## 3 Results  
Major deviations of the hole's center line were noted, leading to concentricity errors between the inlet and outlet edges of the drilled hole. These errors could largely be attributed to the high porosity of the test material, which does not provide sufficient resistance. This allows the tool to follow the path of least resistance, which causes drill bending during the machining process. Such errors could cause significant problems during the assembly process (for example in sandwich panels). To calculate both x and y-axis outlet centers, the foam was face milled on two sides to provide a flat plane from which measurements were taken using a caliper. The measured deviations were then input into a model created in Solidworks 2024 to determine the angular deviation of the hole in two axes. Observing the top-down view in Figure 4, concentricity errors can be easily noted. \begin{table}
\begin{tabular}{c c c c c} \hline \multicolumn{2}{c}{**Controllable Factor**} & **Level 1** & **Level 2** & **Level 3** \\ \hline Cutting speed, \(v_{c}\) & A & 40 & 70 & 100 \\
[m/min] & A & 40 & 70 & 100 \\ \hline Feed rate, \(f_{n}\) & & & & \\
[mm/rev] & B & 0.05 & 0.125 & 0.2 \\ \hline  & & 90\% & 60\% & 30\% \\ \cline{2-5} Valve position [/] & C & & & \\ \hline Outlet temperature [\({}^{\circ}\)C] & \(-2.1\) & \(-23.2\) & \(-27\) \\ \hline Flow rate [L/min] & 382.5 & 255 & 127.5 \\ \hline \end{tabular}
\end{table} 
Table 2: Taguchi L9 controllable factors and levels for the preliminary experiments. Figure 4: Top-down view of the deviated holes. A high amount of built-up edge formation was noted after just one pass of the tool. A digital microscope was used to measure the length and height of the built-up edge. The measurement example is shown in Figure 5.  
Table 4 shows the measured thrust force, deviation of the outlet hole and built-up edge dimensions after each test run. The Taguchi method uses a statistical analysis measure called the S/N ratio, where \"S\" stands for signal, which would be a desirable value of the output parameter, while \"N\" stands for noise, which would indicate an undesirable value of the output parameter. Using the S/N analysis, three types of parameters can be observed: \"largest is best\", \"smallest is best\" and \"nominal is best\". Since the goal of these experiments was to determine an experimental parameter set that would lead to the lowest thrust forces, built-up edge formation and deviation of the hole, the \"smallest is best\" S/N ratio was selected and calculated using the following formula:  
\[\text{S/N}=-10\text{log}\left(\frac{1}{\text{n}_{\text{r}}}\sum_{i=1}^{\text {n}}y_{i}^{2}\right) \tag{1}\]  
where: i--experiment number, yi--measured value of output variable, nr--replicates number [49]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Test No.**} & \(\begin{array}{c}\text{\emph{v}}_{\text{c}}\\ \text{\emph{[m/min]}}\end{array}\) & \(\begin{array}{c}\text{\emph{f}}_{\text{n}}\\ \text{\emph{[mm/rev]}}\end{array}\) & \multirow{2}{*}{_ce [_I_]} & \multirow{2}{*}{_Ft [N]_} & \multirow{2}{*}{_a [\({}^{\circ}\)]} & \(\begin{array}{c}\text{\emph{L}}_{\text{BUE}}\\ \text{\emph{[mm]}}\end{array}\) & \(\begin{array}{c}\text{\emph{H}}_{\text{BUE}}\\ \text{\emph{[mm]}}\end{array}\) \\ \hline
1 & 40 & 0.05 & CCA & 3.3 & 2.46 & 0.38 & 0.19 \\ \hline
2 & 40 & 0.125 & MQL & 9.0 & 3.30 & 0.36 & 0.33 \\ \hline
3 & 40 & 0.2 & CF & 9.4 & 2.99 & 0.20 & 0.21 \\ \hline
4 & 70 & 0.05 & MQL & 4.6 & 1.15 & 0.23 & 0.29 \\ \hline
5 & 70 & 0.125 & CF & 10.4 & 2.99 & 0.19 & 0.28 \\ \hline
6 & 70 & 0.2 & CCA & 14.3 & 4.02 & 0.65 & 0.62 \\ \hline
7 & 100 & 0.05 & CF & 7.4 & 1.91 & 0.43 & 0.33 \\ \hline
8 & 100 & 0.125 & CCA & 11.9 & 2.92 & 0.48 & 0.25 \\ \hline
9 & 100 & 0.2 & MQL & 13.4 & 2.69 & 0.30 & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measurement results obtained using [PERSON]’s L9 orthogonal array. Figure 5: Built-up edge dimensions for Test no. 1 (40 m/min, 0.05 mm/rev, CCA): (a) built-up edge height, (b) built-up edge length.