Electrical discharge machining (spark erosion), EDM,, is a process in which a part is shaped by generating an electric arc between the workpiece and an electrode. EDM is regarded as a type of material-removal machining.
What is EDM?
EDM, or electrical discharge machining, is a material-removal process that uses sparks that are precisely controlled and generated between the graphite electrode (the cutting tool) and the workpiece; the process takes place in the presence of a dielectric fluid. And what is a dielectric fluid? Dielectric fluids are hydrocarbon products characterised by being highly refined or synthetic.
It is important to note that this is not a new technique: EDM dates back to the eighteenth century, when it was discovered that electrical discharges had an erosive effect. It was not until 1943 that Soviet scientists began to use EDM as a machining process. The first EDM system was a circuit with a succession of discharges produced between two conductors kept apart from each other, using a non-conductive liquid called a dielectric.
EDM can be defined as a process in which an electric arc is generated between the part to be machined and an electrode, using a dielectric fluid to bring about material removal; the part and the electrode must be made of conductive materials so that the arc and, consequently, material removal can occur.
Unlike other machining processes, where material is removed by a cutting tool, in EDM there is no direct contact. In other words, the electrode removes the excess material without any physical contact with the part being machined. This is the main advantage of the technique: parts with complex shapes that would be impossible with other techniques can be machined.
Operating principle
EDM uses thermal energy. Thanks to this thermal energy, material can be removed by the action of heat. The heat is produced by sparks formed by the electric current generated between the electrode and the workpiece. For EDM to work, certain basic operating principles must be met.
The process requires a power supply that controls the duration and intensity of the electrical discharges; there must also be a movement performed by the electrode. For the sparks that remove material to form, the electrode must be kept at a specific distance from the part. The distance between the part and the electrode is known as the overcut distance or gap; to maintain this distance, a non-conductive dielectric fluid is required. The dielectric fluid can be paraffin, mineral oil or deionised water, depending on the machine used.
What does the EDM process involve?
The dielectric fluid acts as an insulator until the voltage required to turn it into a conductive liquid is applied. Due to the irregularities on the surface of the electrode and the part, an electric field is created between them. This electric field forms at the points where the part and the electrode are closest, and the discharge takes place between these points. When this bridge forms, the electrons and free ions accelerate and form what is known as an ionised column of dielectric fluid, which conducts the electricity. This is when the sparks form, causing the particles to collide.
One of the phases of the EDM process is the formation of a gas bubble. This causes the pressure to rise steadily until a plasma zone is created. Plasma is considered the fourth state of matter and, in EDM, reaches temperatures of between 8,000 and 12,000 degrees Celsius. This is when vaporisation of part of the material on the surface of the workpiece and the electrode occurs. When the gas bubble implodes as the spark goes out, the dielectric fluid is deionised and the temperature drops. This causes the material to detach and form a cloud in the dielectric fluid. The immediate result is a kind of crater on the eroded surface of the workpiece.
As the cloud cools, it solidifies into small particles known as EDM swarf.
Types of EDM
There are three types of electrical discharge machining:
- Sinker (die-sinking) EDM: the electrode is mounted in the machine head and connected to the positive pole of a pulsed power supply. The part to be machined is connected to the negative pole, and a gap is formed between the part and the electrode.
- Wire EDM: similar to sinker EDM, but the electrode is replaced by a consumable wire, which is electrically charged and controlled by computer numerical control.
- Hole-drilling EDM: a process that precedes wire EDM. It produces small-diameter, very deep holes.