Sharpening a mortice chisel significantly improves performance. We can recommend this pair of specially made diamond coated cones for most Taiwanese and Japanese mortice chisels, and Veritas square hole punches. A coarse grit cone gives an initial grind, followed by a finer honing cone to create a sharp even edge.
Use the cones in a drill press or your morticer if you have a chuck adaptor (a handheld drill is neither suitable nor safe). Clamp the bit vertically and securely, or drill a hole in a block of wood the same diameter as the chisel’s shank. The chisel must be held firmly and perpendicular. Fasten the coarse cone in the chuck and align it with the chisel so it centres exactly in the end of the chisel, do this BEFORE switching on. Running at low speed (150 to 250rpm), gently lower the cone until it just makes contact with the mortice bit. Carefully grind until you obtain a clean surface around the entire bevel. Replace it with the 350-grit cone and repeat the process, lightly honing the inside bevel. Use a fine flat stone lap the sides of the bit to remove any burrs.