A method of precision machining called electrical discharge machining (EDM) removes material from a workpiece using thermal energy rather than mechanical force. A thin, single-strand metal wire and deionized water used to...
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This article will take an in-depth look at EDM Machining.
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This chapter will discuss EDM machining together with its working principle.
Electrical discharge machining, EDM, is a manufacturing process where material is removed from a workpiece using a rapid sequence of electrical current discharges between electrodes submerged in a dielectric liquid. The process is used to manufacture parts that are impossible to machine. Since EDM does not use mechanical force to remove material, it is considered to be non-traditional.
EDM is an exceptionally precise process that is ideal for forming complex and intricate shapes and is capable of shaping hard metals such as titanium. The materials shaped using EDM must be electrically conductive for the process to be successful.
The workpiece electrode, anode, of an EDM machine is connected to the positive terminal of a DC power supply while the tool electrode, cathode, is connected to the negative terminal. The electrodes are submerged in the dielectric fluid and separated by the spark gap. Sparking generates extreme electrothermal heat in the spark gap zone, which melts and vaporizes parts of the workpiece surface, a process known as spark erosion.
Although the principles of EDM machining are the same, there are variations in the process, especially between wired EDM working and sinker EDM working. Both processes have anodes and cathodes used to shape the workpiece to fit the parameters of the produced part. How they complete this process using electrical current is quite different.
With sinker EDM machining, an electrical potential difference is created between the tool and work material, both of which are electrically conductive and submerged in a dielectric fluid such as hydrocarbon oil or deionized water. The spark gap that separates the tool and workpiece is flooded with the dielectric fluid. The created electric field depends on the electric potential difference and the spark gap.
The tool takes the negative terminal while the work material takes the positive terminal of the power generator. Free electrons on the tool are subjected to electrostatic forces the moment the electric field begins. If there is less work function or smaller bonding energy of the electrons, the emission of electrons would be from the tool (assuming that it is connected to the negative terminal). This type of emission of electrons is called cold emission.
Through the dielectric medium, the cold emitted electrons are accelerated towards the work material. As they gain velocity and energy and begin moving towards the work, collisions occur between the electrons and dielectric molecules. The collisions cause the ionization of the dielectric molecules, which depends on the work function or ionization energy of the dielectric molecule and the energy of the electrons. As the electrons accelerate, positive ions and electrons are generated because of the collisions.
This cyclic process increases the electron and ion concentration in the dielectric fluid between the tool and the work material at the site of the spark gap. The concentration becomes so high that the matter in the channel is characterized as “plasma.” The electric resistance of the plasma channel is very low. The large number of electrons flux from the tool to the work with ions moving suddenly from the work to the tool. This motion of electrons is known as an avalanche.
The sudden movement of electrons and ions creates the thermal energy of the spark with a heat range of 8,000°C up to 12,000°C. The rapid motion of the electrons hits the work material and the ions on the tool. The impact of the electrons and ions on the surface of the workpiece is converted into thermal energy or heat flux.
The EDM wire machining process, an alternative to sinker EDM machining, works much like a wood band saw using a wire for the cutting process. The wire, made of copper or brass, has a high voltage electrical discharge passed through it that makes it possible for the wire to cut through the thickness of the workpiece.
The wire in EDM wire machining creates a spark in deionized water where conductivity is precision controlled. The water cools the material and washes away the removed material with clean dielectric fluid being constantly pumped into the process to flush the excess waste away.
The extreme temperatures of the EDM process rapidly removes excess material from the workpiece by vaporization and melting or spark erosion. The molten metal is partially removed. As the electric potential is withdrawn, the plasma channel is no longer sustained and generates pressure or shock waves as it collapses. This evacuates the molten material forming a crater of material that is removed from around the spark site.
Material is removed by the formation of shock waves as the plasma channel collapses due to electric potential discontinuation where the work material is made positive and the tool negative. As the electrons strike the workpiece, craters are formed by the heating, melting, and removal of material as positive ions strike the tool resulting in tool wear.
Electrical discharge machining requires a great deal of power. Generators used for the process must be capable of supplying the necessary power in order for the process to run efficiently and successfully. They are selected in accordance with their ability to generate the power parameters of the process.
There are three categories of EDM machines are sinker, wire, and hole, each of which uses the same
A DC power generator is the power supply for the EDM machining process. The negative terminal is connected to the tool while the positive terminal is connected to the part being machined (i.e. the workpiece). Different types of power generators are used such as:
A workpiece is the part to be machined. It’s fixed in the dielectric container using a fixture and is connected to the positive terminal of the power supply.
The fixture is used for holding the workpiece properly in the dielectric container.
Aspects of dielectric fluid are discussed below.
The dielectric medium plays a key role in the functioning of the EDM Machining process. Usually, the dielectric fluid is hydrocarbon oil with low viscosity. The dielectric serves to separate the workpiece and the electrode. During the EDM machining process, as the spark is produced, a surge of current takes place when the dielectric is ionized to form a column or path in the tool. After this happens, the dielectric medium ruptures when the gap between the tool and work gap is about 0.03 mm at about 7V.
The spark discharge will be at around 10,000°C and thousands of atmospheric pressure in a micro small area. It takes less than a microsecond for this to take place for each spark. As the column of ionized dielectric vapor collapses, a small tiny part of the workpiece vaporizes due to the arc being forced out. The small tiny metal particles are then cooled down into small spheres and the flux of the dielectric fluid sweeps the particles from the area.
The following are essential functions of a dielectric fluid used in the EDM machining process:
The dielectric fluid must possess the following properties to act as a good dielectric medium and meet various functional requirements:
The most commonly used dielectrics are hydrocarbon and mineral oil, which have low viscosity. The different types of dielectric fluids are paraffin oil, lubricating oil, transformer oil, and deionized water. Distilled water is used when higher rates of material removal are required.
Different types of fluids perform differently in EDM machining processes. Distilled water has a medium material removal rate and a low wear ratio. Tetraethylene glycol possesses a high material removal rate and a high wear ratio. The dielectric fluid must be filtered first before reuse so that removed metal from the work material and tool electrode is separated. This ensures efficient performance during the process.
Flushing is the proper circulation of the dielectric fluid at the gap that is between the workpiece and the electrode tool in EDM machining. The efficiency of the cutting process depends on the flushing of the dielectric fluid to a greater extent. To achieve good machining conditions, good flushing is essential in EDM machining.
The dielectric fluid gets contaminated with eroded metal particles during the machining process. The dielectric fluid also gets contaminated with carbon particles resulting from the cracking of the dielectric fluid due to heat. This contamination, in turn, will reduce the insulation strength of the dielectric fluid, leading to early discharge of the spark. If the contamination exceeds the permissible level, then bridges are formed in the tool and the work gap. This will lead to short circuiting and damage to the tool and the work surface. Such bridges are eliminated by proper flushing in the gap.
The following are different methods of flushing in EDM machining:
Pressure flow types are a broadly used method for circulating dielectric fluid in EDM. The fluid is forced to pass through the gap between the tool and the workpiece, by forcing it through holes in the electrode. Under pressure, the dielectric fluid flushes out the solid metal particles and cools the workpiece and the tool electrode. A needle-like work material is left off in the electrode hole. This will then be removed afterward to get a clean machined surface.
In this method of flushing, the dielectric fluid flows from the outside of the electrode tool from the bottom. In both reverse and pressure dielectric flow, a taper is formed at the mouth of the cavity.
This method produces straight holes in the workpiece. A vacuum pump is used to create a vacuum, to draw the dielectric fluid around the tool electrode, which passes through a hole at the center. This type of method leaves a central needle like work material which will be removed afterwards in order to have a clean machined surface.
In this method, the flushing action to the fluid is produced by vibrating the tool. This method is suitable for small tools that can not accommodate a fluid passage for the flux of the fluid. This method is the most suitable for deep hole drilling of small diameter.
A pump is used as a passage for the dielectric fluid to flow from the base of the container to the tool and the part to be machined such that more MRR takes place.
A filter removes any irregularities or dust particles that may be present in the dielectric medium. The filter is situated just above the pump.
The tool holder serves to hold the tool properly.
Between the tool and the part to be machined, a spark is generated, in the presence of a dielectric medium. Therefore, the removal of material occurs from the surface of the workpiece.
Any electrical conducting material can work as an electrode tool in EDM machining. The shape of the tool is transferred in the cavity cut during machining in EDM machining processes. Thus the shape and accuracy of the machined surface are fully dependent on the electrode’s shape and accuracy. Erosion takes place on both the tool and the workpiece during cutting. The wear ratio range is usually between 5: 1 and 100: 1.
The wear of the tool compared to the wear of the workpiece i.e. the material removed is called the wear ratio. The physical and chemical properties of the tool and work material, the fluid used as the dielectric and the operating conditions of the machining process determine the wear ratio. The wear ratio of the workpiece and tool are affected by the melting point of the tool and work.
Wear ratio is given by: Wr=2.25Mt-2.3
Where, Wr is the work/tool wear ratio; and Mt is the Work/tool melting point ratio
The wear ratio of the work and tool reduces as the cross sectional area of the work and tool increases. But the wear ratio is increased by higher cutting rates, when machining sintered hard metals like vanadium and molybdenum steels. Higher wear ratio results in narrow deep sections with sharp corners. To minimize tool wear, reverse the polarity and use copper tools. EDM electrode tools are made by casting, machining or by powder metallurgy. Tools with a diameter as small as 0.1mm have been successfully used in EDM processes.
EDM tools must exhibit high melting and vaporization temperature or high thermal conductivity like graphite or copper respectively. The tool materials must also exhibit other properties like the ease of fabrication or shaping and wear resistance. Cost is another consideration when choosing a material to be used as a tool material in EDM machining.
The following are important factors that determine the suitability of a material for application as an electrode tool in EDM machining:
Although any electrically good conductive material can serve as an electrode material, considering the essential factors and properties for better application in EDM, graphite has been the most used as a tool material. Graphite exhibits a low wear rate with a higher degree of electrical efficiency and it is relatively cheap and easy to fabricate.
Good structured isotropic grain graphite that has fine grains, possessing the same current carrying characteristics, together with wear characteristics in any direction, make an excellent material for the tool electrode in EDM Machining. Copper, copper-tungsten, and brass are also utilized as EDM tool materials. To obtain minimum tool wear in EDM, graphite is used as an electrode with a frequency of around 3000 kHz and a power supply of 30A.
To control the actual wear of the graphite tool, it is coated with steel. But the no-wear machining condition results in a machined surface that is very rough for most applications and a finishing cut is constantly needed. The power supply to the system determines the major operating factors of material removal rate, wear ratio and machining stability. The selection of material for tool electrodes is also governed by the type of machine used, not only on the work material that is to be cut.
A certain amount of clearance must be provided on the tool side to obtain the correct size of the work produced afterwards, although the electrode tool in EDM machining is designed to act as a mirror image of the size and shape of the work to be produced. In designing the tool electrode, this is an important consideration.
The amount of clearance provided primarily depends on the tool material, material removal rate and the workpiece’s material. It is also dependent on the nature of the cut like roughing or finishing operation. The side clearance to be provided in machining hardened steel with brass as a tool material is 0.25 mm; 0.3 mm side clearance with duralumin and for graphite electrodes the side clearance will be 0.35 mm. The clearance to be provided with a high removal rate (rough cut) is 0.5 mm, 0.3 mm for medium rate, and 0.05 mm for the slow rate of cutting (fine surface).
During the course of the EDM machining process, the material is removed from the part to be machined and the electrode tool. This increases the gap that is between the work and the tool. The tool must be fed properly using a device that controls the feed in order to maintain a constant gap and arcing voltage to prevent the occurrence of a short circuit.
It is essential for the system to respond rapidly, with a low inertia drive. If there is any overshooting, it will close the arc gap causing short circuiting. Hence, a rapid reversing feed motion is required. A signal obtained from an electrical sensor that responds to gap voltage or working current, provides actuation of the control.
The voltmeter is a device for measuring voltage. Voltmeters are used to measure voltage in the EDM system.
An ammeter is used to measure or check the flow of current. The ammeter must be connected in order for us to check whether the current is flowing or not.
A servo control mechanism automatically maintains a constant gap that is about the thickness of human hair between the work material and the tool. This mechanism is found in both wire and vertical EDM machines. It is important to ensure that there is no physical contact of the electrode and the work, otherwise arcing would occur. This in turn would result in the damaging of the workpiece and also breaking of the wire. As the operation progresses, the servomechanism advances the electrode into the work material and controls the work-wire to maintain the proper arc gap, by sensing the work-wire. The proper arc gap maintained is essential to a successful operation of the EDM.
The table serves to hold the material to be machined (i.e. the workpiece).
This chapter will discuss two main types of EDM machines.
This type of EDM machine is also known as sinker EDM, die sinking, volume EDM, ram EDM, and cavity-type EDM. This type of EDM is popular because of being suitable for creating complex shapes.
Conventional EDM makes use of machining to create a distinctive shape of an electrode that is sunk deep into the material to be machined. This inverse copy is a negative impression of its shape created by the electrode.
Conventional EDM uses shaped electrodes, therefore it is useful for making dies and molds in particular. It is also suitable for small-batch production (production of prototypes). It has many applications in industries such as automotive and aerospace industries because it can accurately produce complex engine parts. It is also mostly used in a great diversity of industries for injection molding processes
These machines are also known as wire burning, spark EDM, and wire erosion. They use a thin heated wire as an electrode. Hard diamond is used as a guide to hold the wire steady. The wire electrode is moved through the workpiece to craft a particular shape, but only the electrical discharges of the wire do touch the workpiece; the wire itself does not touch the workpiece. In this type of EDM, the wire is put in slow motion.
In wire EDM, the wire is always available for cutting a smooth, uninterrupted form because the wire constantly unspools from an automated feeder. Sometimes, a cut through the middle rather than along the outside is required by a shape. In cases like these, wire EDM is coupled by hole-drilling EDM by machinists. As the name of this technique indicates, hole-drilling EDM involves drilling a small hole through the center of the work material. The wire can then be threaded through the hole to continue its shaping accurately. In cases like these, the electrodes’ shapes are tubes, and a dielectric fluid flows through the electrodes to the hole.
This type of EDM offers a few distinct capabilities and advantages. They provide performance that is robust, reliable, cutting edge, while remaining user-friendly. The following are advantages of wire EDM over conventional EDM:
With traditional EDM, the electrodes are prone to erosion and must be regularly replaced when they become too worn to function. Traditional EDM also needs the machining of electrodes of particular shapes, and this additional pre-machining consumes much time. On the other hand, wire EDM is ready to start as soon as the wire is put in place and does not need the time and expenditure on material, which is associated with pre-machining. It is suitable for applications that are time sensitive and shapes in which the machining of matching electrodes would present challenges. It is also mostly used in extrusion dies.
This chapter will discuss the applications, advantages, and disadvantages of EDM machining. Aspects to consider when choosing the right EDM machining will be discussed.
However, it must be noted that the disadvantages of EDM machining are far outweighed by the advantages.
Electrical discharge machining is a process that consumes high power; hence the generators used in EDM machining must be able to supply a lot of power for the process to run efficiently. Careful considerations must be made when opting for a generator to use in EDM Machining. The type of fluid used as a dielectric fluid is another factor that must be taken into consideration, as it has been seen that different dielectric fluids possess different wear ratios and different material removal rates. A dielectric fluid of high material removal rate is required but also factoring in the wear ratio of the dielectric fluid. EDM machining offers a great diversity of benefits such as high accuracy, good surface finish, though EDM machining has disadvantages that must be noted when applying EDM machining.
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