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Short notes on Unconventional Machining Processes

Unconventional Machining Processes form an important part of the chapter of Machining Processes in the subject of Manufacturing Engineering for GATE Mechanical Engineering. In this short notes section we will cover the first Unconventional Machining Process - Electric Discharge Machining (EDM).

Electric Discharge Machining (EDM):

EDM is also known as Spark Erosion machining or simply Spark Machining. The metal removal takes place due to the erosion caused by the electric spark. This process may be used for machining any material, irrespective of its hardness. The rate of metal removal and the resulting surface finish can be controlled by proper variation in the energy and the duration of spark discharge. A liquid dielectric, like light oil like transformer oil or kerosene oil or more commonly Paraffin is always used in the process. 

Basic principle of EDM:

The EDM setup includes:

  • A power source
  • A dielectric medium
  • Work piece
  • A tool
  • A servomotor
  • Speed reduction gear box
  • A rack and pinion arrangement

The work-piece is connected to the positive terminal and the tool is connected to the negative terminal of the power supply source. (Work-piece = ANODE, Tool = CATHODE)

The principle involved in the process is that the work-piece and the electrode (tool) are separated by a gap, which is called spark gap. This gap is filled with a dielectric medium, which, with the use of a proper voltage breaks down. The spark gap usually varies between 0.005 mm to 0.05 mm. When 50 V to 450 V of circuit voltage is applied, electrons start following from cathode and the gap is ionised. 

The electric spark impinges directly on the work-piece surface. It takes only a few micro seconds to complete the cycle and the spark discharge hits the anode with a considerable force and velocity, which results in the development of very high temperature on the spot of impingement. This causes vaporization of the metal and the metal so vaporised is then carried away by the flowing liquid dielectric due to electrostatic and electromagnetic forces. 

During the operation, the erosion takes place on both tool and the workpiece, but tool gets eroded much less as compared to the work-piece. The reason for this is that the tool tip is subjected to compressive forces due to an electric and magnetic field which results in slower erosion of the tool tip surface. 

The material removal rate depends upon:

  • Discharge current
  • Duration of pulse
  • Rate of pulse repetition

The Servo system controls the gap in the EDM process. The function of this system is to correctly locate the tool with respect to the workpiece surface, maintain a constant gap throughout the operation and sense changes in the gap conditions. 

Applications of EDM:

  • Tool manufacturing
  • Re-sharpening of cutting tools and broaches
  • Trepanning of holes with straight or curved axes
  • Machining of cavities for dies
  • Re-machining of die cavities without annealing

In addition to the above mentioned uses, EDM can be used for almost all kind of machining operations performed by conventional machines. 

Advantages of EDM:

  • High accuracy can be achieved for machining die cavities and tools.
  • Highly delicate portions can be machined without the fear of getting them distorted as there is no direct contact between tool and workpiece.
  • Any material which is a good electrical conductor can be machined, irrespective of its hardness.
  • Fine holes can be easily drilled.
  • Much faster as compared to conventional machining operations, even for harder materials.

Disadvantages of EDM:

  • Only small sized workpieces can be machined.
  • Electrical non-conductors are difficult to machine.
  • Thermal distortion.
  • It is unable to produce sharp corners.

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