Abstract: Automatic object-positioning mechanism, comprising means for supporting the object (1), means for sensing positioning errors, means for moving the means which support the object under control (1), means for activating the means which move the object-supporting means, wherein the means for supporting the object (1) comprise an element (31), a floating plate (3) mounted on a plurality of guides (4) located at the end of two orthogonal axes and to which ball joints (6) provided with threaded bush (7) are associated; wherein the means for moving the means which support the object under control (1) are made up of a plurality of worm screws (8) engaged to electric servomotors (9), said worm screws being oriented parallel to the centering axis (h--h) of the object under control (1), being supported by a fixed frame (5) and engaging the threaded bush (7) of said ball joints (6) so that, by suitably moving the pairs of opposite screws (8), there is obtained a corresponding movement of the object about the center of th