Abstract: In an abrading tool having a cup-shaped outer housing with a shank for attachment to a power-driven chuck and rotatable abrading discs on radial axles inside this housing, a plurality of knocker bars are attached to the outside of this housing. As the rotating housing is moved across the surface to be abraded, these knocker bars appear in succession at its leading side to bite into the material there and overcome the tendency of the abrading discs to ride over this material.