Abstract: Before being coated with the magnetizable material, the substrate plates are machined on a precision turning lathe in such a way that the annular surface provided for the storage area is convexly arched on both sides of the plate. For this purpose, the substrate plate, during machining, is held on the chuck in a concave arch of the annular surface and machined flat with a purely radial free movement of the cutting tool, or held on a flat chuck and turned down under a feed movement of the tool, which feed movement corresponds to the required convex arching of the annular surface. In the first machining method, the elastically deformed substrate plate, after removal from the chuck, loses tension and forms a convexly arched annular surface.