Generation of Predetermined Surface Structures by Intentionally Invoked Self-Excited Tool Vibrations when Milling Free-Formed Surfaces (CRC/TR 73 – SP B3)

Due to the development of better machines, tools, and cutting materials, the milling of dies and molds for sheet metal forming and bulk metal forming has multiple advantages compared to eroding. In addition to shorter machining time and better surface quality, the flexibility of the milling process is most advantageous. Existing molds can easily be modified or adjusted. Especially for small-batch production, milling is a very efficient process. The subject of this project is the creation of dies and molds for sheet-bulk metal forming, which is researched in the Transregio 73. Due to the complex causal effects between workpiece shape, material behavior, and surface quality, the production of process-capable molds is of great importance. In addition to the actual geometric shape of the tool and the dynamic forces, the material flow during the forming process is to be optimized by systematically structuring the surface of the mold.

So far, the structuring of forming tools was performed as an additional machining step. Due to the discontinuous cut, the discrete NC-path spacing, and tool vibrations, the milling process already creates a surface structure. Therefore, this projects aims to develop a software system based on an existing simulation, which can be used to determine the adjustable process parameters for the milling of free-formed surfaces with a given NC program so that a defined surface roughness or structure is created. Regenerative tool vibrations are to be intentionally generated in order to allow a broad spectrum of structures. Hereby, utilizing the surface structure as a constructive element for the milling of free-formed surfaces should be made possible. Several kinds of tools with high cantilever length are to be utilized. By varying the cantilever length and the diameter, the modal parameters can be adjusted in a certain interval so that for chosen process parameters axial and radial immersion, spindle speed and contact situation, a distinct vibration pattern develops. However, the dynamic process behavior and thereby the generated surface structure changes for slight variations of the process parameters at the boundary of two process states (chatter-free and with chatter). Thus, experiments to find the right parameters for a desired surface structure can be time and cost intensive. The process parameters should therefore be chosen by using the milling simulation. Here, the effects of different process parameter settings on the surface quality can be investigated without great effort [1].

Another objective is to investigate which types of surface structures can be generated by the dynamics of the milling process and by the choice of tool type. In prior investigations, it has been shown that it is possible to generate a variety of surface structures with the simulation and in the real process. The simulated and the machined surfaces are often very similar as long as the machining parameters are the same. This could already be shown for aluminum (EN AW 7075), steel (C45E) [2] or hardened high-speed steel (1.3343).

In the progress of the investigation it should be possible to classify the generated surface structures by similarity to generate a database, which maps each topological surface characteristic to the process parameters that can be used for its production. To do this, the effects of the process parameters tool shape, spindle speed, tool length, and diameter, as well as tool inclination and axial and radial immersion, on the surface characteristic are to be analyzed. At first, this is conducted qualitatively for constant engagement situations while validating the results using real experiments. This database is the basis of further investigations of other projects in the Transregio 73. One example is the determination of tribological properties of the structures which are very important for the forming tool designer [3].

Up to now, self-excited tool vibrations have been considered undesirable since they are characterized by high oscillation amplitudes and to some extent high frequencies compared to the tooth engagement frequency and, thus, can be damaging to the spindle system and the cutting edges of the tool. Therefore, another objective is to gather knowledge about the tool life of different cutting tools during a process with chatter. Initial experiments have shown that it is possible to produce large surfaces (100 mm × 70 mm) which are structured in a very regular way without significant tool wear. Only after the run-in phase of the tool, there is a visible change in structure.

Fig. 1: Comparison of surface structures generated by the simulation (top) and by the actual milling process (bottom), 0.28 mm × 0.28 mm area

After achieving the described objectives, the obtained knowledge can be used to adjust the system for the machining of free-formed surfaces. At the end of this project, it should be possible to use the simulation to determine the process parameters of the milling process for the creation of a mold for bulk-sheet metal forming prior to the actual machining so that the milling process directly generates the requested surface qualities on the workpiece.

References

[1]
Biermann, D.; Surmann, T.: Surface Structuring by Milling with Intentionally Invoked Chatter. Proceedings of the CIRP 2nd International Conference Process Machine Interactions, 10.6.-11.6. 2010, Vancouver, BC, Canada, ISBN 978-0-9866331-0-2, digital published
[2]
Biermann, D. et al.: Creating Functional Surface Structures by Milling using Self-Excited Tool Vibrations. Proceedings ASPE 2011 Spring Topical Meeting – Structured and Freeform Surfaces, 6.3.-8.3. 2011, Charlotte, North Carolina, USA, ISBN 978-1-887706-57-5, S. 55-58
[3]
Biermann, D. et al.: Akquise von heuristischem Wissen für die Prädiktion von Oberflächenstrukturen beim Fräsen mit regenerativen Werkzeugschwingungen. 1. Workshop Blechmassivumformung, Tagungsband, 13.10.-13.10. 2011, Erlangen, M. Merklein, Fr.-W. Bach, A. E. Tekkaya (Hrsg.), ISBN 978-3-87525-321-4, S. 77-96

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