Electric Discharge Machining is explained in brief.

1. By : Chandrakanth B

2. Topics covered
 Introduction
 Electric discharge machining – principle of operation
 Mechanism of metal removal
 Spark erosion machining process
 Spark erosion generators

3. Introduction
 Several machining processes involving the application
of intense heat have come into use in recent years.
 In these processes, material is removed by melting or
vaporizing small areas at the surface of the work
piece.

4. Continued…
The processes in which metal removal is based on
thermal principles are;
i. Electric Discharge Machining EDM
ii. Plasma Arc Machining
iii. Electron Beam Machining
iv. Laser Beam Machining
v. Hot Machining

5. Electric Discharge Machining
 In 1970, the English scientist, Priestly, first detected
the erosive effect of electrical discharges on metal.
 The EDM process can be compared with the
conventional cutting process.
 In this case, a suitably shaped tool electrode, with a
precision controlled feed movement is employed in
place of the cutting tool.
 And the cutting energy is provided by means of short
duration electrical impulses.

6. Electric discharge machining (EDM): (a) overall setup, and (b)
close-up view of gap, showing discharge and metal removal
Electric Discharge Machining

7. Principle of operation
 Electrical Discharge Machining (EDM) is a
controlled metal-removal process that is used to
remove metal by means of electric spark erosion.
 In this process an electric spark is used as the
cutting tool to cut (anode) the work piece to
produce the finished part to the desired shape.

8. Continued…

9. Mechanism of Metal removal
 The electro-sparking method of metal working
involves an electric erosion effect which connotes the
breakdown of electrode material accompanying any
form of electric discharge.
 A necessary condition for producing a discharge is the
ionization of the dielectric i.e., splitting up of its
molecules into ions and electrons.

10. Continued…
 Consider the case of a discharge between two
electrodes through a gaseous or liquid medium.
 As soon as suitable voltage is applied across the
electrodes,
 the potential intensity of the electric field between
them builds up, until at some predetermined value,
 The individual electrons break loose from the surface
of the cathode and are impelled towards the anode
under the influence of field forces.

11. Continued…
 While moving in the inter-electrode space, the
electrons collide with the neutral molecules of the
dielectric,
 Detaching electrons from them and causing ionization
becomes such that a narrow channel of continuous
conductivity is formed.
 When this happens, there is considerable flow of
electrons along the channel to the anode, resulting in
a momentary current impulse or discharge.

12. Continued…
 The liberation of energy accompanying the discharge
leads to the generation of extremely high
temperatures, between 8000 to 12000 °C
 This causes fusion or partial vaporization of the metal
and dielectric fluid at the point of discharge.
 The metal in the form of liquid drops is dispersed into
the space surrounding the electrodes by the explosive
pressure of the gaseous products in the discharge.
 This results in the formation of a tiny crater at the
point of discharge in the work piece.

13. Continued…
 Comparatively less metal is eroded from the cathode
tool as compared to the anode work due to following;
i. The momentum with which positive ions strike the
cathode surface is much less than the momentum
with which the electron stream impinges on the
anode surface.
ii. A compressive force is generated on the cathode
surface by the spark which helps reduce tool wear.

14. Continued…
 Most of the EDM operations are conducted with work
immersed in a liquid dielectric.
 The particles removed from the electrodes due to the
discharge fall in the liquid, cool down and
contaminate the area around the electrodes by
forming colloidal suspensions of metal.
 These suspensions, along with the products of
decomposition of the liquid dielectric are drawn into
the space between the electrodes during the initial
part of the discharge process

15. Continued…
 And are distributed along the lines of force, thus
forming current carrying “bridges”.
 Discharge then occurs along one of these bridges as a
result of ionization.
 Spark discharge in liquid leads to an intense ejection
of anode particles into the surrounding space,
 But, discharge in gas results in the partial transfer and
diffusion of detached anode particles into the surface
of cathode.

16. Continued…
In liquid In gas
 drilling, die sinking and
preparation of tool.
 operations connected with
the toughening and building
up of surfaces.

17. Spark Erosion Machining Processes
 EDM is the removal of materials conducting
electricity by electrical discharge between two
electrodes (work piece electrode and tool
electrode)
 A dielectric fluid being used in the process.
 The aim of the process is controlled removal of
material from the work piece.

18. Classification of the spark erosion
processes

19. Sinking by EDM
 In this case, the metal removal is affected by
nonstationary electrical discharges which are
separated from each other both spatially and
temporarily.
 This process include those EDM operations in which
the average relative speed between the tool and work
piece is coincident with the penetration speed in the
work piece.

20. Continued…

21. Cutting by EDM
 It includes those machining operations where the
work piece is cut off or notched.

22. Grinding by EDM
 Spark erosion grinding embraces the machining processes
made with an electrode rotating around an axis in addition
to the normal electrode feed.

23. Thank you