1. Department of Mechanical Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi-shi, Ibaraki 316-8511, Japan
2. Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi-shi, Ibaraki 316-8511, Japan
3. Yoshida Seiko, Ltd., 660 Yakimaki, Namegata-shi, Ibaraki 311-3506, Japan
4. The Yoshida Dental MFG Co., Ltd., 1-3-6 Kotobashi, Sumida-ku, Tokyo 130-8516, Japan