Abstract: A reinforcement rib structure for a support column of a rotary machine tool is disclosed. The reinforcement rib structure includes a first set of ribs which are inclined clockwise with respect to the vertical and a second set of ribs which are inclined counterclockwise with respect to the vertical such that the first and second ribs intersect each other. In the preferred embodiment, the ribs are formed on opposing sidewalls of the support column and form acute angles with respect to a vertical within the range of 30.degree..+-.20%. With this reinforcing rib structure, the rotary machine tool may be operated at higher speeds while maintaining machining accuracy due to the enhanced rigidity of the support column with minimal weight increase.