In order to increase the critical depth of cut, some works on the diamond turning of brittle materials by applying ultrasonic vibration have been undertaken [16–18]. The reported results show that ultrasonic vibration cutting is a quite promising technique for the ultra-precision machining of brittle materials in terms of the machining efficiency and the ease of the operation. Although mirror surfaces have been produced on soda-lime glasses under specially controlled vibration conditions, the mechanism for the ductile machining of brittle materials by applying ultrasonic vibration is not well established. Tool wear is still quite large compared with that in the conventional machining of soft metals. A better understanding of this technique is needed for applying it in practical production.