Vol.2 No.2 2009

19/98

Research paper : High accuracy three-dimensional shape measurements for supporting manufacturing industries (S. Osawa et al.)−99−Synthesiology - English edition Vol.2 No.2 (2009) 3.2.2 Ball plateIn Europe, a ball plate and a hole plate (see Fig. 7) that allow two-dimensional assessments are used more often for assessment and calibration of CMM than end measures that are one-dimensional standards. The central coordinates of a sphere or a cylinder placed in the standards are determined, and the CMM assessment is conducted using these coordinates. Compared to end measures, it is possible to obtain more information and know precise errors within the measurement rage. To make the two-dimensional standards available in Japan, AIST constructed a calibration system for two-dimensional standards[5].Here, the calibration system of a ball plate will be outlined. A ball plate cannot be calibrated using laser distance meters due to its round shape. Therefore, calibration is done using a CMM, but it is impossible to calibrate beyond the accuracy of the CMM when the results of the CMM measurement are used directly. Therefore, central coordinates of each sphere are measured using a method called the inverse method to reduce the geometrical error of the CMM. In measurements using the inverse method, the error of the scale itself (normally, primary tilt factor) remains, and this is corrected using a standard that is traceable to the length standard to calculate the final scale error. Normally, gaugeblocks with differing lengths are used for this correction, but a laser distance meter was used at AIST. Therefore, extremely precise correction data was obtained. By using this system, uncertainty 0.37 µm (95 % confidence interval) was achieved when calibrating the ball plate with measurement length 500 mm. Figure 8 shows the calibration of a ball plate using a laser interferometer. Currently, there are only five laboratories including AIST in the world that use advanced laser interferometer technology for ball plate calibration.AIST also developed a new standard that can be traced to length standards to enable high-precision calibration of a ball plate at laboratories that do not possess an interferometer[6]. This standard is called the ball step gauge, and is in the form of one-dimensionally arranged balls. The sphere which will be measured is positioned at the neutral axis in the cross-sectional secondary moment of the H-shape main plate, and it is designed to keep the changes in relative positions to extremely small levels even when positional changes occur due to distortion by the sphere’s own weight or heat distortion in vertical and horizontal directions. The distance between the spheres is calibrated at AIST using the laser distance meter, and then supplied to the user. To check the efficacy of this standard, and as part of the activities of the Council of Promotion of Industrial Technology Collaboration, comparative measurements of a ball plate were conducted by circulating this standard among regional public research laboratories. As a result, almost all labs agreed at a deviation within 0.5 µm against the AIST calibration value. It was confirmed that this standard was effective for comparative measurement, and that the technological level of the regional public laboratories were sufficiently high[5].3.3 International comparison of standards3.3.1 International comparison of step gaugesThe international comparison (CCL-K5) for calibration of step gauges was conducted from 1999 to 2002, with the participation of major national metrology institutes. The pilot laboratory prepared a step gauge with excellent stability, this gauge was transported to the metrology institutes of various countries to conduct blind measurements, and the results were reported to the pilot laboratory. The nine national Fig. 6 Step gauge calibration system.Fig. 7 Two-dimensional standard.Fig. 8 Ball plate calibration.InterferometerLaser headStep gaugeProbing systemEnlargedHigh-precision inner cylinderHigh-precision sphereEnlarged(b)Hole plate(a)Ball plate

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