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Chatter stability of turning and milling with process damping Eynian, Mahdi


The prediction of chatter instability in machining steel and thermal-resistant alloys at low ‎cutting speeds has been difficult due to unknown process damping contributed by the ‎contact mechanism between tool flank and wavy surface finish. This thesis presents ‎modeling and measurement of process damping coefficients, and the prediction of chatter ‎stability limits for turning and milling operations at low cutting speeds. ‎ The dynamic cutting forces are separated into regenerative and process damping ‎components. The process damping force is expressed as a product of dynamic cutting ‎force coefficient and the ratio of vibration and cutting velocities. It is demonstrated that ‎the dynamic cutting coefficient itself is strongly affected by flank wear land. In ‎measurement of dynamic cutting forces, the regenerative force is eliminated by keeping ‎the inner and outer waves parallel to each other while the tool is oscillated using a piezo ‎actuator during cutting. ‎ Classical chatter stability laws cannot be used in stability prediction for general turning ‎with tools cutting along non-straight cutting edges; where the direction and magnitude of ‎the dynamic forces become dependent on the depth of cut and feed-rate. A new dynamic ‎cutting force model of regeneration of chip area and process damping, which considers ‎tool nose radius, feed–rate, depth of cut, cutting speed and flank wear is presented. The ‎chatter stability is predicted in the frequency domain using Nyquist stability criterion.‎ The process damping is considered in a new dynamic milling model for tools having ‎rotating but asymmetric dynamics. The flexibility of the workpiece is studied in a fixed ‎coordinate system but the flexibility of the tool is studied in a rotating coordinate system. ‎The periodic directional coefficients are averaged, and the stability of the dynamic ‎milling system is determined in the frequency domain using Nyquist stability criterion. ‎ The experimentally proven, proposed stability models are able to predict the critical ‎depth of cut at both low and high cutting speeds.‎

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