Abstract: A shearing device for cutting boards of particulate material, or similar, having a body adapted for fastening to a drill, and first and second cutting blades fixed to the body, each cutting blade having a curved cutting edge extending to a forward tip of its respective cutting blade. A third cutting blade is pivotably mounted to reciprocate in a plane between the first and second cutting blades the third cutting blade having curved cutting edges extending to a forward tip of the third cutting blade so as to cooperate with the first and second cutting blades to perform the cutting operation. The third cutting blade is spaced from the first and second blades by spaces to provide a clearance of 1 to 3 mm.