The Uncertainty of Roughness Parameters

The process capability is an important indicator for stable and reliable manufacturing processes. Hence, the measurement uncertainty and the unambiguous calculation of geometrical parameters play an important role in quality control. A major deficiency pertains to the evaluation of the uncertainty of surface roughness parameters. Already simple roughness parameters have a large statistical variance depending on the manufacturing process and not on the measuring system. For industrial companies it is essential to find rules to express the surface roughness uncertainty, particularly for geometrical tolerancing. By describing a surface parameter as a statistical variable, it will be shown that the uncer-tainty can be expressed more precisely than in usual practice. As an example, the rule of thumb “six times Ra is approximately Rz ” is derived from a statistical model of the surface. Key words: uncertainty, roughness parameters, quality control, tolerancing