1. Overview of wire cut EDM machine processing

The existing CNC wire cut EDM machines are divided into two types: low speed wire cut machines (LS-WEDM) and high speed wire cut machines (HS-WEDM).  Low speed wire cut EDM machine tools generally use brass as the electrode wire. The electrode wire uses a one-way low speed operation and is discarded after one use. The electrode wire of the high speed wire EDM machine uses high speed wire travelling way which enables fast back-and-forth movement, and the electrode wire is used repeatedly during the machining process. This kind of electrode wire is mainly molybdenum wire and the molybdenum wire with a diameter of 0.18mm is commonly used in the market. The wire speed of the fast wire travelling mode is generally more than ten meters per second. If the wire speed is 10m/s, it is equivalent to the electrode wire moving 0.01mm in 1 microsecond. Such a fast speed is conducive to the rapid deionization of the discharge channel at the end of the pulse.

2. Cause analysis of black and white stripes produced by wire cut EDM machining

With the use of high speed reciprocating wire EDM processing, the surface of the processed steel workpiece often has obvious black and white stripes. The black and white cross stripes on the cutting surface are an important issue that affects the macroscopic quality of the processed surface.

Through repeated observation of the wire cut EDM processing and workpiece detection, a rule can be found that in a cutting stroke during the reversal of the wire transport of the drum, the appearance of stripes is related to the direction of movement of the electrode wire. The entrance of the electrode wire is black, and the exit is white. This is caused by different chip removal and cooling conditions. When the electrode wire moves from top to bottom, the working fluid is poured from the upper part and enters the slit from the electrode ribbon, and the discharge product is brought out of the processing area by the electrode wire from the bottom. At this time, the upper working fluid is sufficient, the cooling condition is good, the working fluid at the electrode wire inlet is sufficient, the discharge is carried out in the working fluid (emulsion), the lower working fluid is less, the cooling condition is poor, but the chip removal condition is better than the upper part. The industrial liquid is affected by high temperature and instantaneous high-pressure gas in the discharge area, and rapidly diffuses outward, which makes it difficult to discharge the electrical corrosion products on the upper part. At this time, the carbon black and other substances produced by the discharge aggregate and adhere to the upper processing surface, making it appear black. In the lower part, the chip removal conditions are better, the working fluid is less, and the discharge product contains less carbon black. Moreover, the discharge often occurs in the gas, so the processed surface appears white. In the same way, when the electrode wire moves from bottom to top, the lower part is black and the upper part is white. In this way, the surface of the wire EDM that goes back and forth the wire forms a staggered black and white stripe.

This kind of black and white stripes usually have a certain effect on the surface roughness of the finished products. The working fluid at the entrance of the electrode wire is sufficient, and the discharge is carried out in the working fluid (emulsion). At the outlet of the electrode wire, there is less liquid and more gas. Under the condition of low pressure discharge, the discharge gap in the gas is relatively small. Therefore, the discharge gap at the inlet is larger than that at the outlet, and as a result, the white stripes are more protruding than the black and white stripes. Depending on the stability of the silk transport system, the white stripes will be a few micrometers to tens of micrometers higher than the black and white stripes.

3. The solutions to limit the production of black and white stripes

According to the reason resulting in the production of black and white stripes, it is difficult to absolutely avoid the black and white stripes under the circumstance of molybdenum wire reciprocating movement. But the production practice shows that the black and white stripes vary in depth, some are very obvious, with a difference of tens of microns between the bumps; some are not obvious, and the difference between the bumps and bumps is only a few microns, indicating that the black and white stripes can be limited. The methods to limit black and white stripes are mainly as follows in production practice.

Ensure the stability of the wire transport system. Adjust the molybdenum wire storage drum to make its operation balanced, and the guide wheel has no axial movement and radial runout.

Adopt a more reasonable working fluid injection method. The spiral nozzle is used to spray the working fluid along the axis of the electrode wire, even up and down.
Use a saponification solution with strong washing properties as the working fluid for wire cutting.

According to different processing technology indicators, it is generally in the range of 5%-20% (emulsion 5%-20%, water 95%-80%). Generally configured according to mass ratio, but also roughly according to volume ratio. The concentration ratio of workpieces that require relatively high surface roughness can be appropriately larger, about 15% to 20%. For workpieces that require high cutting speed and large thickness, the concentration can be appropriately lower, 5% to 8%.

Wire cutting processing often uses the cutting method of mold material Cr12. Cr12 is a high-alloy tool steel with high hardenability and wear resistance, small heat treatment deformation, and can withstand greater impact loads. For this kind of material commonly used on molds, it has good wire cutting performance. The working fluid is prepared with distilled water to reduce the concentration of black and white stripes on the surface and make the surface of the workpiece white and uniform.

Ultra-short stroke reciprocating cutting method, the actual cutting distance of each wire reversal is very short, about 1/4 of the wire diameter. The unevenness formed when the electrode wire moves up and down will overlap each other, black and white Streaks will be difficult to see. Although the wire storage drum rotates frequently, the whole cutting process is still performed on the full-length molybdenum wire of the full wire drum, which effectively avoids the problem of wire diameter loss after concentrated discharge.

During the reciprocating movement of the electrode wire, the electrode wire discharges only when it moves in one direction, but does not discharge when it moves in the other two directions (the opposite direction). This can also limit the generation of black and white stripes, but the cutting productivity is lower when moving in one direction.

Through the above 6 methods, black and white stripes can be reduced to a large extent, the surface roughness of high-speed wire cutting can be improved, and the surface of the workpiece can be smooth and uniform.