The instrument has a tapered rod (1) having an active portion (lb) and a helical flute (4). The edges defined by the helical hollow (5) is coincident with the cutting edge (4a) defined by the helical flute. The hollow is provided to reduce the cutting angle of the cutting edge. The cross-section of the active portion of the rod is provided with a convex side belonging to the helical flute. The curvature at any point of the convex side is increased, and is tend to approximate the curvature of the circumscribed circle, and is passed through the cutting edge.
