Manufacturing processes What is Electrical Discharge Machining and how does it work?
When traditional machining methods reach their limit, electric discharge machining can be the answer. The EDM process allows for high accuracy and is applicable for any conductive material.
Electrical discharge machining, or EDM, is a non-traditional method in which material is removed from a workpiece using thermal energy. Much like processes such as laser cutting, EDM does not need mechanical force in the removal process. This is the reason why it is considered non-traditional contrary to, for example, the processing with cutting tools.
In tool and mould making, EDM is very popular due to its applicability especially for hard materials like titanium or for particularly complex shapes that are hard to achieve with milling.
The EDM process
The Ecyclopædia Britannica gives a short explanation of EDM:
“EDM involves the direction of high-frequency electrical spark discharges from a graphite or soft metal tool, which serves as an electrode, to disintegrate electrically conductive materials such as hardened steel or carbide.”
To put it more simply, electrical discharge machining is a manufacturing process that precisely removes material from conductive materials using an electrode. Similar to pushing a form into soft material, the electrode leaves a negative imprint in the workpiece. The physical process is a little more complicated: In a small gap between workpiece and electrode, a discharge occurs that removes material through melting or vaporizing. For this process, the electrode and the workpiece have to be submerged in a dielectric fluid.
The principle behind this process is the ability of controlled electric sparks to erode material. The workpiece and electrode do not touch during this porcess. In between is a gap that is roughly as thick as a human hair. The amount of removed material with a single spark is small, yet the discharge occurs roughly several 100,000 times a second.
While the electrode is moved closer to the workpiece, the electric field in the gap, also known as spark gap, increases until it reaches the breakdown volume. For this process, it is necessary that the fluid in which this discharge occurs is not conductive, or dielectric. The discharge causes strong heating of the material, melting away small amounts of material. This excess material is removed with the steady flow of the dielectric fluid. The liquid is also useful for cooling during the machining. Moreover, it is necessary for controlling the sparks.
Three different types of electrical discharge machining: Wire EDM, sinker EDM and hole drilling EDM
There are three different types of electrical discharge machining. The one described above is called sinker EDM. It is also known as die sinking, cavity type EDM, volume EDM, traditional EDM, or Ram EDM. Using Die sink EDM allows users to produce complex shapes. This method requires electrodes (often made from graphite or copper) that are pre-machined to have the necessary shape. This electrode is then sunk into the workpiece, creating the negative version of its original shape.
The second type of electrical discharge machining is called wire EDM and is also known as wire erosion, wire burning or spark EDM. In Wire EDM a thin wire is used to cut the work piece. In this case, the wire works as the electrode. During the machining, the wire is constantly coming from an automated feed with a spool. If the cut has to be made in the middle rather than the outside of the workpiece, small hole drilling EDM is used to make a hole in the workpiece through which the wire is threaded afterwards.
The wire is held with diamond guides. Usually the liquid is deionized water. The wire is often made from brass or copper.
The following video gives a short explanation on how wire EDM works:
The last type of electrical discharge machining is called hole drilling EDM. As the name suggests, this process is used for drilling holes. Compared with traditional drilling methods, EDM is able to machine extremely small and deep holes. Additionally, EDM drilled holes don't need any deburring. The electrodes in this process are tubular and the dielectric fluid is fed through the electrode itself.
In general, every conductive material can be machined with electrical discharge machining. Common materials include metals or metal alloys such as hardened steel, titanium, and composites.
Typically, the electrodes for die sinking EDM are made of copper or graphite. The main factors that influence the decision for an electrode material are the electrode's conductivity and its resistance to erosion. Graphite has the advantage that it is easier to machine than copper. However, copper is highly conductive and strong. Brass, an alloy of cupper and zink, is often used for wire EDM or small tubular electrodes.
Contrary to electrodes for die sinking, the wire used for wire EDM does not have to offer good resistance characteristics, as new wire is fed constantly duting the cutting.
Advantages: When applying EDM makes sense
The main advantage of electrical discharge machining is that it can be used on any material as long as it is conductive. It is therefore possible to machine workpieces made from tungsten carbide or titanium that are hard to machine with traditional cutting methods. Another advantage of electrical discharge machining is the lack of mechanical force put into the workpiece. Fragile outlines are easier to produce because there is no high cutting force needed to remove the material.
EDM also allows for shapes and depths that are impossible to reach with a cutting tool. Especially deep processing where the tool length to diameter ration would be very high, is a usual application for EDM. Sharp internal corners, deep ribs and narrow slots are other specialities of electrical discharge machining. Another argument for using EDM is that the surface finish is usually better than with traditional methods. Electrical discharge machining produces surfaces with a fine finish and high precision.
Moreover, EDM allows users to machine hardened workpieces. Whereas other machining techniques need to be executed before the workpiece is hardened with heat treatment, electrical discharge machining can be applied on the hardened material as well. Thus, any potential deformation from heat treatment machining can be avoided.
However, there are numerous examples where electrical discharge machining is not the right solutions. EDM is a high precision machining method. EDM is a rather slow method compared to traditional machining. High-volume tasks are therefore not suited for this method. At the same time, the electro thermal process requires high power consumption.
Similar to traditional chipping methods, the tool life in EDM is not endless. In sinker EDM, the electrode is also vulnerable to erosion. Because of the tool wear, the electrode has to be replaced regularly. In sinker EDM, it is also necessary to produce the correctly shaped electrodes before the workpiece can be machined. This is an additional step compared to machining processes with traditional cutting tools.
There are numerous examples of companies that have managed to implement electrical discharge machining for their benefit. In many cases an update to a newer and more modern EDM machine has done the trick. These are some case studies that tell the stories of some successful companies and their experience with electrical discharge machining:
EDM and safety?
How safe is EDM? This is actually a question that worries many technicians coming in touch with EDM for the first time. The need for high voltage and reoccurring sparks make it seem like a safety hazard to inexperienced workers. As long as the machine is used according to manufacturer specification, however, there is no great danger involved.
The Metalforming Magazine gives tips on how to ensure process safety. These are some of the measures that need to be taken to run an EDM machine safely:
- Operators and workers have to be properly trained to work with electrical discharge machining
- Make sure that fire protection measures are installed and regularly maintained.
- Keep an eye on the fluid: The level of the dielectric fluid is of utmost importance. The liquid prevents the discharge from crossing to conductive materials other than the workpiece.
- Proper ventilation can clean the air of gases that might be produced in the fluid due to chemical reactions accompanying the discharge.
- You have to monitor the dielectric fluid when it is circulating to ensure it retains its non-conductive characteristics.
EDM machines and automation
There are numerous suppliers of EDM machines. Among the most well-known producers are Mitsubishi Electric, OPS Ingersoll, Makino, Excetek, Sodick, GF Machining Solutions and Ona.
A current trend is the automation of the EDM process. This does not only include the spark or wire erosion itself but also the machining of the electrodes. Using CNC machine tools to machine electrodes, EDM machines, cleaning and measuring stations and — last but not least — storage areas and robotic units to handle the electrodes, the EDM process can be executed automatically. This ensures that a sparker is always fed quick enough with electrodes. The machining of these electrodes and the erosion process can be programmed to take place over night, thus, making production processes much more time efficient. Additionally, a higher worklode can be processed without additional workforce. Several processes can happen simultaneously, including eroding, measuring, milling, laser engraving and cleaning. Companies that have implemented automation in their production processes have witnessed significant time savings and an increase in turnover.
Companies like Zimmer & Kreim offer automation systems particularly for EDM processes. OPS Ingersoll is another manufacturer that offers a one-stop automation solution designed to fulfill the needs of their clients in the toolmaking industry.
Applications of EDM
Invented in the 1940s, EDM is a very early non-traditional process. Combined with computer numeric controls (CNC), it has become an accurate and reliable machining method that is now standard among more conventional cutting methods. It is particularly popular for small-volume productions such as prototypes. Various processes are possible with electrical discharge machining, including turning, milling, grinding and small hole drilling. Besides die and mould-making, EDM is typically applied in the automotive and aerospace industry, for example, in the production of aircraft engines.
Do you want to know more about EDM? Here are some sources for additional information:
- Reliable EDM, an North American manufacturer, offers a free handbook with practical information: Complete EDM Handbook.
- A user of instructables.com, called alchemistdagger, has build his own homemade EDM machine. The complete user manual and instruction can be downloaded online: DIY EDM machine
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