Quality Magazine - March 2009 - (Page 39) of both the standard and extended measuring uncertainty of the process. First, even before the series production begins, the operator must procure a suitable measuring system. This way its suitability for measuring characteristics key to functionality can be ensured, and quality management standards’ requirements can be met and the risk of false test decisions reduced. The measuring system manufacturer must prove the suitability of the test equipment for the actual measurements to the user. A measuring uncertainty budget offers security for customer commitments and provides notes on minimizing measurement deviations. That is why this process is important to optical gaging. Figure 1 Pinion shafts from the pilot series as evidence of the test equipment and test process suitability. Source: HommelEtamic America TWO WAYS TO SUITABILITY If, for example, the shaft measuring system is used to measure the bearing diameter D of a gear shaft (tolerance Figure 2 Test Equipment Designation: Manufacturer: ID No. Resolution: Optical shaft measuring system ABC Co. Device No. 10100608 0.1 µm field location js7), the standard uncertainty, Udm, of the measuring system is established first from its maximum permissible deviation limits MPE= (2 + D/100) µm, with D in millimeters as seen in Table 1. In a test, the proof was obtained using three hard turned and three ground pinion shafts. Assuming that the rectangular distribution measurement deviations (coefficient b=0.58) of the measuring system are within these “device error limits,” the results for Udm (MPE) shown in Table 1 are obtained. Key measuring conditions are the monitoring of the calibration state using built-in, fine-tuned standards (calibration disks) as references. It also is essential to be able to automatically correct measurement deviations due to fluctuations in environmental temperature and in the workpieces from the reference temperature of 20 C. The metrological properties of the measuring system can quickly become clear. The combined standard uncertainty (Udm) of the test equipment, based on the directly active uncertainty characteristics, is determined through repeat measurements on calibrated step standards. The following are included in the measuring uncertainty budget for the test optical gage for shaft diameter 38.12 js7: • The calibration uncertainty with which the step standard was calibrated. In this case, Ucal = 0.3 µm + 0.8 x 10-6 x D, with D in millimeters. It is specified as the extended measuring uncertainty in the calibration Accepted Measuring Variable Shaft Diameter 38.12 js7 Min. limit Value Max. limit value Tolerance 38.108 38.132 24 µm Test Object Gear Shaft Drawing no. 56-0897-xx Standard/ID number Step-Master 8-140 L211 Standard cert. no. 3D-0511-115 Measuring Conditions Measurement of the workpieces in accordance with requirement calibration of the measuring system including built-in calibrated Step-Master (standard) Automatic compensation of the measurement deviations due to deviations in the temperature of the environment and of the workpieces from the reference temperature of 20 C. Workpieces are free of contamination Standard uncertainty u(ux ) in nm i Limit value a in nm Distribution model Number of measurements n Device error limits Calibration uncertainty Measured value resolution Repetition Systematic deviation Combined standard uncertainty of the test equipment Characteristic tolerance ✓ ✓ ✓ ✓ B Correction factor k: B A A/B 50 50 2 2381 RECTANGLE Calibration uncertainty ukai[nm] 330 0.41 165.0 20.5 59.1 27225 420.3 3493.9 8296 u(x )² in nm i Source uncertainty components u(xi) 50 Triangle 276 Standard 0.33 91.1 . T = 24 μm Tolerance resolution Correction yes no . Request suitability of test process general preset freely selected Actual value for the ratio . Result The test process is suitable! . . Minimal testable tolerance in nm . A measuring uncertainty budget is used to certify the suitability of the test equipment. Source: Hommel-Etamic America www.qualitymag.com March 2009 | QUALITY Part of u(x )2 to i u(y)² 69% 1% 9% 21% Transformation coefficient b Determination method c weighting xi 39 http://www.qualitymag.com
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