Electrical Discharge Machining (EDM): The Process of Cutting Metal Without Touching It—Mechanisms, Three Types, and the Companies Pushing the Limits

In the world of manufacturing, there have always been barriers such as materials being 'too hard to cut' or shapes being 'too complex for cutting tools to reach.' These include hardened steel for molds, superalloys used in aircraft engines, and cemented carbide, which is the material for cutting tools themselves. There is a machining method that can cut these tough opponents without using any blades at all, relying solely on the heat of sparks. This is 'Electrical Discharge Machining (EDM).' Many of the precision items we see in our daily lives—such as molds for forming smartphones, tiny nozzles for fuel injection in automobiles, cooling holes drilled in countless numbers in aircraft engine turbine blades, and precision parts for medical devices—have benefited from this technology at some point.

EDM has a characteristic that may feel surprising when you first hear it: it is not the case that 'the harder the material, the more difficult it is to cut.' As long as the material conducts electricity, it can be cut regardless of its hardness, even if it is a difficult-to-cut material that would make cutting tools cry. Why is such a thing possible? In this article, we will explore the depth of EDM technology and introduce the companies that are pushing its limits, while tracing its mechanism, history, three machining methods, and the factors that determine the 'quality' of the machining.

The idea of cutting metal with sparks—the principle of EDM

The star of EDM is a phenomenon very similar to lightning. ASTEC, a manufacturer of small-hole EDM machines, explains this process by comparing it to 'tiny lightning.' Even materials that do not normally conduct electricity, like air (insulators), can suddenly conduct electricity when a high voltage is applied. This is an electrical discharge, or in nature, lightning. In EDM, this tiny lightning is repeatedly generated between the electrode (the tool side) and the workpiece at an astonishing rate—thousands of times per second, or even tens of thousands of times depending on the conditions.

At the point where the discharge occurs, a localized high temperature is generated in a very short time. The temperature can reach several thousand degrees, with some explanations stating it reaches 6,000 to 7,000°C. Even the material with the highest melting point on Earth has a melting point of approximately 4,000°C. In other words, the heat at the discharge point exceeds the melting point of any metal. That is why, as long as the object is metal, it can be melted or evaporated and removed little by little, regardless of its hardness. What is doing the cutting is not 'force' but 'heat.' This single point is the key to understanding EDM.

Each discharge is a repetition of a small cycle that proceeds at a speed too fast for the eye to follow. When voltage is applied, the insulation of the machining fluid breaks down at the point where the electrode and the workpiece are closest, creating a path for electricity (discharge column, plasma). The heat of the current flowing through it melts a tiny portion of the metal, and some of it evaporates. Then, when the discharge is paused, the machining fluid cools it down all at once, blowing away and removing the particles of molten metal. This is repeated at high speed to carve out the shape little by little. Simply changing the combination of the time the discharge is on (on-time) and the time it is off (off-time) changes the machining speed, surface roughness, and electrode wear. The reason setting these conditions is called a craft is because these countless combinations are optimized for each material.

Another major feature is that it is a 'non-contact' process where machining proceeds without the electrode and the workpiece touching. In cutting processes like lathes or milling machines that press a blade against the material, force (cutting resistance) is inevitably applied to the workpiece, making thin, brittle, or small parts prone to deformation or breakage. Because EDM does not apply force, it can machine thin plates and delicate shapes without distortion.

Furthermore, EDM is generally performed in a liquid called machining fluid. This can be water (ion-exchanged water = pure water) or a specialized oil similar to kerosene. The liquid has three roles: insulation to properly control the discharge, cooling the heat generated by the machining, and washing away the molten residue (sludge, machining chips). Especially when using water, tap water alone contains impurities that conduct electricity, preventing the discharge from forming properly. Therefore, it is passed through ion-exchange resin to become pure water (deionized water), and machining is performed while keeping its electrical resistance (specific resistance) constant. It may be understated, but managing the machining fluid is an important process that determines the finish.

Which of the electrode or the workpiece is set to positive (polarity) also affects the quality of the machining. There is 'positive polarity,' where the electrode is negative and the workpiece is positive, and the opposite 'reverse polarity,' which are used selectively depending on the material of the electrode or workpiece and the machining content. On the other hand, since EDM is a method of melting and removing material little by little, the reality is that for materials that can be cut normally, conventional cutting is faster. It can be said that EDM is a technology chosen not for its 'speed,' but for 'areas that cutting tools cannot reach.'

Technology born from 'destruction'—the history of EDM

The idea for EDM was ironically born from a 'problem.' In the 1940s during World War II, the Lazarenko couple, scientists in the former Soviet Union, were studying the phenomenon where electrical switch contacts gradually wore out due to sparks (discharges). While discharge was a nuisance that damaged contacts, the couple thought that if they could control this destructive action of 'eroding metal,' it could be used for machining. Thus, EDM was born. They turned a destructive force into a force for creating shapes. This is where the inherent fascination of this technology lies.

After the war, the practical application of this technology was eagerly awaited in Japan as a way to machine hard metals and complex shapes that were difficult for cutting tools. Domestic commercialization is said to have been achieved around 1953 by Ikegai Iron Works and the Japan Electrical Discharge Machining Laboratory (JAPAX). However, EDM machines at the time had severe electrode wear and were mainly used only for 'blanking' like punching out plates. It was not until around 1968, when power supply circuits using transistors were commercialized, that it became possible to perform 'bottom-finishing' to carve out recesses with bottoms as we do today. The advancement of power supply technology was the driving force that suddenly expanded the applications of EDM. The history of Sodick, which we will touch upon later, is truly a history of this 'struggle with power supplies.'

Three methods—wire, die-sinking, and small-hole

EDM is broadly divided into three types based on the shape of the electrode and how it is moved. Since each is good at completely different jobs, they are used selectively depending on what you want to make.

Wire EDM—cutting out contours like a jigsaw

Wire EDM is a method where an extremely thin metal wire with a diameter of about 0.05 to 0.3 mm (around 0.02 mm for the thinnest ones) is stretched as an electrode and moved like a jigsaw to cut out contours from the material. Brass is mainly used for the wire, and it can be replaced with tungsten or other materials when higher precision is required. The table moves in the XY direction to change the positional relationship with the workpiece, and the wire, which continues to run from top to bottom, cuts out the shape.

A point often overlooked is that the wire itself melts and wears away due to the electrical discharge. Because the wire becomes thinner and eventually breaks if it continues to cut in the same spot, fresh wire is constantly supplied from above during the process. You can imagine it like a jigsaw blade that wears down as it is used, so it is continuously wound and replaced while cutting.

To improve precision, there is a method where the machining is not completed in one pass but divided into several stages. First, it cuts quickly with a strong current, albeit with a rough surface (first cut), then the current is reduced for semi-finishing (second cut), and further reduced for finishing (third cut), tracing the same contour several times while changing the conditions. The more passes made, the cleaner the surface and the more accurate the dimensions, but this increases processing time and wire consumption. Balancing the finish with the effort and cost is where the skill of the operator comes into play.

Additionally, many wire EDM machines are equipped with a function (UV axis) that moves the upper and lower wire guides independently. This allows for "taper cutting" by tilting the wire, enabling angles of up to about ±5 degrees or the cutting of three-dimensional shapes with different top and bottom profiles. This is a standard method for making press mold parts, and depending on the conditions, dimensional tolerances can reach the precision range of ±several µm (around ±0.01mm). As long as the material is conductive, this method's strength lies in its ability to cut not only stainless steel and cemented carbide but also extremely hard materials like polycrystalline diamond, provided it is made conductive.

Sinker EDM: "Transferring" and Carving Shapes

Sinker EDM (die-sinking) is a method where an electrode that is the "reverse mold" of the desired cavity is prepared in advance, and the shape is carved into the workpiece by applying electrical discharge as if pressing the electrode into it. Because it can reproduce complex three-dimensional indentations and bottomed, bag-like shapes, it is widely used in making various molds for resin, die-casting, forging, and pressing. It might be easier to think of it as taking an impression, like stamping a seal.

The key to this method is the choice of electrode material. Pure copper is the most commonly used in Japan; it has low electrical resistance and dissipates heat easily, resulting in low wear and allowing for finishes as smooth as a mirror (0.3µmRz class). On the other hand, graphite (a form of carbon, like pencil lead, but distinct from lead) has high heat resistance, can be machined with almost no wear, and has the advantage of allowing for higher machining speeds because it can be cut with shorter pulses than copper. Since it is light and does not expand or contract easily with heat, it is suitable for large electrodes or extremely thin pin shapes that are difficult to make with copper. Furthermore, copper-tungsten, a mixture of copper and tungsten, excels in situations requiring surface and dimensional accuracy, or in the machining of cemented carbide. The optimal material is chosen by weighing the target surface roughness, the ease of machining the electrode itself, and the cost.

Because the electrode wears down along with the discharge, spare electrodes are sometimes prepared in anticipation of wear, and the process is carried out while swapping them mid-way. There are also techniques such as giving the electrode a slight circular motion (orbiting) to control the gap, finishing the sides cleanly, or using a single electrode for everything from roughing to finishing. How the electrode is designed, made, and moved—that accumulation of effort determines the final result.

Small-Hole EDM: High-Speed Drilling of Narrow, Deep Holes

Small-hole EDM is a method that uses a pipe-shaped (hollow) electrode to drill narrow, deep holes at high speed while spraying high-pressure machining fluid from the inside. It can drill holes with a diameter of 3mm or less—in some cases, well under 1mm—to depths that drills struggle to reach. A feature of this method is that by spraying fluid from inside the electrode, it forcefully ejects the melted debris, significantly increasing the machining speed.

The difference from a drill is clear. With a drill, burrs almost always appear at the edge of the hole, but with small-hole EDM, there are almost no burrs, which reduces the need for post-processing deburring. Furthermore, because it does not press a cutting tool against the surface, it can drill straight holes even on spherical or slanted surfaces where a drill would slip and deflect. It is also used for opening "start holes" for wire EDM and is widely used in fields such as automotive, electronic components, aerospace, and medical, ranging from mass-produced parts to precision components.

The differences between the three methods can be summarized as follows.

What Determines the "Quality" of Machining: Surface Roughness, Altered Layers, and Electrodes

The quality of EDM cannot be measured solely by "what shape was cut." The state of the cut surface—namely, the surface roughness and the properties of the extremely thin surface layer—also greatly influences the quality of the product. In the past, it was customary in mold making to include a polishing process after EDM to remove the EDM-machined surface. However, due to advancements in power supply technology and methods like "powder-mixed EDM," where fine powder is mixed into the machining fluid, we have reached a point where surfaces finished by EDM can be used directly in products. Being able to produce a mirror-like finish using only EDM was a major leap forward for this technology.

On the other hand, there are weaknesses due to the principle of cutting with heat. The most prominent is the "altered layer (recast layer, white layer)." The surface layer that was melted by the heat of the discharge and then rapidly cooled and solidified by the machining fluid may have a different structure or properties than the original material, and may contain tiny cracks (micro-cracks) or residual stress. While this is usually not a problem, in applications like aerospace engine parts or medical components where even a slight crack can be fatal, how to keep this altered layer as thin as possible becomes the core of quality. The reason EDM manufacturers are competing in power supply technology is precisely to refine this "surface quality." Whether one can design not just the shape, but also the nature of the cut surface—that is where the difference in each company's capability is revealed.

The difficulty of machining also changes depending on the material being cut. For example, cemented carbide, used for cutting tools and molds, has a very high melting point of about 2,900°C for its main component, tungsten carbide (WC), and requires significant energy to cut. Therefore, setting conditions tailored to the material, such as increasing the peak current and shortening the duration of each discharge (pulse width), is essential. Even within EDM, countless "recipes" are used depending on the material and the goal.

To summarize again, EDM excels at cutting difficult-to-cut materials regardless of their hardness, creating complex and minute shapes, and machining delicate items without applying force. Conversely, its weaknesses are that it cannot machine non-conductive materials (such as most resins and ceramics) and that it takes longer to machine compared to cutting. Based on these strengths and weaknesses, it is used in the right place for the right job, in combination with cutting, grinding, and laser processing.

How to use it in combination with other machining methods

The position of electrical discharge machining (EDM) becomes clear when compared to other machining methods. First, if a material can be machined with standard cutting tools, milling or grinding is overwhelmingly faster and more cost-effective. EDM is specifically chosen when cutting is not an option: for hard materials that would wear out tools, for narrow or complex shapes where a cutting tool cannot physically enter, or for delicate parts that would break under pressure. Conversely, it is not a good strategy to use EDM for materials that can be easily cut.

In terms of using heat to remove material, it is often compared to laser machining. Lasers are excellent at cutting thin sheets at high speeds and are used in many workplaces today, but they do create a heat-affected zone, and conditions become difficult with thick materials or narrow internal geometries. In contrast, EDM has the advantage of being able to finish the contours of thick materials with wire, or deep, narrow holes with small-hole EDM, all with high precision and without physical contact. In short, no single method is universal; the practical reality in the field is to intelligently combine milling, grinding, laser, and EDM depending on the material, shape, and required precision. Understanding EDM adds a deeper level to your 'toolbox of combinations'.

Where is it used? — Familiar applications

The stage where EDM has played the most active role is in mold making. Resin molds for plastic products, die-cast molds for pouring aluminum, forging molds for impact forming, and press molds for punching and bending sheets—all require hard mold steel or cemented carbide to be finished into complex shapes with high precision, making EDM indispensable. It is often said that the development of Japanese industry would have been impossible without molds, and EDM is what has supported those molds from behind the scenes.

EDM also plays a crucial role in the fields of electronic components and semiconductors. Precision press molds for punching semiconductor lead frames and motor cores are typical applications for wire EDM. Even the small, precise parts found in smartphones and digital cameras can be traced back to EDM through their molds.

In automobiles, it is used for tiny holes in fuel injection nozzles that spray fuel as a mist. The true value of EDM is perhaps best seen in aircraft engines. Because turbine blades inside an engine are exposed to the intense heat of combustion gases, they must be protected by a film of air through countless tiny cooling holes on their surfaces. Drilling thin, numerous, and accurate holes in hard superalloys is a challenge met by small-hole EDM. In recent years, technology has emerged to create curved holes rather than straight ones, allowing for more efficient cooling flow paths. EDM is even rewriting the common sense that holes must be 'straight'.

Medical devices cannot be overlooked either. Instruments that enter the body or precision surgical tools require fine, complex machining on hard, corrosion-resistant materials, making non-contact EDM, which produces minimal burrs, an ideal choice.

There is also an increasing number of cases where products themselves are finished using EDM without the need for molds. It is not uncommon for EDM to take on one-off parts that 'cannot be made by other methods,' such as precision components for semiconductor manufacturing equipment, nozzles and slits for energy equipment, and prototypes for research and development. The wide range of applications, from mass-production molds to one-off parts, is a testament to the depth of this technology.

Looking at it this way, it is no exaggeration to say that 'EDM is involved in almost all the precision items around us'.

Companies pushing the limits

The world of EDM is driven by two wheels: the machine manufacturers that create the technology itself, and the specialized companies that master that technology to its limits in the field. Let's look at the key players in each.

Major manufacturers supporting the industry

A leading machine manufacturer is Sodick. The company's history is essentially the history of overcoming the weaknesses of EDM. Founder Toshihiko Furukawa studied electrical theory at university while working for an EDM manufacturer (formerly JAPAX) and took on the challenge of electrode wear, a major issue at the time. Based on his own theories, the company notes that they were the first in the world to commercialize a 'non-wear transistor power supply' that prevents electrode wear. They also developed a method (LA-processing) to finish the sides of workpieces with high dimensional accuracy, bringing EDM—which once relied on the intuition of craftsmen—closer to something anyone could perform with high precision. According to public information, Sodick currently holds a top-class global market share in EDM machines, pursuing precision through features like low-friction linear motor drives, while also expanding into areas like metal 3D printers.

Mitsubishi Electric has combined its long-cultivated power and control technology with its 'Maisart' AI technology in recent years, setting a direction where the machine itself supports the adjustment of machining conditions that previously relied on the experience and intuition of experts. In addition, they are focusing on building stability that supports precision from behind the scenes, such as the 'Thermal Buster' which keeps the entire machine at a constant temperature. The philosophy of 'machines supplementing human skills to achieve both the inheritance and evolution of technology' is a hallmark of the company's EDM machines.

In addition, Fanuc, which holds a high market share in numerical control (NC) devices, has developed wire EDM machines (ROBOCUT) based on its control technology. Makino Milling Machine, a long-established company founded in 1937, handles both cutting machines like machining centers and EDM machines, and has strengths in mold making that combines cutting and EDM. By having these major manufacturers compete in machine precision, automation, and intelligence, the foundation of EDM as a whole is being raised.

Specialized companies breaking through 'limits' at a single point

Apart from large machine manufacturers, there are specialized companies that dig deep into specific areas, honing outstanding technology. Their existence is what truly speaks to the depth of EDM.

ASTEC, which specializes in small-hole EDM, is challenging the limits of 'depth'. Their ultra-deep hole unit, the '6Z1800', uses an electrode as long as 1,800mm and claims to be capable of through-hole machining with a diameter of 1mm and a depth of 1,500mm. This is an aspect ratio—the depth of the hole divided by the diameter—of a staggering 1,500 times. Considering that the limit for deep holes with a drill is generally considered to be an aspect ratio of about 30, the gap is clear. Using the power of discharge to accurately penetrate thin, deep holes that would bend with a gun drill—this is made possible by steady, meticulous engineering, such as mechanisms to suppress electrode vibration.

Estro Lab (trade name "Hosoana-ya"), a specialist in small-hole EDM located in Higashiosaka, Osaka, truly embodies the "fineness" and intensity of the shop floor. According to the company's public information, they can drill small holes with an aspect ratio of over 200 times, starting from an electrode diameter of φ0.1mm, even in hard materials such as hardened steel, titanium, molybdenum, and cemented carbide. They handle holes on curved surfaces, sloped surfaces, and angled surfaces, as well as multiple holes requiring high positional accuracy, with a minimum positioning readout unit of 0.001mm. Their internal record is a 30mm deep hole drilled with a 0.15mm diameter electrode. They also emphasize a stance of keeping costs down through ingenuity rather than relying solely on expensive specialized machinery. Furthermore, it is worth noting that this company was founded and is run entirely by women. Rather than competing on scale, they dig deep into the single niche of "thin and deep holes." Their presence serves as a reminder that the strength of a manufacturing company is not determined solely by its size.

There are also companies that challenge the limits of "hardness." For example, Tokyo Byora Koki specializes in "direct cemented carbide engraving," which involves carving directly into cemented carbide. As mentioned earlier, cemented carbide is a difficult material with a high melting point that requires significant energy to process. However, because EDM can cut any material regardless of its hardness as long as it is conductive, it allows for the high-precision finishing of cemented carbide molds. In die-sinking EDM in particular, discharge tends to concentrate at corners and edges, causing the electrode to wear out easily; it is said that the more complex the hole shape, the more critical the electrode design, production, and machining process become. Specialized companies in this field overcome such difficulties through years of electrode expertise and advanced equipment.

Key points when requesting EDM services

Finally, for those in a position to outsource EDM, here are some points to keep in mind to facilitate smoother communication. First, check whether the item you want to machine is a "conductive metal." Most resins and ceramics cannot be processed by EDM as they are. Next, the appropriate method is basically determined by the shape. Wire EDM is for cutting contours out of a plate, die-sinking EDM is for complex cavities with a bottom, and small-hole EDM is for thin, deep holes.

Also, a point often overlooked is the importance of sharing the desired "surface quality" early on. By communicating the required surface roughness and the tolerance for the heat-affected layer, it becomes easier for the machining side to propose appropriate discharge conditions and finishing processes (such as additional finishing passes or the necessity of polishing). Sharing these details along with accuracy, quantity, and delivery deadlines is the surest way to prevent rework and achieve good results. Whether or not you can discuss not just the shape but also the surface quality is one way to evaluate a potential machining partner.

Summary—Traces of sparks are all around us

EDM is a technology that overcomes the barriers of hardness and shape simultaneously through the concept of "cutting with the heat of sparks without touching the material with a blade." Originally born from the problem of contact wear, it has expanded its applications alongside advancements in power supply technology. Today, the three methods—wire, die-sinking, and small-hole—support everything from molds to semiconductors, automobiles, aviation, and medical equipment.

With a proper understanding of its limitations—that it is restricted to conductive materials, is slow, and creates a heat-affected layer—machine manufacturers are making equipment smarter through AI and automation, while specialized companies continue to update the limits by digging deep into their respective niches. The fact that machines are gradually taking over the parameter setting that once relied on the experience of skilled workers has a quiet but certain significance for manufacturing sites struggling with labor shortages and the succession of technical skills. As miniaturization progresses and the need for difficult-to-cut materials grows even stronger, the role of EDM will likely increase. There is still plenty of room to make "what cannot be done yet" possible, such as curved cooling holes, mirror finishes that require no polishing, and combinations with metal 3D printers.

The next time you touch a smartphone or a car, try to imagine that somewhere inside, there is a trace of machining left by a tiny spark. Familiar things might start to look a little different.