**3. Previous Tests on Ti6Al4V Alloy**

Preliminary tests on Ti6Al4V alloy were carried out. This alloy was used firstly because is more economic, easier to buy and has better machinability than the superalloys, such as Inconel 718. Titanium plates were used (200 × 100 × 7.5 mm dimensions); the main aim of these tests was to establish a first approach to the process before carrying it out on Inconel 718. On the other hand, titanium alloys are used not only in engines but in airframe key parts, which join the wings to the airplane body.

Prior to conducting the brushing tests, 80 holes of 10.7 mm diameter were drilled in the plates. The following conditions were used for the drilling: Vc = 35 m/min and f = 0.12 mm/rev. Figure 2 shows the experimental set-up.

**Figure 2.** (**a**) Drilling set-up. (**b**) Brushing set-up.

As shown in Figure 1 and Table 1, five different flexible abrasive tools were used. With each different brush, 16 holes were made at these brushing conditions: Vc = 60 m/min and f = 0.5 mm/rev. Figure 3 shows the results of the roughness measurement, both for the drilled holes and the brushed holes. Firstly, the surface quality obtained in the previous drilling with the conditions used was quite good, averaging around 0.5 μm Ra. The main problem was the results dispersion, with varying Ra roughness parameters from 0.29 to 1.17 μm.

After brushing with the five different flexible abrasive tools, the roughness parameters decreased and the results dispersion was lower. The best brush type for this material in terms of surface roughness was the SC11-400, as it reduced the average Ra roughness up to 0.25 μm, with values between 0.2 μm and 0.3 μm (Figure 3).

**Figure 3.** Ra [μm] and Rz [μm] roughness values after drilling and after finishing Ti6Al4V.

#### **4. Test on Inconel 718 Superalloy**

Based on data from the preliminary tests carried out on Ti6Al4V, we found that that the five different flexible abrasive brushes were able to reduce the roughness parameters. Moreover, it was an easy and economical finishing process that can be carried out using machine tools.

On the other hand, the preliminary tests showed a low cutting capacity. It was difficult to make a chamfer on a hole or deal with large burrs because removing that much material was impossible. However, these brushes could be useful in order to finish surfaces, round edges or carry out cross-hole deburring [9]. For these reasons, experimental tests were carried out on Inconel 718 plates, which is a commonly-used material in aerospace components working at high temperatures. This is a difficult material to machine, so the soft cut of these brushes may have been insufficient.

In this case, Inconel 718 plates were used with the dimensions 200 × 100 × 7.5 mm, similar to those used in the preliminary tests in titanium. The tests were carried out in an Ibarmia ZV25 milling machine, with a spindle with 25 KWs. Regarding the previous drilling, the Table 2 shows the two different conditions used.


**Table 2.** Conditions used for drilling in Inconel 718.
