## 5 Case Study  
For validation of the developed models and the optimization results above, a test part with intersecting stiffeners was fabricated following the steps in Figure 3, as shown in Figure 7. Note that the width and the total length of the stiffeners are 7 mm and 1200 mm respectively, and the ratio of the masses of the plate to the stiffeners is about 4, which are the same as those in Equation (10). Therefore, the corresponding optimal process parameters are \(W_{\text{FR}}\) = 3.8 m/min, \(T_{\text{S}}\) = 0.6 m/min, \(S_{\text{S}}\) = 6694 rpm and \(T_{\text{FR}}\) = 3.1 mm/s according to Table 4. Furthermore, note that the height of each stiffener is 16 mm, which means that the deposition and the milling processes need to alternate twice here. After this stiffened panel was fabricated, the actual surface roughness in the tool-feed direction was measured at different locations. The average value is 1.38 \(\upmu\)m, which is about 7.0% higher than the predicted value, i.e., 1.29 \(\upmu\)m. The actual material utilization is calculated by dividing the final part's mass (1.82 kg) by the sum of the masses of the plate and the raw metal wire (1.46 kg + 0.55 kg). The result is 91%, which is a little lower than the predicted value, i.e., 97%. This is attributed to the fact that the removed mass from the top surface in Step 2 is neglected in the theoretical evaluation. The actual construction time is about 102 min, which is slightly longer than the predicted value (48.35 \(\times\) 2 = 96.7 min). This is because the time for the milling tool to move from one surface to another is not considered in the theoretical evaluation. In all, these developed models (Equations (6), (8) and (9)) are capable of predicting the surface quality, material utilization, and efficiency of HWMP with reasonable accuracy. As a comparison, the performances of the traditional machining for the same stiffened panel are also evaluated. It starts with a thick plate with dimensions 300 mm \(\times\) 300 mm \(\times\) 22 mm and then goes through two stages: roughing and finishing. The roughing stage aims to remove extra mass as quickly as possible by employing a large material removal rate (MRR), whereas the finishing stage aims to refine the part to the required accuracy by employing a small MRR. To make a fair comparison, the MRR selected for the finishing stage is the same as that in HMWP, i.e., \(MRR_{\text{finishing}}\) = 14.88 mm\({}^{3}\)/s. That is to say, the achieved surface quality here is assumed to be the same as that in HWMP. The MRR for the roughing stage, i.e., \(MRR_{\text{roughing}}\), is assumed to be 10 times that of \(MRR_{\text{finishing}}\), i.e., 148.8 mm\({}^{3}\)/s. The machining time for each stage can be calculated by dividing the volume to be removed by the corresponding MRR. The tool-changing time is not considered here. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \(T_{\text{W}}\) & \(W_{\text{FR}}\) & \(T_{\text{S}}\) & \(S_{\text{S}}\) & \(T_{\text{FR}}\) & \(R_{\text{a}}\) & \(T_{\text{C}}\) \\ (mm) & (m/min) & (m/min) & (rpm) & (mm/s) & (\(\upmu\)m) & (m) \\ \hline
6 & 3.5 & 0.6 & 6694 & 3.1 & 1.31 & 96\% & 48.35 \\
7 & 3.8 & 0.6 & 6694 & 3.1 & 1.29 & 97\% & 48.35 \\
8 & 4.2 & 0.6 & 6695 & 3.1 & 1.29 & 97\% & 48.35 \\
9 & 4.6 & 0.6 & 6695 & 3.1 & 1.29 & 98\% & 48.35 \\
10 & 5.0 & 0.6 & 6695 & 3.1 & 1.29 & 98\% & 48.35 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Optimization results under different target widths. Figure 7: (a) Stereogram; (b) photograph of the produced stiffened panel. A detailed comparison between HWMP and the traditional machining is summarized in Table 5. HMWP improves the material utilization from 34% to 91% (by 57%) and reduces the construction time from 166 min to 102 min (by 32%). Significant advantages of HWMP over the traditional machining for the fabrication of stiffened panels are demonstrated. It is also observed that the cooling time accounts for about one third of the total construction time in HWMP.