Generation of Predetermined Surface Structures by Simulation based Process and Tool Design when Milling Free-Formed Surfaces (CRC/TR 73 SP B3)

Due to the development of powerful machines, tools, and cutting materials, the milling of dies and molds for sheet metal forming and bulk metal forming offers multiple advantages compared to EDM processes. In addition to shorter machining times 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 investigated in the Transregio 73. Due to the complex interactions 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 the feed per tooth, the milling process itself already creates a surface structure. A simulation environment which was developed in this project is able to predict the surface structures of the workpiece after milling. With this approach it is possible to determine the process parameters for a NC program that leads to structures with the desired tribological characteristics. In this manner material flow during the forming process can be slowed down or accelerated depending on the structuring. The deficient filling of secondary design elements and functional elements of the mold or undersize of the formed workpiece is counteracted in this way.

By the application of high-feed milling, a wide spectrum of different structures can be produced. Therefore, utilizing the surface structure as a constructive element for the milling of free-formed surfaces should be made possible. Several kinds of tools with special cutting geometries are applied. By varying process parameters like the radial immersion, the feed per tooth and the lead angle distinct pattern develop.

The high feed milling process is characterized by specific tooth geometries, high feeds per tooth and low axial cutting depth. Usually, it is applied for roughing operations and the milling of hardened materials. Working with these tools always leads to the generation of structures. The design of specific milling tools for structuring surfaces and the operational behavior of surface structures on forming tools’ surfaces, are parts of the research activities in this project.

Fig.: Pictures (confocal microscope) of a forming tool with structured surface produced by milling with high feed milling tools