Machine Tools with an Enhanced Ball Screw Drive in Vertical Axis for Shaping of Micro Textures

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1 Proceedings of the euspen International Conference Zurich - May 28 Machine Tools with an Enhanced Ball Screw Drive in Vertical Axis for Shaping of Micro Textures D. Kono 1, T. Fujita 1, A. Matsubara 1, I. Yamaji 1 1 Dept. of Micro Engineering, Graduate School of Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto, , Japan matsubara@prec.kyoto-u.ac.jp Abstract This study describes a three-axes machine tool equipped with a fine motion device in the Z (vertical) axis. The machine motion is basically provided by ordinary ball screw drives. The synchronization of the fine motion with the XY motion to control the depth of cut can produce micro textures on a workpiece surface. First, the closed loop control of the fine motion was tested, and the frequency response was evaluated. Second, in order to improve the response, a second order digital filter was installed in the FF controller. The bandwidth was improved to 13 Hz at the amplitude of 5 nm. Finally, the evaluation in actual machining was conducted. A sweep-sine wave was machined with a diamond bite by the control of the depth of cut. From the experimental result, it was verified that the system properly provided the depth of cut at less than 1 Hz. 1 Introduction In these days, demands are increasing for the machining of micro textures with the size of several hundred micrometers and the accuracy of sub-micrometers in the machining areas of optical parts and precision dies. For the control of the depth of cut in these applications, both the high frequency response with the resolution of ten nanometers and the synchronization with feed motions are required. The combination of an ultra-precision lathe and a fast tool servo system (FTS) has been used to fabricate the micro textures[1,2]. The FTS synchronizes with the feed motions by referring the rotary encoder pulse of the spindle. Although this combination provides good solutions for rotationally symmetric textures, the coordinate transformation from the XY to the polar coordinates are required to fabricate asymmetric ones.

2 Proceedings of the euspen International Conference Zurich - May 28 On the other hand, machine tools that have two horizontal axes for contouring and one vertical axis for the control of the depth of cut, which are called vertical machines, are general to machine asymmetric textures. However the vertical machines have drawbacks in improving the positioning accuracy as well as the frequency response of the vertical axis. Friction-less drive systems with linear motors, which are promising to obtain both the nanometer resolution and the high response [3], also suffer from the cancellation of the gravitational forces on vertical axes. Therefore, devices such as pneumatic cylinders [4] are required, which may be new disturbance recourses. The objective of this research is to develop a three-axes machine tool equipped with a fine motion mechanism with a piezo-electric actuator on the Z (vertical) axis. The synchronization of the fine motion with the XY (horizontal) motions to control the depth of cut can produce micro textures. A PI+feedforward (FF) controller with a second order digital filter was designed to increase the bandwidth of the fine motion. A sweep-sine wave was machined with a diamond bite by the control of the depth of cut for evaluation. 2 Developed machine and closed-loop control of the fine motion system 2.1 Developed machine tool The schematic view of the machine and the fine motion system is shown in Fig.1. This machine has three linear axes. The travels of the X-, the Y- and the Z-axes are 23 mm, 22mm, 2mm, respectively. The maximum feedrate is 1 m/min. A precision ball screw and a servo motor are employed for each drive system. The X- axis employs aerostatic guideways. The Y- and Z-axes employ linear ball guideways. A commercial CNC controller is used to control these coarse drives. The Z-axis is equipped with a fine motion mechanism to control the small depth of cut in the Z direction. The fine motion is provided by the piezo-electric actuator that changes the preload of the support bearings of the ball screw [5]. A PC, independent of the CNC controller, controls the fine motion. For the closed-loop control, a linear encoder with a resolution of 1 nm is used to detect the position of the Z-axis. The fine motion drive has a travel of 5 µm and a positioning repeatability of.1 µm. In order to synchronize the fine motion in the Z direction and the machine motion, the X and the Y position of the table are measured by using linear encoders with a resolution of 1 nm and input to the PC through 32bit counter boards.

3 Proceedings of the euspen International Conference Zurich - May 28 Ball Screw Support bearings Spacer Coarse motion command Piezo-electric actuator Support unit with a fine motion mechanism Tool X-Y table Z Y X Fig.1 Schematic view of the machining system Fine moion command Table position Amp. CNC PC Gain db Gain db Phase deg (a) Without the filter Phase deg (b) With the filter Fig.2 Closed-loop frequency response of the fine motion system 2.2 Closed-loop control of the fine motion system A PI + FF controller was designed for the fine motion. The feedforward gain, the proportional gain and the integral gain were set experimentally. The closed-loop frequency response was measured by using sweep-sine commands to evaluate the fine motion system. Figure 2(a) shows the frequency response from the command voltage to the table displacement. The bandwidth is about 45 Hz. In order to improve the bandwidth, a second order digital filter with a natural frequency of 1 Hz and a damping ratio of.25 was installed in the FF controller. The control gains were also tuned experimentally. Figure 2(b) shows the frequency response from the command voltage to the table displacement with the filter. It can be observed that the bandwidth was improved to 13 Hz.

4 Proceedings of the euspen International Conference Zurich - May 28 When the table position is input to the PC, the command according to the position is output from the table as a reference. In order to drive motions in both the positive and the negative directions, an offset of 2 µm is added to the reference. 3 An application example to machining Shaping with a non-rotational cutting tool was tested to evaluate the system. The depth of cut was controlled by the fine motion mechanism to fabricate sweep-sine waves. The surface profile was measured by a surface raughness measuring machine and compared with the reference signal. A single crystal diamond tool was used, and the workpiece material was aluminum. A pick feed of 8 µm was given toward the positive X direction, and the Y-axis was driven under 3 mm/min toward the positive direction for the machining. The amplitude of the sweep-sine was set as 5 nm, whose frequency varied from.1 Hz to 1 Hz. In the profile measurement, the number of the measurement points was 46 points per 23 mm (.5 µm/point). Figure 3(a) shows the commanded sweep-sine displacement. Figure 3(b) shows the comparison of the profile curves without and with the sweep-sine desplacement. The periodic component with the wavelength of 1 mm can be observed in both profiles in Fig.3(b). This wavelength corresponds to the lead of the ball screw. Comparing the two profile curves, it can be verified that the commanded sweep-sine wave was fabricated on the workpiece. Profile µm Displacement µm Distance mm (a) Commanded displacement 1 With sweep-sine command Without sweep-sine command Distance mm (b) Surface profile of the workpiece Fig.3 Commanded displacement and surface profile of the workpiece

5 Proceedings of the euspen International Conference Zurich - May 28 4 Conclusion A machine tool equipped with a fine motion mechanism on the Z-axis to control the depth of cut was developed and evaluated. A second order digital filter was installed in the FF controller of the fine motion device to improve the bandwidth to 13 Hz under the amplitude of 5 nm. A sweep-sine wave with the frequency of from.1 Hz to 1 Hz was successfully machined with a diamond bite by the control of the depth of cut. Acknowledgement This research is partly supported by JSPS/MEXT Grant-in-Aid for Scientific Research (C) The design and fabrication of the experimental device are supported by NSK Ltd., Mitsubishi Electric Corporation, HEIDENHAIN K.K., and industria Co, Ltd. Authors would like to give special thanks to these supports. References: [1] T.A. Dow, M.H.Miller and P.J.Falter: Application of a fast tool servo for diamond turning of nonrotationally symmetric surfaces, Precision Engineering, 13, 1991, 243 [2] W. Gao T. Araki, S. Kiyono, Y. Okazaki and M. Yamanaka: Precision nanofabrication and evaluation of a large area sinusoidal grid surface for a surface encoder, Precision engineering, 27, 23, 289 [3] H. Shinno and H. Hashizume: High Speed Nanometer Positioning Using a Hybrid Linear Motor, Annals of the CIRP, 5, 21, 243 [4] M.F. Hsieh, C.J. Tung, W.S. Yao, M.C. Wu and Y.S. Liao:Servo design of a vertical axis drive using dual linear motors for high speed electric discharge machining, International Journal of Machine Tools & Manufacture, 47, 27, 546 [5] A. Matsubara, T. Fujita, D. Kono, N, Tanaka and Y. Watanabe: Nano Positioning Drive with Piezoelectric Actuator Integrated into Support Bearing Unit of Ball Screw, Proc. 35th Int. MATADOR Conf, Taipei,Taiwan, 27, 323