In order to build a guitar with a slightly arched top, we need to
arch
the braces that will be glued to the underside of the top. This is
easily done with the nifty jig pictured below. The jig clamps a brace
into a desired
arch, but in reverse - it clamps the brace into a concave arch, with
the ends higher than the middle. The concave top surface of the brace
is then planed flat; when the brace is released from the jig, the brace
springs back so the bottom surface becomes flat again, and the top
surface, which was planed flat while the brace was bent, becomes a
smooth convex arch.
The jig consists of a movable jaw that clamps the brace in position by
squeezing its sides - this way, the top surface is open for planing.
The jaw has sandpaper glued to the face to provide extra gripping power.
In the photo below, a spruce brace is shown clamped in the jig. The
brace is pressed down so it contacts the curved bottom surface of the
jig, and then the jaw is tightened to hold it down.
The photo below shows an end view with the brace clamped in position. This shows the concave top surface of the brace.
The top surface of the brace is now flattened. The brace clamped to the
jig is first run through the bandsaw, to saw the top of the brace
flat...
... and the jig is then run through the planer to smooth the top surface of the brace.
The photo below shows the jig after planing with the brace still
clamped in position. Notice that the top surface of the brace is flat.
When the brace is removed from the jig, it springs back, and the top surface becomes the desired convex arch.
