A disc chipper has thick double-edged chipper blades clamped in rectangular pockets between a knife segment and a knife clamp which are connected to a chipper disc. The blades are precisely positioned for cutting by knife stops selected from a set of knife stops of incremental modular widths. A different width knife stop is used to position the chipper knife blades each time it is sharpened. After each sharpening the knife blade has a selected width which is smaller by a constant incremental value, and it is assembled with a mating knife stop from the set so that the blade edge position remains unchanged.