight]

I get what your saying as it doesn't makes sense that something hollow, with less mass is stiffer than a solid. I can talk [er type] till I am blue and you may still not be convinced. Build one and measure it then report your findings

[Jim.S]

Oh I completely understand how something hollow with less mass can be stiffer than a solid depending on materials and configuration, Carbon fibre box section can be much stiffer than steel solid. Would you have any nomex offcuts that you could post to me (I'm happy to cover cost) and I will make up some beams and defection test them. If you can't I will look for something around here but then I have no idea of the glue technique and may not get the results you would get.

Jim

[Jim.S]

I guess something like covering glass with thin glue then placing the nomex on it to pick up a little glue might be okay.

Jim

[perttime]

Originally Posted by fitness1

No, there is a layer of nomex (sp) between and little or no bracing....completely different than laminated.

Three layers of material blued together? Sounds like the very definition of laminated.

"""""
Lamination is the technique of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance or other properties from the use of differing materials. A laminate is usually permanently assembled by heat, pressure, welding, or adhesives.
"""""
http://en.wikipedia.org/wiki/Lamination

[Jim.S]

Cancel that nomex request Tim but I have another question if you don't mind. What measurements did you take to know it ends up stiffer and are you talking both stiffer long and cross grain?

Jim

[Tim McKnight]

Originally Posted by Jim.S

Cancel that nomex request Tim but I have another question if you don't mind. What measurements did you take to know it ends up stiffer and are you talking both stiffer long and cross grain?

Jim

Measured by deflection, as is all of our tops and backs, both along and across grain.

[stonedtone]

I've never heard of this. I don't get it. Two tops vibrating together?

[Tim McKnight]

Originally Posted by stonedtone

I've never heard of this. I don't get it. Two tops vibrating together?

Its no longer 2 separate tops and a core but one single conjoined top that vibrates as a whole.

[M19]

Originally Posted by rick-slo

Played three double tops - all were forum roadtrip guitars.

McKnight Diamond double top: Italian spruce/Italian Spruce

http://dcoombsguitar.com/Guitar%20Mu...andLexicon.wav

Was expecting to hear you start singing "You put the Lime in the Coconut..."

Cornerstone Guitar Cedar/Englemann double top

http://dcoombsguitar.com/Guitar%20Mu...eightTrain.mp3

Sweet version of this old chestnut! Yea Derek!

[rick-slo]

Originally Posted by M19

Was expecting to hear you start singing "You put the Lime in the Coconut..."

Just checked out this song on youtube. After the intro it must have been quite a surprise when I switched gears.

[Jim.S]

Originally Posted by Tim McKnight

Measured by deflection, as is all of our tops and backs, both along and across grain.

I may be easier to convince with words than you think Tim. If they are stiffer for the same thickness then there must be a mechanism.

The things I have come up with so far after using the phone a friend option are. If you are deflection testing the unbraced board could it come up stiffer because you laminate the nomex into a dome before you brace? Or It may be possible that if enough epoxy soaks into the 0.75mm thick spruce it could account for a gain in stiffness. Any further thoughts?

Jim

[Simon Fay]

Jim you must also consider that the nomex core effectively ties the top together in various directions. Wood is stronger with the grain than across the grain. Think of what happens when you glue a brace that is perpendicular to the grain direction -- the nomex achieves a similar effect. Bear in mind, wood isn't homogenous material like plastic or steel. This is one reason why the double tops are absolutely stronger than regular tops of the same thickness.

As regards the term "laminate" -- I agree that a nomex core top is a type of "laminate" but it is a very unusual one. When we hear "laminate" wood -- it is usually a reference for a plyboard layup of "sheets" with grain running in opposite directions.

[Jim.S]

Originally Posted by Simon Fay

Jim you must also consider that the nomex core effectively ties the top together in various directions. Wood is stronger with the grain than across the grain. Think of what happens when you glue a brace that is perpendicular to the grain direction -- the nomex achieves a similar effect. Bear in mind, wood isn't homogenous material like plastic or steel. This is one reason why the double tops are absolutely stronger than regular tops of the same thickness.
.

Fair enough Simon. so are you saying that the nomex and epoxy are stiffer in sheer than the spruce they replace? Also strength and stiffness are not the same thing.

If, just looking at long grain, and the nomex laminate is stiffer than the solid spruce then the nomex and epoxy need to be stiffer in the long grain direction than the spruce it replaced, so I have to guess that that is the case. It is just hard to see how that floppy nomex could be stiffer so I then guess that the epoxy is the culprit.

Jim

[PWoolson]

Tim has said pretty much everything that can be said on the matter, I think he's become a bit blue.
My /$.02 that I'll add are more WHY rather than HOW I make them. As Simon said, (no pun intended), wood is not a homogenous material, which can be both good and bad. When nomex is introduced into the equation, the top (two layers of wood and one of nomex) does indeed become more of a homogenous material. Much more uniformly stiff. Cross section us usually much closer, in deflection, to that of long-grain stiffness.
If you have ever gone around the top of a heavily braced guitar and tapped it (about 1" in from the perimeter, all the way around), you hear, "bonk, bonk, bonk, thud...etc." The "thud" being when you hit a brace. It is my opinion that these dead spots translate to the overall usefulness, sonically, of the top. Because a double top doesn't need bracing for structural integrity, the very fine braces, that I use, are solely used for tuning the top. When this same tap test is done on a double top, you won't hear the dead "thud" sound of hitting a tall brace that is often needed to support a single top.
Are they better? Well, define "better". I personally prefer the sound of a double top much more than that of a single top. (everything else being equal of course) and I've done LOTS of A/B testing to prove this to myself. In my early double top career, I built the exact same guitar, with only a flat top, for every double top I made. This was simply for my own testing. I wouldn't go to the trouble of finishing them, and they usually ended up as firewood. But the double tops were, in my opinion, much more successful than their single top equals.
Since my first road-trip guitar back in 2004(?) I've only built one flat top. Everything else has been double topped. As Tim stated, it's MUCH more work, but it's a process that I firmly believe in, to the point that I'm willing to stake my whole career on it. Double tops are 100% of my work and I'll keep it that way.
A little more reading if you have the desire:
http://woolsonsoundcraft.com/doubletops/doubletops.html