## 2 Materials and Method
### Experimental Procedure  
The experiments performed on the alumina (Al2O3) substrate were divided into two main tests, (i) tool overlapping test and (ii) tool path strategies test. In the tool overlapping test, the effect of several tool overlapping percentages (0%, 5%, 10%, 15%, 20% and 25%) on the surface quality and profile accuracy of the milled micro-pockets were examined. The tool overlapping was calculated as a percentage of the RUM tool diameter (2 mm). For example, the 10% overlap was equal to 0.2 mm  
\begin{table}
\begin{tabular}{c c} \hline
**Property** & **Value (Unit)** \\ \hline Purity & 99.9 \\ Grain size & \textless{}0.1 (\(\upmu\)m) \\ Bulk density & 3.96 (g/cm\({}^{3}\)) \\ Poisson’s ratio & 0.23 \\ Vickers hardness (HV 0.5) & 2000 \\ Tensile strength & 650 (MPa) \\ Compressive strength & 4000 (MPa) \\ Fracture toughness & 4(MPa-m1/2) \\ Melting point & 2040 (\({}^{\circ}\)C) \\ \hline \end{tabular}
\end{table}
Table 3: Mechanical and thermal properties of Al2O3.  
Figure 2: (**a**) Measuring the workpiece flatness by using dial gauge and (**b**) RUM tool used for pocketing.  
((\(10\%\times 2\) mm)/100). Figure 3 shows the schematic diagram of the tool overlaps used in this test. It should be noted that surface roughness in terms of Ra and Rt, surface morphology and pockets profiles were the three outputs considered during the overlapping test because all the three outputs play a vital role in the micro-pockets functioning [32,33].  
Based on the results of the overlapping test, the experiments of the tool strategies test were carried out by fixing the tool overlapping at 20%, which results in the best surface roughness and morphology as will be explained in the results and discussion section. In this test, the effects of tool path strategies on the surface quality, tool wear and material removal rate (MRR) of the milled pockets were analyzed. The tool path strategies considered in this study are illustrated in Figure 4. As mentioned earlier, the pocket's depth = 0.3 mm can be achieved by removing two passes of 0.15 mm each (depth of cut = 0.15 mm). In Uni-X and Uni-Y strategies, the tool always moves in one direction (\(X\)-axis or \(Y\)-axis) from point a to point b for the first pass and repeat the same path movement for the second pass. Zigzag strategy means the tool moves in bi-direction (both X and \(Y\)-axes). In cross strategy, the tool moves in the \(X\)-axis direction for the first pass (from point a to point b) and then moves in the \(Y\)-axis direction for the second pass (from a1 to b1). In the Mix2 strategy, the first pass of the tool follows the zigzag path (from a to b) while the second pass follows a uni-x strategy (from a1 to b1). The tool movement in the Mix3 strategy follows zigzag X for the first pass (from a to b) and Zigzag Y for the second pass (from a1 to b1). Machine strategy is the strategy built in the RUM Siemens controller for milling the pocket feature. The tool paths of the remaining strategies (Spiral and Mix-1) can be observed by following the direction of the arrow in schematic diagrams in Figure 4. The range of the machining conditions (see Table 4) was selected based on trial experiments and guidance taken from previous studies reported in the machining of alumina materials by using RUM [14,15,17].  
Figure 3: Schematic of tool overlapping (0–25%).