Development of nanometer-order ultra-precision machine for realizing six-degree-of-freedom perfect relative motion between workpiece and tool

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To realize ultra precise machining or measurement, a machine system generating accurately relative motion between its tool and the workpiece is required as well as the accuracy improvements of each element of the machine. This research proposes a machine system using a measurement device for six-degree-of-freedom (6-DOF) motion errors between the tool and the workpiece. A hexapod-type parallel kinematics mechanism installed between the tool spindle and the surface plate is passively actuated by a conventional machine. Then, the hexapod measures the 6-DOF motions regardless with temperature fluctuation and external forces because the mechanism has a compensation device with Super-Invar rods for elastic and thermal errors of the joints and the links. Therefore, the measured 6-DOF errors compensate the tool position and orientation. First, an extensible connecting chain (strut) consisting of spherical joints and a prismatic joint with a linear scale unit has been designed and experimental … More ly manufactured. The spherical joint has been improved to eliminate thermal deformation of its ball shanks. Moreover, a compensation device with Super-Invar rods has been installed in the strut, and tested for investigating effects of the gravity and temperature change. As a result, the gravity and the temperature change have little effect on length measurement accuracy of the strut. Second, a conventional Cartesian-coordinate-geometry mechanism has been. The mechanism employs three ball-screw-driven linear positioning stages equipped with AC servomotors and rotary encoders. Positioning errors and motion errors between the surface plate and the tool spindle have been measured by using a laser interferometer length measurement system. Finally, a calibration method with a redundant passive connecting chain has been investigated for identifying 37 kinematics parameters for the hexapod mechanism. Length changes measured by the scale unit and changes calculated by the kinematics derive seven length errors of strut when the hexapod mechanism moves in its workspace. The least square method using Jacobian matrix corrects the 37 parameters and repeated so that the length errors are minimized. As a result, obtained parameters decreased positioning errors of the mechanism. Less