Table of Contents

EDM is not limited by the strength and hardness of the workpiece material and has the advantage of no macro-cutting force. However, during the EDM process, the cost of manufacturing electrodes is high and the electrodes are easy to wear. At the same time, using kerosene as the working fluid also has safety hazards and pollution problems. With the advancement of machine tools and tool technology, significant progress has been made in directly processing difficult-to-handle materials using cutting methods, especially the development of high-speed milling processing technology, which has gradually replaced traditional EDM.

Electrical discharge machining diagram

In the 1990s, researchers proposed a three-dimensional CNC EDM milling method. This technology uses simple rod-shaped electrodes to scan layer by layer, which effectively reduces processing costs and improves flexibility, but it still brings the problem of environmental pollution. Professor Masanori Kunieda of Tokyo University of Agriculture and Technology in Japan proposed dry electric discharge milling technology, using gas as the dielectric, successfully replacing the traditional kerosene working fluid. The researchers found that under the right conditions, material removal rates from gases can match or exceed those of kerosene working fluids.

Experimenters of in-gas EDM milling have shown that EDM performed in oxygen has higher material removal rates than kerosene operating fluids. In addition, in-air machining can achieve higher dimensional accuracy while the electrode is rotating. Since the molten workpiece material will splash onto the electrode during processing in gas, it can compensate for the loss of the electrode and even cause negative loss. The study also found that the electric corrosion pits formed during gas machining are larger and shallower, the white layer on the surface of the workpiece is thinner, and the surface quality is improved.

Progress of in-gas electric discharge machining

Masanori Kunieda believes that in-gas EDM has two important advantages: minimal reaction force and minimal surface damage to the workpiece, making it particularly suitable for micromachining. He also researched wire electrical discharge cutting in gas (DRY-WEDM). In addition, Zhang Qin and his team from the Institute of Manufacturing Automation of Shandong University proposed a composite processing method that combines dry electric discharge machining with ultrasonic vibration of the workpiece.

Exploration of powder mixing working fluid and bubble technology

After studying the processing characteristics of the powder mixing working fluid, the Special Processing Research Institute of Harbin Institute of Technology attempted to mix gas into kerosene for processing. The results show that adding gas to the working fluid can increase processing efficiency and improve surface quality to a certain extent. However, it is very difficult to mix uniform fine bubbles into kerosene. Bubbles are easy to diffuse and aggregate in kerosene, especially on the electrode surface, affecting the discharge effect. Additionally, this approach still carries the risk of environmental contamination.

To solve these problems, researchers have tried to use new working fluids, such as water-based working fluids. Comparative experiments show that the material removal rate of pure water working fluid is generally lower than that of kerosene working fluid containing organic compounds. However, if organic substances such as polyethylene glycol or glycerin are added to the water-based working fluid, the material removal rate can be improved. Nonetheless, the use of water-based fluids increases machine tool costs, and the disposal costs of water-based fluids are comparable to those of kerosene. At the same time, water-based non-combustible working fluid and equipment operation costs are also higher than that of ordinary kerosene working fluid systems, so it is difficult to promote on a large scale.

EDM milling in oxygen

Overall, EDM milling in oxygen excels in several aspects. It can not only achieve high single-pulse erosion but also has the advantages of low electrode loss rate and good surface quality of the workpiece. This processing method is in line with the development trend of green manufacturing and has great potential.

However, the machining gap of machining in gas is smaller (usually smaller than that of machining in liquid), so the short circuit rate is significantly higher than that of machining in liquid. When the pulse width is less than 60 μs, the short-circuit rate of machining in gas exceeds 85%, while that of machining in liquid is only about 10%. Although the short-circuit rate can be reduced by rotating or shaking the electrode, even so, the short-circuit rate of machining in gas is still much higher than that in liquid, which also limits the further improvement of its machining efficiency.

In addition, the cooling effect of gas on the electrode and workpiece is not as good as that of liquid, so it is difficult to use a higher peak current to improve processing efficiency. During the discharge process, the molten metal produced cannot be cooled and solidified in time, and part of it will adhere to the surface of the workpiece, affecting the processing quality.

Although dry electric discharge machining technology has been around for nearly 8 years, due to the above problems, no practical products have officially entered the market. This is mainly because a high-precision tool electrode feeding servo system and power supply system are required to control the short-circuit rate, which greatly increases equipment costs and reduces market competitiveness. Therefore, although this technology holds great promise, it currently requires further research and improvement.

Proposal of spray EDM milling method

In EDM, how to improve efficiency and quality while reducing manufacturing costs and dependence on working fluid is an important issue currently faced. To do this, we need new research to take full advantage of the advantages of EDM and reduce the impact on the environment. Based on the summary and analysis of previous research results, we proposed a new “mist-jetting-ED-milling” method. The processing principle is shown in Figure 1 (1—workpiece; 2—pulse power supply; 3—automatic feed adjustment device; 4—tool; 5—working fluid; 6—filter; 7—working fluid pump). This method uses high-pressure mist formed by mixing water and gas as the discharge medium to achieve the purpose of integrating the advantages of dry EDM milling, liquid EDM and CNC EDM, and overcoming their respective inherent shortcomings.

Its core elements can be described as:
(1) Use high-pressure water mist as the working medium.
(2) Use simple-shaped hollow tubular electrodes.
(3) Adopt layered scanning CNC EDM milling processing strategy.

Figure 1 Principle diagram of spray EDM

Mechanism analysis of spray EDM

Since the discharge medium that constitutes one of the three major elements of EDM (discharge medium, electrode material pair, and pulse power supply) has undergone important changes, it will inevitably have an important impact on the core of EDM – the mechanism of the discharge process. This is It is necessary to re-understand the characteristics of fog electric discharge machining based on the original understanding of the electric discharge machining mechanism. The various processes and characteristics of EDM in fog medium are analyzed below.

Three major elements that constitute EDM

1. Breakdown voltage when forming discharge channel

 

So far, the electrical machining circles inside and outside China have not yet reached a unified conclusion on the dielectric breakdown and discharge channel formation process during EDM. Generally speaking, research on the mechanism of spark discharge is based on dielectric physics and can be divided into research on the discharge mechanism in pure dielectric media (gas and liquid) and research on the discharge mechanism when the liquid contains additives. Regardless of powder mixing, gas mixing, or EDM milling, they all belong to discharge when containing additives.

Distortion occurs in the electric field, thus changing the breakdown field strength. In most EDM, the gas phase is the continuous phase, and the liquid phase is the dispersed phase. After the droplets are mixed into the gas, the shape of the electric field is changed. Assuming a uniform electric field between the two poles, Gauss’s law can be used to estimate that the maximum electric field intensity on the droplet surface is about 3 (the dielectric coefficients of air and water are 1 and 80 respectively).

It can be seen that during spark discharge in the fog, the breakdown voltage drops to about 1/3 of the breakdown voltage of pure gas. In this way, the discharge gap in the fog is much larger than the discharge gap in the gas, which also means that larger machining processes can be used. Gap to obtain a more stable discharge and reduce the incidence of short circuits. It can be seen that the breakdown voltage of spark discharge in fog is reduced to about 1/3 of the breakdown voltage of pure gas, so the discharge gap in fog is much larger than the discharge gap in gas, which also means that a larger processing gap can be used to achieve Obtain more stable discharge and reduce the incidence of short circuit.

2. Diffusion of discharge channels and size of electric corrosion pits

When performing EDM in fog, since the working medium is a two-phase fluid formed by mixing gas and liquid, the plasma channel during discharge is not subject to the inertial compression of the liquid medium like discharge in liquid. The resistance to the expansion of the discharge channel comes only from the compression effect of the magnetic field generated by the current, so at the beginning of the discharge, the channel cross-section expands much faster than in liquid machining.

(1) Better surface quality: The cross-section of the discharge channel in the mist is larger than that of discharge in liquid, so large and shallow electric corrosion pits are generated during processing, which helps to obtain better surface quality. The processed surface has a thinner white layer and lower surface roughness, which is particularly beneficial in milling.

(2) Lower throwing force: Since there is no channel compression effect of the liquid, the force generated during discharge is smaller, so a smaller processing gap can be used than for liquid discharge, or greater discharge energy can be selected to improve efficiency.

3. Deionization in the discharge channel

During discharge machining in liquid, at the beginning of the pulse discharge, the inertia of the liquid prompts the plasma channel to continue to expand, and the discharge channel will remain in the bubble state for some time. To ensure complete deionization, sufficient pulse interval time is required. During discharge machining in fog, there is no liquid inertia to maintain the discharge channel, and the liquid droplets in the fog vaporize and absorb heat when heated, which is conducive to rapid warming of the plasma and rapid deionization. In addition, the vaporization process will cause plasma fluctuations, further shortening the deionization process. Time, which is conducive to the use of higher pulse frequencies.

4. Erosion of electrode material

During EDM milling in the fog, the droplets are heated by the plasma and vaporized, and the volume will increase by more than 1,000 times. This process produces an explosion-like effect. The strong impact force directly acts on the surface of the tool electrode, thus promoting the throwing of metal in the molten pool. Out. In addition, the vaporization of droplets will cause plasma fluctuations, which will strengthen the magnetic fluid oscillation, thereby promoting the discharge of metal in the electrode molten pool.

Since the high-pressure mist is ejected from the tool electrode, it is beneficial to the cooling of the metal electrode and can reduce the erosion of the tool electrode during discharge, thereby hopefully reducing the relative loss of the tool electrode.

Feasibility test

 

To verify the feasibility of spray EDM milling, the existing TROOP-PNC EDM forming machine was modified and a feasibility verification test was carried out.

Internal spray EDM requires a hollow electrode to spray mist as the processing medium. Therefore, the machine tool has been modified as follows:
(1) Change the clamped-shaped electrode to a hollow replaceable tubular copper electrode.
(2) Use siphon nozzles to generate high-pressure mist.
(3) Close the working fluid system.
(4) Adopt a processing method similar to layer milling,
Figure 2 is a physical photo of the modified machine tool spray device and electrodes.

Figure 2 Actual photos of the spray device and electrodes

In this test, the compressed air pressure used was between 0.4~0.8MPa. The specially made siphon nozzle can make the droplet diameter not larger than 20μm to ensure that no discharge in the liquid is caused by the droplet diameter being too large in the processing gap.

 

The tool electrode uses a copper tube with an outer diameter of 9 mm, an inner diameter of 6 mm, a length of 10 mm, and a wall thickness of 1.5 mm. If the electrode wall thickness is too thin, it will affect the processing efficiency, while if it is too thick, the mist medium will not be evenly distributed on the electrode surface.
The test parameters are shown in Table 1.

Table 1 Spray EDM milling test parameters and results

The SEM photo of the processed surface is shown in Figure 3.

Figure 3 SEM photo of the workpiece surface after processing

The waveform during processing is shown in Figure 4.

The test results show that the spray EDM milling method is feasible. The following phenomena can be initially observed during the test:

(1) The surface roughness value of spray processing is lower than that of processing in oil under the same electrical parameters.
(2) The diameter of the electric corrosion pit on the surface of the workpiece is larger than the diameter of the electric corrosion pit on the surface processed in oil.
(3) When ordinary EDM machine tools are used, the circuit and other parts are not modified, and the electrodes are not rotated, machining in fog can still achieve a low short-circuit rate.
(4) The efficiency of this mist processing test is lower than that of oil processing.

Conclusion

 

Through the above analysis, fog EDM reduces the dependence on working fluid compared to liquid EDM in milling processing, which makes it more in line with the trend of green manufacturing. Simply put, if we deeply study the principles and characteristics of fog EDM and choose appropriate processing conditions, we can maximize its advantages and obtain better workpiece surface quality.

At the same time, the development of a dedicated power supply and control system for fog processing is expected to increase the material removal rate and reduce electrode loss. Adopting a simple electrode design can also significantly reduce the production cost of complex three-dimensional electrodes and shorten the production cycle. This will promote the development of milling processing technology in the direction of high efficiency, low loss, and pollution-free, bringing greater potential to the manufacturing industry.