With this dual-purpose guide, you can rout dadoes or crosscut wide project boards with perfect results and near-zero setup time. To make it, use a table saw to cut a 3-in. wide strip off a 2-ft. wide x 3- to 4-ft. long 1/2-in. piece of plywood, being sure both sides of the strip are dead-straight. Screw the strip along the middle of the wider piece of plywood, then clamp the whole unit so it overhangs your workbench on one side. Cut off the overhanging side of the plywood with a circular saw, running the saw’s base against the strip to create a straight-cut guide for your saw. Now spin the guide around and cut the long slot with your dado bit in the router to create a straight-cut guide for your router and that diameter bit.
To use the guide, lay out the start and ﬁnish marks on both ends of the board to be cut, line up the selected guide’s edges with the marks, clamp tightly, and make the crosscut (second photo) or dado (first photo).