Abstract: A cutting or drilling spindle and a tool unit supportable within the spindle for rotation with the spindle wherein the tool unit is provided with radially extending protrusions which can be received within corresponding inner radially spaced recesses within the spindle. The radial protrusions of the tool unit comprise balls support adjacent the exterior surface of the axial neck of the tool unit by a cage and which operate in such a way that during axial insertion of the tool unit into the spindle and locking, the radially outwardly extending portions of the balls will be engaged in the spindle's recesses. The balls are relatively movable such that if they are not properly aligned with the recesses when the tool unit is inserted into the spindle, the balls facilitate correction so that the protrusions of the tool unit will match the indentations of the spindle.