Metal working machine having polygon tool support bar

A machine tool, comprising:

• a machine base;
• a carrier movably supported on said base;
• a cutting tool support bar journalled in said carrier for rotation about a longitudinal bar axis, said support bar being axially slidable during rotation thereof, and said support bar having
• a cutting tool-receiving first surface and
• a second surface of polygon cross-sectional geometry formed around said longitudial bar axis;
• drive means for rotating said bar, said drive means comprising, in part, means cooperatingly formed to, and matingly engaged with, said second surface of said bar, said drive means located proximal to a carrier face defining a point of exit of said bar with respect to said carrier, and
• means for axially moving said bar with respect to said drive means and said carrier;
• wherein said polygon cross-sectional-geometry of said second surface comprises
• a center, coincident with said longitudinal bar axis;
• a closed perimeter formed, in part, by a plurality of symmetrically spaced, identical, curved segments, at least one of said segments having a radius-of-curvature extending to the segment from a point of origin spaced from said center;
• and
• a pressure angle at the point of contact of said second surface with said drive means during rotation, said pressure angle having a tangent value greater than the coefficient of friction between said second surface of said cutting tool support bar and said drive means.