Exact

Exact-Width Dado Jig

Here's a jig for routing bookcase or cabinet-side dadoes that exactly match the thickness of your shelf stock. Better yet, no special bits are needed. Just use an ordinary straight bit and a guide bushing. (We used a 1" guide bushing and a 1/2" straight bit.) To start, cut a 3/8" rabbet 3/8" deep along the inside edge of both guide rails (A). Then complete the jig as shown in the drawing, below, right. To customize the rails for a different bushing and bit, install them in your router and trim the protruding lip of the rabbet in the guide rails, as shown in the Dadoing Detail. The remaining lip will now match your bushing/bit combo.
To adjust the jig for the exact width of your shelf stock, slip the jig over the shelf stock as shown in the Adjusting the Jig drawing. Pull the guide rails (A) tightly against the stock, and tighten the wing nuts. Slip the jig off the stock, and clamp the jig onto the piece being dadoed, centering the opening between the rails (A) over the marked dado on the side panel. Adjust the depth of cut with your router sitting on top of the rails. Start the router and make one pass with the guide bushing riding against one of the rabbeted rails. Make a second pass riding the bushing against the opposite rabbeted rail.