The suction cups and stops are in turn secured to the work table (smooth and without any grooves of any type) again by means of a vacuum circuit, being connected to another dedicated circuit in which the vacuum is always present; in actual fact the suction cups are symmetrical and so can be used indifferently in one direction or the other. Because of the smooth work table with no grooves there are no limitations of any kind for positioning the suction cups and stops on it.