EDM is a highly specialized machining process that is used in the precision engineering industry. It is a useful method for producing complex mechanical parts with extremely high precision tolerance.
Below we take a closer look at what EDM machining is and how it works.
What is EDM
It was in the 1940s when the Russian-born Lazarenko brothers fortuitously discovered EDM. Their experiments were aimed at studying the wear of electrical contacts, which led them to the creation of the first EDM machine.
Before them, only the English chemist Joseph Priestley understood the potential of electrical discharges, theorizing that they could be used to achieve an erosive effect on metals. This was the second half of the eighteenth century, and the scientist's brilliant insights added to the pioneering studies on electricity that he pursued.
Over time this machining was refined to the degree of precision it is today. We are dealing with spark EDM, also called Electric Discharge Machining (EDM); an advanced technology that exploits the erosive capacity of electrical contacts for the removal of metallic material.
The Functioning of EDM Machining
EDM machining takes place thanks to the combined power of heat and electricity, which working in synergy are able to remove countless metal particles. Special conditions must be met to complete the process.
Having taken a piece of metal to be machined, an electrode and a dielectric liquid, typically composed of fluid oil, are essential to trigger the EDM process.
The Steps of the EDM Process
The process begins with the immersion of the workpiece in the dielectric fluid and subsequent feed with negative polarity. The electrode, on the other hand, is fed with a trigger voltage of 20 to 100 volts with positive polarity and remains at a distance of approximately 25 µm from the workpiece (sparking gap).
What occurs is the liberation of electrons from the cathode, which collide with the atoms of the dielectric fluid to create ions, both positive and negative. There is thus an ionisation of the fluid, which goes from being an insulator to becoming a conductor. Electric discharges pass through it at regular intervals, succeeding each other: at the end of the process, the voltage is cancelled, and then it begins to store energy again, triggering a new cycle. The pause between cycles varies between 1 and 30 micro-seconds.
Under these conditions, the dielectric fluid changes its state, transforming into a plasma (a glowing, ionised substance of free atoms and electrons) capable of melting and vaporising portions of the workpiece. This is how many small holes are created on its surface: hole after hole, the material is removed precisely because of the erosive power of the plasma. The waste of the process is not swarf, as in traditional techniques, but gases and metal spheres.
Electrodes in Detail
We have talked about the electrode, one of the central components in EDM machining. This has special characteristics, such as high resistance to electrical discharge and wear, but can be found in two different materials.
On the one hand, we have graphite, and on the other, copper. The former, in view of the aforementioned qualities, is not suitable when surfaces with low roughness are involved in machining. In the latter case, it is preferable to use copper, which, however, involves higher costs.
Types of Processing
Starting from the same workmanship, there are two ways of putting it into practice, to be used alternately depending on the result to be obtained and the characteristics of the workpiece in front of you.
Wire EDM
The EDM process remains unchanged, but in this case the electrode is a taut conducting wire that cuts the workpiece.
There is a coil that constantly lets go new wire throughout the process: it could not be otherwise, because if the same segment is always used, it would eventually break under the weight of wear and current.
The material of which it is made is generally copper, the conductive power of which is further enhanced by a brass cover. The thickness does not need to be considerable and in fact 0.02/0.03 millimetres is sufficient to operate correctly.
This type of machining makes it possible to work on parts that require a high degree of precision, being able to act on nanometric surfaces down to RA 0.04.
Plunge EDM
Plunge EDM is based on a machining process that exploits the complementarity of the electrode. In fact, the workpiece takes on a complementary shape to the electrode, which must therefore have a precise conformation.
To proceed with die sinking EDM, therefore, it is essential to create an electrode whose structure meets these requirements. For the rest, the procedure remains unchanged.
Advantages and Disadvantages
EDM machining is used in various industries for the production of precision parts. This technique can offer numerous advantages, such as freedom in the choice of metal materials to be machined. Virtually all of them can undergo this process, regardless of hardness: in fact, the only characteristic required is conductivity.
Moreover, it allows figures to be created with a very high degree of precision. The same result cannot be obtained with other machining operations; for example, milling, which does not allow the creation of sharp internal edges. Once the machines have been set up, they can be left on for more than 130 hours, continuing to operate non-stop. The result is a saving in terms of work by the operators, who can devote themselves to other activities.
Like all machining, however, it is not perfectly adaptable to every case. The machined surfaces are characterised by a high degree of roughness, which is not always acceptable.