Machining of Molds with Filigree Structures for Sheet-Bulk Metal Forming (CRC/TR 73 SP B2)

In the CRC/Transregio 73 the manufacturing of complex functional components with secondary elements made from thin sheets is investigated. This forming process is classified as Sheet-Bulk Metal Forming (SBMF). Manufacturing the forming tools required by SBMF represents an enormous challenge for the micromilling processes used in this task. Especially, the mechanisms of hard machining have not yet been examined in detail to manufacture filigree structures from materials of high hardness. The latter and the five-axis machining process, which is required due to technological and efficiency reasons, demand fundamental knowledge of the process. This knowledge is attained in this subproject B2 to achieve a reliable and efficient control of the micromilling process and to manufacture suitable and highly precise complex dies having filigree structures for SBMF. Therefore, the milling of parts having high aspect ratios in combination with the material properties, especially the high hardness, is analyzed for a cold-working steel (1.2379), powder and smelted metallurgical produced high-speed steel (1.3344 and 1.3343) with a hardness > 60 HRC. Process parameters and machining strategies are developed, which allow the milling of dies with filigree structures at the same quality level as it is nowadays achievable for macroscopic dies. Further the SBMF tools require a design which supports the form filling for the secondary form elements. This can be achieved by a material flow control, based on a tool-sided structuring of the surface by micromilling.

The milling of the geometrically complex, filigree secondary elements of the dies demands, among other things, the manufacturing of toothings with radii of R < 0.2 mm by small micromilling tools of d ≤ 1 mm. The application of these small tools and the resulting downscaling of the milling process from the macro to the micro range requires a comprehensive adaptation of the machining process. Especially the strong tool wear and the high tool deflection can have a stronger influence on the process reliability and process accuracy than it is commonly known from the macro range.

In milling processes with small tool diameters, the tool deflection and the resulting dimensional deviations are an important issue as well. Because of their low stiffness, the tools show a low resistance to the applied torques and forces. Especially for the machining of the hard materials described above, up to six times higher cutting forces are expected in comparison to micromilling of titanium alloys or stainless steels, resulting in large deflections. In this context, the magnitude of the tool deflection has to be determined for the machining of simple structures and for five-axis machining of geometries that are more complex as well. In a subsequent step, these investigations are extended to the machining of free-formed surfaces. To produce filigree structures with high aspect ratios, five-axis micromilling strategies, which have not been studied extensively so far, are used. The dynamic properties of the appropriate five-axis machine tool are very important with regard to the overall machining quality. For instance, the rotational axes of the machine tools feature a lower accuracy than the linear axes. Furthermore, because of their low motion speed and acceleration capabilities, the rotational axes are often not able to meet the programmed feed rates. For this reason, five-axis milling strategies are developed in this project, which take into account these inadequacies.

B2 also focuses on the development and production of surface structures. Their design is based on the kinematics of micromilling (technological structures) and on bionically-inspired structures. For the manufacturing process, influencing variables have to be analyzed and suitable process boundary conditions must be defined. Due to the high number of microstructural elements on surfaces of just a few millimeters in size, the calculation of NC paths within the common CAD/CAM process chain is no longer effective (> 6,000 structure elements). Therefore, new repetitive approaches to generate machining programs are developed. Furthermore, a simulation-based process design is developed in order to achieve a higher degree of applicability.

Fig.: a) Micromilled surface, b) micromilling of a tooth cavity, and c) functional bionically-inspired surface structures

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