Using simple geometry, Rockler’s mortise-centering router baseplate can help you make perfectly centered mortises. Rockler makes the baseplate from 1/4″ clear acrylic, so you can see exactly what you’re routing. Two solid brass, 11/16″ pins screw into one of three sets of inserts drilled into the baseplate to accommodate different board thicknesses.
Place the pins in equidistant inserts for dead-on center mortises, or place one pin closer to the center to move the mortise towards one edge of the board or the other. By straddling the board with the two pins as you route the mortise, you’ll get a perfectly straight mortise, centered or not.
Rockler designed the 6-1/2″ diameter baseplate to fit the Porter Cable 690/890 series, Makita 1100-1101 series, Bosch 1617-1618 (fixed base only), and Dewalt DW616/DW618 routers.
Rockler sells this base with the brass pins for $20.
Mortise-Centering Router Base [Rockler]