Abstract: A mounting apparatus for securing alternative grinding wheels to a threaded portion of an output shaft of a portable rotary grinding machine, the mounting apparatus comprising a threaded clamping device which cooperates with the threaded portion of the output shaft; a support hub which rigidly mounts on the output shaft, the support hub having an inner annular contact surface and an outer annular contact surface for backing grinding wheels mounted on the output shaft. The inner and outer annular contact surfaces are disposed concentrically relative to each other as well as relative to the output shaft, and the inner and outer annular contact surfaces are disposed at different axial levels such that the outer annular contact surface protrudes in front of the inner annular contact surface.