Using a straightedge guide is one of the best ways to rout dadoes and grooves. But trying to remember the offset distance from the outside edge of my router base to the cutting edge of my bit is a pain. Even when I do remember, I always feel a little uneasy, hoping that the cut will be in the right place. A quick and simple way to ensure accurate cuts every time is to attach an alignment board to your straightedge guide.
You can see how this works in the drawing. First, attach the alignment board square with the edge guide. Then position your router against the edge guide and rout a dado across the guide board.
You now have an exact reference point for all your cuts. Just line up the dado in the alignment board with the marks on your workpiece.
