TMRP

2. Topics covered
 Introduction
 Electric discharge machining – principle ofoperation
 Mechanism of metal removal
 Spark erosion machiningprocess
 Spark erosiongenerators

3. Introduction
 Several machining processes involving theapplication
of intense heat havecome into use in recent years.
 In these processes, material is removed by meltingor
vaporizing small areas at the surface of the work
piece.

4. Continued…
The processes in which metal removal is based on
thermal principlesare;
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, firstdetected
the erosive effectof 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 cuttingtool.
 And the cutting energy is provided by means of short
duration electrical impulses.

6. Electric discharge machining (EDM): (a) overall setup, and (b)
close-upview of gap, showing dischargeand 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 sparkerosion.
 In this process an electric spark is used as the
cutting tool to cut (anode) the work piece to
produce the finished part to thedesired shape.

8. Continued…

9. Mechanism of Metal removal
 The electro-sparking method of metal working
involvesan electric erosion effectwhich connotes the
breakdown of electrode material accompanying any
form of electricdischarge.
 A necessarycondition forproducing a discharge is the
ionization of the dielectric i.e., splitting up of its
molecules into ions andelectrons.

10. Continued…
 Consider the case of a discharge between two
electrodes through a gaseous or liquid medium.
 As soon as suitablevoltage 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 fieldforces.

11. Continued…
 While moving in the inter-electrode space, the
electrons collidewith the neutral moleculesof the
dielectric,
 Detaching electrons from them and causingionization
becomes such that a narrow channel of continuous
conductivity is formed.
 When this happens, there is considerable flow of
electrons along the channel to theanode, resulting in
a momentarycurrent impulseordischarge.

12. Continued…
 The liberation of energy accompanying thedischarge
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 pointof discharge.
 The metal in the form of liquid drops is dispersed into
the space surrounding the electrodes by the explosive
pressureof the gaseous products in the discharge.
 This results in the formation of a tiny crater at the
point of discharge in thework piece.

13. Continued…
 Comparatively less metal is eroded from the cathode
tool as compared to the anode work due to following;
i. The momentumwith 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…
 Mostof the EDM operationsare conducted with work
immersed in a liquiddielectric.
 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
decompositionof the liquid dielectricare drawn into
the space between the electrodes during the initial
part of the dischargeprocess

15. Continued…
 And are distributed along the linesof force, thus
forming current carrying “bridges”.
 Discharge then occursalong one of these bridges as a
result of ionization.
 Spark discharge in liquid leads toan intenseejection
of anode particles into the surroundingspace,
 But, discharge in gas results in the partial transfer and
diffusion of detached anode particles into the surface
of cathode.

16. Continued…
In liquid Ingas
 drilling, die sinkingand
preparation of tool.
 operations connected with
the toughening andbuilding
up of surfaces.

17. Spark Erosion Machining Processes
 EDM is the removal of materials conducting
electricity by electrical discharge betweentwo
electrodes (work piece electrode and tool
electrode)
 A dielectric fluid being used in theprocess.
 Theaim of theprocess is controlled removal of
material from the workpiece.

18. Classification of the spark erosion
processes

19. Sinking by EDM
 In this case, the metal removal is affected by
nonstationary electrical discharges whichare
separated from each other both spatiallyand
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 operationswhere the
work piece is cut off ornotched.

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

23. Thank you