Shaping Bridge Soles
By
Bob Gleason
Many builders today intentionally put a dome or arch shape into the top of their instruments. This improves the strength of the top, but makes bridge soles more difficult to shape prior to gluing. For a proper fit, the bridge should rest very closely on the top even without clamps. The most common error in fitting the bridge is having a shape that requires the bridge wings to be forced down with clamps. If anything, the wings should rest on the top with a slight space under the bridge center. Then, when a clamp is applied to the center, the wings will be pressed down tight automatically.
A common technique for bridge fitting is to put sandpaper on the instrument top and slide the bridge back and forth to achieve a good fit. I find this awkward and usually the bridge wings still lift slightly off the top. I use this jig that was shown to me by Spanish guitarmaker Jose Romanillos.
My jig as made from a section of 4 x 10 construction lumber. It could just as easily be glued up out of smaller pieces. The important thing is that it be slightly wider than the widest bridge you will want to sand. I made mine just wide enough to accommodate the width of an uncut sheet of sandpaper. The paper is held onto the jig by wedges that fit into slots on the side. Normally I use 120 grit paper with a stiff backing, such as the gold automotive papers.
I have more than one of these jigs because I use different radii on my instruments. The top of the jig can be quickly shaped with scrapers, sandpaper, arched sanding forms, or files to match the radius you want.
I do not make a dome shape. I simply make the arch from side to side. For ukes I use a 30’ radius. Because the jig is solid, I find that I can control the shaping process by the amount of, and the way that I apply, pressure to the bridge as I push it back and forth on the sandpaper. It is a very fast way to shape bridge soles.
The bottom of the jig can be shaped in any way that will conveniently fit your shop vise.
