Uncertainty and capability analysis performed on a CMM

The instability and the strong competitiveness of modern markets and the need for customer satisfaction have severely imposed reliable control quality systems on companies. The growth of quality standards of the products required by the customers implies to delve into methods of control. In the industrial activities less time is spent for control operations, more time and money are saved. This gain can be obtained only if the control is performed with a reliable measuring system and method. This paper focuses on an actual industrial problem: to obtain reliable measurements and, consequently, to develop reliable considerations when an industrial piece is measured on a Coordinate Measuring Machine. In a word, companies aim to obtain effectively reproducible phenomena when the same dimension is measured. Their real need is the possibility to understand and to manage the uncertainty, reducing the probability that good pieces will be considered out of tolerances and the ones really out will be wrongly accepted. The main objective of this paper is to show the importance of these statements, to study and assess the variability of the measurement system by using a CMM. The measurement uncertainty is a quantitative expression of one's belief in the closeness of a measurement value to the "true value" of the measurand. Many factors, according to the standard prescriptions and the specific environment, have to be included in the uncertainty evaluation. In order to evaluate the metrological and industrial points of view, comparing information and results achieved from uncertainty calculation and industrial considerations, a real example is shown. First, the uncertainty of a CMM is considered, following the international standards and including the main sources of variability. Estimating the total variability must consider the difficulty due to the fact that coordinate measuring systems are multipurpose measuring instruments whose factors vary with the task being performed, the measurement, the environment, the operator and the chosen measurement method. Finally a common industrial piece is measured on the same machine and a capability analysis is performed.