# Cutting Force When Machining Hardened Steel and the Surface Roughness Achieved
## 4 Discussion  
The input data for the evaluation of surface roughness in all samples were mainly the measurements by the touch method, where three measurements were made for each sample. This number allows the determination of at least the size of the standard deviation of the measurements for each sample. However, the set of three measurements is too small for the calculation of other statistical variables. The non-contact method was used for four samples to compare the results, and here one measurement was taken for each sample. The experiment conducted was primarily focused on the measurement of the cutting force components on a conventional machine tool. The surface quality data is a secondary output of the experiment and is intended to confirm the feasibility of using a CBN tool. A detailed evaluation of the results on individual samples is as follows. When the cutting speed is set to \(v_{c}=130\) m\(\cdot\)min\({}^{-1}\), there are no significant differences due to changes in the cutting width \(a_{p}\) and feed per revolution \(f\). In general, the resulting surface finish depends mainly on the feed rate \(f\); the lower the feed rate \(f\), the lower the surface finish. The average values of the arithmetic deviation of the profile under consideration, Ra, are between 0.3 and 0.4 \(\upmu\)m. For a cutting speed of \(v_{c}=130\) m\(\cdot\)min\({}^{-1}\), the best combination of cutting conditions appears to be \(a_{p}=0.35\) mm and \(f=0.05\) mm\(\cdot\)rev\({}^{-1}\), with an average Ra value of 0.34 \(\upmu\)m. The best (i.e., the smallest) values of the Ra parameter were obtained for a cutting speed of \(v_{c}=155\) m\(\cdot\)min\({}^{-1}\) in combination with \(a_{p}=0.35\) mm and \(f=0.05\) mm\(\cdot\)rev\({}^{-1}\), where the average Ra value is 0.23 \(\upmu\)m, i.e., sample 6. However, for the same cutting speed \(v_{c}\) and increasing the feed per revolution to \(f=0.1\) mm\(\cdot\)rev\({}^{-1}\) with the same \(a_{p}\) value, the average Ra value was 0.48 \(\upmu\)m, an increase of approximately 209% (for sample 8). These fluctuations may also be due to inaccuracies during the measurement or a crack may appear on the surface of the sample that was measured. These factors can be eliminated by repeating the measurements more often. At a cutting speed of \(v_{c}=180\) m\(\cdot\)min\({}^{-1}\) it can be observed that more stable surface quality values are recorded in connection with the increase of the feed per revolution \(f\) from 0.05 mm to 0.1 mm. The average value of the parameter Ra is at 0.3 \(\upmu\)m. Again, the small number of measurements  
Figure 18: Spatial contrast profile display of the surface: (a) part 5, (b) part 6, (c) part 8, (d) part 12.  
is evident here, specifically for sample 10 (\(f\) = 0.05 mm\(\cdot\)rev\({}^{-1}\) and \(a_{p}\) = 0.35 mm), where the resulting Ra values fluctuate more, as seen in the highest value of the standard deviation. As for the resulting average Ra values, for samples 5, 6, 9, 11 and 12, a value lower than Ra = 0.30 \(\upmu\)m was obtained. These samples have cutting speeds of \(v_{c}\) = 155 m\(\cdot\)min\({}^{-1}\) and \(v_{c}\) = 180 m\(\cdot\)min\({}^{-1}\). In general, based on the results obtained from the contact measurements, it can be stated that out of 12 machined samples with different combinations of cutting conditions, 11 samples were below Ra = 0.4 \(\upmu\)m on average during the measurements. It is difficult to reach this value with conventional finishing tools made of materials other than CNB. It follows that the CNB insert material is able to replace finishing operations such as grinding in certain cases. After performing comparative measurements of selected samples on the Alicona instrument, it can be stated that for samples 5 and 6 almost the same values were obtained as for the touch method. For sample 8, where the touch method measured an average value of Ra = 0.48, the non-contact method measured Ra = 0.328, which also places this sample among the compliant pieces.