Radial Error Feedback Control for Bar Turning in CNC Turning Centers

In-process measurement and control of CNC machines can improve machining accuracy while increasing productivity and eliminating waste in a cost effective way. This paper describes an in-process measurement and control system called radial error feedback control (REFC) for bar turning in CNC turning centers. REFC was designed to compensate for the radial error due to the time-varying bar deflection caused by the cutting force, which moves along the bar at a constant velocity. In a dry cutting environment, a noncontact laser sensor was used to directly measure the radial error of the workpiece while it is being cut. To compensate for the radial error, a gain-scheduled Kalman filter for prediction together with PI control law was designed, based on a time-varying model of the deflection of the bar. Analytical modelling (using classical beam vibration theory) was employed to develop a simple model for the bar bending. For non-uniform bars finite element modelling can be used. Experimental results show that the dimensional and geometric accuracy of the workpiece is substantially improved by this radial error feedback control technique.