Working Principle of EDM process

Working Principle of EDM process
  • It is a metal removal process. It is based on the principle of material removal in which a material is removed by giving an interrupted electric spark discharge between the electrode tool and the work piece. 
  • A potential difference between the tool and work piece is applied in this process. For this process it is essential that both apparatus and work material are to be conductors. Initially apparatus and material are submerged in to a dielectric medium. 
  • Commonly kerosene or de ionized water is utilized as the dielectric medium. It is necessary that a gap is maintained between the apparatus and material. 
  • Depending upon the applied potential difference (50 to 450 V) and the gap between the apparatus and material, an electric field would be established.Normally material is connected to anode and equipment is connected to the cathode of the generator. Cathode is a negative terminal whereas anode is a positive battery.
  • As soon as the electric field is established between the apparatus and material electrostatic forces are applied to the free electrons on the tool.
  • In the situation where the bonding energy of the electrons is less, electrons would be transmitted from the tool. This type of emission of electrons is known as ‘cold emission’. After that cold emitted electrons are then accelerated in the direction of the job by means of dielectric medium.
  • A collision among electrons and dielectric molecules takes place when these cold emitted electrons gain velocity and energy, and start moving towards the job. 
  • As a result of these collisions ionization of the dielectric molecule takes place.
  • Ionization depends on the ionization energy of the dielectric molecule and the energy of the electron. With the increment in electrons speed, more positive ions and electrons are produced due to collisions.
  • As a result of this repeated process the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap is increased. The concentration would be so high that the matter presented in that channel could be mark as “plasma”. It is necessary that the electrical resistance of such plasma channel would be exceptionally few. Thus surprisingly, a large number of electrons will flow from tool to job and ions from job to tool. This is called avalanche motion of electrons. This progress of electrons and ions can be considered as a spark.
  • In this way the electrical energy is exhausted as the thermal energy of the spark. The high-speed electrons then impose on the job and ions on the tool. The kinetic energy of the electrons and ions would be transformed into thermal energy or heat flux depending upon their collision with the surface of the job and tool respectively. As a result of this intense localized heat flux a large immediate and restricted increase in temperature takes place. It would be in excess of 10,000oC.
  • Due to this tremendous increase in temperature the process of material removal starts. 
  • The process of material removal takes place as a result of immediate vaporization of the material as well as due to melting.
  • The molten metal is not absolutely detached but only partially. At the time of withdrawal of potential difference, plasma channel collapses. Due to this compression shock waves are produced on both the electrodes particularly at high spots on work piece surface, which are closest to the tool.
  • As a result, molten material is removed and a hole is created around the site of the spark. The entire cycle of procedure take place within a few microseconds.
  • Therefore, it is said that, generally the removal of material in EDM happens due to development of shock waves as the plasma channel collapse owing to discontinuation of applied potential difference. 
  • Usually the material is made positive and the equipment is made negative. Consequently, the electrons hit the job. As a result, a hole is formed due to high temperature and melting and material removal.
  • In the similar way, the positive ions are impose on the tool. Due to this tool wear take place.
  • In EDM, voltage pulses are applied between the equipment and material with the help of generator. 
  • The applied voltage is irregular. Only sparking is preferred to a certain extent in place of arcing. Due to arcing localized material removal at a particular point take place whereas sparks get distributed all over the tool surface leading to uniform material removal.
Hope, above information is sufficient to clear your doubt about working principle of EDM process.