INTRODUCTION affect the state of consecutive-pulse discharge and

INTRODUCTION 1

Electrical discharge machining
(EDM) is a widely used non-traditional material removal process. This uses
thermal energy to machine electrically conductive parts regardless of hardness which
is one of its most distinctive advantage in manufacturing mould, die,
automotive, aerospace and surgical components. The electrode does not have any
direct contact with the work piece thereby eliminating mechanical stresses,
chatter and vibration problems during machining. In today’s world, an electrode
as small as 0.1 mm can be used to ‘drill’ holes into curved surfaces at steep angles
without drill ‘wander’.

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The basis of EDM starts back in
early 1770, when English chemist Joseph Priestly found the effect of erosion of
electrical discharges or sparks. However, a proper definition was found only in
1943 at the Moscow University where Lazarenko and Lazarenko discovered the destructive
properties of electrical discharges for constructive use. They formed a process
of machining difficult-to-machine metals by vaporising material from the metallic
surface. They used a resistance–capacitance type of power supply, which was used
at the EDM machine during the 1950s.

Many
claims have been made about EDM like a group of Americans came up with the fact
of using electrical charges to remove broken taps and drills from hydraulic
valves. This has since been used as the basis for the vacuum tube EDM machine and
an electronic-circuit servo system that automatically provided the proper
electrode to work piece spacing (spark gap) for sparking, without the electrode
contacting the work piece. In 1980, Computer Numerical Control Machines helped
Electricsl Discharge Machining vastly which helped it to improve its efficiency
to a great extent thereby improving machining operations. CNC has facilitated
total EDM, which implied an automatic and unattended machining from inserting
the electrodes in the tool changer to a finished polished cavity or cavities. These
aspects made it a widely used machining process in the current world.

 

 

 

LITERATURE
SURVEY:

Jin Wang and Fuzhu Hana 2
found that the debris concentration and bubble volume fraction in the bottom
gap between the electrode and work piece affect the state of consecutive-pulse
discharge and the ef?ciency of electrical discharge machining. Thus, debris and
bubble movement mechanisms during consecutive-pulse discharge should be made
understandable. Since it is difficult to interpret them using simulated models,
in their study, they proposed a three-dimensional model of ?ow ?eld with
liquid, gas, and solid phases for machining gap in EDM. The mechanisms of
debris and bubble movement in the machining gap during consecutive-pulse
discharge were analysed through this model. The movement of debris and bubbles
where observed by conducting a series of experiments. The results showed that
the proposed simulation model is feasible. The bubble expansion is the main way
that the bubbles exclude from machining gap. Much debris moves outside the
machining gap following the excluded bubbles, which is the main way that the debris
excludes from machining gap. The bubble expansion becomes strong with the
increase of the discharge current and pulse-on time.

Li Jianzhong, Yin
Guoqiang, Wang Cong, Guo Xuejie and Yu Zuyuan 3 suggested that micro
electrical discharge machining (EDM) can drill micro holes with high accuracy
in metallic materials. The aspect ratio of a micro hole generated by micro EDM
is higher than those by other processes. However, the drilling speed reduces
significantly when the aspect ratio of the hole being drilled reaches certain
value. A theoretical model was proposed based on fluid mechanics and surface
tension to have a better understanding on this phenomenon. Then experiments
were conducted with different machining conditions and the results were
compared with theoretical values obtained. The theoretical values were found to
be in agreement which caused the model to be valid. There was a slight
variation in value which was found to be caused due to the formation of debris
in the machining process, which is ignored.    

In this paper, put
forward by Wang K, Zhang Q, Liu Q, Zhang M, Zhang J, Liu Y 4, a series of
experiments were conducted to find which electrode produces better material
removal rate and also with low tool wear rate. Two electrodes were chosen for
the experiment namely, cylindrical and helical. It was found that no matter
what the tool geometry is, there exist a fluctuation in material removal rate
depending on the open voltage. But it was found that helical tool was much
better than cylindrical tool since it had a low tool wear ratio.

Ramy Nastasi and Philip Koshy 5
tells us that there is a formation of huge amount of debris during machining in
electrical discharge machining. They suggest that this happens because of the inadequate
gap during flushing which affects the quality of the machining process. In
order to avoid that they suggest that we could use conventional flushing
methods which helps us to remove the debris formed during machining. Also he
suggests that we could develop a special electrode which are provided with
features that can facilitate tool rotation through which debris can be removed.
The
corresponding flow fields were modelled numerically to optimize the designs.

In this study, proposed
by Liu HS, Yan BH, Huang FY, Qiu KH 6, they are trying to find a process in
which an efficient and accurate hole can be created on a high nickel alloy so
it can be used to form an optimized design. High nickel alloys have high
magnetic permeability and hence they have a huge prospect in manufacturing
certain important components. But they cannot be machined using traditional
processes due to their high rigid property which causes the tool to get worn
away quite easily. Hence EDM is considered in machining this alloy. Also this
high nickel alloy has the property of preventing interference of magnetic
field. Therefore, in order to manufacture this a tool was developed such that
the bottom has a small diameter and the rest of the tool is shaped as a helical
structure with a larger diameter. Then the first stage of tool with smaller
diameter is used to bore the holes and the second stage of helical tool is used
to give a smooth finish.

In this paper, written by
Wansheng Z, Zhenlong W, Shichun D, Guanxin C, Hongyu W 7, a series of
experiment is conducted by coupling both micro electrical discharge machining and
ultrasonic vibration tool. They found that it is difficult to machine high
rigid materials like tungsten using traditional processes. Hence they divised
such a process where a four axis EDM containing both ultrasonic vibration and
micro EDM was used. It was found that the quality of the holes produced by this
setup has improved to a great extent. The ultrasonic vibration used help the
electrode to produce superior quality holes since it helped it to remove the
debris formed and also helped in better penetration.

Ravinder Kumar and Inderdeep
Singh 8 found that the two factors were causing the micro EDM to have a major
effect on the product that is being machined. They were occurance of arcing and
short circuiting. This is paper mainly relies on improving the process by
eliminating arcing and short circuiting. The authors found that the machining
process can be improved only by using an electrode of suitable tool geometry.
The tool geometry is made in such a way that the debris formed during machining
process can be removed during the time of machining iself. This causes the
machine to remove the debris without using a traditional flushing system. Also
the proposed electrode can produce holes of high aspect ratio. Also the authors
evaluate the electrode by analysing the tool wear rate, material removal rate
and also the aspect ratio of the holes produced.  

 

DISCUSSIONS
AND CONCLUSIONS:

            From
the above literature survey, we find that different parameters affect the
process of EDM in many different ways. Therefore, the authors have put forward
their perspectives of improving the production of micro electrical discharge
machining in an efficient way analysing the different parameters involved and
conducting experiments to find the optimal solutions.

            We
can find that most of the processes are affected due to the formation of debris
during material removal which affects the finish and accuracy of the product
being manufactured. Also it is found that they have come up with a plan of introducing
flushing to remove the debris as and when it is produced. The electrode being
used is changed and experimented to find whether there is improvement in the product
that is being produced. From the experiments it is found that there is a
significant improvement in the product being produced and also the accuracy
with which the product being machined is also increased. R and aspect ratio was
also evaluated. 8 The idea of using an inclined micro slot for eliminating
the accumulation of debris during micro EDD process was found to be successful.
The slotted electrodes generate tool rotation induced debris removal, thereby
eliminating the occurrence of arcing and short circuiting. The proposed
electrode completely eliminates the need of ?ushing and acts as a self-?ushing electrode.
Compared to solid electrode, slotted electrode gives improved output
characteristics i.e. MRR, TWR, taper angle, corner radius and aspect ratio at a
tool speed of 1000 rpm. Type III electrode further improves MRR and TWR 8.

Also it is found that
time is not considered in any of the papers referenced. Since every process
must be time efficient which plays as an important role in manufacturing as the
main purpose of using this is to produce the product in an optimized manner. Therefore,
in the future people can study more about the time factors involved in machining
a part using micro EDM. This might help the future manufacturers to have a
better perception towards the areas where they can use electrical discharge
machining more effectively and efficiently.