Electrical Discharge Machining (EDM) is a relatively new metalworking technique compared to rotary cutting tools, shearing, and forging. High-voltage electrical discharges (sparks) in rapid pulses are passed through the gap between an electrode and a grounded workpiece, removing material from the workpiece by erosion. Even the hardest and most difficult-to-machine materials can be cut and machined with little significant heating and almost no macroscopic force as long as the material is electrically conductive. This results in very low distortion and does not add meaningful residual stress to the workpiece.

 

The first EDM method was drill EDM, developed by Stark, Harding, and Beaver (and Lazarenko in the Soviet Union). Its earliest application was in the mid-1940s for removing broken bolts and taps from high-value aluminum castings.

 

From this relatively simple drilling method, a second branch of EDM was developed: ram EDM or sinker discharge machining. This technique is essentially the same as EDM, except that a complex form can be added to the end of the "drilling" electrode. This process can form the finished, fine, and precise shape of the bottom of a cavity in a single operation.

 

The third type of EDM is wire EDM, which uses a straight wire electrode to cut vertical or angled slots (slightly wider than the wire) as the wire is slowly fed through the kerf to always maintain a "fresh" electrode.

This article will describe Electrical Discharge Machining, discuss the three main EDM methods, emphasize the important technical aspects of each method, and identify the applications of each EDM method. It will also focus on practical applications, the selection of the appropriate method, and the main benefits of each method.

Wire Electrical Discharge Machining, commonly known as wire EDM, uses a wire as the "cutting tool" to erode material through sparks between the wire and the workpiece. The wire electrode discharges and passes through the entire length of the kerf, helping to remove chips. This technique was developed as a way to make 2D cuts in hard materials (cuts can only be made along the x and y axes of the part, and the z axis is the thickness of the material), but its complexity has been increasing. Offsetting the wire rollers from each other allows cuts to be made at a controlled angle relative to the z axis. Later, a rotary axis could also be added to the wire EDM process, resulting in a high-performance 5-axis wire EDM machining center.

This method allows free cutting. When combined with EDM drilling (for a fixed electrode), it is most commonly used in the manufacturing of extrusion tools and punch/die. In wire EDM, brass, galvanized brass, and zinc-diffused copper (layered wire) running on sapphire or diamond guides are used as electrodes. The various conductivities, hardnesses, and tensile strengths of the electrode wires offer different machining characteristics.

Sinker discharge machining (also known as "ram" or "cavity" EDM) uses discharges from a basically 2D electrode. This 2D electrode can have a complex 3D-shaped end, which is repeatedly inserted to arc-erode the workpiece and is raised a small distance in the Z axis to clear debris. The electrode can have any cross-sectional shape and a complex end shape (without undercuts). For sinker discharge machining, a precision copper or graphite electrode is first machined into the desired cavity shape and then used to erode that shape into a hard material.

Ram EDM is widely used in the manufacturing of injection molding and die-casting tools. It is used to create unmachinable complexities and fine cavity features in molds without applying any machining or impact stress to the mold. It also reduces the need for post-processing to improve surface quality or hardening.

Drill EDM was the first developed EDM technique and remains a crucial tool in various fields, including gas discharge control in rocket engines, medical and scientific equipment, and open cooling channels in gas turbine blades. Long, straight, small drill holes can only be cut by EDM drilling. A straight electrode is used to erode a hole that perfectly reflects the tool profile. It maintains precise dimensional accuracy along the entire length of the hole. This method allows the machining of straight holes with small diameters, high aspect ratios, and any depth, with a diameter of 0.0015 inches or larger. EDM drilling allows the drilling of blind and through holes and can drill smooth walls in otherwise unmachinable materials or achieve difficult geometries. The machined surface finish of holes made by EDM is smooth enough to be used as a bearing surface without additional machining steps.

Electrical Discharge Machining (EDM) is a non-contact machining method used to cut unmachinable complex shapes and cavities in very hard, difficult-to-machine materials. When the workpiece and electrode are immersed in a dielectric fluid (usually paraffin or kerosene), a high electrical potential at the electrode finds a ground path through the workpiece. Each spark generates a local temperature peak of up to 12,000°C. This temperature melts/evaporates some of the workpiece (and electrode). Clearing the cutting debris is important to avoid short circuits, so drill and ram tools are Z-cycled, and the electrode feed in wire cutting also achieves the same effect.

 

The EDM process does not produce an applied overall stress in the workpiece and only causes limited, highly localized heating. This results in macroscopically stress-free machining. It can also cut very thin sections and fine features that would otherwise be impossible with machining based on mechanical cutting tools. This technology has revolutionized many aspects of tool manufacturing. It eliminates design limitations due to the need for rotary cutting tools, enabling complex features that were previously unattainable.

Sinker discharge machining is the best EDM type for manufacturing parts with complex cavities. It allows for the complex cavity shapes often required in plastic and die-casting part designs. The limitations on the cavity shapes that sinker discharge EDM can produce have little impact on its value as a tool for mold manufacturing. The "pull line" that prevents undercuts in EDM also applies to plastic molding and die-casting parts that enter the tool cavity in liquid form but leave in solid form.

The various types of EDM are classified according to their operations and applications. For example, wire EDM uses a linearly fed wire electrode to make essentially 2D cuts. The electrode wire is located on diamond or sapphire guides, the dielectric fluid is deionized water, and the electrode is usually made of brass or galvanized brass.

 

On the other hand, sinker discharge EDM allows the use of complex electrode shapes to create equally complex cavities. Graphite or copper electrodes are pre-machined into the desired shape and then sunk into the workpiece by erosion, forming a hole opposite the electrode shape.

 

Drill EDM can provide smaller and deeper holes than traditional drilling. This allows for high precision in diameter and roundness, with good surface finish and no burrs. The electrode can have a complex cross-sectional shape. Drill tools usually have cooling channels, so the medium is pumped through to help clear chips.

EDM can replace traditional machining techniques for hard materials, especially for machining refractory metals and difficult-to-cut shapes. Wire EDM can achieve tolerances that are impossible in precision cutting, and drill EDM can also achieve tolerances in diameter and roundness. Generally, EDM can maintain high precision and very low Ra levels (high-quality surface finish). This feature reduces post-processing and thus usually reduces the cost of difficult-to-manufacture components.

Electrical Discharge Machining (EDM) is used in the manufacturing of molds for plastic injection molding and metal die-casting. It is also used to create complex cavities in pre-hardened materials. EDM is particularly useful because of its ability to create precise and complex cavity shapes and make very deep cuts. The aerospace, automotive, and electronics industries also use EDM for manufacturing prototypes and production parts.

You can use EDM for a variety of applications, including complex cavity shapes, i.e., 3D relief shapes cut on the bottom surface of a basically 2D blind hole; extracting precise 2D shapes from difficult-to-machine materials; thin sections that are prone to distortion; and drilling fine and/or long holes in any metal. EDM basically does not apply stress to the workpiece and can produce one-step drillings of any cross-section with a good surface finish.

The materials most commonly machined by the EDM process are those that are difficult to machine by other means, such as tungsten, molybdenum, and hardened tool steels. Materials that exhibit extreme work hardening during machining, such as titanium and austenitic stainless steel, are also good candidates for EDM. These materials become increasingly difficult to cut during machining because they harden while being machined. This hardening usually reduces the accuracy of part features.

 

At SogaWorks, we deeply understand the significance and potential of these different types of Electrical Discharge Machining. We are constantly exploring how to make the best use of them in our manufacturing processes to improve efficiency and product quality, and strive to stay at the forefront of this advanced machining technology field.