Jig for Routing Dadoes
When building a cabinet, dadoes are often used to ensure that shelves and partitions are spaced correctly. To make a strong joint, the width of the dadoes must exactly match the thickness of the plywood. That can be a challenge because few pieces of plywood are precisely 3/4 in. thick. They're usually 1/32 in. or so less than the “nominal” thickness.
This jig will rout perfect-fitting dadoes for any piece of 3/4-in. plywood regardless of its actual thickness. Use a special top-bearing trim router bit with the jig. The bit is 1/2 in. wide, 1/2 in. long and has a bearing mounted above the cutter. The bearing is flush with the bit's cutting edges.
This routing jig has two long, parallel guides made from 1/2-in. MDF. When you rout a dado, ride the bearing along one guide, then make a second, return pass and ride the bearing along the second guide. This way, the space between the guides determines the width of the dado. Setting up the guides is easy. One guide is fixed to the jig's 3/4-in. cross members; the other is loose and adjustable. Take two small pieces of the plywood and place them against the fixed guide. Then slide the adjustable guide against the pieces and clamp the guide to the cross members. Done!