With some exceptions, steel-string acoustic guitars - often referred to as "flat-top" guitars - actually have arched or domed backs and soundboards, the advantage of which is arguably more structural than tonal. There are those who would argue that where soundboards are concerned there's little benefit in introducing an arch, but I think it's widely accepted that a domed top is better able to flex with variations in humidity thereby reducing the chance of cracks developing over time.
The mating surfaces of the various braces attached to the back and soundboard are curved in such a way that the dome is maintained in the finished instrument. Typically, the top assumes the shape of a section of a sphere having a 25 foot radius, while a 15 foot radius is commonly used for backs.
The following sequence of photos shows the method I use to shape the braces:
After the brace stock has been thicknessed to 7mm, I pencil the radius onto the brace blank using the appropriate acrylic template. In this case, I'm shaping the braces for the guitar back.
A 12-inch disk sander makes short work of the excess wood outside of the pencil line and it's surprising just how accurately the curve can be defined at this stage. A shop vacuum hooked up to the sander takes care of the wood dust pretty effectively.
With very little additional effort, the final arching of the brace can be established. Here, I'm using a hollow form whose radius matches that of the finished brace. Using 180-grit and 240-grit sandpaper in turn, the radius is accurately sanded into the brace. The scrap of wood held in place by my right hand helps prevent the brace from rocking sideways as it's sanded.
The finished back braces, ready for gluing.
Cheers
Pete
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