# Development of an On-Line Defect Detection System for EDM Process
## 1 Introduction  
The electrical discharge machining process applies pulse voltage to perform spark discharge, and uses the consumption that occurs at this time to process. The electrode and the workpiece are not in direct contact, and between the gap voltage is intervened by the electrical discharge fluid. So, the electrical discharge machining is applied for the material, which usually was difficult to machine by traditional cutting with higher hardness and precision. In terms of processing characteristics, [PERSON] et al. [1] pointed out three typical indicators of material removal rate, electrode consumption rate, and surface roughness. The three indicators were affected by peak current, peak voltage, discharge cycle time, polarity, discharge gap, etc. The workpiece in this study is a diamond grinding wheel, which has become a substitute for traditional cutting tools and is widely used in manufacturing, especially for processing alloys and glass. Based on its high hardness, good impact resistance, wear resistance, and excellent heat resistance and thermalconductivity [2], these characteristics mentioned above are suitable for electrical discharge machining. The surface material of the diamond grinding wheel is polycrystalline diamond (PCD). PCD is made of synthetic diamond and then sintered under high temperature and high pressure. [PERSON] et al. [3] proposed that PCD crystals were arranged disorderly, isotropic, and without a cleavage plane. There are two main processing methods for refurbishing PCD. One is cutting and grinding, but it is difficult to achieve tapered edges and complex shapes, and this method has other problems such as the potential for serious wear. The other is EDM, which is simple to process conductive material parts and has low loss. EDM can be used effectively for the processing of complex shapes in PCD. The principle of EDM processing PCD is shown in Figure 1. As the controller controls the feed rate, the gap (L\({}_{\text{g}}\)) between the electrode and the workpiece changes accordingly. The electrode (positive electrode) and the surface of the workpiece (negative electrode) are rich in a large number of cations and anions, which are, respectively, affected by the electric field. As the intensity of the electric field increases, electrons are conducted to the surface of the workpiece to form a loop. [PERSON] et al. [4] proposed that when EDM processed PCD, there was a graphene film on top of the PCD, and the discharge point was not restricted by the metal binder or limited to conductive materials (which was the case of conventional EDM). The discharge point could work on any surfaces of the piece theoretically. Therefore, this increased the occurrence of effective discharge pulses and contributes to an increase in material removal rate. According to [PERSON] et al. [5], when a discharge spark occurred, the electric field strength between the electrodes reaches the strength of the dielectric breakdown. Avalanche ionization and collision dielectric would then occur, and the PCD particles would melt and evaporate. In actual production, high spots (Bumps) are often generated due to uneven distribution of PCD. The phenomenon that causes abnormal processing is called high spots and foreign bodies. This phenomenon causes the gap (L\({}_{\text{g}}\)) to be unable being reduced, and the feedback is given to the controller. The position is not increased, and therefore the machine idles. During the processing, if the PCD arrangement is irregular, the large-scale distribution of diamond grit is uneven, as shown in Figure 2. Obvious abnormalities will occur. Not only will the processing produce large friction and impact and the machine vibrate significantly, but also due to the inability to monitor the machine at any time it takes more time to repair and measure when the problem is found. Therefore, the accuracy of the finished product often fails to meet the requirements. Figure 1: Principle of PCD in EDM processing [4]. In order to detect the processing process, [PERSON] et al.