Occasionally, an existing frame needs the rabbet enlarged, for a painting other than the one the frame was built for. This frame has an inner blind frame building up the area at the rabbet, so I decided to use a router to remove the bulk of the wood. The base plate has a bushing, which will ride against a plywood "template", nailed in position as below. This one is butt and biscuit joined (see previous post) into an L shape to provide an end stop.
Due to the depth of wood to be removed, I did about six passes, then using mallet and chisel cleaned the corners. Alternatively, this can be done all by chisel and mallet, using a knife to "score" the line, then removing the waste, cleaning up with a bull nose plane.