Electrode Material Selection for Electrical Discharge Sawing (EDS)
Like electrical discharge machining (EDM), electrical discharge sawing (EDS) also works on the principle of spark erosion. In EDS machines, instead of a shaped tool, a continuous feed of wire is used as the tool electrode. During electrical discharge cutting, the wire is moved along a cutting path, which creates a slot through the surface of the work piece. The traversing speed and path of the electrode wire can be precisely controlled using CNC or NC systems. EDS electrodes are available in diameters ranging from 0.076 mm to 0.30 mm for applications that require varying kerf widths.
Through electrical discharge sawing, slanting or spherical surfaces can be sawed with minimal scrap. EDS systems can process hard materials more efficiently than conventional sawing machines. Non-contact EDS process does not deform the work piece surface. EDS machines are widely used for making dies, molds, punches, and stripper plates. They are also used for cutting EDM electrodes. The EDS process can be applied to cut hard metals and alloys, regardless of their metallurgical or physical properties. Toolmakers utilize this technique to make tools and dies made of hard materials such as tungsten carbides and hard steels.
The performance efficiency of an EDS machine greatly depends on the type of tool electrode used. For instance, some wire materials would leave great wear on the work piece but the machining process may be fast, other materials may not create any debris but the EDS process would be slow. Electrode materials can be classified into two groups: graphite and metallic electrodes.
Some important factors to be considered during the selection of electrode materials:
- Electrode manufacturing process
- Required kerf width
- Cost of the electrode material
- Expected amount of tool electrode wear
- Amount of wire feed required to do the job
The following table consists of a list of commonly used tool electrode/wire materials for processing specific work piece materials:
Tool Electrode / Wire Material
Zinc Alloy, Steel, Aluminum
Brass, Steel, Aluminum
Copper, Tungsten Carbide, Steel
Low Temperature Alloys
High Temperature Alloys
Graphite electrodes are widely used for processing high temperature alloys. Compared to copper electrodes, graphite exhibits higher resistance to heat in the discharge spark gap. Therefore, it is most frequently used as the electrode material in several EDM applications. At local temperature, graphite electrodes do not melt in the discharge gap. If the temperature surges above 3350°C, a small portion of the electrode evaporates. Graphite electrodes are usually suited for machining steel and all its alloys.
A significant aspect of graphite electrodes is that they can be easily machined and ground. Machined metal electrodes have minute burrs while machined graphite electrodes are free from such burrs.
On the downside, graphite electrodes create black dust when machined. To prevent premature wear of machine ways, machinists use carbide-sawing tools to shape graphite electrodes. Electrode grain structure is critical to the finish accuracy of ED machined components. Graphite electrodes with fine grain structure tend to be more resistant to erosion.
Metallic electrodes are best suited for machining work piece materials with low melting point. Brass, tungsten, aluminum, zinc, and copper are commonly used metallic electrode materials. The key features of metallic tool electrodes are their mechanical integrity and electrical conductivity. Compared to graphite electrodes, EDM machines using metallic electrodes have low cutting speed.
Copper is the most electrically conductive electrode after silver. Though copper is stronger than silver, it has low resistance to oxidation. In terms of cost, copper is more expensive than graphite. Compared to graphite or brass, copper is more difficult to manufacture. Copper electrodes are used in applications that warrant a fine finish. The downside in using copper as the electrode material is its low melting point (approximately 1085°C), which causes high wear-in relation to the substrate material-during machining. Copper electrodes are most commonly used to make female tool electrodes in wire EDM systems. They are also used in EDS machines used for processing materials such as carbide and aerospace alloys.
The melting point of tungsten and copper are almost similar. However, tungsten electrodes are very difficult to machine. They are usually used as pre-forms and as rods or tubing in small hole drilling applications. Low electrical conductivity reduces the cutting speed of tungsten.
Brass, an alloy of zinc and copper, is widely used for making small tubular EDM electrodes and EDM wires. It is more wear resistant than copper or tungsten and easier to machine. For special applications, EDM electrodes can be manufactured from brass through extrusion or die-casting. Brass electrodes are commonly used in high-speed small hole EDM machines.
Structural integrity, purity, and good electrical conductivity are some significant attributes of silver. However, high cost has limited the use of silver in a wide range of EDM applications. Silver electrodes are widely used for making coining dies.
Molybdenum is best suited for making small hole EDM electrodes. Due to its high melting point, molybdenum exhibits high resistance to arc erosion. The material is also known for its high structural integrity.
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