I made a cool little fixture this week that allows me to quickly, easily and accurately contour a variety of curves on the bottom edge of my braces, whether I’m arching my backs and tops to a 25, 30 or 15 foot radius. Because the hollow forms (concave forms) used in arching tops and backs require the braces to have a matching curvature on the bottom, it is critical to really nail the curve on the brace and make sure it is perfectly perpendicular. This fixture is simple to make; mine is birch plywood with two handles. It only took about half an hour to make and saves so much time with perfect results. It also gives me another opportunity to use my new custom made router table!
When placed on a hollow form dish a flat un-braced top will be pushed down into the correct arch by the curved brace bottoms when clamped. When the glue dries and the top is removed from the form, it will be perfectly domed to the desired radius. Arching the brace bottoms by hand is potentially less accurate than with this fixture and more time consuming. I’d rather spend that time carving and voicing the braces. The front and back of the fixture in these photos are 25′ and 30′ arcs respectively. Un-arched brace lumber is double-sided taped to the desired edge just protruding past the front. A bearing guided router bit in the shaping table rides along the fixture edge quickly carving the correct arc in the brace.
It is then placed on the top or back in the hollow form and clamped in this case with go-bars. The braces are fine shaped after the glue is dry, carved, sanded and voiced to lighten the top and maximize the tone of the instrument. This top was light as a feather when I was done and rang like a bell when tapped. You can see the whole process in the photos. In the 2 shots where I’m holding the brace on the form, you can see light under it in the centre before arching. Once arched, it fits the curved form perfectly.