Validating real time compensation: A thermal test piece for 5-axis machine tools to separate thermal errors in Z-direction

The accuracy of 5-axis machine tools is a key factor to manufacture multi-axes machined workpieces. However, thermal deformations of the machine structure often cause significant deviations at the tool centre point. To evaluate the impact of thermal errors on the accuracy of 5-axis machine tools under machining conditions, suitable thermal test pieces are required. Therefore, this paper introduces a 5-axis thermal test piece which is based on the thermal test piece for rotary axes being part of the standard ISO 10791-10:2022. The new thermal test piece identifies ten instead of five thermal errors during eight time steps. The additional features enable an error separation of the table-related thermal error and the thermal error pointing consistently in Z-direction. This error separation is especially important to analyse the accuracy of five-axis milling operations. In total, the developed 5-axis thermal test piece analyses five location, two length and three orientation errors without requiring a kinematic model of the machine tool. The conducted experiments comprise thermal load cases without and with metal working fluid and show a clear agreement between the thermal location errors and the dominant thermal orientation errors of the thermal test piece and an on-machine measurement cycle. On the other hand, the analysed thermal length errors of the thermal test piece indicate a clear effect of the thermal material expansion compared to the thermal positioning errors of the analysed machine tool. Furthermore, the analysed thermal load cases of the linear and rotary axes conducted without and with metal working fluid result in a different thermal behaviour of the machine tool depending on the use of metal working fluid. Finally, the developed 5-axis thermal test piece is used to evaluate a thermally compensated 5-axis machine tool under machining conditions for thermal load cases without and with metal working fluid. The self-learning thermal error compensation reduces the mean of the thermal location errors at the thermal test piece by up to 71%.