## 2 Materials and Methods
### Feasibility Analysis  
The oil film-assisted ECDM technology proposed in this paper has both gas film and liquid film in the process. In order to verify whether the spark discharge between the electrode wire and the electrolyte can breakdown the oil film, the experimental device shown in Figure 3 is designed by cleverly taking advantage of the fact that the density of oil is less than that of water.  
Figure 2: Principle of spark discharge generation by oil film-assisted ECDM during diamond wire sawing: (**a**) The oil film on the electrode wire surface is complete, (**b**) The oil film on the electrode wire surface is defective.  
The electrolyte is placed in a transparent glass beaker, the auxiliary electrode is placed at the bottom of the beaker, and the electrode wire is installed on the fixture of the vertical feeding mechanism. A small voltage is applied between the electrode wire and the auxiliary electrode, while the feed mechanism drives the electrode wire straight down. In this process, the current in the circuit is measured in real time. When the electrode wire does not drop to the electrolyte level in the beaker, the current in the circuit is zero, and when the electrode wire contacts the electrolyte level, the current is generated in the circuit. Thereafter, the feed mechanism drives the electrode wire gradually up at a slow speed until the current in the circuit is just zero, at which time the electrode wire is just free from the electrolyte. Based on this position, the electrode wire continues to move upward to form a gap between the electrode wire and the electrolyte surface, and the distance of the electrode wire moving upward is equal to the size of the gap. Slowly drop the oil into the electrolyte level to ensure that the oil fills the gap between the electrode wire and the electrolyte. At this time, the thickness of the oil between the electrode wire and the electrolyte is equivalent to the thickness of the oil film covering the surface of the electrode wire, and the oil film thickness is equal to the gap distance between the electrode wire and the electrolyte. The voltage between the electrode wire and the auxiliary electrode is gradually increased, the voltage and current signals are collected, and the volt-ampere characteristic curve is established to determine the breakdown voltage threshold.  
Figure 4a shows the experimental device, which is carried out on a high-speed EDM perforator. The electrode wire is connected to the negative electrode of the power supply, and the auxiliary electrode is connected to the positive electrode of the power supply. Figure 4b shows the spark discharge between the end face of the electrode wire and the electrolyte after the voltage is applied. The experiment can prove that the spark discharge between the electrode wire and the electrolyte can indeed breakdown the oil film.  
To sum up, the technology of diamond wire sawing combined with oil film-assisted ECDM proposed in this paper is practical and feasible, which can solve the problem of poor stability or even impossibility of traditional EDM, improve processing efficiency and processing quality, and thus solve the processing problems of hard and brittle insulating materials. At the same time, this method significantly reduces the energy consumption for maintaining the electrochemical reaction to generate hydrogen. It has obvious advantages in energy conservation.  
Figure 3: Schematic diagram of breakdown voltage measurement principle of complete oil film covered by electrode wire.