Electrical Discharge Machining (EDM) Process Guide

High-voltage electric sparks erode the surface of the workpiece or cut out the outline by evaporating materials. This makes electric discharge machining one of the most precise methods for processing metals. This method removes materials from the workpiece and creates a texture at the same time.

Process Introduction

Electric Discharge Machining (EDM) has revolutionized mold making and metal prototyping. This process is highly precise and can be used to machine metals and create surface textures. EDM is the ideal method for making molds used in injection molding and other plastic molding processes. Mold making is the largest application area of this process.

There are two types of EDM processes: die-sinking EDM (also known as spark erosion) and wire EDM (also known as wire erosion). The former can be used for drilling holes, creating surface textures, and machining complex geometries deep within metal parts. The latter works in a similar way to cutting polymer foam with a hot wire and is used to machine internal and external contours with parallel or tapered edges. Together, these processes offer metalworkers plenty of opportunities to showcase their skills.

Typical Applications

DEM equipment is very expensive, so it is limited to applications that require high-precision machining, hardened steel machining, and other metals that are not suitable for CNC machining. It has been widely adopted by the mold-making industry for injection molding, metal casting, and forging. It is also used in model making, prototyping, and small-batch production, usually with no more than 10 parts.

Related Processes

EDM is often used together with CNC machining. The electrodes (molds) used for die-sinking EDM are processed through traditional metalworking techniques (Figure 1).

Figure 1: On these molds for die-sinking EDM, the luster of the black vaporized metal shows the cutting area.

However, complex molds are sometimes cut using wire – EDM, and the two processes may also be combined (Figure 2).

Figure 2: This very small mold for die – sinking EDM was cut using the wire – EDM process. Compared to traditional machining, this part is too small and complex.

In application fields that require precise and complex internal features, die-sinking EDM has replaced CNC machining. EDM is used because it can create geometric shapes that are difficult or even impossible to achieve with other processes. Machining the internal features of workpieces, especially those made of hard metals, is almost infeasible for CNC machining. This is because it requires very precise cutting tools, and these tools wear out quickly.

Alternatives to wire EDM include water jet cutting, laser cutting, and high-energy beam welding for certain forming and drilling processes. All these processes have high equipment costs, so the choice mainly depends on the available equipment. Wire EDM is suitable for parts up to 200 mm thick, and it also requires high dimensional accuracy, parallel cut walls, and controllable surface textures.

Quality

The quality of parts processed by EDM is very high. They can be used to manufacture injection-molding molds without any other finishing operations. The surface roughness is measured according to the VDI scale of the German Engineers Association (Figure 3). The VDI scale is equivalent to an average roughness (Ra) of 0.32 – 18 μm. Many plastic products around us today are made with molds formed and surface-treated by EDM.

Figure 3: The VDI scale is used to measure the surface texture.

The surface quality and the resulting texture are determined by the cutting speed and voltage. High voltage and high cutting speed produce a rough texture. Lower voltage, slower cutting speed, and more passes result in a finer surface texture. Parts can be produced with an accuracy of up to 5μm.

Design Opportunities

For designers, the main advantage of this process is the ability to cut metal and apply texture simultaneously. Textures, both matte and glossy, are usually applied after machining using sandblasting or photo-etching techniques. The fine textures generated by EDM are determined by machine settings and require no further processing. These processes can be used to produce geometries that cannot be achieved by traditional machining. For example, the guide heads on many wire-EDM machines can move independently during cutting. The advantage is that complex tapers of up to 30° can be cut with extreme precision, which is not possible with other machining techniques.

Die-sinking EDM can be used to produce internal geometries of parts that traditional machining cannot achieve. This is because the negative copper electrode can be machined into a shape that does not match the cavity. The negative electrode (the mold) is replicated in the workpiece, creating sharp corners and complex features. The corrosion of the copper electrode (the tool) is much slower than that of the workpiece (0.1%), so small internal radii and complex features are reproduced with the same accuracy as simple geometries.

Wire – EDM is used in a very similar way to cutting polymer foam with a hot wire, although it is more precise and has a longer processing cycle. The equipment requires a high level of operating skills.

During the processing, no stress is applied to the electrode (mold or wire) or the workpiece because the metal is not formed by force. Instead, it is formed by evaporation from the surface through high-voltage sparks. Besides this obvious advantage, this processing technology also has many processing advantages. For example, multiple thin-walled parts can be stacked up for wire EDM, which can reduce processing time.

Technical Description of Die-Sinking EDM

In die-sinking EDM, the electrode (mold) and the workpiece are submerged in a lightweight oil similar to paraffin. This fluid flows continuously. It can keep the temperature of the workpiece stable and wash away the evaporated materials. It is also insulating, which isolates the working area and maintains the electrostatic discharge inside.

The copper electrode (tool) and the metal workpiece come into close contact, triggering spark erosion machining. High-voltage electric sparks leap from the electrode to the metal. The sparks jump back and forth between the closest points on the electrode and the workpiece, resulting in the continuous and uniform removal of surface materials.

Technical Description of Wire – EDM

In this process, the electrode wire, usually made of copper or brass, is placed between the supply spool and the take – up spool. It is charged with high voltage, and electrical discharges occur as the electrode wire passes through the workpiece. Similar to die – sinking EDM, the electrical sparks occur at the smallest gap between the metals. Thousands of sparks are generated per second, evaporating a very small amount of metal from the surface of the workpiece. The electrode wire is not recycled; instead, it is continuously replaced to maintain the precision of the machining process.

This process is carried out while being submerged in de – ionized water at a constant temperature of 20°C. The water flows continuously to flush away the waste material and is recycled through a filtration system.

The upper wire guide can move along the x – axis and y – axis, allowing a maximum cutting angle of up to 30°.

Design Considerations

Even though these processes can create internal radii as small as 30μm, larger radii are often used. This is to avoid stress concentration. In most applications, the inner diameter usually doesn’t need to be more than 500μm. The minimum inner diameter is affected by the thickness of the electrode wire. The diameter of the electrode wire generally ranges from 50 to 300μm.

The thickness of materials that can be processed by wire – EDM varies from 0.1mm to 200mm. This depends on the capabilities of the equipment. For die-sinking EDM machines, the depth they can reach is limited by how easily the vaporized metal (black powder) can be washed away. So, the maximum depth is related to the ratio of depth to diameter. Making very deep contours is possible, but it requires extra flushing, which will make the processing cycle longer.

During operation, no pressure is required. The electrode slowly goes down into the workpiece. The processing speed depends on the surface treatment needed. For a very rough surface treatment, the speed can range from 2mm³ per minute to over 400mm³ per minute. When the mold sinks into the workpiece, it stirs constantly and goes down in a spiral way. This action washes the evaporated materials out of the cutting area and makes sure the material is removed evenly and efficiently.

Suitable Materials

Many metals can be shaped using die-sinking EDM. The hardness of the material doesn’t matter when it comes to using this method. Metals like stainless steel, tool steel, aluminum, titanium, brass, and copper are often shaped this way.

Processing Costs

Wire – EDM doesn’t need a mold. But the electrode wire is used up continuously during the process, so it has to be replaced.

The molds for die-sinking EDM are usually made of copper alloys. They can be made through traditional machining or wire – EDM. The molds are not very expensive, but for precise parts, a new mold is needed for each operation.

Different things can affect the processing cycle of die – sinking EDM. In wire – EDM, the cycle is affected by the thickness of the material. Thicker materials need more power and use more electrode wire. For example, if a 36mm piece of hardened steel is cut at a speed of 1.5mm per minute, it will give a uniform matte finish. Slowing down the cutting speed or doing more cutting passes will result in a finer texture. Making the processing cycle shorter will lead to a rougher surface.

Similarly, the processing cycle of die – sinking EDM is decided by the cutting area and the surface roughness required. Usually, when the processing speed is 4mm³ – 400mm³ per minute, the internal geometry will be rough, which means a rough surface will be produced. If a new mold is used later, the cutting cycle will be much longer. When the processing speed is 2mm³ per minute, a very fine surface can be achieved. So, fine surface textures are better for smaller surface areas.

Environmental Impact

This process requires a large amount of energy to vaporize the metal workpiece and doesn’t need any further treatments like sandblasting or photo – etching.

The dielectric fluid can be continuously recycled, and the metal electrodes are also suitable for recycling. However, the operation process generates smoke, which may pose certain risks.

Case Study

Case of Die – sinking EDM

This process is widely used in mold making. Besides machining the entire cavity for injection molding, it is often used to modify existing molds. It can also be used for drilling holes or engraving surface textures and patterns. In this case, RAPID PROTOS is creating a cavity that is directly formed into the surface of high – carbon steel (Figure 1). Without using die – sinking EDM, it is impossible to produce complex and precise cavities in hard metals.

The copper alloy electrode (mold) is inserted into the chuck (Figure 2). The die – sinking EDM machine is programmed with the settings required for the mold. The black area shows the part of the mold exposed to spark erosion. Each mold can be used 5 times before replacement. If a high – precision level is required, new molds need to be machined for each die – sinking operation, including roughing molds and finishing molds.

The mold and the workpiece are inserted and submerged in a dielectric fluid similar to paraffin (Figure 3). In fact, paraffin was once used as an insulating liquid until this dielectric fluid was developed.

During the rough cutting process, electric sparks and smoke are emitted (Figure 4). The copper electrodes carry an electric current. When they are very close to the workpiece, the current jumps to the oppositely charged workpiece. There are thousands of sparks per second. Each spark evaporates a small piece of surface material.

The arc will jump across the shortest distance between the electrode and the workpiece, ensuring uniform removal of the metal surface. In the rough – cutting stage, about 400mm³ of metal is removed per minute, and the resulting surface is very rough (Figure 5).

The second stage of processing is much slower. In this case, 50mm³ of metal is evaporated per minute (Figure 6). This produces a finer surface texture (Figure 7). This is a relatively shallow cavity. Die – sinking EDM can also be used to form very deep cavities.

Case of Wire – EDM

Unlike die – sinking EDM which is used to form internal grooves, wire – EDM is used to cut both internal and external contours. The wire for EDM is kept under tension to cut straight lines. The guide heads move side – by – side and back – and – forth along the x – axis and y – axis to form contours. They also move independently along the x – axis and another axis (presumably a misspelling, might be z – axis in a 3D context) to form tapers. The wire functions like the copper mold used in die – sinking EDM, and the wire is usually made of a copper alloy.

In this case, there are many cutting operations. This series of figures shows one of the operations, and its principle can be applied to all other wire – EDM operations. The partially machined high – carbon steel workpiece (Figure 1) is placed in the fixture and clamped firmly. A small hole is drilled so that the electrode wire can pass through automatically (Figure 2).

The guide head approaches the workpiece for precise positioning. Both the workpiece and the electrode wire are submerged in de – ionized water which acts as an insulator (Figure 3). Once submerged, the cutting process starts (Figure 4). This is a long process. In this case, cutting a 36 – mm workpiece at a processing speed of 15 mm per minute, this process will produce the desired surface effect.

The entire cutting process takes about 2 hours. After that, the part is taken out and cleaned (Figure 5). The accuracy of the part can be measured with a micrometer (Figure 6). On the finished product, the cut can be identified, that is, from the pre – drilled hole to the place where the wire is cut on the cutting surface (Figure 7). On the left is the part before cutting, in the middle is the finished workpiece, and on the right is the removed material.