# Measuring Procedures for Evaluating the Surface Roughness of Machined Parts
## Appendix A
### The Surface Roughness Standards  
The basis for measuring surface roughness parameters and characteristics is the unit of length, defined in EN ISO standards and implemented by a set of micro-length etalons(line depth and line spacing), whose conventionally true value of line depth is measured by wavelength (on the device--interference microscopes), these etalons are for the area of surface roughness measurement--Primary Surface Roughness Standards.  
Secondary Surface Roughness Standards (Figure A8) are sets of standards that embody the values of roughness parameters--_Ra_, _Rz_, _Ry_, and others. They are used to calibrate working gauges using the direct measurement method, where a measured standard of the same nominal value is directly calibrated using standard gauges.  
The average distance of profile elements _RSm_ can be determined as the average value of individual widths _Xsi_ of profile elements' surface roughness in the range of section length (Figure A9) according to the relationship:  
\[R_{sm}=\frac{1}{n_{pe}}\cdot\sum_{i=1}^{n_{pe}}X_{si}\] (A2)  
where \(n_{pe}\) is the total number of profile elements.  
In the experimental part of this paper, we distinguish the so-called periodic and non-periodic surface roughness profiles.  
**Non-periodic surface roughness profiles** arise mainly during finishing technologies such as grinding, polishing, and lapping, and with these methods, the _Ra_ value reaches a maximum of 0.8 \(\upmu\)m. Grinding technology is implemented in such a way that the material is removed by geometrically undefinable and randomly arranged cutting wedges on the grains of the abrasive material of the grinding disc, which separate the particles of the surface layers of the workpieces with abrasive grains stored in the binder, on the supporting tool or freely. Lapping is a finishing machining method that is used precisely to refine the surface. In principle, this technology belongs to grinding technologies. Lapping is the sanding of the surface of the product with a fine grinding abrasive material that is freely applied in a lapping liquid (e.g., oil, kerosene) or paste (e.g., paraffin, stearin) with dispersed fine abrasive that is applied to the lapping surface. During lapping, the peaks of unevenness are cut off with abrasive grains. Abrasive grain and fine abrasive can be understood as a cutting knife that has a random geometry and orientation, so the surfaces that arise after these types of machining have a non-periodic and randomly oriented character.  
**Periodic profiles of surface roughness** arise during technological operations such as turning, milling, planing, which reach \(Ra\) values of more than 0.8 \(\upmu\)m. With periodic profiles, there is a periodic movement of the tool along the surface of the component and the gradual removal of material, so the waviness of such a surface is more pronounced than with non-periodic profiles. \(Ra\) values greater than 25 \(\upmu\)m are typical for the surfaces of parts that arise during casting and forging technologies.  
When assessing the roughness of the surface, it is also necessary to take into account the direction of the unevenness of the surface, which is related to the machining method used. The unevenness of the profile can be parallel, perpendicular, oblique, or crossed with the contour line of the component. During some machining operations, circular irregularities or irregularities directed from the center of the component or unevenness are random in nature.  
The ISO 21920-3 [39] standard introduces no distinction between periodic and non-periodic profiles compared to the edition of ISO 4288:1996 [34]. Nevertheless, the paper is focused on the analysis of the choice of cut-off filter for individual technologies, which were used to produce individual samples.