# Measuring Procedures for Evaluating the Surface Roughness of Machined Parts
## 3 Experiments
#### 3.2.3 No Discrimination Periodic and Non-Periodic Profiles for Procedure of \(Ra\) Estimation  
It was complicated for users working with unknown surfaces to identify whether it was a periodic profile or a non-periodic one. In some cases, even with the same evaluated surface, it could happen that a profile that was periodically changed to a non-periodic  
Figure 14: Recommendation for the selection of parameters of the cut-off filter \(Lc\) for evaluating the surface roughness of samples according to the ISO 4288:1996 standard. Figure 15: Obtained recommendations for the choice of cut-off filter \(Lc\) and recommended according to ISO 4288/1996. one. This made the strategy of measuring and evaluating surface roughness quantities difficult. Subjective assessment of component surfaces can thus cause errors caused by the operator himself. ISO 21920-1, ISO 21920-2, and ISO 21920-3 standards [37; 38; 39] eliminate these problems. The ISO 21920-3 standard specifies the non-discrimination procedure for periodic and non-periodic profiles for the roughness quantities estimation procedure. This edition of ISO 21920 cancels and replaces the edition of ISO 4287:1997 [33] and ISO 4288:1996 [40], which have been technically revised. ISO 21920-3 introduces no distinction between periodic and non-periodic profiles. The ISO 21920-3 standard introduces a unified strategy for setting measurement parameters using setting classes--Sc1, Sc2, Sc3, Sc4, and Sc5. In cases of missing part surface specifications, one of the options for determining the surface roughness of the examined part is to compare the surface roughness profile with the surface roughness profile of the standard sample whose nominal \(Ra\) value is known. For this purpose, the Comparex monocular comparison microscope (Figure 16) can be used. It is a device that enables the determination of surface roughness on different types of surfaces obtained by all available technologies. The eyepiece is divided into two halves, and both surfaces of the surface roughness are displayed in it, namely the surface of the part and the surface of the reference sample, so it is possible to optically compare both surface profiles. According to these displayed profiles, it is possible to make a fairly reliable estimate of the \(Ra\) of the examined sample. By successively optically comparing the surfaces of the reference samples with the examined surface of the component, it is possible to determine the desired value of \(Ra\) for the surface of the component (Figure 17). In Figure 17 is a comparison with the standard sample \(Ra\) = 1.6 \(\upmu\)m, and from the image of the eyepiece, it can be seen that the standard has a greater surface fragmentation than the examined surface of the part. Then the surface of the component is compared with the reference sample \(Ra\) = 0.8 \(\upmu\)m, and a match is visible (Figure 18), based on which we can estimate the value of \(Ra\) and then subsequently set the cut-off parameter \(Lc\) for the subsequent measurement and evaluation of \(Ra\) using stylus-based surface roughness tester. The ISO 21920-3 standard [39] also provides recommendations for the selection of Setting class settings according to the estimate of the unknown quantity of surface roughness \(Ra\) (Table 10). The graph (Figure 19) shows a comparison of the recommendations of both ISO 4287 [33] and ISO 21920-3 [39] standards and the results of experiments for non-periodic part surface profiles. It results in only slight differences, which can be considered measurement errors and uncertainties. Figure 16: Comparex comparison microscope for determining surface roughness. For periodic profiles, preferred values resulting from experiments and recommended values according to the ISO 4288 standard and according to the ISO 21920-3 standard are shown. Except for planing technology, all values for periodic profiles are in accordance with the recommendations of the ISO 21920-3 standard.