How to Choose Wire for EDM?
Apr 29 - 2022
Tensile strength is the maximum load-bearing capability given to a material based upon its ability to resist stretching and breaking. It is determined by the maximum load in pounds per square inch divided by the cross-sectional area of the wire. High-tensile EDM wire provides better edge straightness and so is a good choice for single-pass parts and in small- and fine-diameter wires to reduce wire breakage. Wirecutting tall parts is another application that requires good straightness and resistance to wire breaks. High-tensile wire is also good for skim cuts, aiding in part straightness and geometric accuracy.
There is no true index or “rating” of fracture resistance, so fracture resistance of an EDM wire might better be described as wire toughness or resilience. It is the ability of the wire to resist the effects of the incredibly dynamic environment of the spark gap.
This is the measure of a material’s ability to carry electrical current. In EDM, the higher the conductivity of the wire, the more power can be delivered to the workpiece. Since higher conductivity is more efficient, increased cutting speeds are the usual results.
A low melting/vaporization temperature of the wire will prove the best for flushability. We want the wire surface to vaporize, to quickly turn into gasses, instead of contaminating the gap with resolidified “chips”. All chips have mass and therefore, will “take up room”. The spark gap is small enough already and we don’t need “chunks of stuff” floating around in it. When using longer on times (roughing), melting will occur. Even though melting produces chips, lowtemp electrode alloys (zinc-coated) will produce larger craters on the wire surface to carry more water and contaminants away from the gap, aiding flushing. Consequently, moly and tungsten wires, with high melt/vaporization temperatures cut slowly and flush poorly, but are satisfactory for skim cuts where speed is not an issue but part straightness and crisp geometry is.
The term hardness is often mistaken for tensile strength. The hardness or temper of a wire refers to the wire’s ductility, or its ability to undergo elongation. In this category, EDM wires are called soft or hard. On closed-guide machines, a hard wire will thread better than a softer wire, but a soft wire will taper-cut better. A hard wire will also provide the best auto-threading reliability.
The first wire used for wire EDM was copper. It’s availability and high conductivity (100% IACS) made it a logical choice for an EDM electrode at the time, but as generators became more powerful, copper’s limited cutting speed and low tensile strength (34,000-60,000psi) quickly revealed it’s limitations. Except in older machines that specify the use of copper wire, this wire is seldom recommended. In these machines, this wire must be used to wire-cut all materials simply because there is no other alternative.
The element zinc is added to copper to produce brass EDM wire, which is the most common EDM wire in use today. Brass wires for EDM are typically an alloy between 63/37 (American and European) to 65/35 (Asian), Cu/Zn ratio. Zinc has a lower melting/vaporization point which makes it a better electrode material than copper, so the more zinc in the surface of an EDM wire, the faster it will cut. However, manufacturing difficulties arise when the volume of zinc approaches 40% and its crystalline structure changes to a gamma phase, causing the wire to become very brittle and difficult to draw. However, there are two wires being produced with a 60/40 Cu/Zn content for faster cutting speeds. Tensile strengths of brass wires range from 54,000-173,000psi, depending upon the composition of the alloy and it’s temper. Brass wires are usually a shiny brass color. Brass wires with a dull, matte finish or with spots or discoloration are indications of oxidation or contamination. Brass wires are a good, all-around utility wire that can be used by most modern machines. Economical cutting of tool steels and most metals is possible with plain brass wire. Available in hardness (elongation) in ranges from <2% to over 30%. Wires with low percentages of elongation will thread reliably but are limited in tapering ability. Obversely, wires with high elongation can taper-cut to 45° but with much reduced threading relaibility.
Coated EDM wires are wires that have had a very thin layer (2-3μm) of pure zinc applied to a brass or copper core. Electro-galvanization is the most precise method of application, depositing pure zinc, atom by atom onto the body of the wire, insuring uniform thickness and surface uniformity. Hot-dipping is another method of coating the wire. It is a quicker and less precise method of coating and these wires will usually be less expensive. These wires are available in tensile strengths from 61,000-130,000psi and can have a pure zinc or zinc oxide coating depending upon the application. Pure zinc coatings will have a shiny, silver color while a zinc-oxide coating will appear dull gray. Zinc coated wires will cut much faster and break less often than plain brass wires and are recommended for fast roughing and finishing of steels. Zinc coated wires will produce a superior surface finish and better surface integrity, especially against carbides and PCD’s.
Wires with high zinc content make a better EDM electrode but EDM wire with zinc percentages approaching 40% are difficult to produce, so a special method of producing wire with a higher zinc surface has been devised. A heavy coating of pure zinc is applied to the outer surface (18-35μm) of a copper or brass core. Undergoing annealing in a special furnace diffuses the pure zinc coating into a mixture of nearly 50% brass and 50% zinc (actually, zinc content is approximately 45-47%), which is higher than the 40% threshold which was the previous manufacturing limitation. Diffusion-annealed wires are available in tensile strengths from 62,000-128,000psi and appear yellowbrown to brown in color. These wires are suited for fast cutting of tall workpieces, volume production and in poor flushing conditions. They can be used against a wide range of materials including tool steels, aluminum and graphite.
This is a very high strength wire with tensile strength in excess of 275,000psi. but because moly wire has very high melting and vaporization temperatures, 4,757°F (2,625°C) and 10,040°F (5,560°C) respectively, it is a reletively poor electrode material. Because of it’s high working temperatures, the EDM craters left on the wire’s surface are very small compared to those left on a brass-based wire and therefore will not flush as efficiently. This wire is often used in the small, (0.006 to 0.004”), and fine, (<0.004”), diameters to meet blueprint requirements of narrow kerfs and near-sharp, inside corner radii. Because of its high tensile strength, moly wire aids in maintaining excellent wall straightness and reducing the number of wire-breaks common with many small and fine brass wires. In special cases, typically in medical and military applications, there can be no copper or zinc contamination of the wirecut surface. This criteria eliminates all copper-family wires and forces the use of a moly or tungsten wire. Note: Use of moly wire must be application-specific because of its high cost, reduced threadability and slow machining times.