Last summer, on the closing day of the Guild of American Luthiers (GAL) convention, Charles Fox gave a slide show presentation on how he makes composite tops for his Ergo guitars using Nomex honeycomb.
His presentation was so straight-forward that John Fitzgerald, a fellow SIMSCal (Stringed Instrument Makers of Southern California) member who was sitting next to me, and I turned to each other and said, "I can do that." And so we did.
Only different.
In a departure from Charles Fox and others, who make their composite tops from 2 tops, routing an excavation into the bottom layer to accept a Nomex cut-out, we made our composite top from three layers. The reason came from the literature: routing an excavation is risky because, unless it is held down securely, as with a vacuum system, the thin floor of the excavation is likely to be sucked-up into the router and ruined. Thus, we chose to laminate a wood middle layer, cut to an outline similar to Charles Fox's unexcavated area, to the bottom layer, which was the same thickness as the floor of the Fox excavation.
Thus, where Charles started with a back plate .065" thick and routed out .035", we thicknessed the back plate of our composite top, to .030" - .035" using an ordinary Ryobi thickness sander.
On top of this, we laminated a wood-and-Nomex middle layer .080" thick, which is the height of the LMI Nomex, on a flat surface in a vacuum clamping fixture. First, we glued the wooden element to the back layer using LMI instrument makers glue, clamped them on a flat surface in the vacuum fixture, and ran the vacuum for a couple of hours.
Then we expoxied the Nomex into the resulting 'excavation' with System Three T-88 epoxy, also on a flat surface, and let that lamination cure for 8 hours under vacuum.
When cured, we carefully sanded the Nomex side of the laminated plate(s) down to .105" - .110" on the thickness sander, using progressively finer grades of sandpaper.
While the middle layer was curing, we thicknessed the top layer, the show face - with rosette installed, but soundhole intact - to about .035". Fortunately, the rosette was set to just a couple of thousandths less than this, so that it was "supported" (or at least covered) on the bottom by the thinnest layer of spruce. This was just dumb luck.
Finally, we applied epoxy to both the middle layer and the show face, and in another departure from Charles Fox and others, placed the stack face down on a 28' radius form that was cut-down to fit in the vacuum fixture, and let it cure for 8 hours under vacuum. When removed, the top retained this radius. And was very, very stiff down the middle.
Of course, this is the short version of our steel string composite top making. Along the way, there was a bit of empirical testing of whether (and how) to apply the epoxy to the Nomex, as Charles and others do, or to apply it to the adjoining solid surfaces. And, as you would expect, a couple of the steps were done more than once.
According to his presentation, Charles Fox uses a 12' radius for his Ergo top and essentially no bracing, allowing for string tension to contribute to and maintain the radius. He recommended to us that a more conventional radius should probably have some light bracing, which we did.
As with many of Charles Fox's post-presentation "what if…" questions – and there were many – he concluded, "I don't know the answer. I've haven't made that many composite top instruments. Give it a try, and let me know how it turns out. "
We plan to.
Larry
ps - I am prep-ing the guitar for finish right now.
If I knew how to post pictures, I would.
I have more details in a presentation that I am putting together for SIMSCal, our guitarmaking club. I'd post that, but for my dial-up modem.
I'm sure that I'll have it figured out by the time the finish is dry - and the bridge is on.
L
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to your mug. Its a lot of work but well worth it.
