This open access paper shows how easily misunderstood uncertainty of product verification can be. This comes from our ground breaking work with Rolls-Royce. We found that the uncertainty of the instrument was only a very small part of the uncertainty of measurement. Before we optimized the process most of the uncertainty came from tooling. After improved tooling was designed the dominant source was thermal expansion of the engine. So we couldn’t improve it any more without temperature control in the factory. The lessons of this study are far reaching. Most importantly, the uncertainty of measurements on a product may be many times higher than the individual coordinate measurements that an instrument records. Tooling, datum structures and thermal expansion may be far more significant than the instrument used.
Paper Title: Uncertainty of measurement for large product verification: evaluation of large aero gas turbine engine datums
Authors: J E Muelaner1, Z Wang1, P S Keogh1, J Brownell2 and D Fisher2
1) University of Bath 2) Rolls-Royce plc
Published in: Measurement Science and Technology, 2016, Issue 27(11): p. 1-12
Abstract: Understanding the uncertainty of dimensional measurements for large products such as aircraft, spacecraft and wind turbines is fundamental to improving efficiency in these products. Much work has been done to ascertain the uncertainty associated with the main types of instruments used, based on laser tracking and photogrammetry, and the propagation of this uncertainty through networked measurements. Unfortunately this is not sufficient to understand the combined uncertainty of industrial measurements, which include secondary tooling and datum structures used to locate the coordinate frame. This paper presents for the first time a complete evaluation of the uncertainty of large scale industrial measurement processes. Generic analysis and design rules are proven through uncertainty evaluation and optimization for the measurement of a large aero gas turbine engine. This shows how the instrument uncertainty can be considered to be negligible. Before optimization the dominant source of uncertainty was the tooling design, after optimization the dominant source was thermal expansion of the engine; meaning that no further improvement can be made without measurement in a temperature controlled environment. These results will have a significant impact on the ability of aircraft and wind turbines to improve efficiency and therefore reduce carbon emissions, as well as the improved reliability of these products.