When you hear about EDM, what comes to mind? Electronic Dance Music? (POV: The nightclubs in Singapore had just reopened when I wrote this article!)
Well, the EDM I mean here may not sound as glamorous as that, but it serves an important role in our lives.
Electrical Discharge Machining is a method of cutting any conductive material very precisely using a series of rapidly recurring sparks, created by high voltages between the material and the cutter. This non-contact process removes material by melting and vaporising it, using temperatures that can reach several thousand degrees celsius. Deionised (distilled) water is then used to rapidly cool down and flush out the waste material.
It may sound like cutting-edge technology (no pun intended), but EDM is more common than you think – it plays a role in manufacturing the next aeroplane you are going to board, and even the toothbrush you use every day!
Why use EDM?
Compared to traditional machining methods, EDM does not create vibrations during the process, creating high accuracy and surface finish. It can also be used to cut extremely hard materials such as tungsten carbide (used to make cutting tools and industrial alloys), and to create geometries that are impossible with traditional machining techniques (e.g. sharp internal corners).
As it can be used to drill holes that are microns in diameter or thread extremely hard material, EDM is commonly used in the mould and dies industry. The plastic products we see in our everyday lives, from toothbrushes to water bottles, are often shaped with moulds created using EDM. In the aerospace industry, EDM is used to drill small holes into turbine blades in jet engines so that they can withstand higher temperatures.
One limitation though: EDM is not the panacea of machining due to its high power consumption, which is a huge problem in the energy crisis we are in. Therefore, despite its advantages, EDM may sometimes be less preferred compared to traditional machining.
Types of EDM
There are 3 main types of EDM. Let’s look at each of them:
In Wire EDM, the workpiece is submerged in deionised water and a thin strand of wire (usually tungsten or brass) is fed through it. The wire is constantly fed from a spool and held by upper and lower guides, made from diamond or tungsten carbide. The wire cuts the workpiece horizontally by means of Computer Numerical Control (CNC). Sometimes, the wire can be tilted sideways to create tapered edges.
The wire is the most important part of the operation, and there are several parameters that can affect its performance.
First, we have its thickness, which can vary from 20 micrometres to 2 millimetres. Thinner wires are used to cut sharper corners. However, thinner wires typically mean lower power settings and therefore decreased cutting speed (i.e. slower cutting). Thus, thicker wires are normally preferred for cutting less delicate features.
The second is the tensile strength of the material. The stronger the material, the better it is at withstanding more aggressive flushing, enabling high voltages without breakage of the wire, This also allows for a higher feed rate.
The last is the wire coating. A zinc coating, for instance, further increases cutting speed typically by 10-15 per cent due to its lower melting point than brass. The zinc absorbs the heat as it boils away and less heat enters the brass wire, allowing it to retain its strength.
In Sinker EDM, an electrode that is machined to an appropriate shape is used as the cutter. The workpiece and the electrode are both submerged in oil, and a large pulsating direct current is used to generate the heat to vaporise the metal, creating a cavity. The machined electrode, with different heights at each point, imprints itself onto the workpiece to create specific shapes.
In superdrilling, an electrode is used to turn the metal into a suspension, and waste material is flushed away using a fluid. As compared to conventional gun drilling which depends on the material’s hardness, superdrilling depends on the material’s melting point as the key factor. This means that superdrilling usually allows for much faster cutting. Furthermore, gun drills require a unique drill bit for every hole size, while superdrilling only requires a thin copper wire as an electrode, thus decreasing the price of operation. Superdrilling is therefore preferred over conventional gun drilling for the creation of small diameters and deep holes
That would be our quick ride through the basics of EDM. While you may not see an EDM setup in your daily life, it not only provides us with the products we use every day, but is also full of interesting engineering concepts. The wire may be small, but its impact on our lives is huge.
Written by Lee Yi Zhi
Illustrations by Lydia Konig