How much does bracing affect geometry?

[sammyjit]

I have a 000OM 24.9 scale top from Martin. This is my second build. I used 28ft radius for the top because that is what i have for a dish. I glued the braces and it makntained that 28 foot contour at the lower bout. I glued the top to the sides and have lost almost all the radius where the bridge would sit. Is this normal? I did sand the sides to a 28 foot radius and i sanded the 1.5 degree neck angle. So why did the radius change? The only thing i did different from my previous build was use the Martin 000OM brace patterns. I used OM brace patterns on my previous build. I know martin's flat top 000OM guitars use a 52 foot radius, and the Xbrace is at a slightly different angle than the OM guitars. Did i do something incorrect, or is this what happens when gluing tops? Since my arch has such a low radius now, almost flat, will i have a compromised structure?

Thanks,
Happy Thanksgiving!

[tippie53]

you should be fine. I don't use 28 for the entire top , as i only use that to set that 1 1/2 degree angle and some dome behind the bridge. So yes the bracing will effect geometry.

[MaineGeezer]

I assume you sanded the 28' radius into the braces. You may just be seeing the effect of a change in humidity, as we get into winter and things are drying out.

Changes in humidity can do amazing things. I had a back go from 15' radius convex to essentially flat and possibly a bit concave, because I put the braces on in a high-humidity environment and then the humidity dropped. The back shrank cross-grain far more than the braces shrank along the grain, so the back pulled the radius right out of the braces.

[tippie53]

Main is 100% right
how the braces work with the top and what the RH is when you glue up the braces also come into play.

[ruby@magpage.com]

If I recall, Martin uses something like a 65 foot radius on the lower bout - essentially flat

Ed

[sammyjit]

Wow. Yeah, i didnt think about the humidity. I did sand the radius into the braces. It is very dry right now. Hasn't rained in weeks. Forest fires are everywhwere. When I glued the braces it was summertime and very humid.

[tippie53]

Martin does use a slight radius about 52 foot and the upper part of the the top is set to a slight angle.
originally they were flat not sure the date when they started the new geometry but it was about the same time they started to cut the dovetail in the necl block after the body was closed.

[MaineGeezer]

I bet if you sealed the top in a plastic garbage bag with a wet sponge for a day or two the top would curve to the radius again, or possibly more.

[sammyjit]

Ill try that and see what happens.

Thanks everyone

[tippie53]

if that doesn't work you may need to pull the braces and start over. Sometime that is the best thing you can do. see if you get the dome back and that it stays.
best of luck