A metal extrusion press for the production of tubular workpieces and/or pipes has been disclosed in the German patent DE 1,227,858 (GB 929,056). There, a mandrel is mounted with the piercing cylinder on the main extrusion piston. The piston of the piercing cylinder is connected to a piercing cross-bar that is guided in the platen of the press in a sliding manner. Alternatively to such inside punching devices, it is known to provide the piercing cylinder outside the main extrusion piston or cylinder of the press.
Since the mandrel in general can have varying lengths, wear has to be taken into consideration, exact adjustment of the mandrel tip in the die opening is carried out by means of threaded spindles and spindle nuts with associated drives when extruding a pipe with a fixed mandrel. These are typically mounted in conjunction with mandrel stroke-limiting rods in the cylinder cross-piece or in the piercing cross-bar. Such an arrangement of a threaded spindle and spindle nut in the piercing cross-bar for limiting the stroke of the mandrel is disclosed for example in the patent mentioned above. The mandrel stroke-limiting rods here are fixed with one end in the cylinder cross-piece and carry stops on the other end facing the pressure plate. The piercing cross-bar is supported against these stops with a nut that limits the stroke and consequently the mandrel and that can be adjusted with the threaded spindle.
In order to be able to position the mandrel guided through the ram in the tool or in the die when extruding tubular workpieces and/or pipes, and in order to maintain this position throughout the extrusion process with high precision, during the practical operation of the extrusion press the mandrel is held in position during operation by means of a piercing cylinder. To allow this position of the mandrel to be maintained in the die, the cylinder has to move the mandrel back at exactly the same speed at which the ram performs the forward movement. Here however disturbances due to forming forces, friction and hydraulic compressibility come into play, which the control system has to compensate for dynamically.
Additionally, it is necessary to cover high speed ranges of 1:120 and greater. Finally, it is important to note that due to the application method considerable, variable tensile forces are applied to the mandrel, which can also reverse at the end of the extruding operation. In order to guarantee positioning, servo valves are used, via which the entire volume for the piercing cylinder is controlled. Since these servo valves can operate only a limited volume range, it is unavoidable that several servo valves in different nominal variables have to be provided parallel to each other for the speed ratio of 1:120.