Supply energy is transferred over the distance from the power feed contact to the actual point of cutting. This distance can be considerable, especially if the job is being cut with “open guides” to clear a work piece obstruction. Low wire conductivity will result in a voltage drop and associated energy loss over the distance from the power feed to the cutting point. This is not insignificant when one considers that the peak current of most modern power supplies often exceeds 100 amps.

Tensile Strength indicates the ability of the wire to withstand the wire tension imposed upon the wire during cutting, in order to make a vertically straight cut. Since all commonly used wire EDMs are imported, tensile strength is most commonly specified in the metric units of N/mm2. EDM wires are considered “hard” at tensile strengths of 900 N/mm2 or above. EDM wires are considered “half hard” at tensile strengths at or about 490 N/mm2. EDM wires are considered “soft” at tensile strengths at or below 440 N/mm2. (Unfortunately, the nomenclature for “half hard” and “soft” has different meanings in Europe and Asia) Hard wires are commonly used for most work, while half hard and soft wires are primarily used for taper cuts where the taper angle is greater than five degrees, since a hard wire will resist bending at the guide pivot and will result in inaccurate taper cutting. Half hard and soft wires are often unsuitable for automatic threading unless the machine is specifically design to work with these wires.

Elongation describes how much the wire “gives” or plastically deforms just before it breaks. Elongation is measured in % of the gauge length used in a given test. It could also be stated that elongation relates to how “brittle” the wire is. Usually, hard wires have elongation that is considerably lower than half hard wires. Elongation is an important property, since EDM wire operates in a hostile environment in which it is under high tension and being “attacked” by thousands of sparks which are eating at its cross-section. A brittle wire might snap at the first overload condition, while a more ductile wire is more likely to accept a temporary overload by “giving” a little and continuing to cut.

Melting Point is not normally specified for a given wire, but is obviously important since EDM is a spark erosion process, and we would prefer that our wire electrode be somewhat resistant to being melted too quickly by all those sparks.

Straightness is another important property of EDM wire that is seldom specified but is critically important to successful auto threading.

Flush ability is normally not specified for a given wire, but is an important property that relates to how well a wire will actually cut. This property is related to the sublimation temperature of the wire’s alloy components that is more in the realm of metallurgy than tool making. Let’s just say that the better the flushablity, the faster the wire will cut.

Cleanliness is a property that is not specified for EDM wires. Wire can be “dirty”, due to contamination by residual metal powder left over from the drawing process,

Drawing lubricant or paraffin added to the wire by some manufacturers prior to spooling. Dirty wire can ruin your day, resulting in clogged guides and power feeds or slipping belts or rollers.

Like most other things in life, finding the best wire for any application means finding an acceptable compromise of the aforementioned properties, since they are often contradictory. For example, high conductivity wires often have low tensile strength.

EDM Brass Wire

Brass wires start out as a continuously cast 20mm diameter rod. This rod is either cold rolled or cold drawn until it is approximately either a 6mm round or hex cross-section. The wire is then annealed and drawn through a series of dies until it is approximately .9mm diameter. In this state, it is commonly called “re-draw” wire. The re-draw wire is subsequently drawn through another series of diamond dies until it is the final size. At final size, the wire is resistively annealed or thermally tempered in an inert atmosphere, cleaned, and then spooled.

Plain Wires

While in the fashion world plain might be a derogatory term, in the EDM wire business “plain” merely means that the wire consists of a single homogeneous component and does not have a coated or composite construction.

Copper Wire was the original EDM wire. At the time it was thought that since Copper wire had high electrical conductivity, it would make the ideal EDM wire. Unfortunately, Copper wire has both low tensile strength and low flush ability. This soon became apparent with the development of the 2nd generation pulse type power supplies and Copper wire was soon supplanted by Brass wires. It should be noted that Copper wires are still used occasionally for applications in which Zinc (contained in brass wires or coated wires) is considered an unacceptable contaminant.

Brass Wire was the successor to Copper wire and is still the most commonly used wire today. Brass, which is an alloy of Copper and Zinc, delivers a powerful combination of low cost, reasonable conductivity, high tensile strength, and improved flush ability. (It should be noted that even a small amount of Zinc added to Copper wire drastically reduces the conductivity. Hard Brass wire typically has conductivity only 20% of Copper wire.) Brass wire is most commonly available in the following alloys: Since it is the Zinc that gives Brass Wire its improved flush ability, some manufacturers now offer a “high zinc” brass which is Cu60%Zn40%. This increase in Zinc content can increase cutting speed up to 5% in some optimized applications, however, many users who are content to use “standard” settings may not see a cutting speed increase. It should also be noted that in certain circumstances, a significant Brass deposit can remain on the work piece after the cut that can prove to be quite difficult to remove.