PURPOSE: To facilitate the grindstone replacing operation and improve the efficiency in grinding process by separately installing two kinds of grindstones onto the shafts supported rotatably at the both edges of a rotary plate and departing one grindstone when the other is at the grinding position on a cutter by the revolution of the rotary plate.

CONSTITUTION: When a motor is revolved normally in rough finishing grinding, a rotary plate 12 turns clockwise to bring a rough finishing grindstone 18 close to a ground cutter W, drawing a circular locus (r), and at the same time, a finishing grindstone 18a is departed over the cutter W, and intermediate gears 27 and 27a revolve fixed gears 26 and 26a reversely. Then, the counterclockwise revolution in equal revolution angle to that of the rotary plate 12 is transmitted to a shaft, and since the shaft does not revolve relatively, and elevation frames 17 and 17a maintain vertical posture, the axis lines of the rough and finishing grindstones 18 and 18a are not changed. When revolution is suspended, the grindstone 18 corresponds to the ground surface of the cutter W at the center of a cutter installation board 4, and grinding is carried-out by controlling the transverse transfer by a handle 3, after adjusting the vertical position by an elevation screw 16. Also in finishing grinding, similar operation is carried-out.