Rapid advances in technology require further development in the manufacturing
of micro-parts and micro-electromechanical systems components. Increasing demand
for micro-parts made micro-machining process more focused and investigated among
the front-end of the technology in recent years. Micro-machining is the basic
manufacturing technology of the miniaturized and smaller parts having size of
millimeter down to submillimeter. EDM is considered to be one of the machining
processes that may have high potential to manufacture small size components.
It is a thermal process that utilizes spark discharges to erode a conductive
material; the tool electrode is almost unloaded, because there is no physical
contact between the tool electrode and the work piece. Therefore, the process
works efficiently, particularly in the machining of difficult-to-cut materials.
When the same process principles are applied to the micron size for micromachining,
the process is called a micro-EDM.
The basic physical characteristics of the micro-EDM process is essentially
similar to that of the conventional EDM process with the main difference being
in the size of the tool used, the power supply of discharge energy and the resolution
of the X-, Y- and Z-axes movement. EDM is widely used in tool room for machining
of dies with fine details and for the production of unusually shaped and/or
sized production work. Fuel injector valves, parts and components for medical
devices, fiber optic connectors, micromachining, micro-mould making, stamping
tools and micro electronics parts are the examples of miniaturized and smaller
size parts produced by the micro-EDM technology.
There are many manufacturing techniques to drill micro holes and micro parts
beside micro EDM. The recently developed methods are Wire Electric Discharge
Grinding (WEDG), Micro- Electrochemical Machining (MECM), Laser-Beam Machining
(LBM), Focused Ion Machining (FIM), Micro milling, Micro Ultrasonic Machining
(MUSM), Electrochemical Discharge Machining (ECDM) and Micro punching. Performances
of these methods are unique, because they have different machining mechanisms.
Growing popularity of micro EDM depends on its advantages including low set-up
cost, high aspect ratio (depth/diameter ratio) of the holes, enhanced precision
and large design freedom. In addition, EDM does not make direct contact between
the tool electrode and work piece material, hence eliminating mechanical stress,
chatter and vibration problems during machining. Therefore, replying on the
above advantages, micro- EDM is very effective to machine any kind of holes
such as small diameter holes down to μm and blind holes with 20 aspect
In spite of many studies on fast EDM hole drilling, the research on making
of small-size holes on aerospace alloys is limited.
In EDM, the machining of conductive materials is performed by sequence of electrical
discharges occurring in an electrically insulated gap between the tool electrode
and work piece. During the discharge pulse, a high temperature plasma channel
is formed in the gap causing evaporation and melting of the work piece. Debris
of material is removed by the resulting explosion pressure, enabling the machining
of the work piece. The characteristics of the electrical discharge pulses are
linked with a set of machining parameters which control the energy and frequency
of discharges and thus the power in the gap. Consequently, the chosen set of
parameters affects the material removal rate MRR and overcut. However, in micro
EDM a number of issues remain to be solved. For instance, the processing time
is significantly higher. This study investigates the influence of various combination
of machining parameters in micro EDM in an attempt to optimize the influence
Characteristics of micro-EDM: The EDM process is based on the thermoelectric
energy created between a work piece and an electrode submerged in a dielectric
fluid. When the work piece and the electrode are separated by a specific small
gap, the so-called spark gap,
a pulsed discharge occurs which removes material from the work piece through
melting and evaporation.
In recent years, numerous developments in EDM have focused on the production
of micro-features. This has become possible due to the availability of the new
CNC systems and advanced spark generators that have helped to improve machined
surface quality. Also, the very small process forces and good repeatability
of the process results have made micro-EDM the best means for achieving high
aspect ratio micro-features.
Current micro-EDM technology used for manufacturing micro-features can be categorized
into four different types:
||Micro-wire EDM, where a wire of diameter down to 0.02 mm is
used to cut through a conductive workpiece
||Die-sinking micro-EDM, where an electrode with micro-features is employed
to produced its mirror image in the work piece
||Micro-EDM drilling, where micro-electrodes are used to drill micro-holes
in the work piece
||Micro-EDM milling, where micro-electrodes are employed to produce 3D cavities
by adopting a movement strategy similar to that in conventional milling
Similar to macro-EDM, micro-EDM is also classified into several manufacturing
configurations by simply scaling down the machining geometries. In general,
micro-wire, die-sinking, milling and drilling EDM are the widely recognized
methods. They can produce a feature size down to a few microns with possible
aspect ratio of 100. In particular, Micro-Wire Electro-discharge Grinding (WEDG),
firstly introduced by in 1985, is also regarded as an important micro- EDM configuration.
This process is broadly used for forming very thin rods with high-aspect ratio
which can be used as tool electrodes for micro-EDM drilling or milling. Moreover,
it also allows shaping the tools into complex geometries which can be applied
directly for fabricating 3D structures and die-sinking. An overview of the capabilities
of these machining variants is listed in Table 1.
|| Micro EDM capabilities
|| Micro-EDM problematic areas
Micro-EDM issues: Figure 1 gives an overall view of
the problems discussed. Special attention is paid to the different sources of
errors directly affecting the accuracy of the EDM process and suggestions for
machining strategies to reduce those errors are made.
Previous literature: Several studies on the manufacturing of micro holes
are published in the literature.
Pham et al. (2005) in their study, they mainly
focused on the various actors that affecting on the final accuracy in Micro
EDM drilling. The main parameters size and position of the hole is discussed.
Techniques for minimized the errors is proposed by Pham
et al. (2004). This study is focused on the EDM process and Electrode
wear problem and also the recent developments in the micro machining field (Cusanelli
et al., 2007). In this study, the holes are achieved from 0.05-1.8
mm in diameter for gasoline nozzles. The most important feature in this process
positive tapered holes is obtained by mechanical setup (Liu
et al., 2009). In this study, the process capability of the micro
EDM is discussed with the discharge pulse ratings that affecting the surface
integrity and the machining geometry (Lajis et al.,
2009). The cutting of the Tungsten Carbide ceramic using Electro-discharge
Machining (EDM) with a graphite electrode by using Taguchi methodology has been
reported (Bigot et al., 2005). The electrode
wear ratio is taken into account by volumetric wear ratio. The suitable electrode
wear compensation method is proposed. This study focuses the parameters optimization
for rough and fine machining in micro EDM (Thillaivanan
et al., 2010). The machining parameters optimized based on Taguchi
and the Artificial neural network.
Test piece and electrode materials: The test piece materials used in this
study were common aerospace super alloys Inconel 718 (IN718) (Cullen
and Freeman, 1965). Due to their specific thermal and physical properties
these materials are preferred for aerospace applications. The chemical composition
of IN718 are given in Table 2 and 3 show
melting points and thermal conductive of base and electrode materials used in
Experimental setup: The experiments were performed on test pieces of
IN718 with predetermined dimensions using Sparkonics micro drilling machine
(Fig. 2). The surface of the test pieces were ground prior
The flat surfaces of two specimens were aligned in order to ensure that mating
surfaces could be secured accurately using a specially designed and manufactured
fixture. The holes were drilled in the test pieces one by one with varying the
process parameters. Table 4 represents the Machining Conditions.
Measurement procedure: The drilling time for the each hole was recorded
using an electronic timer. The test pieces were weighed before and after drilling
using a digital precision scale.
|| Chemical composition of IN718
|| Properties of base and electrodes
|| Machining conditions
||Sparkonics micro-EDM machine, courtesy-covai EDM, Coimbatore
Micro-EDM issues: Miniaturization of the product requires a new approach
to process design. Because so far micro-EDM has tended to be performed using
conventional EDM machines modified to accommodate the micro-manufacturing requirements,
a number of Material Removal Rate (MRR) for each experiment was calculated by
the following formula:
The Electrode wear ratio can be calculated by the following relations in percentage:
The overcut shown in Fig. 4 were measured by using the Scanning
Electron Microscope (SEM)
The purpose of the project is to evaluate the performance of the micro-EDM
on Inconel 718. The Sparkonics micro-EDM machine is shown in Fig.
3. To achieve this objective proper experimental plan is necessary to achieve
good results. This experiments consists of four main elements namely, research
design and data analysis, variables, research procedure and instrumentation.
Figure 5 shows the methodology for this experiment. Taguchi
method using Minitab software was applied as a tool for data analysis. The confirmation
test was also implemented in order to give the reliability of the micro-EDM
results for Inconel 718.
Research design and data analysis: The experimental layout for the machining
parameters using the L9 Orthogonal array was used in this study.
This array consists of three control parameters and three levels as shown in
In the Taguchi method, most all of the observed values are calculated based
on the the higher the better and the smaller the better.
Thus in the study, the observed values of MRR, Overcut were set to maximum and
minimum, respectively. Each experimental trial was performed with three simple
replications at each set values.
Research design variable: The design variables are described into two
main groups which are response parameters and machining parameters. Response
parameters (machining characteristic @ dependent variable) include:
||Material removal rate, MRR
||Electrode Wear Ratio, EWR
|| Design scheme of experiment of parameters and levels
||Sparkonics Micro-EDM machine, (Brass Electrode dia 0.4 mm),
Courtesy- Covai EDM, Coimbatore
|| Overcut measurements
|| Research methodology
|| Research procedure
|| Set of experiments: L9 Orthogonal array
Machining Parameters or also known as independent variables involves in this
||Discharge current: Which gives the highest electric
current that can occur during the discharge (if no capacitor is used)
||Pulse-on time: Which is the duration of the impulse generated by
the impulse generator
||Pulse-off time: Which is the time between two impulses
Machining parameters: As indicated machining parameters were deployed
to obtain the optimized value.
||Pulse on time
||Pulse off time
Output functions: In the proposed experimental procedure (Fig.
6), the main functions are Over cut, Material removal rate, Electrode Wear
ratio was calculated. Digital images of the profiles from their profiles taken
from SEM. The reduction of the electrode length due to the wear was measured
on the machine. This was achieved by assessing the difference electrode tip
and workpiece top surface. In this study, in order to facilitate computation
the corner wear was considered as negligible. Set of experiments of L9
Orthogonal array is presented in Table 6.
RESULTS AND DISCUSSION
In this study, In this project, the investigation of machining characteristics
such as MRR. TWR, OC during micro hole in micro EDM on Inconel 718 using brass
electrode with de-ionized water as dielectric medium. It can be concluded from
this investigation that there is a great influence of using brass electrode
for drilling performance characteristics in micro EDM during micro hole generation
on Inconel 718.
From Fig. 7, discharge current 20 level 3, Pulse on time
9.5 level 2, Pulse off time 5 level 2.Gives the optimum conditions for reducing
|| SN ratio graph for MRR
|| SN ratio graph for TWR
|| SN ratio graph for OC
From Fig. 8, discharge current 20 level 3, Pulse on time
9.5 level 2, Pulse off time 6 level 3.Gives the optimum conditions for reducing
the TWR. From Fig. 9, discharge current 10 level 1, Pulse
on time 10 level 3, Pulse off time 4 level 1. Gives the optimum conditions for
reducing the Overcut.