Chatter Index Formulation: Predicting Chatter and Detecting its Causes During Milling of Impeller Blades and Thin-Walled Structures

Chatter frequently occurs while machining impeller blade type thin-walled structures. It imparts a poor finish to aero-engine impeller blades, thereby affecting the efficiency of aero-engines. It is well known that changes in the workpiece dynamics due to material removal has correlation with the chatter. Therefore, to predict chatter during thin-walled component machining, while simultaneously accounting for in-process workpiece dynamics, we present a chatter index formulation based on the thin plate theory. The chatter indices account for the changes in workpiece dynamics due to machining in terms of natural frequency and mode shapes of the workpiece in process. These indices are evaluated using finite element modal analysis of in-process workpiece geometry and milling experiments performed on a workpiece sample. We experimentally validated the capability of derived chatter indices to predict machining chatter for different machining parameters by comparing the chatter index values with the machined workpiece surface topography and machining force spectrogram. The causes of chatter are also detected using the chatter index formulation in terms of unstable workpiece vibration modes, effect of workpiece stiffness, and tooth passing frequency harmonics that cause the resonance. The proposed method is effective in predicting chatter and in detecting its causes in milling of impeller blade-type thin walled structures.