Analysis and modelling of machine tool dynamics and cutting stability during operation

Özşahin, Orkun
Self-excited vibrations of machine tools during cutting result in process instability, poor surface finish and reduced material removal rate. In order to obtain stability lobe diagrams to avoid chatter vibration, tool point frequency response function (FRF) must be determined. In classical machine tool studies, tool point FRF is obtained experimentally or analytically for the idle state of the machine. However, during cutting operations, discrepancies are frequently observed between the stability diagrams obtained by using FRFs measured at the idle state and the actual stability of the process. These deviations can be attributed to the changes in machine tool dynamics under cutting conditions. In this thesis, effects due to the operational conditions on machine tool dynamics are investigated. For that purpose, machining center subassemblies (spindle, holder and tool) are modeled using Timoshenko beam model vi including gyroscopic effects, and tool point FRF is obtained using structural coupling and modification methods. Using the analytical model, effects of operating conditions on machine tool dynamics are investigated for different spindle – holder – tool assemblies and cutting speeds. In addition to the analytical modeling, variations of machine tool dynamics during operation are also investigated experimentally. A new identification method is proposed for the identification of in process tool point FRFs. Then, experimentally and analytically obtained FRFs are used in the identification of the spindle bearing parameters under cutting conditions. Finally, for a real machining center, tool point FRFs under operating conditions are determined using the identified speed dependent bearing dynamics and the analytical model proposed. Analytically calculated tool point FRFs are verified through chatter tests.