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CHAPTER 2
LITERATURE REVIEW
This chapter discussed the theory about the study in detailed. The previous researches and study also summarized in a table.
2.1 Electro Discharge Machining EDM Die Sinking
Electrical discharge machining EDM is one of the most extensively used non- conventional material removal processes. Its unique feature of using thermal energy
to machine electrically conductive parts regardless of hardness has been its distinctive advantage in the manufacture of mould, die, automotive, aerospace and
surgical components. In addition, EDM does not make direct contact between the electrode and the workpiece eliminating mechanical stresses, chatter and vibration
problems during machining. Today, an electrode as small as 0.1 mm can be used to ‗drill‘ holes into curved surfaces at steep angles without drill ‗wander‘ [8].
2.1.1 History Background
The first EDM application was carried out by Mr. and Mrs. Lazarenko in the Technical Institute of Moscow during the Second World War
[9] . The first of the two
important improvements, also carried out by these Soviet scientists, who make it feasible to elevate this electrical technique to the category of manufacturing process,
was the RC relaxation circuit Fig. 2.1
, which provided the first consistent dependable control of pulse times. The second innovation consisted of adding a
simple servo control circuit in order to find and hold a given gap automatically. In
6 spite of these first trials and innovations, EDM technology got nearly unknown until
the 1950s. At this time, this technique began to be interesting for the industrial marketing mainly in the USA. Some of the causes that eased a much more
widespread use of the EDM process were the vacuum tubes, its combination with the basic RC relaxation circuit and finally, the development of the transistor. These solid
state devices were able to provide high currents and a really much faster switch on and off than the previous vacuum tubes. Nowadays, EDM is widely used both in the
European market and in the American market.
Figure 2.1: The basic RC relaxation circuit
2.1.2 EDM Process
Electrical-discharge machining EDM removes electrically conductive material by means of rapid, repetitive spark discharges from a pulsating direct-current power
supply with dielectric flow between the workpiece and the tool. The shaped tool electrode is fed into the workpiece under servo control. A spark discharge then
breaks down the dielectric fluid. The servo control maintains a constant gap between the tool and the workpiece whilst advancing the electrode. The dielectric oil cools
and flushes out the vaporised and condensed material whilst re-establishing insulation in the gap [10]. Several theories were proposed by early investigators to
account for the erosion mechanism of the EDM process [11]. It has been accepted generally that the metal removal phenomenon is predominantly thermal in nature [2].
The best explanation of EDM phenomena has been established by extensive experimental studies [12, 13]
.
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Figure 2.1: EDM machining, JLS Machine Tool, Inc. 2005.
EDM machining is performed, therefore, by means of electric sparks which jump between two electrodes subjected to a given voltage which are submerged in an
insulating liquid dielectric fluid. Since the two electrodes are in a dielectric or
insulating medium, the voltage applied to them must be sufficient to create an electric field which is greater than the dielectric rigidity of the fluid.
a b
Figure 2.2: a Free positive ions and electrons are accelerated b A channel of plasma is thus formed
As a result of the action of this electrical field, free positive ions and electrons are accelerated, creating a discharge channel which becomes a conductor, and it is
precisely at this point where the spark jumps. This causes collisions between the ions + and the electrons -. A channel of plasma is thus formed.
a b
Figure 2.3 a Collision happened b heat generated
8 These collisions create high temperatures in both poles and a ball of gas is formed
around the plasma channel, which begins to grow. At the same time, the high temperatures in the two poles melt and vaporise part of the material of the part, while
the electrode itself suffers only very slight wear.
a b
Figure 2.4 a Implode explode inwards b EDM splinter
In this situation large ball of gas and molten material at both poles the electric current is turned off. The plasma channel collapses and the spark disappears. The
dielectric fluid then breaks the ball of gas making it implodes explode inwards. This creates forces which force the molten material by forming two craters on the
surfaces. The molten material solidifies and is carried away in the form of balls by the dielectric fluid to form what we might call the EDM splinter[14].
Some of the most important parameters implicated in the EDM manufacturing process are the following ones:
i. On-time pulse time or
t
i
: is the duration of time µs the current is allowed to flow per cycle. Material removal is directly proportional to the amount of
energy applied during this on-time. This energy is really controlled by the peak current and the length of the on-time.
ii. Off-time pause time or
t
: is the duration of time µs between the sparks that is to say, on-time. This time allows the molten material to solidify and
to be wash out of the arc gap. This parameter is to affect the speed and the stability of the cut. Thus, if the off-time is too short, it will cause sparks to be
unstable.
9 iii.
Arc gap or gap: is the distance between the electrode and the part during the process of EDM. It may be called as spark gap.
iv.
Duty cycle: is a percentage of the on-time relative to the total cycle time. This parameter is calculated by dividing the on-time by the total cycle time on-
time plus off-time.
The result is multiplied by 100 for the percentage of efficiency or the so called duty cycle.
v. Intensity
I
: points out the different levels of power that can be supplied by the generator of the EDM machine.
I
represents the mean value of the discharge current intensity [15].
2.1.3 EDM Application
