In recent years, with continuous improvement of difficulty of workpiece processing, complexity of mold parts, part accuracy and machined surface quality requirements, requirements for processing machine and equipment are also getting higher and higher, performance requirements for slow wire EDM products have also become higher and higher. Enterprises producing complex workpieces usually have many transition connections of different sizes and angles, such as sharp corners, rounded corners or obtuse angles. These transition connections are collectively called corners. Since angular workpieces need to be bent and cannot be cut on a straight line, complex workpieces are difficult to process. Therefore, how to achieve high-precision and high-surface-quality wire EDM processing of complex multi-corner workpieces has always been a technical problem in this field.

Wire electrical discharge machining (WEDM) is a cutting technology based on principle of electrical discharge machining that uses a wire-shaped tool electrode, that is, an electrode wire, for precision machining. It mainly uses discharge cutting to process various multi-corner complex workpieces.
Due to short pulse discharge time of EDM, accompanied by complex physical processes such as electricity, magnetism, heat, and chemical corrosion, a discharge channel exists between two electrodes in a short period of time, forming local transient high-temperature energy in channel, which directly evaporates or liquefies workpiece material. Based on above processing principles, wire cutting processing has characteristics of small macro force, non-contact processing, wide material applicability, and high processing quality. It is especially suitable for processing multi-corner parts, difficult-to-machine mold parts and complex-shaped workpieces.
Existing technology shows that cracks on electrode wire will be beneficial to cutting, that is, breakage of surface layer covering surface of electrode wire core material can increase cutting speed, but it will also reduce tensile strength of electrode wire, making electrode wire easily broken and unable to continue cutting. Solution to this technical problem is usually to increase diameter of core and add a continuous transition layer between core and broken skin. However, above-mentioned electrode wire can only cut 1 and 2 cuts on difficult-to-machine workpieces. That is, same electrode wire can only cut up to 3 cuts on workpiece during slow wire processing, of which the first cut is rough, the second and third cut are fine trimming. Electrode wire is easily broken at the fourth cut, and there will be an obvious pause during cutting process, corners of complex workpieces cannot be cut. Therefore, a high-precision coated wire for wire EDM processing is designed to achieve efficient cutting of difficult-to-machine workpieces with corners.

Coated wire design

 

As shown in Figure 1, designed coated wire includes a wire-shaped core material and a surface layer covering the surface of core material. At the same time, surface layer includes a base layer attached to surface of core material. Base layer includes a plurality of irregular polygonal large blocks intermittently attached to surface of core material, first cracks are provided between adjacent large blocks; it also includes a surface layer attached to surface of base layer. Outer surface of each large block is attached to a plurality of intermittent polygonal small blocks. There are irregular second cracks between adjacent small blocks, all small pieces and all second cracks together constitute surface layer.
Now we will specifically analyze how to design coated wire core material, base layer and surface layer structure from following three aspects.
(1) Coated wire core material needs to withstand large voltages and high current densities, which limits electrode wire loss and thermal softening, reduces strength. Electrode wire with its surface structure effectively reduces explosive force and water pressure during fine discharge, reduces pressure difference thrust in processing gap to rear of processing path, thereby slowing down hysteresis effect of electrode wire and improving processing quality of small arcs and corners. When performing slow wire processing, workpiece can be cut with multiple cut. The first cut is rough, followed by multiple cut for fine trimming. Multiple cut for fine trimming are repeatedly trimmed with same electrode wire. Electrode wire does not break easily, and there is no pause during cutting process. More complex workpieces can be cut efficiently at corners.
(2) Surface of coated wire core material is attached to base layer of surface layer and base layer is broken. Base layer includes a plurality of irregular polygonal blocks separated by intervals. There are first cracks between adjacent large blocks, there is no continuous transition layer between core material and surface layer. That is, part of core material surface will be exposed to air through the first crack, but this structure does not reduce tensile strength of electrode wire. Not only is electrode wire not easily broken, but it can cut workpiece multiple times, even cut 1 and repair 10, especially when cutting corners of difficult-to-machine workpieces. This is because corner accuracy of difficult-to-machine workpieces is affected by hysteresis caused by explosive force of electrode wire during cutting discharge process and pressure differential thrust of high-pressure water on processing gap to rear of processing path. When discharge current is larger, explosive force in machining discharge gap is greater, and reverse pushing force on electrode wire is also greater; the higher water pressure, the greater pressure difference force in machining gap to rear of machining path. This hysteresis effect is reflected in collapse of corners when cutting small arcs. This fractured base structure can effectively slow down discharge explosion force and high-pressure water flushing, thereby improving corner processing accuracy and surface quality.
(3) Large pieces of surface layer and the first cracks, small pieces of surface layer and second cracks together constitute surface layer, which has a network structure of two-level cracks. By directly arranging a cracked network structure with double-level cracks on the outer surface of core material, problem of reduced tensile strength of electrode wire due to fracture can be better solved. That is, network crack structure on the surface layer will not excessively reduce tensile strength of electrode wire, electrode wire will not break when cutting difficult-to-process workpieces multiple times. In this way, there is no need to increase diameter of core material, nor to add a continuous transition layer between the core material and the broken surface layer, and there is no need to consume too much metal raw materials to prepare electrode wire.

Since the first crack on the surface of base layer is not a longitudinal crack, there is no pause during cutting process, complex and difficult-to-machine workpieces can be cut smoothly at the corners. In addition, compared with regular and orderly surface structure, irregular network fracture structure is more beneficial to cutting difficult-to-machine workpieces. Because wire EDM is processed by discharge heat, surface material of workpiece melts due to high temperature generated by discharge, then rapidly cools to form a deteriorated layer. Deteriorated layer is prone to micro cracks, which are formed because electrode gap does not fully eliminate ionization interference, discharge points are not dispersed, and multiple consecutive discharges occur in same cutting. However, this criss-crossed irregular network fracture structure effectively reduces high energy release caused by capacitive effect and concentrated discharge caused by electrical corrosion products (such as liquid slag, etc.) in working fluid. Especially when cutting 1 to 6 or more fine trims, electrode wire of this structure can fully eliminate ionization interference in electrode gap, uniformly disperse discharge points, and have a stable discharge process, thereby obtaining excellent surface quality. Designed coated electrode wire cuts difficult-to-machine workpieces and after polishing, there are basically no line marks on the surface, which improves surface quality of processed workpieces.

 

Table 1 Processing machine tool conditions

Table 2 Processing cases of various wire electrode cutting workpieces

Coating wire designed above is used to perform wire EDM on difficult-to-machine workpieces as an implementation case. Original technology gamma coating wire, galvanized electrode wire and brass electrode wire are used as a comparison case, workpiece made of SKH-9 was performed on Mitsubishi MV1200R machine tool in Japan. Workpiece is shown in Figure 2. Cut workpieces are subjected to precision testing and surface testing respectively, including workpiece dimensional accuracy, workpiece corner accuracy, polished surface line marks and workpiece surface roughness. Processing machine tool conditions are shown in Table 1, processing cases of various wire electrode cutting workpieces are shown in Table 2. According to Table 2, application cases and test results of various wire electrode cutting workpieces are summarized and analyzed as follows.
(1) In implementation case, dimensional accuracy of difficult-to-machine workpiece cut by coated wire is ±(2~3) μm, which is smaller than ±(5~7) μm in comparison case, indicating that size of cut workpiece has high precision.
(2) In implementation case, corner accuracy of difficult-to-machine workpiece cut by coated wire is ± (0.5~1) μm, which is much smaller than ± (2~4) μm in comparison case, indicating that corner accuracy of cut workpiece is high.
(3) Line marks on the surface of workpiece after polishing represent surface roughness of workpiece and are used to characterize surface quality of workpiece after cutting. In implementation case, after polishing workpiece that is difficult to machine with coated wire cutting, there are basically no line marks on the surface of workpiece, and there is only one line at most, which effectively improves surface quality of workpiece. In comparison case, after polishing difficult-to-cut workpiece, there were 3 to 5 line marks on the surface of workpiece, which seriously reduced surface quality of processed workpiece.
(4) Surface roughness of workpiece is used to characterize quality of workpiece surface. The smaller roughness value, the higher surface quality. In implementation case, surface roughness value of workpiece when cutting difficult-to-machine workpieces with coated wire is Ra0.345~0.352μm, which is smaller than 0.385~0.485μm in comparison case, indicating that surface quality of designed coated wire is used to cut complex multi-corner workpieces.
Coated wire EDM wire-cutting of multi-corner complex workpieces using above design has achieved good results in process tests and actual production by users. Compared with original technology, dimensional accuracy, corner accuracy, surface line marks and surface quality of workpieces that are difficult to machine using wire electrodes for wire EDM processing have all been significantly improved, which has improved accuracy and surface quality of processed workpieces and created greater economic benefits for enterprises.