Electrical Discharge Machining uses electrical spark discharges to shape and form metal components by erosion.
What is Electrical Discharge Machining
Electrical Discharge Machining (EDM) is a manufacturing process that shapes and forms metal parts using electrical discharges or sparks to remove material from a workpiece made of conductive material. EDM is sometimes known as electro-discharge or spark-erosion machining.
The process occurs in a dielectric fluid, which acts as an insulator, a cooling medium, and a flushing medium.
EDM is commonly used to produce intricate shapes or features that are difficult or impossible to create with traditional methods. Also, several industries prefer it for cutting hardened materials. Examples include:
- Making dies for forging, extrusion, die casting, injection moulding
- Machining small deep holes – uses tungsten wire as an electrode
- Very narrow slots in parts
- Cooling holes in turbine blades
EDM is a high-precision method that can remove approximately 10-6 to 10-4 mm ( of material per spark discharge and produce parts with extremely tight tolerances, thin walls, and small features. Also, EDM results in a high-quality surface finish, sometimes even mirror-like, thus requiring reduced post-processing.
Various industries apply this manufacturing process, including but not limited to:
- Aerospace
- Medical
- Automotive
How does Electrical Discharge Machining work?
EDM consists of a machining tool known as the electrode, the workpiece, a DC power supply, and a dielectric fluid.
Tool and workpiece preparation
The process requires two following initial steps. One is the connection of the electrode and the workpiece to the DC power supply. The second is ensuring the dielectric fluid covers the electrode and workpiece completely.
Material removal
Then, the machine feeds the tool electrode. When the tool electrode is close enough to the workpiece, it generates a potential difference that is high enough to break the dielectric down and create a discharge that results in a cavity on the workpiece. Then, the dielectric fluid flow removes the material from the cavity.
The process involves repeated discharges, typically at a 200 to 500 KHz frequency, with voltages of 50 to 380 V and currents of 0.1 to 500 A.
The material of the workpiece must be electrically conductive, with the material’s melting point and latent heat of melting as the properties that will determine the amount of material removed per discharge.
The most popular electrodes for EDM are graphite electrodes. However, it is essential to highlight that electrodes made of brass, copper, and copper-tungsten alloys are also available.
Type of EDM
There are four types of EDM. These are:
Sinker or die-sinking EDM
Die-sinking EDM is considered the first type of EDM that ever existed. Sinker EDM uses an electrode that mirrors the shape wanted on the workpiece, similar to a stamping die. Therefore, it can achieve the required shape in one step, which makes it a highly efficient process.
Manufacturers often apply this type of EDM to create blind holes with specific geometries, which is why the main applications include gears, dies, turbine blades, and air compressors.
Wire EDM
Wire EDM or electrical-discharge wire cutting is a type of EDM that involves a wire moving slowly at a constant speed along a path provided by a CNC program to achieve the desired shape. The wire removes material along the path and at a constant kerf whose size depends on the wire diameter.
The applications of wire EDM generally include the production of plates, punches, dies, tools, and components for electronics testing and assembly.
The wire used in this process is usually placed between two spools to ensure the portion actively cutting the material is continuously changed. This feature ensures that the wire does not break due to excessive erosion.
EDM wires are typically made of tungsten, copper, brass, or molybdenum. However, alternatives such as steel-cored, multi-coated, zinc-coated, and brass-coated wires are also available.
Since the kerf size depends on the wire diameter, wires come in different diameters for different applications, ranging from 0.02 mm to 0.3 mm, with 0.25mm being the most commonly used diameter.
Electrical-discharge grinding
EDG is an electrical-discharge machining (EDM) type that uses a grinding wheel as the electrode. Graphite and brass are the most common materials for the grinding wheel. Another feature of the grinding wheel is that it contains no abrasive element. As with a regular electrode, the process generates sparks between the grinding wheel and the workpiece to remove material. The main characteristic of EDG is that there is a chemical action where the sparks separate the oxide layer from the surface of the workpiece, thus cleaning the surface as it happens in regular grinding. EDG provides a high-quality surface finish to carbide tools, dies, and surgical needles, among other applications.
Hole-drilling EDM
As the name suggests, this EDM type focuses on creating holes in the workpiece.
Hole-drilling EDM involves a rotating tool electrode that can create minuscule holes ranging from 0.25mm to 6.1mm in diameter. The holes created with this process need no deburring after drilling since the dielectric fluid cleans the hole from any remaining debris. This characteristic makes this hole-drilling method excel above traditional ones.
Common applications include:
- Internal cooling for turbine blades.
- Medical equipment.
- Tools and moulds for semiconductor production.
Advantages of EDM
- Machining accuracy – EDM can achieve tight tolerances within the millionths of an inch range.
- Machining capabilities – EDM can work with a wide range of material hardness and brittleness. Therefore, manufacturers can process any conductive material, including titanium and nickel alloys.
- Complex geometry – EDM can create complex shapes with intricate features that may be difficult or impossible to achieve with conventional machining or additive manufacturing methods
- Surface finish – EDM can achieve high-quality surface finishes without post-processing. Manufacturers can benefit from obtaining end products with remarkable aesthetics and performance.
- Manufacturing efficiency – EDM is a highly efficient production process since it can reduce or eliminate the need for post-processing. Therefore, manufacturers use it to save the cost and time required by those extra processes.
- Limited distortion –EDM can limit internal stress and distortion on the surface since there is no heat accumulation, tearing, or fracture.
Disadvantages of EDM
- Material limitations – EDM can work only on conductive materials. So, manufacturers cannot use it to machine plastics and composites.
- Electrode wear – EDM tool electrodes can wear out during the process, and changing them is often required. As a result, manufacturers see their operational costs move higher.
- Machining speed –EDM cannot remove material faster than other manufacturing processes. So, manufacturers find it more costly than alternatives on a per-part basis.
- Rework requirements – The workpiece can sometimes require rework due to electrode burn-off.
What material can be EDM?
As described above, EDM can process any conductive material. It means all metals can be electrical discharge machined.
The most popular material options include:
- Hardened steels
- Stainless steels
- Titanium
- Tungsten
- Molybdenum
It is worth highlighting that soft metals can be EDM, but it is not cost-effective because it is cheaper to use traditional methods. EDM becomes an option for these materials only when the part requires single-stage machining or when the application requires avoiding extra heat on the piece. Examples of materials that fall in this group are:
- Non-hardened steels
- Copper
- Bronze
- Brass
- Aluminium alloys