You can make round tenons easily with this scrap-made jig and a bottom-bearing pattern bit or mortising bit. To make the jig, cut a hole in the base that’s twice the bit’s diameter plus the thickness of the workpiece. Attach a tall fence to the jig so the center of your workpiece will be centered over the hole in the base — this requires the fence to be offset on the hole by half the thickness of the workpiece.
Clamp the wood squarely to the fence with its bottom edge even with the base. Set the jig on your router table and sandwich the bit’s bearing between the base hole and the front of the workpiece (the bit should rotate freely). Then, start the router and move the jig counterclockwise, keeping the bearing pressed against the wall of the hole. Cut the tenon in a few passes of increasing depth until you reach the length you need.
– Anthony Farmwald