The Dulcimer's Internal Structure
The internal structure of a well designed mountain dulcimer is a little complex.
First to be installed are the top linings. These are made of spruce top material, about 1/16x3/16, with the bottom inner edge rounded into a "shoe molding" shape. They are first fitted and cut to shape, then glued and clamped into place with spring clothespins.
The brace stock is straight-grained quartersawn walnut scrap, cut to 1"x3/8", with a 5° angle cut into the sides (to help start the parabolic cross-section they will be shaped into). While the top linings' glue is setting the braces are first measured and cut to length, carefully fitted to the angle of the sides. Next the braces' arch is marked out, using a flexible ruler which has been bowed in a vise, as in the illustration. This yields a parabola. The parabola is an interesting engineering structure, as its mass diminishes outward from the center in exact proportion to the amount of a uniform load bearing at any distance from the center. Acoustically, this means no extra wood to slow down the energy, and maximum strength from the wood that is there. Next, the braces are cut, with the saw table tilted 5° (to match the angle used to cut the brace stock sides).
Next, each brace (there are four in the hourglass, three in the teardrop model) is carved, using a miniature draw knife so the cross section is approximately a parabola too.
Now the braces given a slight rounding on the bottom surface--1/32" sagitta is plenty--and tested together to have the same bottom profile. Then they are glued into the bottom of the body. These joint are critical. Of all musical instrument repairs, loose braces are the most preventable.
Last, the center linings go in. These are re-inforcements for the bookmatch joint. The linings are made of rejected redwood tops, milled to about .050" thick and sliced at 45°, in a bias cut. This makes them equally strong down the length and across their width. Their top edges are champfered to avoid any sharp edges in the interior, and then are tightly fitted between the braces, tailblock and peghead. They are glued in and clamped using risers, so the go-bar deck's bars are of the right height.