How much does bracing affect geometry?

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sammyjit
Posts: 61
Joined: Fri May 22, 2015 7:43 pm

Re: How much does bracing affect geometry?

Post by sammyjit »

I haven't done anything to the guitar yet. The humidity is higher today. It has been raining like crazy. I checked the arch and found that it gained back about 3/4 of its original arch. I would assume that it would continue to swell back more. I would hate to dismantle the guitar since the top and the back are already glued to the sides. I'll just have to watch the arch while I set the neck/saddle.
Dan Bombliss
Posts: 219
Joined: Wed Jan 30, 2008 11:36 pm

Re: How much does bracing affect geometry?

Post by Dan Bombliss »

This an old post, but after reading I figured I'd chime in.

When you sand your braces on the radius dish, make sure you aren't pushing down and forcing the braces to form to a 28' radius. If you were to pencil mark the bottom of your brace and sand it in the dish, you can easily push the brace hard enough to sand off all of the pencil marks and get a false read. The result would be a flatter top. If you are concerned about hitting the radius exact, always be sure to let the brace sit on the dish with now force, and see how the contour matches up. If this is what you had done "wrong", I would not be concerned. The string tension is going to inherently pull a radius in your top, and these are technically "flat" top guitars and flat doesn't necessarily hurt them, it's just a little different.

However... if your guitar top is flat (or even concave) as a result to humidity, start over. Humidity doesn't necessarily hurt an instrument. The variance in humidity does. If you build your guitar with pieces of wood with varying moisture contents, you just built yourself a guitar that will never stop moving.

Technically, building a guitar in a climate that is too humid, or too dry, isn't the problem. If you built an instrument in 80% RH, and could constantly maintain that RH year round, you'll probably be fine. If you build a guitar in 25% RH and maintained 25% RH year round, you'd probably be fine. The problems happen when you build an instrument in an unrealistic annual climate, so every season it literally tears it's self apart.

When your part is "too" humid, your part is too small (it will shrink down after you cut it, meaning you have less material than you need). When your part is "too" dry, your part is too big (it will expand and become bigger than you need, meaning you have too much material). With this logic, if you glue a top onto the sides that is too humid, it's going to shrink down to the appropriate size, pulling itself apart from the ribs. When the boundary that is glued in place doesn't give, your top will crack. Because curves and arches are technically longer than straight lines, when you're "arched top" shrinks (shortens), your top will flatten/concave (Implode). On the flip side, if you glue a top on that is too dry, when it hits the appropriate humidity, it will expand and buckle/crack/distort, trying to get to the size it's suppose to be, but the edges are glued all around the body, so it can not.

This is probably more information than you need, but absolutely a helpful thing to understand so that you can make your own educated assessments based on your specific situation. I saw John post that you may need to start over, and I would agree, but only when it's RH related. If you "accidentally" made it flat, roll with it. It will still be a stable, good playing, instrument. No harm no foul. If RH is to blame, you just build a volatile instrument that is on the road for future issues.

-Dan
tippie53
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Location: Hegins, Pa
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Re: How much does bracing affect geometry?

Post by tippie53 »

good points Dan . I have seem so many things that can go wrong in 20 years , and I agree flat is fine but when it starts to go potato chip you should take off the redo the braces.
Before I had good RH control I had to do this once or twice. Control the variables for a better process.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
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