The use of mechanical treatments is difficult for economic and / or technological reasons. As a consequence, new techniques have been developed, including electrical discharge machining (EDM – Electrical Discharge Machining). Material removal from the workpiece surface occurs as a result of the influence of electricity (periodic spark discharges or in an electric arc initiated between the workpiece surfaces and the tool occurring in a working center with dielectric properties). The types of EDM are briefly outlined below.
Types of electrical discharge machining
There are the following types of EDM machining:
- die sinking EDM;
- wire cut EDM – WEDM;
- hole drilling EDM.
Die sinking EDM
The electrode-shaped tools sink into the workpiece (figure 1). Machined surfaces obtain a shape that reflects the shape of the electrode. Application: plastic injection molds, casting molds and dies. Micro processing: electronic connectors, medical industry, watchmaking.
Figure 2 shows copper working electrodes. The inclusion of additional rotation around the electrode axis enables machining of curvilinear surfaces, including threads.
Copper is not the only material used for working electrodes. Unlike copper electrodes, graphite has a lower degree of wear. However, copper and graphite machining varies. Figure 3 shows a set of two electrodes used to processing the same surfaces.
Wire cutting (WEDM)
In the case of wire cutting (WEDM – Wire Electro Discharge Manufacturing), the role of the electrode is played by a metal wire (fig. 4), which cuts the workpiece through, which requires the implementation of the so-called start hole or machining starts from the edge of the workpiece. It is also possible to lead the wire at an angle, which allows you to get different shapes at the top and bottom of the workpiece (fig. 5). The wire is standard made of brass or coated copper (diameter varies from Ø0.02 to Ø0.33 mm; the speed of unwinding the wire from the spool is 2.5÷150 mm/s).
Technology for industrial making holes in hard materials (hardened steel, cemented carbides) – figure 6. The length of the hole can be 200 mm. Electrodes are used for holes with a diameter of Ø0.3 to Ø3 mm. They are also used to make starting holes for electroerosive wire cutting.
Non-conductive or very little conductive substances / material are used as a dielectric (the discharge is limited only to electrically conductive materials – the electrode and the workpiece). The dielectrics used are: distilled water, crude oil, kerosene, transformer oil. An electric discharge is accompanied by the formation of a plasma channel surrounded by a gas bubble that increases in diameter. The local increase in temperature (about 13.725 ºC) causes melting along with partial evaporation of a certain volume of the material. The pressure drops after the electric discharge is finished. This phenomenon enhances the process of vaporization of the molten material (the possibility of the so-called microblasts). Pressure changes are rapid, which causes the ejection of liquid material, which usually solidifies in the form of balls. Part of the molten material solidifies in the resulting crater (properties and structure different than before treatment). The discharges occur at sites more favorable to their occurrence, and rarely occur at the same site. The machining allowance is from 0.01 to 0.6 mm.
When there is a need to efficiently remove the digestion effects from the working gap (there is no mechanical contact between the electrode and the workpiece), kerosene is used, and distilled water is used for wire cutting (WEDM). The need to ensure the dielectric properties of the working medium requires the use of forced circulation filtration.
EDM processing effects
The qualitative effects (geometric accuracy, surface layer condition, including surface roughness) of the EDM treatment depend on:
- energy and time of electrical discharges (including the type and real capabilities of the generator) – affects the amount of stresses;
- kinematics, including positioning accuracy;
- real properties of the dielectric.
Modern EDM machines allow for machining errors of 2 μm and the positioning accuracy to be kept at the level of 0.001 mm .
In order to improve the tolerance of geometric dimensions, multi-stage processing is used, i.e. roughing, precise (shaping) and finishing. At each stage, electrodes of different geometrical dimensions are used. An important role in improving geometric accuracy is also played by increasing the flow of the dielectric through the working gap, as well as causing the electrodes to vibrate and reducing the working feed. Figure 6 shows an overview of the top layer layout.
The white layer is the melted workpiece material containing also particles of the working electrode itself, as well as products resulting from the pyrolysis of the dielectric itself. The layer with properties obtained as a result of the action of thermal energy, in the case of steel has a martensitic structure. The martensitic structure is the tempered layer, which has a hardness greater than the parent material of the workpiece. The shrinkage of the previously melted material is the source of the so-called tensile stress, which decreases as it penetrates the workpiece. Microcracks are a consequence of the stresses created in the surface layer. Their removal requires additional technological operations after EDM treatment (e.g. electrochemical ECM treatment or lapping). Lapping allows to achieve the accuracy class at the level IT5 ÷ IT7 and surface roughness Ra0.16 ÷ Ra0.01.
An important issue is the symbiosis with EDM shape machining of high-speed machining from the HSM family. The aim of such an amalgamation is to reduce the share of EDM machining, due to the costs related to e.g. with fast-wearing electrodes. Preparation of a technological process that takes into account the use of HSM and EDM techniques requires individual identification and assessment of the advisability of such activities.
Modern applications of EDM treatments constitute a significant extension of the classically understood manufacturing techniques and play a role in various types of hybrid treatments. EDM is used for:
- hollowing cavities;
- hollowing holes (cylindrical and shaped) – the electrode can perform not only plunge work but also additionally so-called planetary movement;
- making more complex shapes (CNC EDM machines);
- making threads (the shaped electrode in the form of a threaded shaft performs a helical working movement).
The photos used above were taken by courtesy of GF Machining Solutions.
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- Materiały informacyjne firmy GF Machining Solutions