In-process monitoring and control of thickness error in machining hollow shafts

This paper presents the capability of a new ultrasonic system for in-process monitoring and control of part wall thickness during machining. The sensor is non-contact, requires access to the workpiece surface from one side only and provides continuous high speed monitoring. The cutting fluid is used as the medium to transmit the sound waves to and from the machined surface. The system is based on a multi-probe setup in order to compensate for sensor mis-alignment errors. The design of the ultrasonic transducers is optimized to provide high accuracy and a practical working range. The performance of the proposed system is evaluated by adapting it to a CNC lathe retrofitted with an Open Architecture Controller (OAC). The system is applied to monitor and control the external turning of slender hollow shafts. The thickness data obtained during cutting is used in a closed feedback control loop to govern the tool position and compensate for thickness errors in real time. The experimental results demonstrate the accuracy and robustness of the inspection system. The control results show a significant reduction in thickness variation along the cut, especially that due to workpiece flexure.