The cold work applications like aluminium extrusion, it allows complex shapes to be formed with very smooth surfaces and it is popular for visible architectural applications such as window frames, door frames, roofs, and sign frames should have high
edm
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UNIVERSITI MALAYSIA PERLIS SCHOOL OF MANUFACTURING ENGINEERING
CNC EDM WIRE CUT MACHINE LABORATORY 1 MANUFACTURING PROCESS II (DPT 213/2) GROUP
Theory of Operation EDM is an acronym for Electrical Discharge Machining. The machine makes use of the erosive effects of electrical discharges, first observed in the year 1770. Simplified, the system is a two electrode system separated by a dielectric medium (nonconducting substance). machined is the other.
The wire is one electrode and the workpiece being
EDM machining utilizes a voltage across these two
electrodes which is greater than the breakdown voltage across the gap between the workpiece and the wire.
This breakdown voltage is a function of the distance
between the wire and workpiece, the insulating properties of the dielectric (fluid separating the electrodes), and the degree of pollution of the gap. Now imagine that there is a point on the workpiece which protrudes (to push or thrust outward) farther than the rest of the workpiece surface. When a voltage is applied, the electric field is strongest at this location and will thus be the location of a discharge. This discharge is the culmination of the following process:
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Manufacturing Process II DPT213/2
School of Manufacturing 2008
The electric field causes electrons and positive free ions to be accelerated to high velocities between the wire and the high point on the workpiece. An ionized channel is formed across the gap between this point and the wire. At this stage, current can flow and the spark takes place between the electrodes.
Figure 2: EDM Wire Cut Sparking Area
A bubble of gas forms due to vaporization of the electrodes and the dielectric. The pressure caused by the bubble rises until it becomes very high. A plasma zone is formed, which very quickly reaches extremely high temperatures (~8000-12000 degrees C). This causes instantaneous local melting of a certain amount of material at the surface of both the wire and the workpiece. The voltage is now dropped. The sudden reduction in temperature causes implosion of the bubble.
This implosion creates dynamic forces which pull the
melted material away from the two conductors. This eroded material then resolidifies in the dielectric and is swept away. This process occurs over and over about once every 2 microseconds. The initial voltage can be set as high as 200 V. At the beginning of each pulse, the machine applies a high voltage to create the bubble. It then drops the voltage to implode the bubble. It maintains the lowered voltage as the material is ejected. Finally, the voltage is returned to ground state and the process is begun again.
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Manufacturing Process II DPT213/2
School of Manufacturing 2008
The rate of cut, the amount of material removed, and the surface finish are dependent on the shape of these voltage pulses.
Different pulse shapes are
required for various combinations of wire and material. The pulse shape must also be modified according to material thickness. The EDM moves the wire through the workpiece eroding material away. There is always a gap present between the wire and the workpiece, thus there is no contact and very little force on the workpiece. The pulse shape has a big impact on the size of the gap. If we seek to machine to high dimensional accuracy, it is very important to control the size of the gap and to be able to offset the wire the correct amount so that path edge (with gap) left by the moving wire is the desired shape and size.
Figure 3: EDM Wire Cut moves through the workpiece
