Sheet Incremental Forming: Advantages of Robotised Cells vs. CNC Machines

摘要

The incremental forming of metal sheet parts can be an interesting alternative to manual forging of prototypes and pre-series blanks or to the manufacturing of shells for resin dies used for the production of small batches. Such characteristics of small-volume production would call for an increase in the level of flexibility and automation, possibly leading to the use of CNC machines or robotised cells able to produce or complete the parts. In particular, the use of robotised cells, with automatic tool change, can dramatically reduce the process time since on the same fixture it is possible to deform the part, cut the part, bend or flange the borders, load/unload the part, etc. The present contribution has described the crossed experiments performed at the Polytechnic University of Marche in Ancona and at the Universities of Brescia and Bergamo to assess the feasibility of the automated processing by using both a traditional CNC machine and an industrial robot. It is noted that the research and industrial processes of incremental forming realised so far have ever used 3-axes CNC milling machines, apart from the hammering process patented by the Fraunhofer Institute for Manufacturing Engineering and Automation of Stuttgart (Shaefer & Schraft, 2005). Unfortunately the conventional serial robots do not have the required stiffness and are not able to apply the necessary forces to deform incrementally the blank, but the rather new family of parallel robots has characteristics similar to CNC machining centres, while still keeping the versatility of a robot. The complex kinematics of the machine needed the development of a special purpose simulation environment to design beforehand the experiments and assess their feasibility. The necessary force at tool tip has been evaluated both analytically, with a simplified approach based on a plane strain state, and numerically,