Rotary cutting blade

A rotary cutting blade, comprising:

• a leaf comprising two opposite surfaces forming between them a smooth cutting edge extending along at least a portion of their periphery;
• a single smooth inclined cutting surface extending inwardly and inclined at varying angles from the cutting edge at one of the two opposite surfaces, the inclination varying from at least one minimum angle to at least one maximum angle, wherein a distance between said at least one minimum angle to a closest maximum angle, measured along the cutting edge, is larger than any radial distance from the cutting edge to a center of the blade;
• a smooth inclined surface extending inwardly from the cutting surface, said smooth inclined surface being disposed on the same one of the two opposite surfaces as the cutting surface;
• and
• a substantially planar surface extending inwardly from the inclined surface and provided with means for mounting the blade for rotation about an axis disposed at a predetermined relationship to the cutting edge.