## 3 Results and Analysis
### Tool Path Strategies Results  
To further improve the roughness values obtained in the overlapping test, different tool path strategies have been tested. Table 5 presents the experimental results of the significance of tool path strategies on the surface roughness and MRR of the milled pockets. Again, the machining conditions (see Table 4) were kept constant in this test. The three outputs (Ra, Rt and MRR) of the pockets milled at ten tool path strategies can be also shown in the form of bar charts in Figure 10. Again, in this test, the values of Ra and Rt have been taken across the tool feed for all the machined pockets. Figure 11 presents the 2D profiles of the surface roughness for six pockets machined by using six different tool path strategies. The selected profiles in Figure 11 were arranged to start from a smooth pocket surface machined with the cross path strategy to the final with rough surface machined under the Zigzag-Y strategy. Figure 12 shows the microscopic images of the machined pockets. It can be seen that pockets machined at the zigzag milling strategy have a rough surface (higher values of Ra and Rt) comparing to the pockets milled at either Uni-X or Uni-Y strategies in which smooth surfaces can be obtained (see Figure 12a,b). This is because in the zigzag milling strategy two types of milling modes take place; (i) up milling in which the tool rotates at the opposite direction of the feed and (ii) down milling in  
Figure 9: Pockets surface morphology machined at **(a)** 0% tool overlapping, **(b)** 10% tool overlapping and (c) 20% tool overlapping. which the tool rotates along with the feed direction (see Figure 13). The alternate change of cutting mode, which is along with the spindle direction and then versus it, resulted in a deviation in cutting speed and accordingly produces a rougher surface. This observation also matches with the results documented in [32]. Due to the aforementioned reason, the uniform surface quality can be observed at the spiral and cross path strategies in which the cutting speed was kept uniform and results in a more homogeneous surface by using only one direction cutting. In contrast, a rough surface with un-machined regions (high value of Rt = 4.31 \(\upmu\)m) was observed on the surface of the pockets machined at Mix-1 strategy, which was a mix between the spiral and zigzag strategy (see Figure 12c). The second pass of the Mix-2 strategy was carried out in one direction (see Figure 4), this results in a smooth surface comparing to rough surface obtained at Mix-3 and Machine strategies in which the cutting was carried out at different modes. Although the smooth surface of the milled pockets can be produced at Uni-X and Uni- Y strategies, they have the lowest MRR. This is because of more time consumed in dry runs (the tool always moves up and goes to the beginning of the line, see Figure 4). Moreover, the un-machined regions (see Figure 12a) were excluded from the volume of the machined materials. The maximum MRR (0.49 mm\({}^{3}\)/min) can be achieved at the spiral and Mix-1 strategies. It can be found that the values of Ra and Rt in both cross strategy (0.162 \(\upmu\)m and 1.643 \(\upmu\)m) and Mix-2 strategy (0.155 and 1.432 \(\upmu\)m) were close to each other. This is because both strategies had the same finishing path (second pass). Among all tool path strategies, the minimum value of Ra and Rt can be achieved at the Mix-2 strategy, in which the zigzag path was applied to the first pass, while the second pass was carried out by using Uni-X strategy. Figure 14a,b show samples of the surface profile and the surface morphology of the pockets machined at the Mix-2 strategy. Figure 10: (**a**) Effects of tool path strategies on Ra and Rt and (**b**) effects of tool path strategies on MRR. Figure 11: 2D surface roughness profiles of the machined pockets.