At present, many molds are processed by fast thread cutting, and the surface roughness of the workpiece after wire cutting is Ra ≥ 2.5 μm, and the hardness distribution and internal stress state are all very poor.
When the wire cutting machine processes, the current density in the discharge area is as high as 10000A/mm2, the temperature is as high as 10000°C-12000°C, and the filled medium liquid is rapidly cooled, so that the surface hardness of the cut surface is only about 20HRC, and the hardness of the internal quench layer is as high as 70HRC or more. The heat-affected zone is followed by the original hardness zone.
Particularly serious is that the internal quenching of the raw material is tensile stress, and the thermal stress generated by the wire cutting is also tensile stress. The superposition of the two stresses can easily reach the material strength limit to generate micro-cracking, thus greatly shortening the die life. , Wire cutting can not be used as the final processing of punch and die.
At present, most die-molding companies cut off the on-line cutting and remove the gray layer (ie, white layer) of the surface 20HRC by grinding. Although this method can eliminate the white layer with low hardness, it does not change the stress state in the stress region caused by wire cutting. Even if the grinding margin after wire cutting is increased, the high hardness of the hard layer (up to 70HRC) makes grinding difficult. An excessively large amount of grinding easily breaks the geometry of the part, so the high hardness layer produced by the wire cutting does not improve the die life, because its brittleness is the root cause of cracks and chipping.
After the wire cutting grinding process, the low hardness white layer and high hard layer can be removed to increase the die life. Because the thermal stress generated during grinding is also tensile stress, and superimposed with the thermal stress generated by wire cutting, it will undoubtedly aggravate die damage. If after grinding, low-temperature aging treatment is performed, the effect of stress can be eliminated, the toughness of the die can be significantly improved, and die life can be improved. Because most of the punches with complex geometrical shapes are processed by wire cutting, the grinding of complex-shaped punches must use expensive coordinate grinding machines and optical curve grinding machines. These two types of equipment are not generally available for wire cutting manufacturers, so it is difficult to promote them. .
In high-speed wire EDM machining, the working fluid is a pulsed discharge medium and has a great influence on processing. The working fluid generally requires certain insulation properties, better washing performance, better cooling performance and no pollution to the environment, and no harm to the human body. The configuration of the working fluid is also very important. When it is too thick or too light, it will cause broken wires, and the water-liquid concentration ratio will be in the range of 1:10 to 1:20. When the cutting speed is required to be high or when the workpiece is cut with a large thickness, the concentration is appropriately lower, so that the processing is relatively stable and it is not easy to break the filament. However, the concentration of the working fluid should not be too low. Otherwise, the insulating performance of the working fluid will be reduced, the resistivity will be reduced, and the cooling capacity will be enhanced. This will reduce the lubricating effect on the workpiece, and it is not conducive to chip evacuation and easy wire breakage.
In order to ensure that the working fluid channel is not obstructed, the size of the working fluid flow should also be properly controlled. Generally, when the cutting is completed, the workpiece is cut by the cutting section under the working hydraulic force, the cutting part falls off, and the electrode wire is easily pinched off. At this time, the working fluid should be maintained at a certain pressure and flow rate, and the corrosives should be discharged in time to prevent the occurrence of broken filaments. When cutting soft materials such as aluminum and copper, especially when the cutting material is relatively thick, in order to improve the processing efficiency and surface finish and reduce the broken wire, some special coolants can be used, which is also conducive to environmental protection.
In the high-speed wire-cut WEDM process, there are also certain requirements for the operator in order to efficiently process workpieces that meet the requirements. The operator must correctly understand the technical requirements of the drawings. The working fluid should be replaced in time to maintain a certain degree of cleanliness, to ensure that the upper and lower nozzles are not blocked and the flow rate is appropriate. The electrode wire is calibrated vertically and the workpiece is clamped correctly. Reasonably select the pulse power supply parameters, when the processing is unstable, adjust the frequency feedrate and so on. When switching function keys, do not perform during processing, so as to avoid short circuit caused by poor contact of switch or burn wire, and even broken wires. When the cutting is completed, the cutting part will fall under the action of gravity, and the electrode wire can be easily pinched off. It can be fixed in time by means of magnet adsorption, clamping of the clamping plate, etc. to avoid broken wires.
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