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I've been adjusting bracing height after fully finishing my guitars through the soundhole (checking bridge rotation as I go) to obtain what sounds to me like a good balance between bass and trebles response, but it's A) really hard on the wrists and B) leaves bracing not as well shaped as I would like it to be (parabolic).
So. I've been kicking about an idea for a project for several days.
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, before removing the bridge and continue to finish construction/finishing.
(A variation on this could be using the Somogyi idea of trying different bracing arrangements using masking tape CA'd back to back)

Would anyone like to offer up any holes or considerations they can see in this?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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.

I might argue that any difference in strength would only make a difference in the rise of the top with string tension, not the bridge rotation.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

My thought is that this trial "method" may eliminate most of those variables you have mentioned by direct measurement
.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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.

Totally understand the motivation.

I am struggling to see that locking the backless box into the mould would closely simulate the structure of the closed box though I can't quite put my finger on why.

I think Clay's idea sounds good - not tape but a temporary glue job.

Clay the point about the 2 degree test is to establish that the top is sufficiently robust to withstand string tension, but not too stiff, so should answer your concern.

Cheers Dave

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, repeat?


Sent from my iPhone using Tapatalk Pro

Guitar completed.
Strings to tension , body held in a holder.
Mount a digital inclinometer on the bridge, zero, dump the string tension, note inclination.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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.

Why go to all that palaver?
After gluing the top on, it's in the mold anyway.
2 screws though the mold into the neck and tail blocks (holes hidden later by neck mortice and tail strip) and a few clamps holding the sides to the molds (I use laminated sides so they are very stiff)
I'm not interested in testing the body lift, I've got a handle on that already.
And I'm not necessarily going for the full 2 degrees of rotation, so far I'm finding that 1.4 - 1.6 has produced roughly that balance I'm looking for.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Not just old ones - he visited shops and got permission to test guitars in store I believe.-?

Well, building guitars is "fraught with difficulty" anyway, it's not as if this would take much time after jig construction, and I will of course be checking the correlation of the results after finishing the builds..

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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 can see that could be a way to go, one would have to devise/find an method of deflection testing a braced top, but it could of course depend on the size and shape of the top and would require building up data over a number of guitars of all models being built.
I'm thinking what I'm suggesting would be less time consuming.
I'm all for instant gratification....

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

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.

I've been doing the same, deflection testing, bracing on the heavy side, but I have large hands and small soundholes (to compensate for the soundports).
I think 5 guitars should verify the method, I have 2 planned at the moment, different models, one my largest, the other my smallest.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

That sounds like a lot of work gluing and ungluing the top from the rim for every test. Just spool clamp it on without the mold and string it up. You can of course pluck strings and see how it sounds too, but it doesn't do much good since the mass of the clamps changes the tone, and it takes time to settle into a meaningful representation of the long-term tone anyway.

Colin wrote:
"Why go to all that palaver"

Indeed! I still rely on the tips of my fingers, as did Tony Torres.

We all take things as far as necessary for our imagination.

I'm another tips of the fingers guy but I say give a go if you like, you may develop a technique that we all want to start copying next year

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Steve Smith
"Music is what feelings sound like"

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. Knowing where to stop is a skill you develop over several guitars as you see where they end up after what you did. When it ends up too rotated, you know not to make it that loose the next time.

I have had very good luck with my free plate tuning providing the appropriate rotation over time. It’s almost magic: when you get it right, it’s all right. I played with Alan’s Chladni pattern tuning of the tops for a while. I found that I landed in the same place with my tapping.

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.

DennisK - I believe the term is plastic deformation.
bobgramann - agreed, I had noticed this and if you read what I've written, I said "And I'm not necessarily going for the full 2 degrees of rotation, so far I'm finding that 1.4 - 1.6 has produced roughly that balance I'm looking for."
My last 2 were large lower bout guitars (16").but an earlier L-0 based guitar (14") was 2 degrees, so it may be the degree of rotation for "my" balance may differ - a sample of 3 guitars is not definitive.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Bob Gramman wrote:
"I have had very good luck with my free plate tuning providing the appropriate rotation over time. It’s almost magic: when you get it right, it’s all right. I played with Alan’s Chladni pattern tuning of the tops for a while. I found that I landed in the same place with my tapping."

My understanding is that Gore's 2 degree measure is the short term deflection. If I read it right, the FAA guidelines on wood aircraft structures imply that the long-term deflection under max design loads will be about three times the initial movement, due to the cold creep. At six degrees of rotation the peeling stress along the back of the bridge is probably getting critical, so you want to avoid that; hence 2 degrees of short term rotation.

Chladni patterns are just the 'tech' version of tap tones. The patterns take more time, but give you more information from the shapes of the modes.

I have not worried much about deflection. I measured a few when Gore's books first came out, but since they were right in the ball park I stopped bothering. The mode pitches along with the overall mass of the top give you a lot of indirect information about the stiffness, and, of course, I still flex with my hands. IMO, more information is always better.

BTW, Dave Hurd gave a lot of information about using static deflection measurements in his 'Left Brain Lutherie'. He tested the top on my 'corker' guitar, and found it was exactly what he was going for with his system. We're all going for the same things, and any system that works is probably getting you at least part way there.

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

I did consider this, other people already do similar for tuning their tops, tapping and shaving/sanding - e.g. Somogyi, I remember reading? something.

So, perhaps I chose this to try this because I take long enough to build a guitar without adding another step and another making more fixtures, one for each type of guitar I build and which I would have to find space for in my 3 already very crowded small work spaces, and then add the testing/adjusting.

My suggested way would only require gluing on and removing the bridge (mainly waiting time when I can be doing something else) plus the testing/adjusting.

So, 2 reasons - because it suits my circumstances and it's relatively easy to try for "proof of concept."

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.