MIMF

Ryan, what sort of bridge are you using? The reason i ask is related to an experience I had some 30 years ago. I had an old Martin D18 (60s) and a Gibson jumbo from the 50s, both needing work and bridges. They both sounded "great" (big sound, mellow) with the original bridges that had been shaved down. When I strung them up with new bridges that big sound was missing. I was sure they would sound better after all that work but they were rather disappointing, so I got to looking closely at any detectable differences and the only thing I could figure was the wings of the new bridges were thicker than the old ones. So I got out my scraper and started scraping away material from the wings, and the sound began to improve little bit. With this encouragement I continued scraping the wings thinner and the old sound came back. The internal bracing was still factory stock straight bracing with tapered ends.

Now, keep in mind that all things being equal . . . . are never equal. You need some structural integrity in the over all design, so if one area gets weakened (shaved) the stress needs to be taken up by something else. It seems the wings on Martin bridges are a bit thicker over the past 15 years or so than they used to be, but you can bet the factory has taken the interior structure into consideration to balance it all.

Just some stuff to think about, or not.

Ryan, are you testing the completed guitars strung and tuned to pitch? In addition to the sides constraining the top, a completed guitar will have the bridge which is a fairly large brace (though probably effecting stiffness a cross the grain more than along it). If it is strung and tuned, you also have the strings pulling the top up against the weight.

Michael,
this is my basic bridge that I put on all my guitars. I align the bridge pins better now. this was made off a bad original template I made, so ignore that. It's walnut, so it's already lighter than most bridges, 5/16" thick and taper down to about 1/16 at the tips. Bryan- I have not done much in the way of testing. I don't really know how to do most testing. So far I just build them and have liked the way most of them play and sound.
I want to understand more of the sciences behind it. I want to understand Young's Modulus, Chladni patterns, etc... how to measure and then how to use the data it gives me to build a better guitar. but I also don't want to remove my own intuition and end up with a guitar made by science rather than a human. but I wouldn't mind a little more science in my process.

It's not possible to take the 'art' out of this: what you like for sound is, after all, an artistic decision. What the science can do for you is to help keep you out of trouble, and maybe help a bit to move you in the direction you want to go.

Michael Lewis wrote:
"....but you can bet the factory has taken the interior structure into consideration to balance it all."

I bet they didn't. It has been convincingly argued by folks who are 'way more familiar than I am with old Martins that most of the changes they have made since the 30's or so have been done to reduce warranty claims. They went to heavier bracing, for example, when folks started to put heavier strings on to get more sound. Some of the changes may have been due to material availability and/or cost, of course, and some were no doubt made to enhance productivity.

Very thoughtful and informative discussion. I will print this thread out and put it in my Bulging Binder of Luthiery Stuff and save it for the day when I actually start carving braces.

One question -- when removing braces to start over, should one remove the brace down to the back surface of the top, or can one plane braces down to about 1/8", glue on more spruce and start over? Would an additional glue line within a brace make a perceptible difference in the sound of the finished guitar?

Remove the brace entirely...actually pretty easy with a double-bevel chisel. Trying to perfect yet another joint is just more work and another precision clamping task to accomplish...strip the braces and start over.

Because the runout in well chosen brace stock is minimal, the carve-off can go quite fast. Get the brace down to a few thou left and soften up the remainder with hot water and paper towel. If hide glue was used, a little light action with a safer razor blade cleans any residue. If in doubt, take a scrap top and some scrap brace wood and rehearse...my students all end up doing at least a few braces...if only to ensure they understand how quick and easy removal can be when needs be.

"Get the brace down to a few thou left and soften up the remainder with hot water and paper towel."

Braces can also be removed by wetting the glue line with vinegar to soften most organically based glues (hide, titebond, elmers,etc.)

Ditto Clay's comment ...if using vinegar, white leaves less odor behind than and will not stain. De-Glu-Goo, which is just 5% acetic acid in a methyl cellulose base (gel) is easier to use but much more expensive. That said, the stuff stays put and the gel inhibits evaporation, so I use a few bottles a year, mostly for Devil's Glue (aka, Titebond) removal.

I've found deflection testing to be very worthwhile. It helps quantify a whole world of specific understanding about the top that here-to-fore was only possible by many years of experience, trial and error and a fair amount of hokus pokus. Keep in mind the difference between a good guitar and a great one can be just a thousandth of an inch in deflection, so deflection tests have to be standardized, highly accurate and consistent. I've been deflection testing good examples and my own guitars for about four years now and here are few things I've learned along the way.

Make a good deflection measuring tool. Here is a pic my Mark III version. Yep, it took that many tries to get it right, but this jig is highly useable and accurate.
The weight pushes down on the platform spindle which pushes directly on the top portion of the dial indicator spindle, which in turn pushes on the top. The dial indicator measures the displacement. Note the feet sit on the top, this takes any flex in the sides or back out of the measurement, plus makes it easy to set up (10 seconds). The feet have a cork pad at either end and pivot on a single screw so they can be quickly set to a stable sitting (important). The dial indicator is metric and reads to .01mm (.0005"). The weight (load) is 1Kg (lead fishing weights in a cup).

By going metric, you can plug your findings into formulas outlined in the Gore/Gillet book. Along with peak monopole resonance of the top, you can then calculate the specific (monopole) mobility of the top which is very helpful in maximizing volume and a certain voice you might be shooting for. The deflection alone is a very significant and useful measurement though, even without all the math.

Very important to standardize your measurement and to be sure of what you are measuring. Flex in other parts of the guitar and jig can easily give you errant readings. (This is why I'm on Mark III Readings should be repeatable AND consistent with common sense. If you find a guitar that reads way low or high and the construction isn't consistent with the readings, ya gotta ask why.

With measurements this fine, you will likely find friction in the system (hysteresis) to be a factor, so it is important to repeat the measurements a number of times, resetting the dial until you get a consistent measurement. This is one area I'd like to improve on mine, I think most of the hysteresis is in the dial. I have a ~$60 Mitutoyo dial indicator, not bad and I'd probably have to spend a lot more to get something much better.

To compare deflection measurements of good examples with your own builds, you have to measure them the same way, in particular with the top attached to the sides. Al's suggestion of double sided tape might well work, but I just changed my assembly process. I glue the top to the sides first, before the back, so I can adjust braces and top thickness until I get the deflection (and other things) where I want, then glue the back. You might wonder if gluing the back then changes the deflection, but in my experience, it does not, at least measurably. (Deflection at least. Gluing the back certainly does change elements of the voice, but has no appreciable effect on top deflection)

If you are dealing with an acoustic guitar that has a large enough sound hole to get your hand in, you can shave braces on the fully assembled box, at least to some extent, but if not, the glue the top first is the way to go. (I build Selmer style and the hole is way too small). Even if the sound hole is big enough, I'd argue gluing the top first give better access to all the braces for shaving or adding back in if you go too far.

Deflection alone is very useful. If you are interested in more about what you can do with deflection than just that alone, the Gore/Gillet books (2) are outstanding.