What is EDM? ...
EDM or Electrical Discharge Machining , is a non-traditional machining method primarily used for cutting complex or odd shapes in hardened metals or materials that would be impossible to cut with traditional machining methods. The main limitation of Electrical Discharge Machining is that it can only cut materials that are electrically conductive. EDM is especially well suited for cutting intricate contours or delicate cavities that would be difficult to produce with a mill, grinder or other conventional machine tool.
How does it work? ...
Anyone who has ever seen what happens when a bolt of lightening strikes the ground has a fair idea of the process of Electrical Discharge Machining. EDM removes material by producing a rapid series of repetitive direct-current (DC) electrical discharges. These electrical discharges are passed between an electrode and the piece of metal or material being machined. The EDM cutting action works by bombarding the work piece with short but intense bursts (sparks) of electricity. Each discharged spark leaves a tiny crater or pit in the workspace. A die-electric oil or fluid causes the melted material to cool and harden before it can re-adhere to the workspace. The electrical pluses are very short in duration, approximately 1 microsecond to 1 millisecond each. This process repeats its self over and over. With thousands to millions of these craters or pits being created every second, the EDM cutting action results. The small amount of material that is removed from the work piece with each electrical discharge (pulse) is flushed away with the continuously flowing die-electric oil or fluid. The repeating discharges create a set of successively deeper pits or craters in the work piece until the final shape is produced. If you were to observe the EDM process under a microscope, you would discover that the electrode itself does not actually touch the material being cut. EDM is sometimes known as "spark machining" because of this action.
Who developed the EDM process? ...
The history of the EDM process dates back to the days of World War I and II. Two Soviet scientists, Doctor B.R. Lazarenko and Doctor N.I. Lazarenko were studying why electrical breaker and contact points were suffering degradation from material transfer. Through their investigation, it was discovered that material transfer could controlled by varying the electrical properties and materials. With this understanding in hand, they made the first attempts to remove material with the cutting action of electricity. The initial problems included as much electrode material being removed as was material being removed from the work piece as well as the manual feed mechanism they were using lead to more short circuits (DC arcing) than actual cutting discharges. At the beginning few saw any potential benefits of this process and support of this primitive technology was scarce. It was during this time the vibrating circuit (dither) came into being, this represented the first attempt toward controlling the actual spark and spark gap. Vibrating the electrode allowed material to be removed more effectively. Doctors B.R. and N.I. Lazarenko are credited for the invention of both the pulse circuit and a simple servo controller that helped maintain the gap width between the tool and the work piece. This greatly reduced DC arcing (short circuits) and made the EDM cut more effective. This was the turning point in the history of the Electrical Discharge Machining process.
How did it become a commercial machining process? ...
During World War II, a United States company, The Ex-Cell-O Corporation, acquired the research findings from the Soviet Union and started a secret wartime program called "Method X" in the 1940's. Through further research, refinement and development, Ex-Cell-O was the first company to offer a commercially available and effective machine tool in the 1950's. It was called the XLO EDM Machine Tool and it turned EDM into the viable and profitable machining process we know today.
What are the primary methods? ...
There are two primary EDM methods: Conventional EDM , also known as "Ram", "Sinker" or "Die-Sinker" EDM and Wire EDM . The primary difference between the two involves the type of "electrode" that is used to perform the machining.
Conventional EDM ...
In a typical Conventional EDM application, a graphite or copper electrode is machined with traditional machining tools. The now specially shaped electrode is connected to the power source (power supply), attached to a hydraulic or servo-electric ram, and slowly fed into the work piece. The entire machining operation is usually performed while submerged in a fluid bath of die-electric fluid or oil. The die-electric fluid serves the following three purposes: It acts as a conductor for the current to pass between the electrode and the work piece, it serves as a coolant to minimize the heat affected zone (preventing potential damage to the work piece) and hardening the cut chip as it flushes the material cuttings away.
Wire EDM ...
In wire EDM, a very thin moving wire serves as the electrode, much like a band saw. Specially made brass or copper wires are typically used and the wire is slowly fed through the material. Wire EDM is usually performed in a bath of de-ionized water. The path of the wire is typically controlled by a computer, which allows extremely complex shapes to be produced. The best way to explain Wire EDM is to use this analogy- Imagine stretching a thin metal wire between your hands and sliding it though a block of modeling clay cutting any shape you want. You can alter the positions of your hands on either side of the modeling clay to define complex and curved shapes. Wire EDM works in a similar fashion, except Electrical Discharge Machining can handle some of the hardest materials used in industry.
The main parameters involved in EDM machining ...
- Electrical Pulse On Time -
The amount of time current runs into the gap before it is turned off. The typical "On Time" ranges from a millionth to a thousandth of a second. The more "On Time", the larger the craters or pits, the faster the cut and the rougher the EDM'd surface is.
- Electrical Pulse Off Time -
The amount of time the current is off after making a single crater or pit. The typical "Off Time" ranges from a few millionths to a thousandth of a second.
- Peak Current -
The "Peak Current" is the intensity of the electricity as it flows into the work piece forming the crater or pit. A higher "Peak Current" makes the crater or pit form faster and larger, hence rougher and faster.
- The Servo System -
The "Servo System" automatically positions the electrode just the right distance from the work piece to maintain the proper gap during the EDM process. This gap is normally .0005 to .05 inches, depending on the main parameters. As the electrode cuts the work piece, the "Servo System" advances and retracts to maintain the spark gap. Because of the continued production of waste material in the gap, the "Servo System" must continually adjust the gap to minimize DC arcs (short circuits), created by the waste particles.
Final notes ...
EDM is one of the earliest non-traditional manufacturing processes. With the advent of ever-improving electronic circuitry, advance motion techniques, computer numeric controls (CNC) and other modern controlling mechanism's, EDM machine tools have become extremely reliable, accurate and dependable. As the Electrical Discharge Machining industry matured, it has become a standard method of producing complex shaped metal prototypes and even some production parts, particularly in low volume applications.
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