What is Electrical Discharge Machining (EDM)?

Electrical discharge machining (EDM) is a method used to shape parts by removing metal through erosion. This erosion is caused by the extremely high temperature instantly generated by a pulse discharge that occurs between a tool electrode and a workpiece electrode.

The diagram illustrates the principle of an EDM setup. The two poles of a pulse generator are connected to the tool electrode and the workpiece, respectively. When the two poles are brought close together within the working fluid, the voltage across the gap breaks down the fluid, creating a spark discharge. This discharge instantly generates a massive amount of heat in the discharge channel, reaching very high temperatures (over 10,000 °C). This intense heat causes tiny, local areas of the part and tool surfaces to melt or even vaporize, resulting in erosion that leaves a minute crater.

As this discharge process repeats many times, the surface of the part is covered with countless very small craters. The electrode is continuously fed downward, and the outline of the tool electrode is essentially replicated onto the workpiece, completing the machining of the part.

Components of an EDM Machine

An EDM machine consists of a pulse power supply, an automatic feed control system, the machine body (the main structure), and a working fluid circulation and filtration system.

Pulse Power Supply 

The role of the pulse power supply is to convert standard 50Hz AC electricity into high-frequency pulsed power. This power is applied to the tool electrode and the workpiece to provide the necessary energy for the electrical discharge. The pulse generator shown in the diagram is a fundamental type, made up of a resistor (R) and a capacitor (C). A DC power source (E) charges the capacitor (C) through the resistor (R). As the voltage across the capacitor increases, it reaches a specific limit where the gap between the tool electrode (cathode) and the workpiece (anode) is broken down, resulting in a spark discharge. During the spark, the capacitor releases its stored energy, the voltage between the electrodes suddenly drops, and the working fluid regains its insulating properties. The power supply then immediately begins to recharge the capacitor. This cycle continuously repeats, generating thousands to tens of pulse discharges per second.

EDM must use pulse discharge. During the pulse interval between each discharge, the liquid medium between the electrodes must have enough time to restore its insulating state. This is essential so that the next pulse will break down the gap and discharge at a different, relatively closest point, preventing continuous discharge at the same spot, which would form a stable electric arc. A stable electric arc lasts too long, creates a deeper melted layer, and would only result in welding or cutting, making it impossible to achieve the necessary accuracy and smooth finish for dimensional machining.

In the EDM process, both the workpiece and the tool electrode are eroded. However, the rate of erosion is different between the anode (connected to the positive terminal) and the cathode (connected to the negative terminal). This phenomenon is called the “pole effect.” To reduce tool electrode wear and improve machining accuracy and production efficiency, it is always desirable to maximize the pole effect: the part should erode as fast as possible, and the tool should erode as slowly as possible. Therefore, a DC pulse power supply is the required choice for EDM. If an AC pulse power supply were used, the polarity of the workpiece and the tool would constantly change, making the total pole effect zero. The pole effect is typically related to factors like pulse width, electrode material, and the energy of a single pulse, which collectively determine the optimal polarity selection for a given job.

Automatic Feed Control System 

The automatic feed control system adjusts the feed speed of the tool electrode to maintain the required discharge gap between the tool and the workpiece, ensuring that the pulse discharge process operates correctly.

Machine Body 

The machine body is the mechanical device used to clamp, secure, and move the tool electrode and the workpiece.

Working Fluid Circulation and Filtration System 

The working fluid circulation and filtration system forces clean working fluid through the gap between the tool electrode and the workpiece under pressure. This promptly flushes away the eroded particles and then filters the fluid for continuous reuse. Kerosene or machine oil is the most common working fluid used today.

EDM machines are available in a range of products. Based on the machining method, they can be categorized into two main types:

1. Sinker EDM (Die Sinking EDM): This machine uses a specially shaped tool electrode to machine the corresponding features on a part.
2. Wire EDM (Wire-Cut EDM): This machine uses a wire electrode to machine two-dimensional profile shapes on a part.

Wire EDM cuts a part using electrical erosion created by a pulse discharge between a continuously moving metal wire (the tool electrode) and the workpiece. The electrode wire used for wire-cutting is very thin (typically 0.04mm to 0.25mm in diameter) and made of molybdenum, tungsten, or copper. This process can offer a high level of accuracy, with tolerances as tight as ±0.005mm, and a surface roughness (Ra) between 3.2μm and 1.6μm. It is capable of machining precise, narrow, and complex cavities and is used in the production of molds and forming tools.

The diagram shows a Wire EDM device. The wire spool alternates its rotation direction to feed the wire. The pulse power supply provides the machining energy, causing discharges to occur between the continuously moving wire and the workpiece. The CNC worktable holding the workpiece can move independently along the X and Y coordinate axes, which are combined to create various motion paths and machine the part into the desired shape.

Compared to Sinker EDM, Wire EDM does not require a specialized tool electrode. Furthermore, the metal wire, which acts as the electrode, is constantly moving during machining, resulting in virtually no wear. For machining the same part, the total amount of material removed by wire EDM is less than with standard sinker EDM, which means a much higher production efficiency while allowing the machine to operate with lower power.

Features and Applications of EDM

EDM is suitable for machining metal materials that are good electrical conductors, regardless of the material’s strength, hardness, toughness, or melting point. This makes it an effective method for machining materials that are challenging to traditional manufacturing, such as heat-resistant steel, hardened steel, and cemented carbide.

Since there is no direct contact between the tool and the workpiece during the process, there are no cutting forces. This means the tool electrode can be made from softer materials like pure copper or graphite. It also allows for the machining of thin-walled parts, small holes, and narrow slots without the concern that the tool or workpiece rigidity is too low for the operation. Additionally, it can be used for the single-step machining of various complex cavities and three-dimensional curved surfaces without the worry that a large machining area will result in excessive cutting forces.

In the electrical discharge machining service, a set of tuned electrical parameters, such as voltage, current, frequency, and pulse width, is called a machining specification. These specifications are divided into two types: roughing and finishing, to suit different processing needs. The selection of the machining specification is closely related to the required dimensional accuracy and surface roughness of the part. Generally, the dimensional error for hole drilling using the finishing specification can be as low as 0.05mm, and about 0.1mm for cavity machining, with a surface roughness (Ra) value ranging from 3.2μm to 0.8μm. EDM has a very wide range of applications, including:

• Machining various types of holes, such as holes for punching dies, wire drawing dies, and spinnerets.
• Machining 3-dimensional curved cavities, such as the cavities in forging dies, die-casting molds, and plastic molds.
• Performing cutting, sectioning, surface strengthening, engraving, and marking nameplates and tags.

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