The double dado joint connects two dadoes, one dado on the inside face of one board and the other dado—with one tongue shortened—on the end of the mating piece. The joint is stronger than a standard through dado because it provides more gluing surface. It is an ideal choice for joining boards of different thicknesses, such as attaching a drawer front to the sides, and provides good resistance to tension and racking. The setup shown in the steps below and on the following page will join a 3/j-inch-thick drawer front to a 1/2-inch-thick drawer side. The three cuts can all be made with the same bit—a three-wing slotting cutter. In this case, a ‘/4-inch bit is used; the shim attached to the auxiliary fence is also Ч4 inch thick. By varying the sizes of the cutter and shim, you can cut the same joint in boards of different thicknesses.
are using double dadoes to assemble a drawer, cut the dadoes with the shortened tongue on the ends of the drawer front. Start by installing a three – wing slotting cutter in a router and mounting the tool in a table. Cut a notch for the bit through an 8-inch-high auxiliary fence and attach the fence in place; the high fence is essential for feeding stock across the table on end (steps 2 and 3). Position the fence in line with the outer edge of the bit pilot bearing and parallel to the miter slot, then set the cutting height by butting the workpiece against the bit and centering the cutter on the end of the board. Keeping the face of the board flat on the table and the end pressed against the fence, feed it into the cutter using the miter gauge (right).
2 Dadoing the drawer side
Cut a notch in a wood shim for the cutter and screw it to the auxiliary fence. The shim should be as long as the fence and equal in thickness to the difference in thickness between the drawer front and sides. To rout the dado in the drawer side, hold its end flat on the table and its inside face flush against the shim as you feed it across the table (left). Be sure to keep your hands clear of the cutter.
3 Trimming the inside tongues on the drawer front
To complete the joint, you need to shorten the inside tongue of each dado you routed in step 1. Lower the cutting height of the bit so the bottom edge of the cutter is just above the tabletop. Then feed the drawer front across the table as in step 2, holding the inside face against the shim (below).