EDM Wire Covers All the Angles

Advances in EDM-wire technology help today’s shops cut taper angles more efficiently and profitably.

When conventional machining processes cannot successfully cut taper angles up to 45° or higher, wire EDM can. This capability depends as much on the type of wire as it does on the machine and the operator’s level of expertise. The proper wire for taper cutting is a balance of what is ideal and what is practical for the machine, the workpiece, and the requirements for automatic wire threading. The wire must have the right combination of electrical and mechanical properties for it to be a viable and cost-effective tool for cutting high- or low-taper angles.

Plain copper wire once seemed like a good choice for wire-EDM applications. After all, copper is used as a benchmark for measuring electrical conductivity—materials are rated according to the international annealed copper standard (IACS). High conductivity has its advantages, but advances in EDM technology have long since made plain copper wire obsolete for more than 99% of today’s applications.

Selection of wire for cutting tapers greater than 7° involves many considerations and tradeoffs. For example, high-taper cutting requires a wire with an adequate amount of stretch or elongation. Wire elongation is caused by tensile forces and heat during EDMing. When entering and exiting a typical guide during high-taper cutting, high-elongation wire follows the intended path more closely than low-elongation wire does.

However, the higher a wire’s elongation, the lower its tensile strength. Tensile strength measures the wire’s ability to resist breaking when it is placed under a longitudinal load. Wire with tensile strength below 100,000 psi generally isn’t reliable for use on most automatic-threading EDMs.

Furthermore, wire with high elongation is typically soft, which means it has difficulty resuming its original straightness when it is deflected. Although this is not critical during cutting, it creates problems during wire threading. Because soft wire stays bent coming off the spool, it must be manually threaded through the upper and lower wire guides.

Although harder wire has better resistance to deflection, very hard wire with less than 3% elongation can cause vibrations during taper cutting as the wire travels between the upper and lower wire guides. In addition, hard wire’s ability to resume its original straightness when deflected fights the true programmed wire path during high-taper cutting, which bends the wire. This often results in poor surface finish or wire breakage, which leaves intolerable marks on the workpiece and wastes material and production time.

EDM-wire manufacturers have tried to combine the advantages that make hard wire suitable for use on automatic-threading EDMs with the features that make soft wire ideal for taper cutting. The ideal EDM wire would have reasonably high conductivity for greater cutting speed, adequate elongation for high-taper requirements, enough tensile strength to facilitate automatic threading, and sufficient hardness to resist the tensile forces and heat of the cutting process.

Wire manufacturers have also tried to minimize or eliminate some of copper’s undesirable characteristics. Plain copper wire is no longer practical for EDMing primarily because of its poor flushability. An EDM wire should have a low melting point and high vapor pressure to flush debris from the gap. Because plain copper wire is difficult to vaporize, pArticles of molten material rather than gases accumulate in the cutting zone. To improve flushability, manufacturers alloyed copper with zinc to create the first “engineered” wire: brass.

Brass Wire
Because zinc has a lower melting point than copper (786° F vs. 1980° F), it enhances the flushing process, since there are fewer pArticles to flush away. When the zinc vaporizes, it also helps cool the cut and allows the delivery of more usable energy to the work zone.

A common type of brass wire contains 63% copper and 37% zinc. This plain brass wire is available in tensile strengths from 54,000 to 130,000 psi to suit various application requirements. Wire elongation ranges from less than 2% (hard brass) to greater than 30% (soft brass) for applications involving high tapers, low tapers, and automatic wire threading.

Figure 1: These wire-EDMed slugs from an extrusion die have 45° tapers. The complex opening had to be programmed and cut in three steps to remove the slugs.

For challenging part configurations, EDM Wirecraft, Fort Branch, IN, found plain brass wire to be an effective solution. The shop, which is a division of Warren Jones Tool & Die Inc., tackles wire-EDM jobs that demand very close tolerances or require complex high-taper cuts. An example of this difficult work is shown in Figure 1. These wire-EDMed slugs are from two large steel extrusion dies. Each die is 18.5" long, 6.5" wide, and 9.2" thick, and the slugs are actually cut in three steps.

EDM Wirecraft’s primary concern was to produce tapers up to 45° with a good surface finish. To handle these larger taper requirements, the shop chose a soft brass wire with greater than 34% elongation. All three main cuts were made with only one setup required to cut each die. EDM Wirecraft was then able to perform skim cutting on both dies at reduced power to improve part accuracy and finish. Although the machine only allowed the production of tapers up to 30°, this experienced shop used the high elongation of this soft brass wire to achieve 45° tapers on the left and right sides of the extrusion dies.

High cutting speed was not critical for this particular job, but it has become a top priority for a growing number of wire-EDM applications. Neither plain brass nor plain copper wire can handle today’s high cutting speeds of 28 to 30 sq. in./hr. Wire manufacturers knew that zinc in a brass wire’s core greatly improves flushability, which allows faster cutting. When the zinc content of the wire exceeds 40%, however, the material becomes brittle and unsuitable for drawing into fine wire diameters. Since it wasn’t practical to increase the amount of zinc in the wire’s core, manufacturers put the zinc on the outside of the wire. With the development of zinc coatings, both brass- and copper-core wire can cut at the higher speeds that many wire-EDM jobs demand.

Zinc-Coated Wire
During the cut, the zinc coating boils off at a lower temperature than the brass or copper core. This helps cool the cut and reduce thermal stress, allowing more power to be used during cutting. The result is increased cutting speed.

Besides faster cutting, zinc-coated wire has a higher resistance to breakage and a slightly rougher finish that helps flush EDM residue out of the gap. This improved flushing performance is made possible by the microcracks in the wire’s surface layer. The rough coating makes the wire suitable for rough-cut applications, but not for skim cutting.

Zinc-coated brass wire is available for taper cuts up to 20°. With a tensile strength of 70,000 psi, coated brass wire provides high precision in the cut. It also has greater than 20% elongation for high-taper cutting at greater speeds than plain brass wire.

The evolution of EDMs has led to the development of advanced wires that enhance productivity and solve application problems, such as cutting graphite. An example of this cooperation between machine and wire manufacturers is the development of a copper-magnesium (CuMg) wire with a zinc-oxide coating. The magnesium added to the wire’s copper core boosts tensile strength to 65,000 psi. For a wire with elongation in the 3% range, it is an unusually good conductor (62% IACS). This high conductivity makes the CuMg-core wire particularly suitable for deep cuts in aluminum, in which heat energy is easily dissipated. The wire’s zinc-oxide coating also allows high-speed cutting of aluminum and graphite.

The coated CuMg wire was just what Applegate EDM Inc., Dallas, needed to fabricate high-taper pieces in stainless steel and aluminum. Often involved in large profile work, the job shop had problems with wire breakage when it tried to produce tapers up to 20° on a 2.5"-thick aluminum transitional die.

Applegate EDM was using wires with a tensile strength of 100,000 psi or more. When cutting taper angles larger than 7°, however, these wires often broke because they were being bent at severe angles entering and exiting the machine’s round diamond guides.

The shop found that the coated CuMg wire had enough strength to optimize production and enough stretch to handle high-taper work on the aluminum die. Although the coated copper product costs approximately three times as much as plain brass wire, it cut 30% faster than plain brass wire, boosting the productivity of Applegate EDM’s machines.

There is a limit to the cutting speeds that zinc-coated wires can handle, however. The zinc coating can completely vaporize after 1" to 3" of cutting height, and it is not possible to simply add more zinc to the surface of the wire due to manufacturing limitations in the wire-drawing process.

Although a brass wire with more than 40% zinc is not practical to draw, wire manufacturers found that a thick, thermally diffused surface coating of 50% copper/50% zinc allowed the zinc content to be dramatically increased for improved flushability and, therefore, greater cutting speed. The result was diffusion-annealed wire.

Diffusion-Annealed Wire
The wire’s thick copper/zinc coating has been thermally annealed and partially absorbed into the outer surface of the wire’s core. This makes diffusion-annealed wire suitable for cutting at high speeds. Diffusion-annealed brass and copper wires also maximize productivity for applications involving taper cuts up to 7°, precision skim cutting, tall workpieces, poor flushing, or difficult-to-machine materials.

Figure 2: This automotive-grille electrode, which has 149 openings with 3° tapers, was EDMed with diffusion-annealed wire.

EDM Wirecraft discovered that the high electrical conductivity (77% IACS) of diffusion-annealed copper wire made it ideal for cutting graphite. The shop selected this wire to cut 3° tapers on a graphite die-sinker electrode for an automotive-grille detail (Figure 2). Although the wire is too hard (less than 3% elongation) for high-taper cutting, its adequate tensile strength (72,000 psi) made it suitable for automatic wire threading on the shop’s Charmilles EDM. 

Figure 3: Cutting 1"-thick Poco EDM-1 graphite on a Fanuc W1 EDM with various types of 0.010"-dia. wire.

Tests conducted in 12 different grades of EDM materials on five brands of wire EDMs confirm the improved productivity of diffusion-annealed wire. Figure 3 shows the dramatic increases in machining speed and productivity when EDMing graphite with diffusion-annealed wire compared to plain brass and zinc-coated brass wires.

Since the effect of wire choice on profitable production is so critical, inexperienced shops would be well advised to find some time to experiment with today’s high-performance EDM wires. Understanding the advanced technology behind these wires will allow shops to keep pushing the envelope of wire EDMing from prototype quantities to long production runs.

About the Author
Christopher Wilkins is product manager, EDM wire, for GISCO Equipment Inc., Hauppauge, NY.