Electrical Discharge Machining: A Process for Hard Materials and Complex Design

The state of the art now includes automatic tool changing, wire threading, and part programming, as well as extremely fine surface finishes

By Mark Shortt
Editorial Director, Design-2-Part Magazine

Since its development in the 1940s, electrical discharge machining (EDM) - or spark-erosion machining - has gained wide acceptance among manufacturers, especially those in the aircraft/aerospace and medical industries. A process that precisely removes metal from electrically conductive materials, both soft and hard, EDM has long been thought of as a nontraditional or unconventional machining method. Known as ram or sinker EDM when it was introduced in the 1940s, the process was first employed primarily in the tool and die industry to remove broken taps and drills from dies being manufactured.

But EDM has come a long way from its origins in the manufacture of precision tooling and dies. Currently, it is just as likely to be used for production quantities of aircraft, medical, and electronic parts, as for tooling and prototypes. Turbine disks for aircraft engines, and airfoil shapes for ground-based turbines are among its many current applications, which also encompass die cavities for large automotive body components; narrow slots, turbine blades, and various intricate shapes (Manufacturing Engineering and Technology (Kalpakjian, 3rd ed., p. 837). Other major uses of the process include machining carbide stock and producing metal molds and dies for stamping, forging, and jewelry manufacture. (Amstead, et al., Manufacturing Processes, 8th ed., p. 629).

"For certain applications, like burning through hardened steel and other difficult-to-machine alloys like carbide, it can't be beat," said Kevin Dawson, president and owner of American Wire EDM, Inc., Orange, California. "EDM also gives excellent surface finish and high precision with near-sharp inside corners, along with tapering capabilities," he said.

Widely seen as a reliable and precise machining technology, EDM provides burr-free, multi-axis machining of parts that, because of hardness or shape, may be difficult or impossible to machine by other methods. And although the process is slower than laser machining - its speeds are described in inches per hour as opposed to the inches per second that characterize laser cutting - it is capable of holding tighter tolerances.

Whereas a laser's depth of cut is limited by the thickness of the workpiece material, EDM cuts deep into hard, thick metals with no loss of precision. Exotic alloys such as Waspalloy, Hastelloy, and Inconel (as well as carbide, tool steels, stainless steel, aluminum, and copper) are machinable by the process. So are molybdenum, tungsten, and titanium.

Recent technological advances have permitted finer surface finishes, automatic tool changing, and computer-controlled table movements, according to Mr. Speth. However, EDM is not cost effective for operations where an investment in tooling and setup would allow numerous parts to be completed in short machining cycles. An example would be the broaching of an irregular keyway in several hundred parts or more, Mr. Speth said.

Automatic tool changing and computer-controlled table movements have also made life easier for EDM job shops. However, EDM is not cost effective for operations where an investment in tooling and setup would allow numerous parts to be completed in short machining cycles. An example would be the broaching of an irregular keyway in several hundred parts or more.

How EDM Works

Machining is accomplished by the action of rapidly occurring electrical discharges, or sparks, which erode small pieces of metal from the workpiece. The cutting tool is an electrode that is shaped to the contour of the required cut. Both the workpiece and the electrode are submerged in an electrically nonconducting (dielectric) fluid and connected to a dc power supply. Usually consisting of mineral oil or kerosene, the dielectric fluid functions as an insulator, coolant, and medium for flushing away debris from the tool and the workpiece.

Sent from the electrode to the workpiece at a rate of thousands per second, the sparks travel through the nonconductive fluid to reach the workpiece. The electrode vaporizes the metal without ever touching the workpiece, which is by necessity made of electrically conductive material. Electrodes are commonly made of graphite or copper tungsten; however, the range of constituent materials also includes tungsten carbide, copper, brass, and zinc alloys.

Although rates of metal removal are slower with electrical discharge machining than with other commercial machining methods, the slower removal produces better surface finishes. Higher rates of metal removal are known to produce rougher finishes that have a molten and resolidified (recast) structure with poor surface integrity and low fatigue properties (Kalpakjian, p. 838).

Certain parts can only be produced using sinker EDM, which, unlike wire EDM, does not cut all the way through the part. As its name implies, sinker EDM literally "sinks" a required shape into the workpiece.

Wire EDM

EDM EDM uses a thin piece of wire - typically made of brass, copper, or tungsten - as the electrode. A variation of conventional ram or sinker EDM, the process is said to be similar to contour cutting with a band saw. Because it uses the heat of an electrical conductor to vaporize material without the aid of cutting forces, it is well suited to machining parts that have complex geometries. Today's 5-axis machines are capable of producing geometries "which cannot be produced with any other machining techniques," according to Bob Tarantino, president, New Jersey Precision Technologies, Mountainside, New Jersey.

The process is also used to cut thick metal plates and produce a variety of tools, punches, and dies from hard metals. Gears, rotors, turbine blades, and intricate electronics components are all produced by means of wire EDM, also known as electrical-discharge wire cutting.

A vertically oriented wire, constantly renewed as it travels from a supply spool to a take-up spool, is fed into the workpiece. A power supply provides voltage between the wire and the workpiece; deionized water is used as a flush/coolant.

Maximum cutting speeds of wire EDM increased dramatically during the 1980s, going from approximately 3 square inches per hour to between 18 and 20 square inches per hour, according to Larry McNellis, president, AcuCut Inc., Southington, Connecticut. Since then, speeds have increased to about 25 square inches per hour, depending on the workpiece material and thickness, Mr. McNellis said.

Wires have typical diameters of 0.010-0.012 inch. As wire diameters have decreased through the years, narrower cuts have become possible, but at significantly higher costs. Today, wires can be as thin as 0.001-0.003 inch in diameter.

"Years ago, we cut something using 8/10-inch-diameter wire," said Mr. McNellis. "Now, the normal size is 10-to-12 thousandths diameter. We've also cut parts over 36 inches thick," he said.

One of the keys to maintaining this versatility is the ability to change tooling to suit the different requirements of each job. Most of today's CNC machines are equipped with automatic tool changers, which enable companies to do just that.

Today's multi-axis, CNC wire EDM machines have incorporated a number of process enhancements that have delivered greater automation. In addition to automatic tool changers, capabilities include automatic self-threading of the wire in case of breakage, multiheads that permit cutting of more than one part at a time, and programmed machining strategies.

Some companies, such as AcuCut, also use a central filtering system to filter water through all of their wire EDM machines. The technology is economical because it eliminates the need for filters at each machine.

Contract EDM Shops

AcuCut Inc. runs 18 wire and seven sinker EDM machines at its 30,000-square-foot facility in Southington, Connecticut. In addition to wire and CNC sinker EDM, the company performs CNC small-hole EDM drilling and laser cutting.

"Just over 50% of our work is in aircraft parts, including various engine parts for disks," Larry McNellis said. "We do a lot of work for companies that manufacture land-based generating units used for electrical power generation. We also do some work for arms manufacturers and medical companies."

During a recent tour of the AcuCut facility, Mr. McNellis pointed out some of the parts, including airfoil shapes used in ground-based turbines, currently being machined by the firm. A wire EDM machine, using a deionized water coolant, was operating unattended as it methodically machined an aerospace component. Looping through a series of pulleys were thin threads of brass wire, 0.010 inch in diameter, which functioned as the electrode.

Some of the firm's parts are run on more than one type of EDM. "One part came in as a ring," Mr. McNellis said. "we wire-EDM'd it first into segments. Then, on the sinker machines, we put in seal slots. We then drilled a series of holes in the part on a small-hole EDM machine," he explained.

Sinker EDM, according to Mr. McNellis, is well suited to machining of seal slots in aerospace components. It is also used on components for ground-based turbines, joint replacement parts for the medical industry, and for hardened alloys. With the process, AcuCut reportedly achieves "low micro-inch surface finishes" and tolerances to +/- 0.0002 inch.

Sinker EDM machines were originally used for mold work, but AcuCut now uses them almost exclusively for production jobs. "Years ago, 90% of our work was tool and die components, whereas now, 90% is production work," said Mr. McNellis. "We've found our niche in production EDM."

EDMIts 48 high-speed EDM machines reportedly give Adron Tool Corporation, Menomonee Falls, Wis., the distinction of being the largest wire EDM shop in the United States. The company, which produces precision parts, prototypes, and tooling, supplies wire EDM services to OEM customers throughout the United States and in virtually every industry. Manufacturers in the aerospace, defense, medical, and nuclear industries are among the firm's customers, which also include companies in the electronics, automotive, and consumer products sectors.

Adron, an ISO 9002-certified company, employs 50 people and runs three shifts at its 30,000-sq-ft facility. Typical production EDM jobs include hydraulic valve parts, SSC parts for coilwinders, parts for automotive gear racks, and small carbide draw dies. In addition to wire EDM machinery, the firm's equipment lineup includes submersible, 5-axis conventional, and small-hole EDM drills. Adron's largest wire machine handles workpieces up to 10,000 lbs. and 27 1/2 inches thick.

Reliable EDM, Inc., Houston, Texas, performs EDM and wire EDM for manufacturers in the aerospace, defense, petroleum, plastics, electronics, and medical industries. According to Carl Sommer, president, wire EDM is the core business of the company, reported to be "the largest wire EDM job shop west of the Mississippi."

"We can wire EDM parts that are 38 inches tall, and cut a single-hole cavity with wire EDM in a tube up to 22 inches deep," said Mr. Sommer. "Our machines can also cut parts 18 inches tall submersed, cut 45-degree tapers up to 15 3/4 inches tall, and cut parts weighing over 3000 pounds. We can cut parts to +/- 0.0001 inch accuracy," he said.

Reliable EDM runs 24 wire EDM machines at its 12,500-square-foot facility in Houston. The firm, which assists in tool design, engineering, and application support, can receive AutoCAD, DXF, and IGES files through modem or e-mail.

"We get many jobs that can be done with conventional machines, but because of the efficiency of wire EDM in cutting any type of electrically conductive material, many manufacturers use wire EDM and outsource the work," he explained.

One of the major technological advances in the EDM process today, Mr. Sommer said, is the ready availability of AC anti-electrolysis machining capabilities. Cutting with AC permits more heat to be absorbed by the wire rather than the workpiece, he noted.

"These power supplies improve the surface finish of parts by reducing rust and oxidizing effects of wire EDM," he explained. "Also, less cobalt binder depletion occurs when cutting carbide, and it eliminates the production of blue lines when cutting titanium," he added.

Another advance is that today's machines produce minimal recast layers, unlike some of the older wire EDM machines, which typically left deeper recast layers. Because of this improvement, wire EDM can now be used for cutting high-speed turbine blades, Mr. Sommer said.

It's not unusual for the company to modify a machine beyond its normal capabilities in order to accommodate a job. "A customer of ours came to us and asked us to cut four keyways into a part measuring 37 inches," Mr. Sommer explained. "Our machines did not have that capability." But after considering how to solve the problem, Mr. Sommer - an experienced tool and die maker and machinist - knew the answer. The company modified one of its machines to cut 38 inches, and completed the job.

"Another customer wanted to cut one-hole cavities in tubes that had carbide inserts," he added. "We modified a machine that enables us to cut a single hole cavity to 22 inches deep."

References

  1. Amstead, B.H., Ostwald, P.F., and Begeman, M.L. 1987. Manufacturing Processes, p. 658. 8th ed. New York: John Wiley & Sons.

  2. Kalpakjian, Serope. 1995. Manufacturing Engineering and Technology, p. 1028. 3d edition. Reading, Massachusetts: Addison-Wesley Publishing Co.

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