Introduction
Electro Discharge Machining (EDM) can be an electro-thermal and non-conventional machining process, where materials is removed scheduled to thermal energy of the spark. EDM uses electrical energy to generate electric spark which travels by using a dielectric liquid at a manipulated distance. Electrically conductive parts regardless of their hardness, shape and toughness can be machined by EDM because the material removal rate relates to the melting point of the metallic to be machined. It is one of the prominent methods used in die developing and has a good accuracy and detail with no direct physical contact between the machine tool electrode and the task piece so that mechanical stress is not exerted on the work piece.
Electric Discharge Machining Process
Working Principle: Electrode improvements into the steel die materials. On supplying the electric energy, spark is produced between tool electrode and expire material creating temperature which melts the die metallic and vaporizes it. The molten and vaporized steel is removed away from the spark difference (gap between your electrode and material) [1] by the circulation of dielectric substance. The heat range is handled by regulating the spark difference between the electrode and the metallic. Both electrode and the perish metal should have good electrical conductivity to create the good spark. The grade of the surface end and the materials removal rate are managed by the regularity and the spark intensity. Generally to obtain better surface surface finish of the pass away, high regularity and low current are used [1].
Figure1: working priciple of EDM[1]
There are two main types of EDMs, the Sinker and the Wire-EDM. Each is employed for machining very small and detail parts and large stuff like automotive stamping dies as well.
In Sinker EDM machines, the tool/electrode is fixed to the ram which is linked to positive terminal; the die metal is linked to the negative terminal of your pulsed power supply. The material is then positioned in such a way that there surely is gap between steel and the tool. Dielectric fluid is flooded in the spark gap. By supplying the power, current impulses of high frequency pass through the gap, starting the machining process. The temperature selection of the sparks produced is from 14, 000 to 21, 000 F [1]. As the metallic erosion continues, the tool/electrode advances into the metallic while maintaining a constant distance from the steel. The shape of the electrode is the reflection image of the finished die metallic cavity. After machining, the pass away material cavity is bigger than the electrode when measured. This dimensional difference is termed as the overcut or kerf.
Figure2: Schematic illustration of the electrical discharge machining process [10]
Following will be the sub-systems of Sinker EDM:
DC power with controls for voltage, current, rate of recurrence, and polarity
Dielectric system to flush away work and electrode dirt, this fluid is mostly a hydrocarbon or silicone based oil
Servo system to control feed, tool journey of the electrode and also to maintain distance maintenance
The Wire EDM process has an electrically charged line with diameter ranging from 0. 002 to 0. 013[1] inches used for machining complex, intricate profiles and cuts. This type is specifically useful in reducing fine details in pre-hardened blanking dies and pre-hardened steels. An electric line drive system constantly releases out the new wire for long periods without operator attention to cut the metals so that it does not have any wear issue of tool electrode. These wiring will create a kerf just a little greater than their own diameter. A. 012 cable will leave a 0. 015 kerf, just 0. 003[1] ins larger. The four basic subsystems include:
DC electricity supply
Dielectric system
Wire feeding system
Work setting system
Different techniques are employed in order to enhance the material removal rate MMR. Following are different techniques discussed to boost MRR.
Electrode account: Computer numerical handled EDM machines which are now available can generate complicated designs within less time, economically without using regular and traditional 3-D complex shaped Electrodes. To really have the higher MRR structure type tooling is utilized for the pass away designs having linear or axi-symmetrical swept areas. The plate type tool has better material removal rate in comparison to a 3-D form tool.
Figure 3 shows differnces between 3D-form tool, structure tool, and planes type tool[1].
Controlling process guidelines: The MRR can be controlled and improved by handling process variables like discharge voltage. This parameter is normally related to dielectric strength and spar difference. Higher voltage arranging ends up with higher spark space, increases flushing conditions which in the end improves in both MRR and tool wear. Flushing conditions can be improved upon by introducing electrode rotation, increasing flushing pressure, and tube electrode design and work piece rotation.
EDM Versions: MMR can be increased greatly by putting into action ultrasonic vibration. MRR is higher for a discharge pulse-on time for ultrasonic vibration aided machining. Ultrasonic vibration EDM is mainly used to set-up small and profound slots products.
Figure 4: Vibratory, rotary and vibro-rotary electrode [1]
Powder Mixed Dielectrics: Powder merged electric discharge machining (PMEDM) has improved upon capacities of the EDM process. In this technique, a suitable material in fine powder is properly combined into the dielectric fluid to improve its malfunction characteristics. This ends in lessening the insulating durability of the dielectric substance, and finally resulting in the upsurge in the spark gap. Increased spark distance distance makes the flushing of dirt uniform, thus bettering MRR and surface surface finish.
Figure 5: principle of powder blended EDM [1]
Dry EDM is a process which gas is the dielectric medium. Because of thermally activated substance reaction between the gas and die material the discharge electricity density on the work surface exceeds a certain threshold limit which increases the MRR. The MRR of dried up EDM milling is about 6 times greater than that of oil EDM milling.
Figure 6: process of dried up EDM [1]
Some Other Techniques: Usage of the multiple electrodes simultaneously increases the MRR and relative electrode wear proportion. We are able to use multiple electrodes either in linear orientation or perhaps like a globe gear model in which planets are electrodes and products is the pass away or work part metal. This is usually used for lowering pipes etc.
Figure7: Multi Spark EDM, Difference between standard & multi electrode EDM[1]
The Results on Surface Quality
Electrical release machining (EDM) increases importance nowadays due to its ability of reliability and complicated patterns reducing. EDM machine is evaluated in terms of its effectiveness of materials removal rate, surface roughness of the work piece, and comparative wear ratio.
The electrical release machining process consists of interaction of mechanised, thermal, substance and electronic phenomena. The type discharge energy influences the machining
characteristics such as surface roughness.
During the process, electricity is changed into warmth energy which bases after heat copy equations. Then your fraction of the energy transfers to the task piece.
Figure8. The electric powered sparks fuse the surface of the die
Models are created for EDM tests showing that the trials can proficiently make material removal rate and parameter of surface roughness with problems up to 94% precision. The optimum parameters of heat copy and the optimum utilization of type discharge energy can help improving the technical performance.
Figure9. Different tool parts made by EDM machine
Surface modification of steel perish is important when doing electrical discharge machining (EDM). Work piece of the steel expire surface may be provided certain type of material through using powder-mixed dielectric or the eroding tool electrode. Some other factors such as break down of the hydrocarbon dielectric also cause steel surface changes.
As recent research provided, machining processing parameters to discover the best value of micro-hardness of steel die piece are located to be same.
Figure10. Surface view of the task piece after electrical discharge machining
Material Selection
When it involves selecting materials for EDM, two functions of the materials are incredibly important. Those functions will be the conductivity and erosion amount of resistance of the materials. The material in this case is also called the electrode. Because the process calls for the utilization of a power current, the slicing efficiency is dependent of the conductivity. Erosion amount of resistance is one factor of melting point, hardness, and structural integrity of electrode. Strong qualities provides more use from the electrode and for that reason, and therefore, fewer times that the electrode needs swapping. These properties are the factors looked at when choosing an electrode, which makes choosing the right material important. The type of EDM process and the materials chosen then need to be put on the processing of metallic dies.
There are several different types of materials to choose from that could work sufficiently for EDM. These materials include but aren't limited to brass, graphite, copper, and tungsten. Both types of materials being considered here are graphite and copper.
The properties of these two materials as well as the sort of EDM process used factor in to choosing. For instance, when using Cable EDM, an electrode with high ductility is looked for. This is in comparison to Sinker EDM, where an electrode that conducts heats well and it is easily machinable is most effective to use. Copper is an excellent use for Wire EDM while graphite is an excellent use for Sinker EDM.
Graphite is very trusted and its own many advantages are the reason why. It has a much lower denseness than copper leading to lighter electrodes used. It is very easily machined. The electrical resistivity is very low which means the conductivity to warmth is high. Graphite does not have a melting point; it skips the liquid point out and goes immediately from a solid to a gas at high temperature. Many of these attributes are adding factors to lessen the wear amount of resistance of graphite. Graphite is relatively inexpensive, too.
Copper is also frequently used. Copper has good power and conductivity, making it more wear immune than materials such as brass. There is absolutely no assessment to graphite for level of resistance, though. Unlike graphite, copper does not leave any dust particles behind when being machined and therefore, is a much cleaner process. This will keep the machines much cleaner, too.
Table1. Physical properties of graphite and copper electrode [26]
Physical properties
Graphite
Copper
Electrical resistivity (/cm)
0. 12
1. 96
Electrical conductivity compared with silver (%)
0. 11
92
Thermal conductivity (W/m K)
160
380. 7
Melting point (C)
455
1083
Specific heating (cal/g C)
0. 17-0. 2
0. 092
Specific gravity at 20 C (g/cm3)
1. 75
8. 9
Coefficient of thermal growth (-10 '6 C '1)
7. 8
6. 6
To conclude, the assessment of graphite and copper when it comes to manufacturing steel dies has its advantages and disadvantages. Utilizing a copper electrode will produce a more rough carry out, whereas, with a graphite electrode a finer surface finish is achieved. Copper has a greater materials removal rate than graphite and graphite has an increased wear resistance than copper. Graphite is the materials of choice here.
Limitations of the Material
Despite most of its advantages, graphite has its restrictions. One major restriction in the tooling of the electrode is that graphite has very poor ductility, removing it from account when choosing Cable EDM. Another less inhibiting limitation is how soiled graphite can be. Having an effective process in location to remove the dirt during the machining process will solve this issue, but it is an concern nonetheless. Graphite particles can be harmful to the people working with the machines also to the machines themselves. That is why an effective particles removal process is essential to have set up.
Figure11. Graphite dirt collection machine [28]
As with graphite, copper has some constraints as well. Copper is a lot more challenging to machine than graphite is due to it being so versatile. Copper electrodes also carry on a much shorter time than graphite electrodes do. To raise the degree of machinability, tellurium is put into copper. Although, this solves issue of machinability, wear level of resistance and material removal rate is lower in the process. Copper is more costly than graphite is, too.
Cost Consideration
The cost involved in any creation process is one of the main factors a making engineer has to consider while selecting the procedure. The cost involved in the EDM process comprises of the price tag on the electrode, the set up and the dielectric substance. Of these, the main is the price involved with fabrication of the electrode. Inside the die-sinking EDM process, the form of the electrode is a reflection image of the account to be produced on the pass away, so the price of fabrication of the electrode will depend on the geometric complexity of the pass away to be produced and also on the sort of the electrode materials. For the die sinking EDM process, since we have chosen graphite as the electrode material, the fabrication of graphite, as everybody knows is difficult, therefore the cost involved with fabricating the graphite electrode could be more. Among the techniques that could be employed to decrease the cost is to use the 'worn out' electrode which was used in the previous finishing operations, to perform roughing cut in the next operation or the next component. In this way, a considerable cost savings could be obtained in the full total cost, especially regarding mass development.
The cost of the installation involves the price tag on the equipment, maintenance, amount of automation and labor cost (if relevant). That is one of the major down sides of the die-sinker EDM process. As we know that in the die-sinker EDM process, the workpiece has to be immersed in to the Dielectric substance prior to machining. The tank should be emptied before mounting a new workpiece, and the same is usually to be done while eliminating the machined part. This hampers the continuity of the procedure and makes it highly discontinuous and intermittent. This further leads to longer cycle time and hence increases the cost. The dielectric has to be pumped in and from the tank, this escalates the energy consumption and therefore the cost.
The cost of the dielectric will involve the cost incurred in buying and preserving the dielectric liquid. If we use kerosene as our dielectric liquid, then there is likely to be yet another cost in taking the safety measures as to prevent the fire risks. Also the conductivity of the dielectric should be carefully preserved.
Environmental Impact
The EDM process has many environmental issues; a few of them are the following:
The high pressure in the space leads to the generation of smoke cigars, vapours and aerosols that happen to be hazardous in characteristics.
If the dielectric smooth used in the EDM process is a hydrocarbon, then it has an adverse influence on the skin.
There may be a possible fire threat, if the dielectric smooth used is kerosene (as inside our circumstance).
In some conditions, explosions could also occur.
The electromagnetic radiations emitted through the process may also have a detrimental effect on the skin.
The EDM process, the total amount of the vapors released may even exceed 5 mg/m3 [9] if proper procedures are not taken. The amount of fumes and aerosols released be based upon many factors like the process, the electrode, and the dielectric substance involved. These are produced more regarding die-sinking EDM than in cable EDM process. Special treatment should take if nickel exists in virtually any of the components involved in the process (either the workpiece or the electrode). This is due to toxic characteristics of nickel. The quantity of fumes released also depend on the viscosity of the dielectric liquid engaged, lower the viscosity, less are the fumes. As a rule of thumb, the level of the dielectric substance should be at least 40mm (80mm is preferred) [8] higher than the erosion area, so a considerable part of the fumes gets condensed and assimilated in the dielectric fluid itself. But again disposal of the dielectric is also an issue.
A several unsafe fumes produced during the pass away sinking EDM process are: Polycyclic Aromatic Hydrocarbons (PAH), Benzene, mineral oil vapor, and mineral aerosols etc.
Apart out of this, the metal rubble from the interelectrode difference from the workpiece surface is also gathered in the dielectric fluid, which needs proper removal in order to avoid environmental pollution.
Conclusion
The use of pass away sinking electric release machining for the manufacture of material dies is mentioned. We conclude that, even though the die sinking EDM process has its advantages as well as cons, it can be employed to effectively manufacture material dies.
