Machining operations in automated manufacturing centers are under-performing by 20-80%. Optimizing these machining operations requires on-line knowledge about the cutting tool's condition and the process state. Currently, this information is either not reliable or not available in a timely manner. This is due to the lack of suitable sensors, which must measure on-line directly and accurately one or more of the relevant tool and process information sources in the hostile machining environment. A direct, active, ultrasonic method for on-line sensing of the tool condition and process state in turning operations was developed. Sensing is achieved by using an ultrasonic transducer operating at 10 MHz in a pulse-echo mode to send pulses through the tool. The amplitude and propagation time of the reflected pulses are modulated by the tool nose, flank, temperature, and by the material in contact with the tool. The reflected pulses are received and processed by a high speed digital signal processing system. This method has the potential to directly and accurately measure on-line several relevant processes and cutting tool parameters through the use of a single sensor. These parameters are tool-workpiece contact, tool wear, tool chipping, temperature and chatter. / Ph. D.