back not holding curve

[Mark Scrivener]

After gluing my back braces (using a radiused dish, go-bar deck, etc), I've noticed the back springs to nearly flat when the go bars are removed. Shop humidity has been fairly steady around 45% for the entire process, and I even let the back sit clamped up in the dish but not glued for weeks before finally getting around to gluing it. Is this normal? Should I wet the back wood and leaved it clamped in the dish to dry so it holds the curve better? What did I do wrong?

Thanks,
Mark

[AluminumTop]

Just guessing, were the back cross braces contoured? Does the back pickup the contour if placed into the dish? how much pressure to refit into the dish.?

[Mark Scrivener]

Yes, the back braces are curved, same as the dish. Upon closer inspection it appears to be holding the curve across the back, but not lengthwise, which makes sense as there are no curved lengthwise braces (only the flat center strip). It does not require a lot of pressure to make it sit flush in the dish. Should I be concerned about the contour lengthwise?

[phavriluk]

The only guitars I know of that have a domed, not cylindrical back are those whose backs were made as a domed part, like Taylor GS Minis and (I hear) some Guilds. The rest of us, I think, take that cylindrical back and attach it to the domed sides. There's some longitudinal arch but I think it's not as pronounced as the lateral arch the braces hold.

[Mark Scrivener]

Thanks!

[tippie53]

Martins are domed

[Diane Kauffmds]

I had the problem when I made the back braces too low, too far from the edge, if that makes any sense. We thin the braces as they approach the kerfing, but one time I started too far away from the edge. I made parabolic braces and took too much off. The tension caused by back plate pulling on the braces helped flatten the back somewhat. The other cause is humidity and wood shrinkage.

If it's flattened too much your best bet is to unglue the back braces and reglue. Use a flexible metal spatula with heat to get up under the braces to melt the glue. They'll come off.

[tippie53]

also remember that humidity will play a part. Seldom will the back remain in a perfect dome as compared to the braces. Humidity will cause the arch to change .
When you first glue the braces they will not dome until they are glued to the back and you create that geometry.