# 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
Plaring technology does not agree with these recommendations, and the reasons for this are probably found in the essence of the technology itself. It is important to note that reference samples were not available for ranges with small \(Ra\) values, which is related to the nature of these technologies, which are used to achieve larger \(Ra\) values, and thus it was not possible to assess the recommendations of the standards for smaller \(Ra\) values. Interestingly, however, the experimental optimum settings for the periodic profiles are in agreement with the recommendations of the previous ISO 4288 standard. With the exception of planing technology, however, we can state that the recommendations of the ISO 21920-3 standard can be applied to periodic and non-periodic profiles of the surface of components (Figure 20), and therefore the investigation of planing technology will be the subject of further research. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Setting Class Sc** & **Sc1** & **Sc2** & **Sc3** & **Sc4** & **Sc5** \\ \hline Parameter \(Ra\) (\(\mu\)m) & \(Ra\leq 0.012\) & \(0.012<Ra\leq 0.006\) & \(0.06<Ra\leq 1.2\) & \(1.2<Ra\leq 6\) & \(Ra>6\) \\ \hline Profile L-Filter nesting index & & & & & \\ _Nic_ (cut-of \(Ac\) (\(Lc\)) for \(R\)-parameters) (mm) & 0.08 & 0.25 & 0.8 & 2.5 & 8 \\ \hline Evaluation length \(lm\) (mm) & 0.4 & 1.25 & 4 & 12.5 & 40 \\ \hline Profile S-filter nesting index & & & & & \\ _Nis_ (cut-of \(\lambda\)) (mm) & 0.8 & 0.8 & 2.5 & 8 & 25 \\ \hline Maximum sampling interval & & & & & \\ \(\Delta t\) (\(\mu\)m) & 0.15 & 0.15 & 0.5 & 1.5 & 5 \\ \hline Maximum tip radius & & & & & \\ \(r_{sp}\) (\(\mu\)m) & 2 & 2 & 2 & 5 & 10 \\ \hline Section length \(l\)c (mm) & 0.08 & 0.25 & 0.8 & 2.5 & 8 \\ \hline Number of sections nsc & 5 & 5 & 5 & 5 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Setting classes—recommended settings for cases of missing specification (ISO 21920-3) [39]. Figure 17: Comparison of the surface roughness profile of the part and the reference sample with a nominal value of \(Ra\) = 1.6 \(\mu\)m.  
Figure 18: Comparison of the surface roughness profile of the part and the reference sample with a nominal value of \(Ra\) = 0.8 \(\mu\)m.  
A comparison of the validity of using \(RSm\) to select the cut-off filter \(Lc\) according to ISO 4288 and using \(Ra\) according to ISO 21920 can be realized from the evaluation of the previous graphs (Figure 20). According to this evaluation, it can be concluded that better compliance results were achieved using the \(RSm\) quantity according to ISO 4288. At the same time, however, it can be seen that there were not enough samples available for a reliable evaluation, mainly with low \(Ra\) values, which follows from the nature of these technologies, as they are designed to achieve surfaces with larger \(Ra\) values. We will further verify these results in future research. Identification of the Minimum Number of Measurements for Assessing the Surface Roughness of the Part  
To define the problem, it is necessary to determine the minimum number of measurements needed to assess the surface roughness. It is known from common metrological practice that one measurement is insufficient and, especially for quantities with a high degree of variability, it is necessary to carry out repeated measurements and an estimate of the measured quantity together with an expression of the uncertainty of the determination of this estimate of the measured quantity. To solve this problem, a method of successive evaluation of the standard deviation of repeated measurements and evaluation of the quantity \(Ra\) is proposed. After each additional measurement is added, the standard deviation will be recalculated with the addition of a new value to the set of measured data. We called it the cumulative standard deviation, which will capture the effect of the number of performed measurements on the value of the standard deviation of the quantity \(Ra\).