Trapdoor Jig

In making the trapdoor frame and door, Schneider fabricated a fixture that was 4 1/2" deep, a little over 4" tall (without spacers and veneers to be glued), and about 9" to 10" wide. The blocks are each made of six vertical pieces of 3/4" maple, 9" to 10" wide. A seventh piece is larger and made as a pattern to which each of the six pieces can be attached and the curve routed on a router table. I made a pattern for each block with drill bushings to drill locating holes, and then using a flush cut router bit, cut the radius on an edge. The several pieces, with a radius at the top were glued together to form the top and bottom blocks. The pieces are flat on the bottom, which becomes the top and bottom of the blocks. The locating holes then provide a place to tap in a dowel as the pieces are glued, so the built-up radius will be a smooth curve. The blocks get pretty deformed from the clamping pressure.

The stack of veneers is pretty stiff, so you want the guidance from the rods for the entire travel of the top block. The finished door frame is approximately 150MM wide, and as tall as the sides at the centerline of the lower bout, minus the thickness of the top and back. The door is approximately 84MM wide, and as tall as the distance between the inside of the binding/side purfling at top and bottom. In other words, the trapdoor replaces what would otherwise be the sides.

The upward face of the lower block has exactly the same radius as the inside radius of the sides at about 4" on each side of the centerline of the butt of the guitar, because the trapdoor frame is glued to the inside of the side, and the trapdoor has an overlay that is the same thickness as the side. Note that the radius of the top block is tighter than the radius of the bottom block, due the thickness of the stacked veneers between the blocks, which will become the trapdoor and trapdoor frame.

Two 1/4" drill bushing are installed on the upper edge of the upper block, for drilling two guide holes through the top block and into the trapdoor frame or door. The holes are in the center of the trapdoor and door frame, about two inches apart, for mounting the door and door frame to a base to be used in a drill press or as Schneider did, in a pin router after laminating to shape the inside edge.

The bottom of the upper block is curved to match the radius of what would be the outside of the bottom block (facing up) when they are separated by roughly 32 veneers (0.0295" thick) that make up the door frame. With each additional veneer, the radius gets tighter. In order to get a good fit, veneers can be added to the upper and lower blocks to alter the radius. However, one should try the veneers dry, before committing to gluing them together.

The outer shape of the door itself, must match the radius of the trapdoor, and it will also have a veneer the thickness of the sides bent and glued to it. Again, the radius of the doorframe and door comes from the lower block. In making the door itself, comprised of roughly 22 pieces of veneer, it requires a spacer temporarily attached to the top block approximately ten veneers thick, to maintain the same outside radius as the trapdoor frame. Veneers can be added to the top block to get a good fit between the outside of the top block and the inside of the door.

Once everything fits well in the dry fit, put epoxy on each veneer. What Schneider did, which has worked well for me, is to use a credit card to apply the epoxy, stack the veneers, wrap in wax paper, and insert in the fixture, then with bar clamps, clamp it tightly, paying attention to have enough pressure to eliminate gaps between the veneers. Before removing, the part (trapdoor frame or trapdoor) drill the holes for later mounting of the part on a fixture for shaping.

After the process described above, both the door frame and door are now in the shape of a flattened “C.” The two holes in the middle of the “C” are used to screw each of them to a base, such that the screws are well below the surface. Again, the inside of the “C”, which faces the inside of the guitar, needs to be flat. Wood can be first wasted on a band saw. I have an X-Y table on my drill press, and mount the base on it. Using a large straight router bit, I take off just enough wood so that the inner surfaces are flat from edge to edge. At the centerline, the trapdoor frame is approximately 17MM thick. It gets significantly thinner toward the bass and treble ends. The trapdoor is approximately 11MM thick including the veneer glued to the outside of the trapdoor, to either match or contrast with the sides.

The space for the trapdoor is routed into the frame. Part of the space is cut completely though the trapdoor frame, and a partial cut to create a ledge. The ledge onto which the trapdoor sits, is approximately 6MM wide. I cut the space for the door while the frame is still mounted to the base of scrap wood. First, I cut the ledge, and then move in 6MM and cut all the way through. The door is bolted on with 6-32 screws and tee-nuts in each corner of the door frame. I made a plastic pattern for drilling the holes in the proper place. Then, flat side of the door is sanded so the outside of door and a thin cork gasket is the same height as the door frame.