Tool wear and chatter detection using the coherence function of two crossed accelerations

Tool wear and chatter have been found to be the main causes of rejects in the machining of super alloys. A novel detection technique to identify both tool wear and chatter in turning a nickel-based super alloy is introduced. It uses the coherence function between two crossed accelerations from the bending vibration of the tool shank. The value of the coherence function at the chatter frequency reaches unity at the onset of chatter. Its values at the first natural frequencies of the tool shank approach unity in the severe tool wear stage. The results are interpreted using the analysis of the coherence function for a single input-two output model. The advantage of using this method is that the thresholds for detecting severe tool wear and chatter can be easily set, since values of coherence function are normalized to a range of between zero and unity, and are also not so susceptible to changing cutting conditions, because the value of the coherence function is close to unity at the onset of chatter and severe tool wear.