*4.2. "B" Conditions*

In this section, the previous drilling conditions were Vc = 25 m/min and f = 0.06 mm/rev. In this way, the roughness results obtained after drilling were worse than in the previous cases. However, brushing could be useful in this case. Figure 5 shows the roughness results. The roughness values observed before brushing were around Ra 0.9 μm, with a large dispersion of results. After brushing, the roughness parameters decreased to values lower than 0.65 μm Ra. In this case, the BC11 brush provided the lowest roughness values and the lowest deviation of the results, thus this was the most convenient brush. Besides, the tool wear was not critical in this case.

**Figure 5.** Ra [μm] and Rz [μm] roughness values after drilling and after finishing Inconel 718.

It is noted that the results showed that the cutting ability of these brushes was limited, especially when cutting materials with low machinability such as Inconel 718. Therefore, a large amount of deburring and chamfering of holes was impossible. However, once the hole was chamfered, rounding

the edges and finishing the surface was achieved using the flexible abrasive brushes [11]. Figure 6 shows one of the drilled and brushed holes. The process involved drilling, chamfering and brushing. Figure 6 and Table 3 set out the rounding edge produced by the brushes.

**Figure 6.** (**a**) Hole section and detail of the chamfer and rounding edge. Image 1 (5×), Image 2 (10×), Image 3 (10×). (**b**) Angle detail measured with optical means of the rounding edge of a hole section (5×).

**Table 3.** Angle detail—hole section of the rounding edge.


For years, the edge finishing process has been by hand in many areas, but now the tendency is to try to automate these finishing processes [12]. One automation possibility uses flexible abrasive brushes. However, others are possible, such as using shape tools. In this case, the main drawback is the correct tool positioning and also the tangents to the surface. In addition, it is necessary to consider the fact that many of these holes are placed in curved areas or in areas that are difficult to access for a conventional milling tool. The main problem with the brushes is the lack of repeatability and the rapid wear suffered.

Figure 7 shows some photographs of the brushes following their use. As mentioned above, despite achieving the best results, the SC11-400 brush is one of the most expensive, along with the diamond brush. Besides, tool wear on these brushes is greater than the other brushes. To conclude, regarding tool wear, BC11 is the most appropriate option for materials such as Inconel 718. Moreover, in some cases it is also the best option in terms of the surface quality achieved.

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**Figure 7.** Tool wear on three different brushes. New tool; after Inconel 718; after Ti6Al4V (3×).
