When discussing machining as a form of manufacturing, one typically thinks of milling, drilling, and turning. These techniques, though widespread, represent only one area of machining—often referred to as traditional or conventional machining—and are defined by their use of mechanical forces to remove material from a workpiece to produce a part.
There is also an entire category of non-traditional machining, comprising methods like laser beam machining, electrochemical machining (ECM), and—our topic for this article—electrical discharge machining (EDM). Non-traditional machining processes like electrical discharge machining are characterized by their use of energy (including light, chemical, thermal, and electrical energy) to form components from metal workpieces.
As the name implies, electrical discharge machining or EDM machining uses electrical discharges to shape metal parts. The process, which we’ll dive into in further detail below, is characterized by its high precision and the ability to easily machine hard materials like hardened steel or carbide. For these reasons, electrical discharge machining has found applications in demanding industries such as aerospace and medical.
Electrical discharge machining, also colloquially known as spark machining, wire erosion or spark eroding, is a non-traditional machining process that relies on electrical discharge (or electric sparks) to remove particles of material from a workpiece made from a conductive material. The process is especially well suited for producing complex or fine holes and features in metal components, as EDM machining is capable of achieving tolerances within the range of +/- 0.005 mm, and therefore offers high accuracy.
Unlike other machining techniques, EDM machining is characterized as a no-contact machining process. This means that the tool never comes into physical contact with the workpiece. Instead, a continuous series of electric charges are sent from a power source through the tool (called the electrode), which creates electric arcs between the electrode and the workpiece. The sparks generated through this electric reaction are highly targeted and erode particles from the workpiece in a controlled way. These particles are safely flushed away from the electrode and workpiece surface thanks to submersion in a dielectric fluid.
The no-contact nature of EDM comes with a number of benefits. For one, it eliminates the risk of tool marks and burrs. It also enables the machining of small parts or components with thin, delicate walls, which are at risk of being damaged by direct contact with tools.
EDM is often used in combination with other machining or manufacturing processes. For instance, in the aerospace industry, metal engine turbines are produced using conventional machining processes and then undergo EDM for the addition of fine features, such as narrow cooling holes. EDM also has the distinct advantage of being able to machine pre-hardened steel and other heat-treated metals without changing their properties or hardness.
Despite its extensive capabilities, EDM is no more difficult to operate than any other CNC machine.
The EDM process is made up of a few key components: the workpiece, the tool electrode, dielectric fluid, and a power source. We’ll take a closer look at each of these pieces to understand how the EDM process works as a whole.
Every machining process requires a workpiece. In short, it is the piece of material that is undergoing forming. In EDM, there are few requirements for the workpiece. It must simply be a material with electrical conductivity properties. Another important thing to note—and a major advantage to using EDM—is that the process is not influenced by the machinability of a metal. In other words, while conventional machining processes can more easily process certain metals, the nature of EDM means it can just as easily machine the hardest of metals. The workpiece must be held securely in place by a jig or fixture.
The tool electrode, or electrode, is the tool used to create a spark when it comes into near contact with the workpiece. The material of the tool electrode varies depending on the EDM technique. Common materials for the electrode are graphite, copper, tungsten, brass, and their alloys. The type of tool electrode also varies depending on the technique used. For example, in wire EDM, the electrode is copper or brass wire, while fast hole EDM uses a tubular electrode. The tool electrode is controlled by a servo motor, which moves the tool (following CNC pathways) and ensures that it does not touch the workpiece throughout the EDM process.
It is imperative that the EDM process be conducted in a tank of dielectric fluid, such as do-ionized water or non-conductive lubricating oil. Without the liquid, the molten particles removed from the workpiece could be pulled towards the tool electrode and weld themselves to it, which would disrupt the process and ruin the tool. The vat of dielectric fluid (which is kept in continuous motion) helps to not only cool the particles as they are removed from the workpiece but to move them away from the electrode. The fluid also controls the electrical discharge occurring between the electrode and the workpiece.
The power supply controls the release of the electrical charges during the EDM process, tuning the frequency and strength of the electrical current based on the properties of the job at hand. These series of charges go through the electrode to react with the workpiece. This reaction generates sparks at the precise spot where the electric field is strongest (where the workpiece and electrode are closest). At a given moment, thousands of sparks can be eroding the workpiece. EDM is notable as a machining process that consumes a lot of energy.
It is important to note that electrical discharge machining is itself a category of machining processes. There are three sub-types of EDM: Wire EDM, Plunge EDM, and Fast Hole EDM. While the processes have much in common, including the need for dielectric fluid, conductive workpiece, and power supply, their main distinguishing feature is the type of electrode tool used.
Wire EDM (WEDM) is a specific type of EDM that feeds a spool of wire (usually brass or copper) through a workpiece to make precise cuts and slots. The electrically charged wire is often likened to a bandsaw or cheese cutter because it cuts through metal seamlessly by removing tiny particles from the workpieces as it passes through it. The metal wire used in WEDM ranges in diameter between 0.05 and 0.35mm and must be replaced frequently during the process using an automated threading mechanism. WEDM is especially beneficial for parts that require incredibly tight tolerances or small features.
Also known as sinker EDM, die sinking EDM or ram EDM, plunge EDM is a method that uses graphite, copper, or tungsten tool electrodes to produce complex and three dimensional cavities in a workpiece. With this approach, the electrode has the negative shape of the final desired cavity. The electrode moves towards the workpiece vertically, eroding the particles from the workpiece to create a final shape. Plunge EDM is particularly well suited to the production of complex holes or shapes.
Fast hole EDM, or hole drilling EDM, uses a rotating tube-shaped electrode—usually made from copper or brass—to produce holes in the workpiece. Compared to conventional drilling, fast hole EDM is ideal for producing very precise, small and deep holes without the need for deburring. Fast hole EDM is particularly well suited to producing deep holes measuring up to 250 times the tool’s diameter. (Electrode diameters come in a range of sizes, from 0.25 mm to 4.7 mm.) Using this technique, dielectric fluid goes through the tubular electrode to flush out any particles eroded from the workpiece. Applications for fast hole EDM include cooling holes for aerospace turbines and medical parts that require small holes.
Like any manufacturing process, there is a time and a place for electric discharge machining, so it is important to understand when the process is most beneficial or necessary. For instance, a part that requires holes with a standard diameter and shallow depth can be machined easily using a drill and does not require EDM. EDM is therefore most effective when conventional machining processes fall short.
Parts that require fine cuts, narrow, deep holes, or complex shapes with very tight tolerances are good use cases for EDM. The process is also advantageous for machining thin-walled or small parts that are typically challenging to machine using conventional techniques. This is because EDM does not require any physical contact with the workpiece, so there is no risk of the part being damaged or distorted by the tool. EDM is also a good option for achieving tight tolerances with some of the hardest conductive materials, which can be challenging to process using conventional methods.
EDM is today used across many industries, including in manufacturing for the production of dies and molds, in aerospace for the creation of precision holes in turbines, and in medical for making devices with fine features and details. Applications for the process extend beyond these three examples as well, and vary depending on the type of EDM used. In general, however, EDM is well suited for low-volume production needs, such as toolmaking, prototyping, and small series production.
Electrical discharge machining is a powerful fabrication method that complements other manufacturing and conventional machining processes. The electrical, no-contact nature of EDM creates opportunities that traditional machining cannot achieve, especially when it comes to extremely tight tolerances for hard metals, precise holes, thin-walled parts, and complex cavities. On top of that, another argument for using EDM machining is that the surface finish is usually better than with traditional methods.
Based on the 18th century scientific discovery of electrical discharge’s erosive effects, the EDM process was further developed in the 1960s, and has evolved significantly in the past half a century. Today’s iteration of the technology is a far cry from early EDM machines, but there is still more growth to come. The latest trend gaining steam with EDM—and all manufacturing—is Industry 4.0. Specifically, we are seeing the development of new automation features and software improvements geared towards streamlining and improving the EDM machining process and adapting it for an increasingly digital age.
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