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When Should You EDM?

By Harry Moser

GF AgieCharmilles

Introduction

In the 1995 version of this article, I suggested that EDM could also stand for Exact Difficult Machining because the applications best suited for this process were characterized by extremely exacting tolerances in situations it would be extremely difficult or impossible to handle with any other method of machining. Increasingly, due to the dramatic improvements in EDM machine performance and automation, two other sets of words could 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 U.S., because the EDM process is so much less labor intensive than almost any other machining process.

Electrical discharge machining (EDM) has been a growing force in North American tool, die and mold making shops since the 1950s. During 2003, the sale of wire and ram (diesinker) electrical discharge machines (EDMs) represented 7.3 percent of total metal-cutting machine tool sales dollars in the United States. EDMs are used to produce tooling (molds, stamping dies, extrusion dies, forging dies, fixtures and gauges) and parts for the aircraft, medical and other industries. Over the last ten years, from 1994 to 2004, the relative size of these two categories has shifted dramatically.

EDM unit sales rose consistently through 1998 and then were approximately halved by 2003. With the subsequent recovery, sales in 2005 will be back close to 1994 levels. However, the mix has changed from being overwhelmingly tooling to being more evenly split between tooling and parts. The purchase of EDM machines for tooling fell sharply due to the loss of tooling and plastic parts production to Southeast Asian manufacturers. In contrast, the medical and other markets have grown strongly.

EDM (Efficient Domestic Manufacturing™)

The trend to the usage of EDM for parts production has several causes:

1. The EDM cutting process has improved dramatically with higher speeds.
   
   
2. Automation has dramatically improved, significantly cutting labor content. For example, the percentage of wire EDMs sold with automatic wire threading has gone from 40% in 1994 to about 90% in 2004. In addition, modern threaders are much more reliable than earlier versions.
   
   
3. With a strong trend towards robotic workpiece and, for the diesinkers, tool loading and unloading, it is now possible to man one shift and cut on three.

An Overview Of EDM

The origin of electrical discharge machining 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 and developing a controlled process for machining materials that are conductors of electricity.

With that idea, the EDM process was born. The Lazarenkos perfected the electrical discharge process, which consisted of a succession of discharges made to take place between two conductors separated from each other by a non-conducting liquid, called a dielectric. The Lazarenkos achieved a form of immortality with this circuit, which today bears their name. Today, many EDMs 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 electrode and the workpiece are held by the machine tool, which also contains the dielectric. 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 is initiated. Under the effect of this field, electrons and positive free ions are accelerated to high velocities and rapidly form an ionized channel that conducts electricity. At this stage current can flow and the spark forms between the electrode and workpiece, causing a great number of collisions between the particles. During this process a bubble of gas develops and its pressure rises very steadily until a plasma zone is formed. The plasma zone quickly reaches very high temperatures, in the region of 8,000 to 12,000° Centigrade, due to the effect of the ever-increasing number of collisions. This causes 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, which projects the melted material away from the workpiece, leaving a tiny crater. The eroded material then resolidifies 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 equipment 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.

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, particularly when it comes to heights and tapers. Wire EDMs can cut parts 16 inches tall with a straightness of ±0.0005 inch per side! Cutting to 24" tall is now also available. As mentioned, the growth of EDM is now primarily in the production of parts, with the fastest growing segment being medical.

EDM (Every Design is Manufacturable™)

In the past, EDM was used to produce the parts that were difficult to produce with conventional processes. The growth in EDM in the last ten years has been more from producing parts that have been designed to take advantage of the EDM process. Thus, EDM is not the "last choice" for manufacturers, it is the "first choice" for the design/manufacturing team.

What has changed from 1994 to 2004?

To revise this article, I first listed below what has happened in the EDM process and in the market of the companies that use EDM:

EDM Process

• Much faster.
• Much more automated - Automatic wire threading, robots, automatic slug eject, easier programming and maintenance.
• More accurate.
• Smaller diameter wires.
• Machine prices and operating costs down.
• Better surface finish and surface integrity.
• Carbide: 0% cobalt depletion on wire, better cutting on sinker.
• Simultaneous wire EDMing and workpiece rotation.
• No external flushing required on sinker.
• More effective in difficult flushing conditions.
• Much more user friendly (less training needed, less programming time).
• Better training and customer support (easier for first time user).
• The cost and quality of complex graphite electrodes much improved due to HSM.
• Operating cost is down absolutely and relative to other machine processes- cost of wire per part produced is down in contrast to the milling and turning processes in which the tool cost per part is up dramatically to obtain shorter cycle times.

Users' Market

• Faster delivery, smaller lot sizes. (JIT)
• Lower prices, low wage competition (SE Asia).
• MEMS (small features, difficult-to-fixture parts)

Handy Checklist of When to Consider EDM

The following checklist summarizes the characteristics that would suggest that you consider EDM. The more of these characteristics that are present, the more likely that EDM is the right solution.

Geometry

• Very thin walls
• Small internal radii
• High depth:diameter ratios
• Very small, hard to fixture

Material

• Hard
• Tough
• Leaves burrs
• Has to be heat treated

By Process Replaced

• Eliminates multiple processes, fixturing, handling
• Broaching
• Short-run stamping

Other

Want 24-hour production from 1 manned shift.

Want a process that is not labor intense.

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

When Should You EDM?

Whether you're a mold shop owner looking to replace contour form grinding for core and cavity details or a process engineer who wants to explore how EDM can streamline design and production capabilities, what criteria should you use to determine "When to EDM™?"

This question was posed to manufacturing engineers, shop owners and our own technical support specialists. They identified a range of appropriate workpiece materials and geometries, plus the processes EDM can replace. For this article, our recommendations are sorted based on three physical characteristics of the EDM process:

• No force between tool and workpiece.
• Workpiece is vaporized not cut.
• No rotation of tool or workpiece.

No Force

Since EDM does not involve workpiece/tool contact or forces like a mill or grinder, it is possible to EDM shapes that would break conventional cutting tools or be broken by them. Thus, EDM is well-suited for making frail or fragile parts that cannot take the stress of machining. The type of parts that fit this profile includes printer hammers in dot matrix printers, graphite electrodes and any part that features tough-to-machine honeycomb shapes.

Conventional machining has trouble with thin walls. EDM, on the other hand, is ideal for this application because the process does not involve force, contact or deformation. Accordingly, a wire EDM is capable of making walls as thin as 0.005 inch. A ram EDM can produce walls as thin as 0.002 inch. EDMs designed for making very small holes can create walls as thin as 0.0002 inch. Examples of thin-walled parts produced by EDM include surgical tools, microwave horns and the satellite structural components.

Consider using EDM when parts have high ratios of cavity depth to width, such as slots and ribs. Since there is no force between the tool and the workpiece, you can use very long electrodes to make extremely intricate ribs. Wire and ram EDMs are used to make fixtures, collets, jet engine blade slots, mold cooling ribs and reinforcing ribs.

If you have a difficult recessed cut to make, you'll probably need to use a ram EDM. 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? Since the electrode does not come in contact with the material, there's no adhesion of the workpiece to the tool. This fact makes wire and ram EDMs ideal for making magnetic reader heads for missiles, turbine blades and car engine prototypes.

Vaporizing

Do you have an extremely hard material to cut? EDM is not influenced by hardness of material, so it's ideal for cutting materials that have a hardness above Rc 38, including hardened steel, Stellite and tungsten carbide. Since the EDM process vaporizes material, instead of cutting it, the hardness of material is not a factor. That's why wire and ram EDMs are used to create complex dies and other tools from extremely hard materials.

If you're making a part or product with a material that tends to leave tough burrs when traditional machining is used, EDM can solve the problem for you. With traditional machining, burrs are typically left along workpiece edges. In contrast, the EDM process leaves no burrs and the vaporized material is flushed away by the dielectric. By eliminating the deburring process, EDM eliminates extra operations and the potential of causing dimensional changes that may occur during the deburring process. As a result, 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 an EDM is as small as the spark gap, plus the radius of the wire or the electrode corner. In milling and turning, tools or workpieces rotate. The smallest workpiece radius is equal to the tool radius. In contrast, EDM electrodes generally don't rotate and since there is no force, very small, long tools can be used. In cutting a 1" thick piece of D2 the limiting radius would be:

• EDM about 0.0025" with a 0.004" wire.
• HSM about 0.050" with a 0.1" milling cutter.

The minimum dimensions of features that can be EDMd are approximately:

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

Low Cost Tooling

A final consideration for when it's appropriate to EDM is when a part otherwise requires special/unique conventional cutting tools. 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 product produced because of EDM's ability to eliminate special cutting tools is the actuator housing for a missile shown in Figure 3. EDM eliminated the need for expensive broaches 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. That's why EDM is used to make sewing machine components and prototypes. Instead of using expensive broaches, EDM is a very low-cost tooling way to make parts. This is a reason companies use EDM to produce splines and gear teeth.

Limitations Of EDM

Clearly, the benefits of EDM are considerable, and it is often appropriate to EDM instead of using conventional manufacturing processes. But not always. What are some of the restrictions of EDM? Here are some to consider for popular models of standard EDMs:

• Maximum workpiece dimensions for wire EDM are about Y of 59 inches, Z of 24 inches and an X of no limit. For ram EDM, workpiece maximums are about Y of 59 inches, Z of 17 inches and X of 98 inches.
  
  
• Wire EDM tapering 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 sinker EDMs.
  
  
• The accuracy of an EDM is limited to about 0.00002 inch for wire EDM and ±0.0001 inch for ram EDMs.
  
  
• Surface finish is about VDI of 0 (4 microinch) for wire and VDI of -5(2 microinch) for sinkers.
  
  
• Finally, surface integrity is 20 millionths of an inch recast layer thickness for wire and ram EDMs and 20 millionths micro crack length for wire and ram EDMs. The result 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 help you find new EDM applications, I have summarized the guidelines of "When to EDM" by materials and geometry in Tables I and 11.