Many woodworking instructors such as Toshio Odate and David Charlesworth advocate a slight convex radius on the cutting edge of plane irons. This curved or cambered edge prevents ridgelines when smoothing, promotes rapid stock removal during flattening and produces gapless seams when joining.

These diamond-covered, cast iron plates are manufactured slightly concave (.0025" hollow) across their width to rapidly shape this type of cutting edge. They measure 3-1/4" wide x 7-1/8" long x 1/4" thick and will accommodate almost any plane iron.

The 600x (25mic) plate is a fine abrasive. 220x (60mic), 325x (45mic) and 1200x (25mic) plates are also available. If you do your rough sharpening on a coarse diamond or waterstone and can only get one crowning plate, make it the 600x (25mic). Once the plate forms the radius, then you can hone by alternating your finger pressure over the iron on a medium stone (1000-1200 grit) and finish up on a polishing stone (6000-10,000 grit).