Development of Ultra Precision Machine Tool Made of Ceramics

Summary Most of the structural members of the developed machine consist of Alumina Ceramics, whose high specific rigidity and small coefficient of thermal expansion are expected to suit for ultra precise requirements. In addition, the ceramics are very easy to finish to a complete flatness by lapping operation. On the straight slideway obtained in this way, the slider must be traversed rigidly without any friction. For this purpose, a surface restricted type aerostatic slideway was adopted at the present design. The principle is that the air-restriction is given by narrow and very shallow grooves instead of annular oriffices. The characteristics of this type of bearing were solved by the help of computer and tested for several sliding models, and the one which was adopted is as follows; the clearance is 5 am, the rigidity is realized up to 200 N/am. The motion of machine should be as simple as possible, and only the linear motions are composed to a planer type machine tool, where X axis gives a cutting motion by a wire tension mechanism. Y axis presents a cross feed motion by laser feed back. friction drive mechanism, where two shafts cross each other, supported by aerostatic bearings at each end, and the rotational motion given by the servo motor to the one shaft is transformed to a linear motion to the other at the contact point. This system can afford positioning up to 0.08 μm. Z axis gives the tool engagement, where fine positioning is by a piezo-electric element. Here, the hysteresis loop is memorized in a computer and the response is linearized. This mechanism could realize a resolution of 0.03 μm. Some cutting tests were performed with a single crystal diamond tool. The test had shown the result of less than 0.05 μm of surface roughness.