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Hi!
I'm on my first acoustic guitar build, and am fast approaching bracing the top and back.Humidity in my shop has reached 43%, and has stayed there for an appreciable time.
I read on many guitar forums, and some manufaturer's sites, that such and such guitar is braced for medium string gauge, or light gauge strings, etc.
I am building an OM-ish from Grellier plans. Body is 19. 24" long, width of top at lower bout is 15". Thickness at the sound hole is around .120". The soundhole is not cut out yet, so measuring through the center hole used to lay out the rosette channel. The top graduates to between .098" and .100" around the perimeter. It is a Sitka A top from LMII. Sides are .070" Sapele. Body thickness at tail block will be around 4.25"
I want to use Bluegrass gauge (D'Addario EJ 19). Medium on the bottom, Lights on the top. These are my preferred strings.
Would appreciate some guidance RE bracing for this instrument. X braces will most likely be 5/16 wide.
Radius for the top and back will follow Cumpiano.
Thank you for any help.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

There are some who are a LOT more experienced than I am, but I can start the conversation:)
Top stiffness is really what you’re talking about here. The bracing and the actual soundboard all play into this. It’s one of those things you kinda just have to learn a feel for. A lot of people use deflection testing to figure out how stiff the bracing needs to be too.
You mentioned the width of the x brace but keep in mind that the height of the brace is what adds / removes stiffness.
The way I do it is glue the xbraces in , along with most others, with the braces being only rough shaped if not UNshaped. After glue up I’ll begin flexing / tapping the top and removing material in the appropriate places until I get the ring I’m looking for (still very much learning). How much flex? It’s either a deflection or a feel thing.

Also, unless the top is particularly floppy, .120 is pretty thick by anything I’ve dealt with (only 10 guitars in myself).

I hope I haven’t only confused you in some way.
I’m certain others will have a shot too.
B


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For medium bottom /light top EJ19 strings that doesn't sound too bad for a top thickness - just under an eighth inch around the sound hole and a tenth of an inch around the perimeter. Some people might suggest using 1/4 inch thick braces X 5 /8ths tall but others recommend 5/16ths thick, which I often use on bigger guitars. "A grade" Sitka can be as stiff or stiffer than "Master grade" Sitka, as soundboards are primarily graded on cosmetics. For a first guitar I would do what the factories do - stick to the middle ground on dimensions - and it will probably turn out O.K.

Thank you, Gentlemen. Much appreciated.

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.