Red cedar student boo-boo - too thin?

[Dave Bagwill]

Tim - yep, them's the measurements.

[Hans Mattes]

About non-traditional bracing patterns: Can't get much more divergent from the Martin/Gibson clones than Edwinson Guitars. The uniqueness of their design always teased me.

I've built a couple of 000/OM guitars using Edwinson bracing (http://edwinsonguitar.com/Design_Philos ... cture.html).
They don't sound a lot like Martins or Gibsons (or my Larrivee), but they do sound very nice -- resonant and musical, with great sustain. I think that comes from the cross dipole as the bridge and bridge plate are the only transverse elements below the soundhole. The X-bracing can be made as strong as desired, at least from the upper bout to the bridge plate (I run it under the lower corners of the bridge itself), so the "thin" cedar top needn't be a structural problem. This could make a very nice instrument.

[Dave Bagwill]

Thanks Hans. I did read your web page on bracing philosophy, and the pictures were very helpful.
Why do you use epoxy for the fan-bridgeplate joint?
Do you really feel that the notched-in braces are any more effective than good butt-joints?
Do you have a method (that can perhaps be shared?) for testing the top post-bracing to determine if thinning, shaving etc. is necessary?

Thanks again
Dave

[Hans Mattes]

I'll try to respond to your questions, but have to preface any remarks by saying that that's not my webpage. It's the webpage of Stephen Sheriff, who, I believe is Edwin Sheriff's son. Ergo, Edwinson.

I don't use epoxy for the fan-bridgeplate joint, though Sheriff recommends it. Certainly epoxy has a couple of desirable properties: gap filling and good adhesion to most materials, but I don't see the need for it at that joint.

Regarding notched-in braces, Sheriff seem motivated by an admiration for the long-lived carpentry of old Japanese temples. I'm more motivated by a concern for glue being sucked out of joints by open end-grain, so I use notched-in joints when practical -- especially on the central fan-brace.

Regarding testing the top, post-bracing, to determine thinning, shaving, etc., I have nothing from Sheriff, but I have developed two processes inspired by the Gore/Gilet books.

PROCESS 1 -- Gore/Gilet noted that they found that the bridges on the guitars that sounded best (to their ears) would rotate by around 2 degrees when the guitars were strung up. So I build by gluing the top, with initially carved braces, to the sides, leaving the back open and putting the open-backed box in a mold. Then I clamp a fixture that I've built to the guitar/mold combo. The fixture is comprised of a rectangle of steel tubing that surrounds, without touching, the lower bout of the mold, a wooden "bridge" that is epoxied to the tubing and presses against the top in the expected bridge location, and a wooden caul that is pressed hard against back of the bridge plate via 2 long screws which run through nuts welded to the frame of tubing. The fixture is now clamped to the top of the guitar, touching only at the faux bridge on the front and the caul forced against the bridge plate. The mold is then clamped vertically in a vise, with the upper bout of the box pointed down, and a 24" rod is inserted into a holder in the tubing, centered on the faux bridge and extending directly out from the face of the box. Weights are positioned on the rod to provide 90 inch-pounds of torque to the fixture. (The 90 inch-pounds figure comes from light strings, which generate 180 pounds of tension at pitch, being ½ inch above the sound board.) I then carve the braces (particularly the X-brace from the upper bout to the bridge plate) until the deflection is 2 degrees. That's easy to measure as a deflection 13/16" at the end of the 24" rod means a rotation of 2 degrees. And high precision is really not required here in any case; I figure that +/- 10% is close enough.

PROCESS 2 -- Again following the suggestions of Gore/Gilet, I tap the bridge area of the open-back box and record the results with a microphone connected to a laptop running Audacity. I use the results to look for the frequency of the top resonance and then carve the fan braces and the lower X-braces and thin the top, particularly around the lower bout, to get the top resonance below 190 - 200 hz.

Both processes are iterative and time consuming. I can't carve the braces while the guitar is in the Process 1 fixture, so the box and mold are repeatedly mounted to the fixture, measured, unmounted, carved, remounted, and remeasured. The Process 2 iteration is simpler, but still takes some time. I would note, however, that both of these processes inherently account for the stiffness of the brace wood and the top wood (as well as the effectiveness of the top/side connection), so have a level of refinement that building to fixed dimensions does not provide.

I hope this is useful. I realize that my description of the fixture may not be clear. PM me if you'd like a picture via e-mail.

[Dave Bagwill]

Very useful, thanks Hans. Sorry for the mis-attribution - I was focused on that bracing scheme and did not notice much else.

PM sent.

[John Link]

This is an interesting situation. Happenstance has taken you to a place you would not ordinarily go, and so is something of an opportunity, as well as the obvious risk. I have several cedar tops that I would not be afraid to try 0.10 as their thickness. However, you say this one is "floppy" and mine are exceptionally "stiff" both ways. If the top was not exceptional in the first place, there may be no where to take it. That said, I once made a dread out of cedar that was stiff with the grain but not so stiff across and it turned out loud and mellow. But I thinned it to 0.12, not 0.10, though it was a full 16 inch dread. That top featured 100 grains to the inch (which explained its with the grain stiffness) but was comparatively weaaker across. Trees have the final say, I guess.

It might be worth it to try bracing it and seeing what happens, waiting until then to decide if the top is usable. I would use Guild-style braces (no scallops) thin and tall, so that wood could and would be easily removed, but leaving plenty of room in which to wiggle (and sweat blood). David Russell Young style bracing under the fingerboard in the upper bout would be safer than Martin style - it would help prevent collapse across the sound hole, combined with tall thin braces on each side of the soundhole.

The small body size certainly is on the side of stability and the smaller gits have a very appealing sound. This problematic top could be the start of one of the loudest guitars you ever made, of any size. Seems worth it to try, at least to the point of bracing it. It is a very good looking piece of wood as well.

[Dave Bagwill]

Thanks for that encouragement, John. I am going to brace it up and see what I can do with it; not sure whether to go X or Ladder yet.