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The calibration of screw-thread gauges is a conventional procedure in calibration laboratories and National Metrology Institutes. Proper measurement and reliable uncertainty analysis of the screw thread gauge calibration is a complex and demanding task. Different technical documents and international standards in screw metrology define calibration categories, explain measurement plans, measuring elements, nominal diameters and their tolerances, propose guidelines for the measurement uncertainty estimation, etc. Therefore, the first part of this paper provides a detailed overview of different technical documents and international standards in screw-thread metrology. The second part of this paper aims to draw attention to critical points of the pitch diameter calibration procedure, which leads to a significant increase in measurement uncertainty. Most calibration laboratories do not measure thread angle during pitch diameter calibration. Therefore, the selection of the measurement wires can become the main contribution to the measurement uncertainty. Even though the international standards propose the best-size wire diameter to avoid this problem, many manufacturers produce measuring wires according to the DIN 2269 (Zeiss). In this paper, a simulation of the uncertainty contribution of measuring wires to the pitch diameter calibration of metric M threads is made. Simulation is created for each thread gauge according to the ISO 1502. critical threads are highlighted where a significant increase in the measurement uncertainty is expected. The authors gave a simple solution to avoid this unwanted increase in measurement uncertainty for pitch diameter calibration by replacing the DIN 2269 (Zeiss) wires with best-size wires just for the critical threads. |
