## 2 Materials and Methods  
### Oil Film-Assisted ECDM  
ECDM wire cutting is a kind of cutting method for hard and brittle insulating materials which has been increasingly studied. The processing principle is shown in Figure 1. Generating a complete hydrogen film on the electrode wire surface is one of the necessary conditions for ECDM discharge [28]. However, due to the combined effect of the buoyancy of the electrolyte and the motion of the electrode wire, the hydrogen generated by the electrochemical reaction easily leaves the surface of the electrode wire, which brings difficulties to the formation of the hydrogen film. With the increase of cutting thickness, the length of electrode wire required to generate discharge increases correspondingly, and the length of electrode wire required to be covered by hydrogen film also increases. Hydrogen film formation is, therefore, more difficult, and even if it can be formed, it is extremely vulnerable to damage, resulting in difficult or even impossible discharge formation. In addition, the need to avoid deterioration of the hydrogen film makes it difficult to use effective means (such as forced flushing) to improve the electrolyte circulation in the processing area. Therefore, the discharge stability of ECDM machining is poor, the processing efficiency is low, and only two-dimensional cutting processing of hard and brittle insulating materials with small thickness (about 1.0 mm) can be achieved at present. Therefore, hydrogen film formation is difficult and it is easily destroyed, which is an urgent problem to be solved in ECDM wire cutting technology. It is difficult to solve the problem that hydrogen film formation is difficult and it is easily destroyed in ECDM. In this paper, an oil coating device is used to coat a uniform oil film on the electrode wire in advance to replace the hydrogen film. Thus, the electrical insulation conditions between the electrode wire and the electrolyte can be met, and then discharge can be generated to remove the workpiece material. This is the basic principle of  
Figure 1: Schematic diagram of electrical discharge wire cutting of insulating materials. the new technology of oil film-assisted ECDM machining proposed in this paper. As shown in Figure 2a, the electrode wire is first coated with oil by the oiling device in the air, and a complete oil film is formed on the surface. Then, the electrode wire enters the electrolyte. Due to the existence of the oil film on the surface, the electrode wire and the electrolyte are in a state of electrical insulation. No electrochemical reaction occurs, and the current in the circuit is zero. At this time, the main function of the electrolyte is to conduct the potential of the anode (auxiliary electrode) to the vicinity of the oil film, thus forming a potential difference on both sides of the oil film, the size of which is equal to the output voltage of the pulse power supply. When the voltage exceeds the critical breakdown voltage of the oil film, a spark discharge is formed between the electrode wire and the electrolyte. The potential difference between the electrode wire and the electrolyte is suddenly reduced, the discharge current is generated in the circuit, and the workpiece material is removed under the high temperature condition of discharge. As shown in Figure 2b, after discharge the oil film at the discharge position will be damaged by high temperature and have a slight defect. When the electrode wire contacts with the electrolyte, an electrochemical reaction will occur to produce hydrogen. Therefore, after discharge, the insulating film on the surface of the electrode wire is an oil-hydrogen two-phase dielectric film with oil film as the main layer and hydrogen film as the auxiliary layer. The two-phase dielectric film is in a state of \"breakdown discharge--damage--electrochemical reaction repair--breakdown discharge....\".