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Abstract

Turning operations are commonly limited by chatter vibrations, in which some combinations of spindle speed, feed rate, and depth of cut cause the tool or workpiece to vibrate uncontrollably. Chatter limits material removal rates, can damage the tool and workpiece, and is highly undesirable. This thesis presents a novel actuator that passively damps workpiece vibrations when turning highly flexible workpieces, such as a long slender rod, or a hollow tube, where the flexibility of the workpiece is the dominant source of chatter. Traditionally, CNC machines and tools are designed to be highly rigid, ostensibly to minimize chatter. However, for flexible workpieces, a rigid tool is not optimal for chatter minimization. This thesis proposes that a flexible tool with a high damping ratio can significantly improve the chatter stability of the tool-workpiece system when the tool’s natural frequency and stiffness are tuned optimally. The proposed actuator has two subsystems, comprising a primary compliant damped tool tip stage, mounted on a secondary linear actuation stage. The tool compliance is delivered by stacking a number of plate springs, which leads to tool flexibility only in the flexible radial direction of the workpiece. This is the direction where the regenerative chip thickness causing chatter is produced. The high damping ratio is delivered by closely-spaced plates in an oil bath, where shearing a thin oil film between plates provides linear damping with no static friction. A stepper motor linear actuator compensates for the static deflection errors from the flexible tool deforming under cutting loads. The mathematical model of the system consists of a workpiece and tool each with single degree of freedom dynamics, and a digital controller that compensates for the deflection errors. The actuator and supporting electronics have been designed, manufactured, and instrumented with vibration sensors. The performance of the actuator has been experimentally evaluated by turning a steel test workpiece. It is shown that the chatter stability is improved by at least 10 fold, and the static deflections within the bandwidth of the controller are compensated with a tool-tip position variation within ±3𝜇m at steady state.