Active and adaptive damping of chatter vibrations in the boring process using magnetorheological damper

Chatter is a limiting factor during boring of deep holes with long slender boring bars. This thesis introduces a new magnetorheological (MR) damper to suppress the chatter and increase the stability of the boring process. A novel design that utilizes a minimal amount of MR fluid filled in a sponge layer surrounding the boring bar is presented. The MR fluid layer and the electromagnetic circuit are externally applied to the boring bar, which allows easy installation and adjustability of the overhang length of the bar. A custom made, bidisperse MR fluid which has an improved sedimentation resistance property is used to ensure a long lifetime of the damper. A new sliding mode control with variable gain super twisting algorithm is designed and implemented for active damping of chatter vibrations in the boring process using the developed MR damper. Simulations of the controller show its fast response and robustness against disturbances and parameters uncertainties. The proposed controller outperforms the commonly used PID controller for disturbance rejection at wide range of frequencies. Experimental validations of the developed system were performed on CNC turning center. The modal analysis of the boring bar with the new MR damper shows improvements in both the damping and the dynamic stiffness of the system. This enhancement significantly increases the chatter-free depth of cut on the stability lobe diagrams. Validation cutting tests were performed under various machining conditions on boring of Al7075 and Inconel 718 workpieces which are materials widely used in many aerospace applications. The improvements achieved by this work are illustrated in terms of measured acceleration values in both time and frequency domains, in addition to the quality of the machined surfaces.