PURPOSE: To accurately, speedily and stably position a polishing wheel with respect to a rotary blade by providing wheels driven respectively by drive means mutually connected to monitoring and control means programwise controllable according to wear or loss speed of edge of the rotary blade and operating automatically.

CONSTITUTION: Polishing wheels 1 are respectively mounted to supports 3 movable passing along a linear locus parallel with a plane dominated by a blade 2. These wheels 1 are respectively operated by drive means 4 mutually engaged with automatic monitoring and controlling means programwise controllable according to wear or loss speed of an edge of the blade 2 at least when it moves in the direction of the blade 2.