Primary Chatter and Limiting Chip Load in Turning Under Negative Process Damping

This paper presents an analysis of primary chatter under velocity-induced negative process damping in the peripheral outer diameter turning of medium carbon steel. A first-order approximation model of the instant specific cutting force with respect to dynamic cutting speed was established and the slope was defined as the specific process damping coefficient (SPDC) to investigate the negative process damping with respect to cutting speed, depth of cut, and chip thickness. The process damping coefficient was defined as the product of the specific process damping coefficient and chip load. The total system damping coefficient as the sum of the process damping coefficient and structural damping coefficient determines the system stability and predict primary chatter. The SPDCs were obtained through experiments under various speeds, feeds, and depths of cut by using a tool system with force sensors and accelerometers. The SPDCs were insensitive to cutting speeds of 2.5 to 5.5 m/sec and ranged from −1514 and −716 MPa·s/m for feeds per revolution of 0.058 to 0.118 mm, respectively. The higher negative SPDC at smaller chip thickness reduces the limiting stable chip load. Equations for the limiting chip load and limiting depth of cut were derived and validated by experiments. Stability diagrams of limiting chip load and limiting depth with respect to feed per revolution were created to provide guidance on preventing primary chatter.