# Measuring Procedures for Evaluating the Surface Roughness of Machined Parts
## 3 Experiments
#### 3.2.1 Effect of Cut-Off Filter Setting for Non-Periodic Surface Roughness Profiles  
The goal is to identify the effect of setting the cut-off filter _Lc_ on _Ra_ values for individual types of reference samples processed by different technologies. The standard (EN ISO 4288 and ISO 21920-3) [34; 39] recommends setting the cut-off parameter _Lc_ or section length _lsc_ and evaluation length _lm_ according to the _Ra_ value. Evaluation length is recommended as five times the section length _lsc_. Here the aforementioned problem arises concerning how to set the L-filter or section length and evaluation length according to an unknown _Ra_ value. This problem does not make sense and creates a challenge with the measurement procedure defined in this way because it is the choice of the basic length that has a significant impact on the result of the _Ra_ evaluation process. This paradoxical recommendation can significantly influence the result of the surface roughness evaluation. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Arithmetical Mean Height** & **Section Length of Profile** & **Evaluation Length** \\
**of the Roughness Profile _Ra_ (mm)** & _lsc_ (\(\mu\)m)** & **of Profile _lm_ (mm)** \\ \hline \(Ra<0.02\) & 0.08 & 0.4 \\ \(0.02<Ra\leq 0.1\) & 0.25 & 1.25 \\ \(0.1<Ra\leq 2\) & 0.8 & 4 \\ \(2<Ra\leq 10\) & 2.5 & 12.5 \\ \(10<Ra\leq 80\) & 8 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Roughness length section for measurement and curves and corresponding parameters for non-periodic profiles (EN ISO 4288, ISO 21920-3) [34; 39]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Cut-Off \(\lambda c\) (mm)** & **Sensor Feed Speed (mm/s)** \\ \hline
0.08 & 0.25; 0.5 \\
0.25 & 0.25; 0.5 \\
0.8 & 0.25; 0.5 \\
2.5 & 0.25; 0.5; 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Recommended measuring speed values for _Lc_ (ISO 11562; ISO 3274) [35; 40]. Some standards for the evaluation of surface roughness quantities differ in these recommendations, stating different values of the parameter \(Lc\) for the values of \(Ra\). These differences are shown in the following Figure 10. The JIS B0601-1982 [50] standard, for example, recommends using a cut-off filter with a value of \(Lc\) = 0.8 mm for values of \(Ra\) < 12.5 \(\upmu\)m. For all larger \(Ra\) values, it is recommended to use \(Lc\) = 2.5 mm. Other standards for evaluation use more finely graded cut-off filters, which complicate the situation when setting the cut-off \(Lc\) filter during measurement  
Measurements were carried out for each setting of the cut-off filter \(Lc\) and for each sample at ten randomly selected locations in the central region of this sample. The average values were shown in graphs (Appendix B), also showing the value of \(Ra\) for each reference sample. To assess suitability, it is necessary to observe the deviation of the average value from the \(Ra\) value of the reference sample. We were looking for a setting of the cut-off filter \(Lc\) for which the average value has the smallest deviation with respect to the reference nominal value \(Ra\). For experimentally assessed non-periodic surface roughness profiles (lapping, peripheral grinding, and face grinding), the optimal setting of the cut-off filter \(Lc\) was observed for some reference samples, which is recommended in the ISO standard [33; 34; 35; 36; 37; 38; 39; 40]. However, for some samples, other recommended settings of the cut-off filter \(Lc\) (for samples with larger \(Ra\) values) were obtained. For the mentioned non-periodic samples with values up to \(Ra=0.1\)\(\upmu\)m, it is appropriate to use \(Lc=0.25\) mm. Then, for values from \(Ra=0.1\)\(\upmu\)m to \(Ra=0.4\)\(\upmu\)m, it is appropriate to use \(Lc=0.8\) mm. From a value of \(Ra=0.4\)\(\upmu\)m, it is advisable to use \(Lc=2.5\) mm. It is shown more vividly in the following Figure 12, while for comparison, the recommended settings of the cut-off \(Lc\) filter according to the ISO standard [33; 34; 35; 36; 37; 38; 39; 40] are also shown.