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Hi,

Does anyone know how thick builders typically make the two top plates (the "bread" in the sandwich)? Specifically when using something like Nomex, I wonder how thin a plate can be before

1. the top becomes too fragile for practical use (i.e., the player could accidentally break through the top plate with rough handling), or
2. the inner material telegraphs through the top.

Of course, I know there's no "typically" with this type of construction, but I'm hoping someone here has experimented with these.

Thanks,
JohnJohn B38832.5674189815

As of last summer C Fox was using for his ERGO model a .090" top "face" with .060" routed out to accept the nomex core. He would then "skin" the back with another .030" layer. I'm not sure what others are using though...

1/32 or .7mm are measurements I have seen used.

My skins are also in .03 range, pretty much the same as Charles is doing, which I did not know. I think some others may go as low as .024 on the classical side of the house. My skin dimension is for a steel string.

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Jim Watts
http://jameswattsguitars.com

I used .5mm (.020") spruce on my first one, and that was too thin, as the Nomex telegraphed through. When I replace it I'll go with redwood, probably about .7mm (.028" or so).

I have a curiosity question about the nomex sandwich type of top. How does one apply the glue to that thin webbing of the nomex without filling up all the honeycombs with glue? Seems like you'd have to be pretty careful or you'd wind up with a top that was full of glue.

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Dave Rector
Rector Guitars

welcome aboard Adam!

Serge

Dave,
I picked up a tip from Randy Reynolds on this. I apply a even film of adhesive to a board place the core in that let it set for some time, I use 10 min. with T-88 epoxy, pull up the core and the adhesive transfers to the cell edges.
Run many pratice pieces to get a feel for the appropriate amount of adhesive. Try peeling the core off of your practice piece. You should get core failure or wood failure put it should not come off clean!
You want a nice fillet of adhesive forming on each cell wall.

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Jim Watts
http://jameswattsguitars.com

Thanks Jim! Man, I was scratching my head trying to figure out a way to do that. Maybe that's where all my hair went.....

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Dave Rector
Rector Guitars

[QUOTE=Dave Rector] I have a curiosity question about the nomex sandwich type of top. How does one apply the glue to that thin webbing of the nomex without filling up all the honeycombs with glue? Seems like you'd have to be pretty careful or you'd wind up with a top that was full of glue.[/QUOTE]

You might try a 'brayer' which is a rubber roller used by printers and artists to ink plates for printing. Found in art supply stores. Technique similar to inking a woodblock, or linoleum block might work with some practice. Get a piece of plate glass to 'roll out' the adhesive, and load the roller, use a medium hard roller, and a light touch. Tack down the edges of the nomex honeycomb.

CrowDuck

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Chris Nielsen
Soquel, CA.

Chis has good sugestion about the brayer. I use one to roll out my adhesive to an even film.

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Jim Watts
http://jameswattsguitars.com

I remember them talking about using a brayer for glue at Healdsburg too..

I still have the paperwork from the double top seminar around here somewhere. Charles said he was using .03 for the spruce layers and .06 core..

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CVT Inc.
A tour of our shop (Somewhat outdated)
My Personal eBay profile. BCWoodBug

Sounds like it would be fun to honeycomb the front and back plates for an archtop. CNC of course.
Nelson

I used a hard breyer on mine. I found it helpful to put a little color into the glue so I could see it, using a dye. Also, go over the whole thing several times using a light load on the breyer each time, and be systematic; think of the way you'd thickness a top with a hand plane. That way you will get an even glue load. I used about 3gm per side of the Nomex for a top. Use the slowest setting epoxy you can get.

This is the real 'trick' I think. It's easy to get a good bond if you put on lots of glue, but that negates the weight savings of the Nomex core if you aren't careful. On the other hand, you _need_ a good bond.

I made a pair of small test plates using Nomex and sliced mahogany veneer to check out the issue of poor glue lines. On one of the plates I marked out circles of various sizes on the side that I was going to glue, and transferred the marking to the outside so that I'd know where they were. Then I waxed the ones on the gluing face with parrafin, well rubbed in. The idea was to make it impossible for the epoxy to stick properly in known areas.

When I first glued the plates together I could not tell where the bad joints were other than by looking at the marks. The stiffness of the two plate, as found by vibrating them, was the same. I then clamped one end of the 'bad' plate in a parallel clamp, which was clamped to the bench, glued a strip of wood along the other edge, and hooked it up to a 'stinger' driver which I could drive with my signal generator at about 30w of input power.   

After about a week of pushing it very hard at several different frequencies I removed the added wood and tested it again. There was very little, if any, change in the mode frequencies, indicating that the stiffness was still high. However, I noted in flexing the plate in my hands that when I pushed on a spot that was over a 'bad spot' I got a little 'crunch' sound, as if there was motion between the skin and the core. Further driving caused the crunchy spots to expand. Without tearing the plates apart to be sure, it seems as though the core and skin are peeling apart starting at the bad spots.

It may well be that the plate would continue to show little, if any, change in the stiffness until it failed catastrophically when the loose area reached the edge. The 'crunch' might get to be a problem long before that, though, and I've heard at least one report of this sort of failure in a sandwich top guitar.

I will note that this sort of thing is easy to fix if you can find it, so long as the problem is not due to big gaps or something like wax on the surface. All you have to do is find the area, drill a small hole through the inside skin, and inject a little thin CA. The trick is finding the problem area.

film glueWould something like this work for glueing the sandwich?
Nelson

Thought I'd chime back in and say thanks for all of the info and discussion - exactly what I was wondering.

Now carry on, if there's more to discuss

npalen:
I've heard of people using something of that sort. I seem to remember it weighing about 15gm: much more than rolled on epoxy.

I've often thought, on the other hand, that one could roll on HHG, allow it to gel, and then warm up the sandwich after it's clamped to get the glue to flow. You could even use paper honeycomb, which would stick great. Whether it would work better than Nomex/epoxy I can't say.

Nelson, Alan and all who are interested.
Most honeycomb panels are constructed with a film adhesive as as Nelson suggest. Cell edge adhesives like we're doing don't really exist for industrial applications, although the industry would love them, too risky for critical applications such as aircraft.
Most major matrerial suppliers supply film adhesives and have typical weights in the 150-300 grams/sq meter range. They make great bonds and all that I'm aware of require an elevated cure. I think there may some that will cure below the 200F point however.
I think it would be very interestinting to make a hide glue film. I bet it would stick great to the nomex, but don't really know.
maybe one of us should try it if we ever have time!Jim_W38835.0399537037

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Jim Watts
http://jameswattsguitars.com

[QUOTE=Alan Carruth] npalen:
I've heard of people using something of that sort. I seem to remember it weighing about 15gm: much more than rolled on epoxy.

I've often thought, on the other hand, that one could roll on HHG, allow it to gel, and then warm up the sandwich after it's clamped to get the glue to flow. You could even use paper honeycomb, which would stick great. Whether it would work better than Nomex/epoxy I can't say. [/QUOTE]

To use HHG, again I'd use a 'printing technique'. Years ago I used to do 'fine art' printing.

I would do a HHG 'transfer' onto the honeycomb material by using a large 'hotplate', one large enough for a top/back. While maintaining a hotplate temp that will keep the HHG from gelling, with a brayer, roll an even film/layer of HHG over the entire hotplate surface. Then lay one side of the honeycomb material down across the wet HHG, and using another clean roller/brayer, gently roll/press the honeycomb material into the HHG. That will 'transfer' HHG to one side of the honycomb material, hopefull in an even consistant layer that doesn't flood the cells of the honeycomb. That's sort of how fine art printmakers would do a 'transfer print'.

That would be 'Step 1' in the process. You'd still have to 'transfer glue' the other side of the honeycomb, as well as, sandwich the woods, under heat and some pressure, all without flooding the cells. An interesting idea...........at least to me. I might play around with it if I can find some of that Nomex material.

CrowDuck

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Chris Nielsen
Soquel, CA.

It is interesting to me that this nomex sandwich is such a hit. Laminated top guitars (with wood cores) have been such anathma to the serious builders for so long that they have become the symbol of a second rate instrument. But nomex cores change all that. I understand the weight vs strength issue, (I brace my tops with that in mind) and how nomex increases that value, but nomex in itself is not a very well acoustically endowed material, so I wonder if its inclusion in the top inproves speed of sound transmission, internal damping, and tonal balance.

I am just being a contrarion, finally giving vent to some questions I have had regarding this innovation. I am certainly no expert on the topic, but the few double topped guitars I have played were not impressive instruments.

Chris,
I could probably share some small cut offs and such if you'd share your results.
Jim

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Jim Watts
http://jameswattsguitars.com

[QUOTE=Jim_W] Chris,
I could probably share some small cut offs and such if you'd share your results.
Jim[/QUOTE]

Jim,

If you can also supply some cut off 'skins'. I don't know how to make anything that thin yet. I've got the equipment, and will gladly share the results.

CrowDuck

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Chris Nielsen
Soquel, CA.