Which cut-off should I use?

In general, you select a roughness cut-off in order to assess the characteristics of the surface you require. These are usually the process marks or witness marks produced by the machining process. To produce a good statistical analysis of these process marks, you would normally select a cut-off in the order of 10 times the wavelengths under consideration. These wavelengths may be the turning marks on the component. Warning; cut-offs should be determined by the nature of the component and not by the length of the component. Choosing the wrong cut-off will in some cases severely effect the outcome of the result.