Analysis of Measurement Uncertainty of (Simple) Pitch Diameter Calibration when Selected Sizes of Zeiss Wires are Used

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 are complex and demanding tasks. Different technical documents and international standards in screw metrology define calibration categories, explain measurement plans, measuring elements, nominal diameters and their tolerances, and propose guidelines for the measurement uncertainty estimation. 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 that lead to a significant increase in measurement uncertainty. Most calibration laboratories do not measure the thread angle during the pitch diameter calibration. Therefore, the selection of the measurement wires can contribute greatly to the measurement uncertainty. Even though the international standards propose the best size wires to avoid this problem, many manufacturers produce measuring wires according to DIN 2269 (Zeiss). In this paper, the authors do simulations of the contribution of measuring wires to the uncertainty in the calibration of the pitch diameter of metric M threads. A simulation is created for each thread gauge according to ISO 1502, and critical threads, in which a significant increase in measurement uncertainty is expected, are highlighted. The authors propose a simple solution for avoiding this unwanted increase in measurement uncertainty of pitch diameter calibration; the solution involves replacing the DIN 2269 (Zeiss) wires with the best-size wires only for the highlighted critical threads.