ER fluid is an agile material, its viscosity and yield stress can be changed when exposed to an electric field. This feature provides simple, quiet, rapid response interfaces between electronic controls and mechanical systems. ER fluid dampers are relatively new semi-actives that utilize ER fluids to provide controllable damping forces. In this paper, following the introduction of the essential characteristics of ER fluids and ER fluid dampers, the theoretical model is presented to describe the ER fluid. A damper based on a shear model is designed to control a cutting chatter. The excitation t...