When using a router, you want to avoid taking too deep a cut. It can create excess chipout and, in some situations, can be dangerous. So when I want to rout a dado or groove that's more than a 1/4" deep, I take multiple passes. I know. It's easy enough to change the height of the bit between passes. But with some routers, changing the height of the bit also slightly changes the position of the bit relative to the base. (This means you could end up with an unwanted shoulder inside the dado.) And besides, I've come up with a way of using depth stops that lets me avoid the extra setups. To rout a deep dado with one setup, I use a set ofdepth stops or plates that are carpet taped to the base of the router, refer to Fig. 2 at right. Two of these plates are made from 1/8" hardboard, and a third is 1/16"-thick plastic laminate. All three stops have a hole drilled in the center for the bit to fit through. Before you add the depth stops to the base plate, the first thing to do is adjust the height of the router bit to match the finished depth of the dado, as in Fig. 1. (Once the bit is set, leave it alone.) Then carpet tape the auxiliary stops to the base of the router (Fig. 2). After the first pass, simply "lower" the bit during successive passes by removing one stop (Fig. 3). The final pass (after removing the laminate depth stop) will be a 1/16"-deep skim cut that will leave a clean dado at just the right depth.
Ted's Woodworking - Guide eBook