# Optimization of Machining Parameters for Corner Accuracy Improvement for WEDM Processing
## 2 Methodology
### Experiment Condition and Equipment  
The wire bending can result in overcutting during corner cutting [14]. Therefore, adjustment of wire tension (WT) can effectively reduce the corner error during machining a corner shape. During corner cutting, the wire direction is tuned, and meanwhile the discharge time (pulse-on time) at the corner is increased due to the slowdown of the cutting speed. This increase in discharge time will result in overcutting. To avoid the interference of wire bending, a workpiece thickness of 10 mm was used in this study. Furthermore, the strategy to reduce the wire deflection was to modify the cutting parameters such as pulse-on time, pulse-off time, and peak current [2; 14]. The power control could increase accuracy at corners during rough cutting, and decrease the processing time [8; 9]. To understand the trend and influence of each parameter on corner error during the WEDM corner cutting process, the parameters of open-circuit voltage (OV), pulse-on time (ON), pulse-off time (OFF), servo voltage (SV), flushing pressure (WA), and wire tension (WT) were selected for experiments based on the literature and the preliminary investigation. According to the preliminary experiment investigation, the larger deviation of uncut or overcut happened on smaller corner angles. Thus, three kinds of corner degrees (30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\)) were selected for the experiments as shown in Figure 1. Table 1 shows the experiment designed with three levels of parameter settings and actual values. The machining operation was designed to comprise rough cutting followed by a trim cutting. The WEDM machine model AL-400 SA with 0.1 \(\upmu\)m resolution of linear motor manufactured by Accutex Co. (Taichung City, Taiwan) was used. SKD11 (alloy tool steel) was used as the workpiece material, and a brass wire electrode with diameter of 0.25 mm was used in this experiment.  
Figure 1: Corner error measurement (**a**) 30\({}^{\circ}\), (**b**) 60\({}^{\circ}\), (**c**) 90\({}^{\circ}\).  
Figure 2 shows the flow chart of the experiment. After selecting the process parameters, they were brought into the Minitab software to design the experiment based on the RSM method. The parameter range was designed according to the standard parameter values suggested by Accutex Co. The suggested standard parameter values were used as middle values, and the smaller values were obtained by reducing the standard parameter values two points. On the other hand, the higher values were obtained by adding two points to the standard parameter values. For instance, the suggested standard parameter value of pulse-on time (ON) was 10, then this value was determined as the middle value. Based on that, the low value was 8 (=\(10-2\)), and high value was 12 (=\(10+2\)), as shown in Table 1. After machining, the corner error and machining time were measured. To accurately measure the corner error, an optical inspection method was chosen. The optical inspection measurement instrument (model of VM-2515) with resolution of 0.001 mm made by Power Assist Instrument Scientific Co. was used. The corner error was defined as the distance from the corner vertex to the machined workpiece plane (X) as shown in Figure 1.