Safety of slim tool extensions for milling operations

The development of 5-axis machine tools (MT) allows complete machining of complex workpiece geometries. In order to counteract lacking operation space and to improve the accessibility of the workpiece, slim tool extensions (STE) are applied. Operating errors, e.g. by crash, can cause plastic deformation of STE during machining operation and therefore lead to an increased moment of inertia, and thus to rotational energy due to the spindle speed control of machine tools. Currently applied machine tool enclosures are not designed for such failures with corresponding kinetic energies EKIN. The described failure implies a risk potential for operators and a high damage potential for machine tools. In this paper, the failure scenario is identified and modeled. This includes the calculation of elastoplastic deformations of STE based on finite element analysis and analytical calculation of kinetic energies EKIN considering deformed STE. Based on the described model a parameter study is carried out considering the geometry of the STE as well as the spindle speed nS. The safety of existing machine tool enclosures is evaluated according to DIN EN 12417 in order to identify safe operating conditions. Finally, the authors suggest possible solutions addressing both, the STE and machine tool enclosure. The research presented in this paper is funded by the German Machine Tool Builders Association (VDW).