## 3 Results and Discussion
### Surface Quality  
Surface quality has been evaluated through the arithmetic mean roughness parameter (Ra) because it is the parameter used by the aerospace sector, generally with Ra below than 1.6 \(\upmu\)m [36].  
The value used for the analysis is the average of two measurements taken in each hole. Figure 8 shows the values obtained in the three repetitions of each combination of parameters. It shows a visual differentiation between the up-milling and down-milling tests.  
In spite of the observed wear, the repeatability of the tests is high, with the largest deviation being found in test 5 with a standard deviation value of 0.30 \(\upmu\)m.  
The values obtained are within the ranges observed in other investigations, for example [14] found roughness between 0.4 and 0.8 \(\upmu\)m, relating this increase with the tool wear and reaching a maximum of 0.96 \(\upmu\)m when tool failure. Some research corroborates that the results in helical milling are better than those obtained in conventional drilling obtaining an average of 0.61 \(\upmu\)m in [11], and values between 0.2 and 0.7 in [22; 32], nevertheless [PERSON] in [15] found better results in conventional drilling. All these values are in line with the values obtained in this experiment.  
The result obtained have been statistically processed in Minitab(r) 18.1 (Minitab, LLC, Pennsylvania State, PA, USA) software. The influence of each parameter or combination of them on Ra is reflected in the Pareto Chart of the Standardized Effects graph (Figure 9), where it is verified that the direction of rotation (Down or Up milling) is the parameter with the greatest influence, followed by the combination of two factors, and to a lesser extent the tangential feed rate per tooth.  
Figure 8: Graph of roughness results.  
Figure 9: Pareto chart of the standardized effects for Ra.  
Figure 10 shows the main effects of the parameters on roughness, where it is observed that the down-milling strategy provides lower roughness values than those obtained with up-milling, and to a lesser extent an increase in the tangential feed rate per tooth (f\({}_{\text{z4}}\)) improves surface quality.  
The trends are not exactly the same as the results obtained in [32], where they coincide in the trend in the tangential feed rate but not in the axial feed rate. In both, an increase in parameters leads to an increase in roughness. On the other hand, an influence of cutting speed in the case of dry machining was observed associated with the adhesion caused by high temperature; nevertheless in the present study this parameter presents the least influence on roughness.  
This effect was already known in peripheral milling, and in helical milling it is shown to have a very similar behaviour. Therefore, the fact that better qualities appear in down milling may be due to the same reasons, among which are that no burnishing or reworking of the chips appears.  
Table 10 shows the results of the P value and F value for the test parameters. Observing the data shows that both the strategy, tangential advance and the combination of two parameters are below 0.05 (95% confidence level), with the direction of rotation being the most evident next to a higher f-value (indicating its significance). The strategy is the most relevant factors in the variation of Ra, obtaining a \(p\)-value = 0.000, lower than the level of significance (\(\alpha\) = 0.05). In addition, \(f\)-value is greater than the rest of the parameters (\(f\) = 68.15), confirming once again the importance of the direction of rotation in Ra during the helical milling process.