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In the 1995 version of this article, I suggested that EDM (electrical discharge machining) could also stand for Exact Difficult Machining because the applications best suited for this process were characterized by extremely exacting tolerances that were difficult or impossible to achieve with any other method of machining. Because of the dramatic improvements in EDM machine performance and automation, two other sets of words could now fit the acronym "EDM." Every Design is Manufacturable reflects the huge benefit that EDM has brought to design engineers, allowing them to relax traditional manufacturability constraints and design products with optimum functionality. Efficient Domestic Manufacturing reflects the fact that EDM is, increasingly, the process of choice for manufacturers in high-wage countries such as the United States, because the EDM process is so much less labor intensive than almost any other machining process.

EDM has been a growing force in North American tool, die and moldmaking shops since the 1950s. In 2003, the sale of wire and ram (die sinker) EDM machines represented 7.3 percent of total metalcutting machine tool sales dollars in the United States.

EDM machines are used to produce tooling (molds, stamping dies, extrusion dies, forging dies, fixtures and gages) and parts for the aircraft, medical and other industries. From 1994 to 2004, the relative size of the tooling and part-making categories has shifted dramatically.

EDM unit sales rose consistently through 1998 and then dropped to approximately half that level in 2003. With the subsequent recovery, sales in 2005 will be back near 1994 level. However, the mix has changed. Previously, the majority of EDM machines were purchased for tooling, but today, purchases are more evenly split between tooling and part making. The purchase of EDM machines for tooling fell sharply because of the loss of tooling and plastic parts production to manufacturers in Southeast Asia. In contrast, medical and other markets have grown strongly in North America.

EDM (Efficient Domestic Manufacturing)

The trend to use EDM for parts production has several causes. For one, the EDM process has improved dramatically because of higher cutting speeds. For another, EDM automation has dramatically improved, significantly cutting labor content. As evidence, the percentage of wire EDM machines sold with automatic wire threading has gone from 40 percent in 1994 to about 90 percent in 2004. In addition, modern threaders are much more reliable than earlier versions.

With a strong trend towards robotic handling of workpieces and, for ram machines, loading and unloading tools with robots, it is now possible to man one shift and cut on three. As a result of EDM's intrinsic efficiency, the ratio of cutting hours to labor hours for EDM is two or three times as high as that for almost any other machining process. Thus, high U.S. labor costs have less impact on U.S. competitiveness in EDM than in other machining processes.

An Overview Of EDM

The origin of EDM goes back to 1770, when English scientist Joseph Priestly discovered the erosive effect of electrical discharges. In 1943, Russian scientists B. Lazarenko and N. Lazarenko had the idea of exploiting the destructive effect of an electrical discharge to develop a controlled process for machining electrically conductive materials.

With that idea, the EDM process was born. The Lazarenkos perfected a process by which a succession of discharges took place between two conductors separated by a non-conducting liquid, called a dielectric. The electrical circuit that makes this happen bears their name. Today, many EDM units use an advanced version of the Lazarenko circuit.

How It Works

During the EDM process, a series of non-stationary, timed electrical pulses remove material from a workpiece. The machine tool, which also contains the dielectric, holds both the electrode and the workpiece. A power supply controls the timing and intensity of the electrical charges and the movement of the electrode in relation to the workpiece.

At the spot where the electric field is strongest, a discharge begins to form. Under the effect of this field, electrons and positive free ions accelerate to high velocities and rapidly form an ionized channel that conducts electricity. At this stage, current can flow as a spark forms between the electrode and workpiece, causing a great number of collisions between the particles. During this process, a bubble of gas develops. Its pressure rises very steadily until a plasma zone forms. This plasma zone quickly reaches very high temperatures, in the range of 8,000 to 12,000° C, as a result of the ever-increasing number of particle collisions. These high temperatures cause instantaneous local melting of a certain amount of the material at the surface of the two conductors. When the current is cut off, the sudden reduction in temperature causes the bubble to implode, blowing away the melted material from the workpiece surface and leaving a tiny crater. The melted material then solidifies in the dielectric in the form of small spheres and is removed by the dielectric.

Growth Of EDM

EDM has earned its place alongside turning, milling and grinding as a proactive, mainstream technology. EDM is best known for its ability to machine complex shapes in very hard metals. The most common use of EDM was traditionally machining dies, tools and molds made of hardened steel, tungsten carbide, high speed steel and other workpiece materials that are difficult to machine by "traditional" methods. The process has also solved a number of problems related to the machining of "exotic" materials such as Hastelloy, Nitralloy, Waspaloy and Nimonic, which are used on a large scale in the aeronautical and aerospace industries.

Fig. 1 - Parts involving complex geometry or thin walls are excellent candidates for EDM. This satellite structural component was wire cut from solid CAL-4V titanium by Numerical Precision, Inc., Wheeling, Illinois.

Because of technical advances in electrode wear, accuracies and speed, EDM has replaced many of the traditional processes in some applications. Another factor contributing to the growing use of EDM is the expansion of the work envelope on EDM machines, particularly when it comes to heights and tapers. Wire EDM machines can cut parts as much as 16 inches tall, with a straightness of ±0.0005 inch per side. Cutting as high as 24 inches tall is now also available on certain machines.

EDM (Every Design is Manufacturable)

In the past, EDM was used primarily to produce those parts that were difficult to produce with conventional processes. The growth in EDM in the last ten years can be attributed to producing parts that have been designed to take advantage of the EDM process in the first place. Thus, EDM is no longer the last choice for manufacturers; rather, it is the first choice for the design/manufacturing team. What has changed from 1994 to 2004?

The EDM process has changed. The market of the companies that use EDM has also changed. Here is a list of significant changes to the EDM process.

• It's much faster.
• It's more automated. Today's machines can be equipped with automatic wire threading, robots and automatic slug eject. The machines are easier to program and maintain.
• It's more accurate.
• It uses smaller diameter wires on wire machines.
• Its operating cost is down. Prices of machines are lower.
• It produces better surface finish and surface integrity.
• It cuts carbide with no depletion of the cobalt content when using wire EDM. Ram machines cut carbide better, too.
• Wire EDM and workpiece rotation can occur simultaneously.
• Ram EDM requires no external flushing.
• EDM is more effective in difficult flushing conditions.
• EDM is much more user-friendly. Less training and less programming time are needed.
• EDM machine builders offer better training and customer support. This makes it easier for the first-time user.
• The cost and quality of complex graphite electrodes for ram EDM is much improved because of advances in high speed milling.
• The cost of operating EDM is down, both absolutely and relative to other machining processes. For example, the cost of wire per part produced is lower than the cost of cutting tools per part in the milling and turning processes, especially in cases where cutting tools have been upgraded to obtain shorter cycle times.

Fig. 2 - This fitting for a satellite deployment structure was wire cut from solid GAL-4V titanium barstock by Numerical Precision Inc.

Users of EDM are facing many changes in their markets that encourage greater use of this process.

• Customers are demanding faster delivery.
• Lot sizes are smaller. Just-in-time (JIT) production is more common.
• Customers are demanding lower prices. Low-wage competition from Southeast Asia is forcing down prices.
• Customers have parts with small features or parts that are difficult to fixture. The growing use of MEMS (microelectromechanical systems) is generating many of these parts.

Handy Checklist

The following checklist summarizes the characteristics that indicate the use of EDM. The more of these characteristics that are present, the more likely that EDM is the right solution.

Consider EDM when workpiece geometry features

• Very thin walls
• Small internal radii
• High depth to diameter ratios
• Very small dimensions
• Fixturing difficulties

Consider EDM when the workpiece material

• Is hard.
• Is tough.
• Leaves burrs.
• Has to be heat treated.

Consider EDM when the process it replaces involves

• Multiple setups, multiple operations, multiple handling
• Broaching
• Short-run stamping

Consider EDM when for other reasons such as

• You want 24-hour production with only one shift for an operator.
• You want a process that is not labor intense.

The potential for running EDM unattended also makes it cost-effective for a three-shift-per-day operation without adding manpower. Because of the fast turnaround time in EDM for small lots of parts, the process also helps shops reduce inventory and shorten deliveries, two factors which contribute to improved cash flow and lower operating expenses.

If you have a difficult recessed cut to make, you'll probably need to use a ram EDM machine. In many cases, traditional cutting tools cannot reach cutting areas and apply the required force.

Why is EDM the preferred process for tough materials such as Inconel, Monel, Hastelloy, Nitralloy, Waspaloy, Nimonic and Udimet? Because the electrode does not come in contact with the material, there is no adhesion of the workpiece to the tool. This fact makes wire and ram EDM machines useful for making magnetic reader heads for missiles, turbine blades and car engine prototypes.

Vaporizing

EDM is not influenced by the hardness of the workpiece material, so it's useful for cutting materials that have a hardness above 38Rc. These include hardened steel, Stellite and tungsten carbide. Because the EDM process vaporizes material instead of cutting it, the hardness of material is not a factor. That's why wire and ram EDM machines are used to create complex dies and other tools from extremely hard materials.

If a part or product involves a material that tends to leave tough burrs when traditional machining is used, EDM can solve the problem. EDM leaves no burrs, and the vaporized material is flushed away by the dielectric. EDM makes deburring unnecessary and eliminates dimensional changes that may occur during the deburring process. For this reason, EDM is often used to make surgical tools and copper electrodes.

Another time to consider using EDM is when you are making a part with accuracies that are difficult to maintain after heat treating. With EDM, you can cut conductive material of any hardness.

No Rotation

Another area where EDM shines is producing sharp internal corners. Conventional machining has problems with internal radii less than or equal to 1/32 inch that are parallel to the tool axis. The internal radius cut by EDM is as small as the spark gap, plus the radius of the wire or the electrode corner.

In milling and turning, either the cutting tools or the workpiece must rotate. The smallest workpiece radius is equal to the tool radius. In contrast, EDM electrodes generally don't rotate, and because EDM applies no cutting forces, very small, long tools can be used.

When cutting a 1-inch thick piece of D2 tool steel, the limiting radius would be: For EDM, about 0.0025 inch with a 0.004-inch diameter wire.

For high speed milling, about 0.050 inch with a 0.1-inch milling cutter. The approximate minimum dimensions of features that can be produced by EDM are given in the following table:

Features Internal radius External radius Hole diameter Slot width

Wire 0.0007" sharp 0.0016" 0.0016"

Ram 0.001" sharp 0.0006" 0.0004"

That's why wire and ram EDM machines are used to make fuel metering valves, printer components, molds and mold repairs.

Fig. 3 - Numerical Precision also produced this fin deployment actuator. Using EDM eliminated the need for expensive broach tooling to form the through T-slot configuration into this forging.

Low Cost Tooling

A final consideration for using EDM is when a part otherwise requires conventional cutting tools that are special or unique. Electrodes are easy to machine, unlike carbide. Equally important, the wire used by a wire EDM is available as a standard, off-the-shelf tool. An example of a workpiece produced because EDM eliminated special cutting tools is the actuator housing for a missile shown in Figure 3. Thanks to EDM, expensive broaches were not needed in this instance.

EDM is a low-cost tooling option when you need short run stamping (under 5,000 pieces) and low-volume broaching. With EDM, there's no need to make a die set. EDM is used to make sewing machine components and prototypes in this way. Likewise, as an alternative to expensive broaches, EDM is often used to produce splines and gear teeth.

Limitations Of EDM

Applications of EDM may be restricted by the size of the worktank on an EDM machine. Here are size figures derived from popular models of standard EDM machines:

Maximum workpiece dimensions for wire EDM are about 59 inches in Y, 24 inches in Z and no limit in X. For ram EDM, workpiece maximums are about 59 inches in Y, 17 inches in Z and 98 inches in X.

Tapering on a wire EDM machine is another consideration. The maximum taper angle is ±45 degrees, although some shops report successfully producing tapers in excess of ±50 degrees. The maximum angle/height combination is 30 degrees at 16 inches high. The maximum electrical resistance for workpiece and fixture is approximately 0.5-5.0 ohm/centimeter for both wire and ram EDM machines.

The accuracy of an EDM machine is limited to about 0.00002 inch for wire EDM machines and ±0.0001 inch for ram EDM machines. Surface finish is about VDI of 0 (4 microinch) for wire and VDI of -5 (2 microinch) for ram. Finally, surface integrity is 20 millionths of an inch recast layer thickness for wire and ram EDM and 20 millionths micro crack length for wire and ram EDM. The results can be as good or better than a ground surface.

A Challenge

North American manufacturers have only begun to discover the many ways EDM can improve their operations. To find new EDM applications, the guidelines presented in Table I and Table II summarize when to use EDM according to workpiece geometry and workpiece material.

For many applications in manufacturing today, there is a trend to design parts that are exact and difficult to machine. EDM will become an increasingly viable option for manufacturing operations throughout the world, but especially in high-wage countries such as the United States.

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