I considered this approach a few years ago but decided that not having the back glued on would make a significant strength difference even with the body in a mold. The only way to get close to the rigidity of a closed body would be to clamp the sides to the form all around the perimeter with maybe 10 clamps but there would still be a loss in rigidity.

there is a fact the the top will rotate as the string load is applied to the top and bridge structure. It is impossible to know exactly how much as the variables of , Bridge design , Saddle placement in relation to the pins , bridge plate thickness and stiffness.
As a rule of thumb it is often in the 2 degree area for the most part.

I was just going to post a question on another thread about this. I read about the two degrees of rotation, but didn’t understand how you could measure it.

I assumed we were talking a top that was still a free plate. I understand that usually Gore is credited with this observation. Was he doing his measurement with the box fully closed and the neck attached? What is the “standard” technique, string up a closed box, bring it to pitch, measure the rotation, take off strings, shave braces through sound hole, rinse and repeat say 3 times.


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Attaching a false back (3mm plywood) with either tape or dabs of hot melt glue might provide a way to get similar stiffness to the closed box, and by extending the false back beyond the guitar body provide a way to attach it to a holding fixture. A "donut hole" could be made in the false back and holding fixture to allow the braces to be adjusted while the guitar top is under load.
If you are going this far, why not attach a false neck to the fixture and "string up" the body and measure the rotation and the amount the top pulls up under string tension as well? Leaving it under tension for a week or more might give a good measure of what it will do in the future.
I understand hand builders desire to build "responsive" instruments and the willingness to test for things beyond what factories are likely to do, but I sometimes wonder what the longevity will be on the guitars built this way.

Gore developed the 2 degree rule-of-thumb by testing mostly old vintage guitars that had a good tone. He did not use this to guide brace shaving. Mostly just to see if his completed guitars hit the target. This is why I think testing an open box is fraught with difficulty.

Bridge rotation in the assembled guitar should be mostly a function of top stiffness. One would think that a deflection test on a 'free' top with bracing could produce a number that could be compared with the bridge rotation on the assembled guitar later, and guidelines developed from there.

I use deflection testing and after collecting a bunch of data it seems to be working out good. I always inch towards the heavier side when carving braces. Once the guitar is done I have no problem reaching in through the sound hole to make adjustments.

It's a clever proposal and the only way to prove it works is to go for it. I'd guess you would have to build at least 5 guitars before getting any real conclusio.

Carving braces until you get 2 degrees rotation will get you a top that is way too loose. It takes several weeks for the full rotation to develop after you string up the guitar.

True, but in my experience that settling deformation is permanent. It will still spring back by the same number of degrees as it did when first strung. And since the rule was developed by measuring spring-back on old guitars, they presumably were also rotated more than 2 degrees from construction time.

The idea is to glue the braced top (unfinished) to the sides, secure in the mold, position vertically and use an appropriate weight on a lever to rotate the glued on bridge (using HHG)
Calcs -Expected Saddle height = 0.5", lever length 12", string tension for lights = 180 lbs, I work out at torque at 90 lbs / 1 ", equivalent to 7.5 lbs at 12" (to be adjusted as required for scale length/string gauge)
I was thinking to secure the lever on top of the bridge using the outer bridge pin holes to bolt a right angled aluminium section, extended from the saddle position out to the 12"
I can then adjust the bracing heights/shape to obtain a desired rotation,

......
Colin,
Why couldn't you build a fixture to test your tops on.
Rather than gluing the top to the sides in the mold, why not make a rim that you can clamp your braced top on. Your "lever" on the bridge can be clamped on the bridge using a caul under the bridge that would just bolt it through the pin holes. You could easily flip the fixture over to shave your braces.
It seems that you wouldn't have to glue anything to your sides.
Or, maybe I am not understanding?
Cheers,
Bob