Electrical Discharge Machining ( EDM MACHINING )
What is Electrical Discharge Machining?
To know about Electrical Discharge Machining, you need to know about ElectroChemical Machining
Electrical Discharge Machining is a versatile process among all the electrical machining methods. This process is also named by Spark Erosion Machining or Spark over initiated Discharge Machining. This Electrical Discharge Machining is so called because the removal of metal occurs due to the erosion caused by electric spark.
The basic principle involved in this process is the described below. Electrical Discharge Machining comprises of a dielectric medium, a tool, a source power supply of around 50-450 Volts, a speed reduction gear box, a rack and pinion and an electric circuit. The job is connected to the positive terminal of source, it acts as anode and the tool which is connected to negative terminal acts as a cathode. Both the job and the tool are separated by a small distance known as spark gap. When the appropriate amount of current is passed between the tool and the job, a dielectric solution, which occupies the gap causes to break down. As a result, the electrons initiate to flow from cathode to fill and ionize the gap.
An electrical breakdown takes place when there is a consistent drop in resistance and discharge of electrical energy. An electric spark with appropriate force and velocity hits the surface of the job. Thus a very high temperature of about 10,000 degrees centigrade is generated on the spot of the hit by electric spark discharge and this completes the cycle in few microseconds. This high temperature melts the metal as well as vapourises it. The melted or vapourised particles of the metal are thrown into the gap by electromagnetic and electrostatic forces and are finally driven away by dielectric flowing fluid.
The main parts or components involved in the Electrical Discharge Machining process are as follows-
Parts of Electrical Discharge Machine-
1. Servo meter
2. Reduction Gear Box
6. D C Power Supply
7. Tool Head
9. Di electric
12. Work Table
In every process of this type, the work piece is attached with the base with the help of fixtures or any other means. The base is then connected to the table firmly. The table must be balanced on a flat rigid floor or any medium. The main aim of the base to to keep the work piece rigid to give undisturbed contact to the tool.
Requirements of Electrical Discharge Machining-
1. In Electrical Discharge Machining, the material sued for both the component and the tool must be effectively conductive which forms as an electrolyte.
2. The component should be held firmly and in exact position with respect to the tool.
3. A predefined gap of about 0.025 mm is to be maintained in between the tool and the component.
4. A pressurized dielectric fluid is required in the arc gap to direct the discharge electric current. It also acts as coolant for tool and component.
5. A power supply provides pulsating direct current to the tool and the component.
6. It requires a 5 micro meters unit filter for general applications. For some applications 2 micro meters unit filter may be also used.
7. Frequency of current flow should be higher in order to obtain better surface finish.
8. Safety measures are to be followed to avoid damage to the machine and man power.
There are especially 4 factors that influence accuracy in Electrical Discharge Machining.
1. Taper Effect-
In Electrical Discharge Machining, the hole produced gets tapered due to the presence of side sparks in the middle of suspended particle. However, due to this effect, a hole with an accuracy of about 0.05 mm can be easily produced.
2. Over cut-
The component experiences over cuts due to the presence of side sparks. The size of the over cut usually depends up on the gap length and the crater size. The voltage applied to the equipment also has an appropriate effect on it. The over cut of about 5 micro meters to 100 micro meters is required depending up on the roughing or finishing operation.
3. Surface Finish-
The surfaces machined by electrical discharge machining process contains microscopic craters. The surface finish of the component mainly depends on and amperage, frequency and energy per pulse. If the energy per pulse is high then the depth of crater increases which in turn produces better surface finish. The surface roughness in Electrical Discharge Machining process is inversely proportional to frequency of cutting. Therefore, the frequency of cutting is inverse of ( Sparking time + Charging time )
4. Heat Affected Zone-
In electrical discharge machining process, the molten metal is not completely removed from the parent metal. A part of it solidifies on the parent metal in the form of thin layers of about 2 micro meters to 10 micro meters. In this heat affected zone, fine cracks with plastic deformation and thermal stresses are induced.
Merits of Electrical Discharge Machining-
1. Electrical Discharge Machining process provides good accuracy for tools and dies, since it facilitates easy machining for hard materials.
2. This process is also applicable to all electrically conducting metals and alloys, irrespective to strength and hardness of the materials.
3. Complicated shape imparted to the tool gives exact replica to the work piece.
4. Machining time of Electrical Discharge Machining is less compared to conventional machining method, even for hard materials.
5. As there is no physical contact between the tool and the work piece, the sections of either weak or delicate materials can be machined easily.
6. Electrical Discharge Machining process allows easy drilling of fine holes.
Demerits of Electrical Discharge Machining-
1. Machining of sharp corners namely complex contours are difficult to produce.
2. Electrical Discharge Machining process is not suitable for electrical non-conducting materials.
3. Electrical Discharge Machining process is confined to only small work pieces and produces thermal distortions.
4. In Electrical Discharge Machining, it is difficult to employ more than one tool as it requires separate power generators and servo mechanisms.
Applications of Electrical Discharge Machining-
1. Manufacturing of tools is a wide area where Electrical Discharge Machining is mostly employed.
2. Resharpening of cutting tools, broaches, etc and re-machining the die cavities without the aid of annealing.
3. Machining of cavities for dies.
4. Trepanning of holes with straight or curved axes.