Process control for ultrasonic hole-drilling in hard brittle materials

Abstract

Ultrasonic drilling offers a solution to bore sub-millimetre holes of high diameter-to depth ratio in difficult to machine brittle materials. A key aspect of the process is to maintain a proper gap between tool and work piece. This paper presents the successful solution developed to address this issue. In our approach, we vibrate the work piece and control the motion of the tool, which is constrained by an aerostatic bearing and generated by a voice coil actuator. We use the voice coil to measure the force acting between tool and work piece. We measure the tool displacement with an Eddy current sensor. Using the two sensor measurements, we developed a control algorithm in which the tool is displaced in such a way as to maintain the tool-work piece interaction force constant. The paper presents experimental details of the characterization of the control loop and results of drilling experiments.