Electrical Discharge Machining

Electrical Discharge Machining (EDM) is an advanced subtractive manufacturing process that uses electrical discharges machine conductive workpieces. This machining technology can create highly complex geometries and machine extremely hard materials that are difficult to achieve via conventional CNC machining processes.

This article explores the fundamentals of EDM, its key processes, advantages, limitations, and applications across various industries, provides valuable insights into how EDM can enhance modern production capabilities.

What is Electrical Discharge Machining

Electrical Discharge Machining (EDM) is a non-contact subtractive manufacturing technology that creates parts by progressively eroding portions of a workpiece using electrical-generated thermal energy. Also known as spark machining, this technology uses a series of precisely generated, positioned, and controlled sparks (electrical discharges) to remove material from specific areas of a workpiece. Unlike conventional CNC machining services such as milling and turning that use contact cutting tools and mechanical energy, EDM machines utilise electrical and thermal energy for material removal. In addition, there is no contact with the workpiece throughout the machining process.

In the electrical discharge machining process, the tool and workpiece act as electrodes and are connected to a DC power supply. The electrodes are submerged in a dielectric fluid and positioned with a slight gap between them. When DC electricity is passed through the electrodes, an electrical discharge forms in the gap between them, creating intense thermal energy that melts or vapourises a localised portion of the workpiece.

EDM machining is an electrical process that relies on materials' conductivities to work. Therefore, it is compatible with electrically conductive materials. However, several techniques are under development for machining non-conductive materials using EDM manufacturing.

Electrical discharge machining is primarily employed for machining extremely hard materials, such as tungsten, titanium alloys, and hardened tool steel. This highly precise technique is also used to create certain complex geometries that are not achievable using conventional machining or advanced 5 axis CNC machining. These geometries include straight internal corners, curved holes, and internal cavities.

How Does Electrical Discharge Machining Work?

Electrical discharge machining utilises a process known as electro-thermal erosion or spark erosion, a process of material removal through localised melting and vaporisation caused by high-temperature electrical discharges between a tool electrode and a workpiece, both submerged in a dielectric fluid. In this process, an electrically conductive tool and the workpiece to be machined are connected in an open circuit to a DC power supply. The tool material may be copper, graphite, or brass, among other conductive materials. Both the tool and the workpiece serve as electrodes, with the tool as the cathode (-ve) and the workpiece as the anode (+ve) in most practical EDM cutting applications. Note that the electrode polarities may be reversed to control parameters, such as surface finish and tool wear rate.

The electrodes (tool and workpiece) are submerged in a dielectric fluid with insulating properties. They are positioned to prevent contact between them while maintaining a predetermined distance called the spark gap. Since the electrodes are submerged, the dielectric fluid fills the spark gap.

The power supply sends a pulse of DC electricity between the electrodes creating an intense electric field that polarises the initially insulating dielectric fluid. At points where the spark gap is smallest (due to surface roughness or imperfections) the electric field becomes intense enough to cause electrons to pull out of the tool electrode due to electrostatic forces and quantum tunnelling, in a process called field emission. The electric field accelerates the emitted electrons causing them to gain significant kinetic energy as they travel through the spark gap. These high-energy electrons collide with polarised but neutral atoms or molecules of the dielectric fluid, causing ionisation - the splitting of neutral molecules into positive ions and free electrons. The free dielectric electrons collide with other neutral molecules, causing a cyclic ripple secondary electron emission effect where the number of free charges (ions and electrons) multiplies exponentially.

As ionisation continues the concentration of ions and electrons increases to the point of dielectric breakdown, where the dielectric fluid loses its insulation and the spark gap becomes electrically conductive. This creates a plasma channel between the tool electrode and workpiece, allowing a high current to pass through the channel for microseconds. This flow of current, i.e., the rapid movement of ions to the tool and electrons to the workpiece is characterised by an electric spark, heating up the channel to up to 12,000ºC and creating immense pressure in a tiny concerntraded area.

The intense heat causes localised melting and vaporisation of the workpiece and the tool electrode, removing small portions of material. As the electrical pulse ends, the plasma channel collapses. Rapid cooling occurs, solidifying some molten material and vaporised material. The rapid expansion of vaporised material creates micro-explosions in the spark gap, ejecting molten material into the dielectric fluid as debris.

Lastly, the dielectric fluid flushes away debris and cools the gap, restoring insulation between the electrodes. The EDM machine raises the tool electrode and brings it down, moving it deeper by a distance equivalent to the material removed, in order to maintain the spark gap.

The entire electric discharge machining process, from the power supply providing a pulse to the flushing of the dielectric liquid is nearly instantaneous, occurring in milliseconds to microseconds. The entire process repeats thousands of times per second, with each cycle removing a minuscule amount of material. The EDM machine continuously moves the tool electrode in an oscillating manner, reversing it and progressively moving the tool electrode deeper into the workpiece to maintain the spark gap as material removal occurs.

The choice of specific components, such as tool material, dielectric fluid, and power supply, as well as parameters, such as voltage, spark gap, and electrode polarity differ in different EDM machining processes, and determine the outcomes of the process.

Types of Electrical Discharge Machining

Similar to its CNC machining counterpart, EDM machining exists in various forms. These forms vary by the type of EDM machine, their primary applications, capabilities, and limitations.

Sinker electrical discharge machining

The sinker EDM process utilises a precisely shaped tool that "sinks" into the workpiece to create the desired cavity or feature. In this electrical discharge machining technique, the tool electrode is fabricated in the negative or positive shape of the desired geometry, similar to a stamp or die. The tool electrode is typically custom-made and is exclusive to the desired geometry. For highly complex shapes, multiple electrodes may be required to achieve the final geometry. During the sinker EDM machining process, the tool remains stationary in the X and Y axes, moving only in the Z axes. As a result of this motion, this EDM process is also referred to as Ram EDM machining. A form of sinker EDM machining exists in which the tool can move around the surface of the workpiece in a technique similar to CNC milling. This technique is called EDM milling.

Sinker EDM machining is also commonly referred to as conventional EDM machining as it is the most common electrical discharge machining technique and has the most applications. This technique can create cubic, parametric, cylindrical, circular, and many more geometries with little to no restrictions. On the flip side, sinker EDM machining is relatively slow and involves significant set up cost and time to create custom tool electrodes.

Wire electrical discharge machining

Wire EDM uses a continuously fed thin wire (usually brass, copper, or tungsten) as the electrode to cut through a workpiece in a process similar to using a wire cheese cutter. The wire is electrically charged and positioned to pass through the workpiece in a precise path determined by a computer-controlled system. The electrical discharges between the wire and the workpiece remove material, allowing for intricate, high-precision cuts.

Wire EDM machines are typically used to cut through thick metal and to produce punches, tools, and dies from hard metals that are challenging to machine using conventional methods. The wire, which is constantly fed from a spool, is held between upper and lower CNC-controlled guides. Both guides move in the x-y plane. However, the upper guide in most EDM machines can move in multiple additional axes, giving wire electrical discharge machining the ability to cut transitioning shapes, as well as tapered and circular shapes typically obtained via CNC turning.

Hole drilling electrical discharge machining

A hole Drilling EDM machine is specifically designed to create precise, small-diameter holes in hard and tough materials. In this process, a fine, tubular electrode is used to generate electrical discharges, which erode the material at the workpiece's surface to form the hole. The electrode is guided into the workpiece, and the dielectric fluid flushes away the debris created by the discharge, ensuring a clean hole. Hole Drilling EDM process is often used for applications that require deep, narrow holes that are difficult to achieve with traditional drilling methods. Common industries include aerospace, where cooling holes in turbine blades or fuel injection nozzles are needed. This technique allows for high precision, even in hard-to-machine materials like titanium, Inconel, and hardened steel.

Fast Hole Drilling and Small Hole Drilling EDM are variations designed to enhance speed and precision. Fast Hole Drilling EDM increases material removal rates for deep, small holes by using higher current or voltage, speeding up the process while maintaining accuracy. Small Hole Drilling EDM is focused on creating extremely fine holes (below 0.1 mm),and is ideal for applications like microelectronics and medical devices.

Components of an EDM machine

An electrical discharge machining setup or EDM machine comprises the following components that play various crucial roles in the EDM process. These components include:

DC Power supply
Tool electrode
Dielectric Fluid
Tool holder and fixture
Servo tool control system
Pump and filter

DC power supply

Also known as the power generator, the DC power supply provides the electrical pulse between the electrodes. In most electrical discharge machining applications, the negative terminal connects to the tool electrode and the positive terminal connects to the workpiece. The power generator may be resistance-capacitance (RC), transistor-controlled, hybrid, or impulse type, with the difference between them being current, frequency, and control.

Tool electrode

The workpiece and tool both act as electrodes. However, in most electrical discharge machining contexts, the term “electrode” refers to the tool electrode. Two crucial considerations for tool electrodes are the tool material and shape.

Tool material

Technically speaking, any conductive material can serve as the electrode tool in an EDM machine. However, the choice of the material is critical as it impacts various parameters and controls the outcome of the EDM process. It also depends on the workpiece material.

Common electric discharge machining tool electrode materials include:

Graphite: High thermal resistance, high electrical efficiency, and low wear rate. Ideal for most electrical discharge machining applications, including intricate and fine-detail machining.
Copper: Excellent electrical conductivity. Preferred for high precision and smooth surface finishes.
Copper-Tungsten Alloy: Combines copper’s conductivity with tungsten’s durability. Suitable for high-temperature applications.
Brass: Commonly used in wire EDM cutting. Offers good machinability and conductivity.
Tungsten: Very high melting point and wear resistance. Used for demanding applications requiring high durability

The key considerations for determining a material's suitabity for tool electrodes are electrical conductivity/efficiency, thermal stability/heat resistance, and wear resistance. The tool electrode’s material properties impact many aspects of the EDM machining process, one of which is tool wear.

During electrical discharge machining, the spark erodes not only the workpiece, but also the tool, albeit to a lesser extent. The ratio of workpiece erosion to tool erosion is the wear ratio and is a measure of the tool’s efficiency. Similarly the tool wear rate is the rate at which the tool electrode material is eroded during the EDM process, without considering workpiece erosion. Typically measured in terms of volume or weight loss per unit of time, wear rate determines the electrode's lifespan. Harder materials with higher melting points resist wear better. Note that various other factors including polarity, type of dielectric field, and cutting rate also impact tool wear.

In addition to wear, material properties such as conductivity and thermal stability affect material removal rate (MRR), surface finish, and machining precision. Copper and graphite, for example, provide higher MRR due to their higher conductivity. Likewise, tungsten and graphite resist high temperatures, ensuring dimensional stability.

Tool shape

Another vital aspect of the tool electrode in EDM manufacturing is the tool shape. Three categories of tool shapes exist in electrical discharge machining, and these shapes depend on the type of EDM machining. In sinker EDM machining, the tool is crafted, via machining or casting, in the shape of the geometry to be machined, similar to a stamp. For example, as in the image below, the tool electrode may be a negative or positive impression of the intended geometry.

Image

As the machining progresses, the EDM machine moves the electrode deeper into the workpiece, progressively “digging” out the intended geometry. The tool shape can range from simple (such as a knife for slicing through rods) to very complex. In certain applications, multiple electrodes and repositioning the workpiece may be necessary to accomplish the final product geometry. Wire EDM cutting utilises a wire held between two guides as the tool electrode.

Tool electrodes may feature a passage that allows dielectric fluid to flow through it to flush the spark gap. The fluid flow and corresponding flushing action may be via suction, where the debris flows up through and out of the tool electrode passage, or ejection, where the dielectric fluid flows forcefully downwards, ejecting the debris into the surrounding fluid and out through an exit. In both scenarios, a pump provides the necessary force.

Servo control system

The servo control mechanism in an electrical discharge machine maintains a critical gap between the tool electrode and the workpiece. This mechanism is essential in both wire and vertical EDM machines to prevent physical contact between the electrode and the workpiece, as such contact can lead to arcing, damage to the workpiece, or breakage of the wire. Throughout the EDM machining operation, the servo system adjusts the electrode’s position relative to the workpiece, ensuring the correct arc gap is maintained by continuously sensing the tool-to-workpiece distance.

As the EDM process removes material from both the tool and the workpiece, the gap naturally increases. To maintain a constant arc gap and voltage, the system uses a feed control device that adjusts the tool’s position. The system must respond quickly and with low inertia to avoid overshooting, which could close the gap and result in short circuits. The servo mechanism, regulated by signals from an electrical sensor monitoring gap voltage or working current, executes a rapid reversing feed motion. This precise and responsive control mechanism is vital for the effective functioning of an EDM machine.

Tool holder and fixture

The tool holder and the fixture are holding devices in an EDM machine. As the name suggests, the tool holder holds the tool electrode in place and positions it correctly, relative to the workplace. It also connects the tool to the control mechanism. In conventional electrical discharge machining and hole drilling electrical discharge machining, the tool holder has clamping or attachment features that hold the tool above the workpiece. In wire EDM cutting, on the other hand, fixtures, known as guides, serve as the tool holder. These CNC-controlled guides hold the wire in place and precisely control its position and movement in various axes.

On the other hand, the fixture holds the workpiece in place. It may be generic for simpler geometries or custom-made for more complex ones. Depending on the product's complexity, different fixtures may be necessary to reposition the workpiece during the EDM machining process.

Dielectric fluid

The dielectric fluid performs several critical functions in electrical discharge machining, and is primarily responsible for providing the charged particles (ions and electrons) that create the eroding spark. It maintains insulation between the tool electrode and the workpiece electrode until the breakdown voltage. When electric pulse is supplied, the fluid polarises and subsequently ionises, creating a conductive path between the electrodes. After the spark, the dieletric fluid deionises the sparkgap, quenching the spark and insulating the gap. Subsequently, the fluid flushes away the removed debris. In addition to these, the dieletric fluid also serves as a cooling medium.

The key property of these fluids is their dielectric property - the combined ability to resist electrical conductivity (maintain insulation until breakdown voltage is reached) and also polarise in an electric field. While various types of dielectric fluids exist, hydrocarbon oils and deionised water are ubiquitous in electrical discharge machining. Some key properties of an ideal dielectric fluid are as follows:

High Electrical Strength: Ensures effective insulation against electrical breakdown.
High Fire and Flash Points: Minimises fire hazards during operation.
Low Viscosity with Good Wetting Properties: Facilitates efficient flushing of debris.
Chemical Neutrality: Prevents corrosion of machine components and workpieces.
Non-Toxicity: Ensures safe handling and minimal environmental impact.
Low Decomposition Rate: Increases fluid longevity and reduces maintenance frequency.
Cost-Effectiveness: Maintains affordability for industrial applications.
Effective Quenching Properties: Efficiently dissipates heat to control machining temperatures.

Pump and filter

The pump and filter system enables the flushing and reuse of the dielectric fluid in the electrical discharge machining process. The pump circulates the dielectric fluid by suctioning out the debris-containing fluid, passing it through the filter to remove the debris, and pumping the clean fluid back into the tank, usually through a nozzle in or around the tool electrode. The pump continuously reticulates the dielectric fluid, effectively removing debris, cooling the spark gap, and maintaining fluid flow.

Important Parameters and Considerations in Electrical Discharge Machining

EDM machining involves several key parameters that influence its quality, efficiency, accuracy, and outcome. Understanding and optimising these parameters is necessary to achieving the desired material removal rate, surface finish, tool wear, and overall process stability.

Pulse On-Time (Ton)

In electrical discharge machining, this parameter refers to the duration the electrical pulse remains on during each discharge. It determines the amount of energy delivered to the workpiece. Longer on-times increase material removal rates but can lead to higher tool wear and rougher surface finishes. Typical Ton values in EDM manufacturing range from 5 to 30 microseconds, with the optimal setting depending on material and surface quality.

Pulse Off-Time (Toff)

Pulse Off-Time (Toff) is the duration between electrical pulses when the discharge is off, allowing for cooling and debris removal. Shorter off-times can increase material removal rates but may lead to higher tool wear and surface roughness, while longer off-times improve cooling and flushing. The typical range for Toff in electrical discharge machining is between 20 microseconds and several milliseconds.

Spark Gap

The spark gap is the distance between the tool electrode and the workpiece during electrical discharge machining. A smaller gap increases the risk of short circuits, while a larger gap can reduce discharge efficiency, leading to inefficient material removal. The typical spark gap ranges from 0.01 mm to 0.1 mm, and it must be maintained precisely to ensure consistent discharge and avoid damaging the workpiece.

Peak Current (Ip)

Peak Current (Ip) is the maximum current applied during the discharge pulse. This parameter directly impacts the rate of material removal. Higher peak currents accelerate material removal but can increase tool wear and surface roughness. Depending on the material and the required machining conditions, typical Ip values range from 1 to 100 A, depending on the material and desired results of the electrical discharge machining process.

Gap Voltage (V)

Gap Voltage (V) is the voltage across the tool electrode and workpiece when a discharge occurs. It must be high enough to initiate the spark but not too high to cause inefficient discharge. If the voltage is too low, the spark might not form, slowing down the machining process. If the voltage is too high, it may cause excessive tool wear and surface roughness. Gap voltage usually ranges from 20 to 100 V, and precise control is necessary to achieve the desired electrical discharge machining results.

Material Removal Rate (MRR)

Material Removal Rate (MRR) is the rate at which material is removed from the workpiece during electrical discharge machining. It depends on several factors, including pulse on-time, peak current, and spark gap. Higher MRR means faster electrical discharge machining, but it can also result in rougher surfaces and higher tool wear. MRR can range from 0.01 mm³/min to several hundred mm³/min, depending on the EDM machining conditions and material properties.

Tool Wear Rate (TWR)

Tool Wear Rate (TWR) is the rate at which the tool electrode material wears down during the machining process. A lower TWR indicates longer tool life and less frequent tool changes, crucial for cost-effective EDM operations. Higher peak currents and longer pulse on-times tend to increase TWR. Typical values for TWR are often below 0.1 mm³/min for optimal machining conditions.

Polarity

Polarity in electrical discharge machining refers to the direction of current flow between the tool and the workpiece. Positive polarity (the tool as the cathode (-ve) and the workpiece as the anode (+ve)) removes material from the workpiece. The reverse polarity is used when very high surface roughness is required. Material removal will be far more gradual on the workpiece but more intense on the tool.

Applications of Electrical Discharge Machining

EDM is widely used across various industries for precision machining of hard materials and complex geometries. Some key applications include:

Tool and Die Manufacturing: EDM machining is extensively used to create intricate shapes in molds, dies, and tooling for the automotive, aerospace, and manufacturing industries. It allows for the production of cavities, cores, and other complex features with high accuracy.
Aerospace Components: Electrical discharge machining is used to machine hard, heat-resistant materials like titanium and Inconel, which are commonly found in aerospace parts such as turbine blades, nozzles, and cooling holes.
Medical Devices: For precision cutting of small, complex parts used in medical devices, such as stents, surgical instruments, and implants, EDM's ability to machine hard materials with fine tolerances is invaluable.
Microelectronics: EDM manufacturing is employed in the production of small components like connectors, sensors, and micro switches, where precision is critical.
Automotive Industry: The EDM process is used for producing components such as fuel injectors, gears, and valve guides. It is also used in high-precision punching and forming tools.
Rapid Prototyping: EDM manufacturing allows for fast and accurate prototyping of complex parts, especially in materials that are difficult to machine with conventional methods.
Jewelry and Watchmaking: Electrical discharge machining can be used to create intricate designs and fine details in metals such as gold, silver, and stainless steel for jewelry and watch components.
Cutting of Hard Metals: Due to its ability to machine hard materials such as carbide, tool steels, and hardened stainless steel, EDM cutting is often to cut through thick pieces of metal.

These applications highlight EDM's versatility, especially in industries where high precision, fine tolerances, and the ability to machine difficult materials are critical.

Advantages of Electrical Discharge Machining

Electrical discharge machining offers various advantages that make it the ideal manufacturing technique for numerous applications. Some of these advantages are as follows:

Complex geometries: EDM machining can machine highly complex shapes, geometries, and features that are difficult or impossible to machine using conventional machining technologies
Material hardness: EDM machines extremely hard materials with ease, including tungsten and hardened tool steel
Tolerance: EDM machining is highly accurate, with a tolerance of 0.005 mm
Surface finish: Produces a smooth surface finish up to 0.2 micrometres
Micromachining: Can machine extremely small parts and features
Mechanical stresses: Unlike CNC machining, that subjects the workpiece to stresses, EDM machining is a non-contact process and leaves no residual mechanical stress in parts

Disadvantages of Electrical Discharge Machining

While EDM has numerous advantages that make it suitable for many applications, it has some inherent disadvantages that make it less than ideal. The following are some of these disadvantages:

Set up cost and time: Sinker EDM machining requires additional setup cost and time to manufacture custom tool electrodes.
Availability: EDM machining is limited to industrial applications, as the machines are only available in industrial settings; unlike 3D printers and CNC machines that have desktop and hobbyists versions.
Consumables: The EDM process uses consumables such as the tool electrode and the dielectric fluid which may require replacement over time.
Material restriction: EDM machining is restricted to conductive materials, and cannot cut work, glass, plastic, or ceramics.
Power consumption: The EDM machining process requires a significant amout of power