# Measuring Procedures for Evaluating the Surface Roughness of Machined Parts
## 4 Discussion
The actual measurement of the surface structure usually takes only a few seconds and is determined by the feed speed and the scanned length, and other settings, but the measurement preparation time is considerably longer, on which the success of the measurement depends. Before the measurement and evaluation process, it is necessary to set several input parameters for the measurement, which can have a very significant impact on the actual measurement and evaluation of individual surface roughness quantities. The effort was to solve this problem, and thus the influence of individual settings of the surface tester on the process of measurement and evaluation was investigated; however, due to the difficulty of this issue, this article focuses mainly on the quantity \(Ra\), which is evaluated by the used surface tester. The effect of the selection of the evaluation standard, the effect of the speed of the stylus movement, and the effect of the cut-off filter value on the resulting \(Ra\) value were monitored for different types of machined parts surfaces, where standard samples with a known \(Ra\) value were used. To investigate the influence of the stylus speed and the choice of standard, experimental measurements were carried out on a standard sample of surface roughness for grinding with nominal values of \(Ra=1.6\)\(\upmu\)m and 3.2 \(\upmu\)m, where for each combination of the selected speed of the sensor and the evaluation standard, ten measurements were carried out on it in a certain place to exclude the variability of the standard sample. The influence of the choice of the evaluation standard and the choice of the sensor movement speed can have a significant impact on the resulting \(Ra\) value, and it can be concluded that for some standards, this influence is negligible, but for some standards, it can be seen that increasing the sensor movement speed tends to reduce the resulting \(Ra\) value and this influence it is mainly visible in samples with a larger \(Ra\) value. A good choice seems to be to use a speed of movement of the sensor with a value of 0.5 mm/s, and the evaluation standard can be selected according to the country where the evaluation is carried out. For our conditions, it was preferred in the experiments and then also the evaluation according to the ISO 1997 standard. To assess the impact of the choice of the evaluation standard and the choice of speed of movement of the sensor, the variability of the measured and evaluated \(Ra\) data obtained under the same conditions is also an interesting indicator. The variability of the data, depending on the choice of the evaluation standard and the speed of the sensor movement, is expressed as a percentage with respect to the average evaluated \(Ra\) values, and from its evaluation, it follows that the variability of the \(Ra\) data is a maximum of 18% in percentage terms. Another monitored and problematic setting is the setting of the filter cut-off value \(Lc\), and this can be seen from the conducted experiments, where it is clear that if, for example, we choose a too-small \(Lc\) value for the sample, then the result is \(Ra\) with an almost zero size. However, if we choose a value of \(Lc\) that is too large, then the value of \(Ra\) may also include the influence of waviness, and this may distort the value of \(Ra\), so the errors that we make with a bad free cut-off may be huge and may have a fatal impact on the result of evaluating the roughness of the surface being assessed parts. The ISO 3274:1996 [35] standard contains guidelines on how to set the cut-off value of the \(Lc\) filter for non-periodic surface roughness profiles, while the EN ISO 4288 [34] standard recommends setting the cut-off parameter \(Lc\) or of the basic length \(lr\) and the evaluated length \(\ln\) according to the \(Ra\) value, while the evaluated length \(ln\) is recommended as five times the basic length \(lr\).