I use a router and a jig to mortise hinges in doors and casement windows. By using a 1/2-in. diameter hinge-mortising bit and a standard round-cornered hinge, I achieve a consistently clean fit that would be difficult and slow to get by hand.
A piece of 3/8-in. plywood forms the base of the jig. After carefully measuring the size of the hinge and the router baseplate, I nail 1×1 fences to the base to guide the router. As the router is passed inside the bounds of the fences, the bit will cut the hinge shape into the plywood base. Once the mortise is cut out of the base, I lower the bit to adjust the depth of cut in the door.
A 1×2 edge guide nailed and glued to the base aligns the jig on the edge of the work. To keep the jig steady while using the router, I tack 6d nails through the plywood base into the edge of the door or window jamb.
Steve Larson, Santa Cruz, CA