## 3 Experimental
### Groove-Machining Experiment  
In order to further study the influences of tools A and B on the groove machining, a set of contrast tests was carried out and the main test parameters are listed in Table 3.  
Figure 7 shows the grooves machined by different tools. For tool A, the material removal in the processing area is non-uniform, and obvious depressions can be observed in the middle of the groove. The reason is that the high flow rate region of the electrolyte is concentrated in the location of the liquid outlet. Even if the tool cathode rotates at different angles, the high-flow-rate area always acts on the center of the tool cathode, so the middle of the groove corrosion is rapid. As such, the processing depth of this region is deeper. Therefore, the adoption of tool B can obtain a relatively smooth machining surface. Based on the flow-field simulation and experimental results, tool B was selected to perform the next ECM testing study. In the cathode ECM process of bar tools, the feed rate has an important effect on the machining depth, machining efficiency, and surface quality. A feeding speed of 10-30 mm/min was selected to carry out the machining test, and the other main test parameters were the same as Table 3.  
At 0.6 MPa electrolyte pressure, the flow rate of the electrolyte is about 5.53 L/min, and the product can be almost conveyed from the machined surface at this flow rate. Figures 8 and 9 show cross-sectional photographs and profiles, respectively, of grooves machined using a variety of feed rates. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Electrolyte & NaCl aqueous solution \\ Concentration (wt\%) & 20\% \\ Electrolyte temperature (\({}^{\circ}\)C) & 30 \\ Spindle speed (rpm) & 500 \\ Processing voltage (V) & 30 \\ Electrolyte pressure (MPa) & 0.6 \\ Initial machining gap (mm) & 0.2 \\ Feed rate (mm/min) & 30 \\ \hline \end{tabular}
\end{table} 
Table 3: Main parameters of groove-machining experiment. Figure 7: Grooves machined by different tool A and tool B. As can be observed from Figure 9, the machined slots gradually decrease with the increase in feed rate. When the feed rate was increased from 10 to 30 mm/min, the maximum groove depth decreased from 1.216 to 0.669 mm. However, a deeper processing depth does not necessarily imply a higher MRR, which is defined as follows:  
\[MRR=\frac{\Delta M}{\rho t}, \tag{3}\]  
where \(\Delta M\) is the quality removed during machining, \(t\) is the total machining time, and \(\rho\) is the density of material, about 2.9g/cm\({}^{3}\). The workpieces were ultrasonically cleaned before and after each experiment, and after drying, they were weighed with an analytical balance (accuracy 0.01 g) to calculate the weight of the quality. Figure 10 shows how the _MRR_ and surface roughness vary with the feed rate. The MRR increases from 94.4 mm\({}^{3}\)/min to 168.6 mm\({}^{3}\)/min as the feed rate increases from 10 mm/min to 30 mm/min. The surface roughness (_Ra_) decreases from 5.787 um to 3.329 um over the same feed rate range. It can be concluded that increasing the feed rate within the 10 mm/min to 30 mm/min range serves to improve the processing efficiency and surface roughness. Figure 8: Cross-sectional photographs of machined grooves under different feed rates. Figure 10: Material removal rate and surface roughness with the feed rate. Figure 9: Cross-sectional profiles of machined grooves under different feed rates. As can be observed from Figures 10 and 11, when the transverse movement was 6 mm, the machining mark was concave. This is because the second processing of the first processing groove for the second corrosion caused excessive material removal and destroyed the bottom surface of the first processing groove. When the transverse movement was 9 mm, the machining mark had a convex shape. Because of the large transverse displacement, part of the anode workpiece material between the two feed paths cannot be completely covered by the tool cathode surface.