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CHAPTER 1

INTRODUCTION

1.1 NEEDS FOR MICROMACHINING

There is a growing demand for industrial products with increased

number of functions and of reduced dimensions. Micro-machining is the most

basic technology for the production of such miniature parts and components.

Micro machining is defined as the ability to produce features with the dimensions

from 1 m to 999 m or when the volume of the material removed is at the micro

level. Lithography based micro-machining technology uses silicon as material to

produce integrated circuitry components and microstructures. However, these

methods, in general, lack the ability of machining three-dimensional shapes

because of poor machining control in the Z axis. Fabrication using hard and

difficult-to-machine materials such as tool steels, composites, super alloys,

ceramics, carbides, heat resistant steels and complex geometries for demanding

aerospace, mechanical or biomedical applications requires alternative novel

methods.

1.2 APPLICATIONS OF MICROMACHINING

In recent years, manufacturing industry has witnessed a rapid increase

in demand for micro-products and micro-components in many industrial sectors

including the electronics, optics, medical, biotechnology and automotive sectors.

Examples of applications include medical implants, drug delivery systems,

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diagnostic devices, connectors, switches, micro-reactors, micro-engines, micro-

pumps and printing heads. These micro-system-based products represent key

value-adding elements for many companies and, thus, an important contributor to

a sustainable economy (Brousseau et al. 2010). As a result of the current trend

towards product miniaturization, there is a demand for advances in micro- and

nano- manufacturing technologies and their integration in new manufacturing

platforms. These platforms must enable both function integration (i.e.

combination of different functions) and length-scale integration (i.e. mixing of

the macro-, micro- and nano-dimensions) in existing and new products and, at the

same time, their cost effective manufacture in a wide range of materials.

1.2.1 Production of Micro-Compressor

The example presented here is a micro-compressor as shown in Figure

1.1. It is a two-and-a-half dimensional structure, machined on the tip of a 1-mm

diameter cylinder. The Materials used for electrode and work-piece are tungsten

carbide and stainless steel, respectively. The centre hole with diameter 0.3 mm is

micro-EDM drilled directly with a purchased electrode. The blades are micro-

EDM milled with the same tool after it is reduced to a diameter of 40 m by

WEDG. The incremental depth for each micro-EDM milling layer is 0.5 m. All

the blades show good consistency, and the examined shape accuracy is less than

1 m. Furthermore, no taper angle and obvious path mark on the side wall of the

blades is observed.

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Figure 1.1 A micro-compressor on a Ø1 mm cylinder: top and detail views(Liu et al.(2010))

1.2.2 Production of Micro-Turbine Impeller

Another application example is micro-EDM milling of Si3N4-TiN

ceramic composite into a miniature three dimensional gas turbine impeller which

serves as a key component for a micro power generation system. A close view of

the manufacturing process is illustrated in Figure 1.2. The milling of each cavity

starts with a Ø1mm WC tool for pocketing using the roughing regime to remove

the bulk of the material, then followed by a Ø 0.7 mm tool for wall finishing

using the semi-finishing regime. Due to the extremely low machining speed of

the finishing regime, only two steps are performed. The same layer-by-layer

machining strategy is applied and the layer thickness for pocketing and wall

finishing are 8 and 3 m, respectively. The shape and geometrical accuracy of the

cavity is measured on a Coordinate Measuring Machine. All the machined

products show good consistency compared to the original CAD model. However,

even with rather high MRR, the total machining time for one cavity takes around

20 hours.

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Figure 1.2 Close views of the Micro-EDM milling process and a finishedturbine impeller (Liu et al.(2010))

1.2.3 Nozzle for Diesel Fuel Injectors

Machining capability of micro-EDM, in conductive materials with

high precision regardless of material hardness, creates a wide range of

application area with the increasing demand for miniaturized parts and

components such as holes, nozzles, and gears. Fuel injector nozzles machined by

EDM is shown in Figure 1.3.

Figure 1.3 Fuel Injection Nozzle drilling

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1.2.4 Inkjet Printer Manufacturing

Electroformed nozzles are presently used in number of commercially

produced ink-jet printers. The nozzle plate of ink-jet printer head is

electroformed. The electroformed nozzles are formed by plating nickel on a

mandrel (mould), which defines the pattern of the nozzle, and then the finished

product is removed (Bhattacharyya et al 2004). The pulsating current/voltage

holds better control over EMM of thin films and foils in the applications of micro

fabrication. Through-mask EMM is used to fabricate a series of flat-bottomed

conical nozzles in a metal foil as shown in Figure 1.4. The process is applicable

to various materials including high strength corrosion resistant materials such as

conducting ceramics.

Figure 1.4 Inkjet Printer Manufacturing

1.2.5 Cooling Holes in Turbine Blades

The main objective of turbine blade cooling is usually to achieve

maximum heat transfer coefficients while minimizing the coolant flow rate.

Usually the turbine blades are made of a super alloy with a very high melting

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temperature. By using the lost wax method, these cavities are cast. With this

technique, it is possible to produce cavities with a serpentine shape. The other

method employs drilling to provide the blades with cooling holes. These holes

are placed in the span wise direction of the turbine blades. In both the cases, the

walls of the passage have ribbed surfaces. Although the technique of casting has

been improved tremendously over the years, it is still very difficult to develop

cavities over large parts. Therefore blades with longitudinal holes are produced

using the drilling technique. Since the holes have such a complex shape and their

diameter is of only a few millimeters, conventional drilling techniques are not

suitable. Moreover the surface of the hole has to be ribbed and the material is

very hard. Therefore an electrochemical drilling technique is preferred. Figure

1.5 show the high accuracy microholes in turbine blades for generating cooling

effect.

Figure 1.5 Cooling holes in turbine blades

1.2.6 Production of Complex Shapes

The productions of microgrooves for self-acting fluid film bearings

and complex shapes (Figure 1.6) are some of the applications of EMM process.

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Figure 1.6 Shaving razor head (Kozak 2004)

1.3 MICROMACHINING TECHNIQUES

There are various techniques available to fabricate micro components

and structures. However some of the important techniques are discussed in this

section.

1.3.1. Laser Technology

Laser technology has been qualified as a capable micro manufacturing

technology because of its high lateral resolution with the minimized focus-ability

down to a few microns, or even smaller, low heat input and high flexibility (Qin

et al. 2010). One major advantage is its capability of processing various materials

which are increasingly needed for manufacturing micro-products. Some

examples of laser applications are micro cutting, micro drilling, micro welding,

soldering, selective bonding of silicon and glass, micro structuring and forming.

Femto-second laser micromachining is a new technique emerging in MEMS area

in recent years, and some promising results have been shown in micromachining

and micro system applications, including industrial material processing,

biomedicine, photonics and semiconductors.

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1.3.2 Micro-Ultrasonic Machining

Micro Ultrasonic Machining (micro-USM) is capable of making

almost any three dimensional microstructure with high aspect ratio on most of

materials, mainly on brittle materials. The USM uses a tool Ultrasonic Vibration

with combination of favorable abrasive slurry to create accurate cavities of any

shape through the impact grinding of fine grains. The machining process is non-

thermal, non-chemical, non-electrical and thus produces high quality surface

finish. However, the micro-USM is not capable of drilling of micro holes smaller

than 100 m for lack of corresponding co-axial micro tools.

1.3.3 Mechanical Micromachining

In these processes, the tools which are in direct contact with

workpiece are able to machine 2D and 3D microstructures in a variety of

materials with higher MRR than the others. Micromachining processes as

grinding, drilling, turning and milling are mechanical processes, which are not

just downscaled versions of the existing conventional processes.

1.3.4 Micro-Electrochemical Machining

Electrochemical machining (ECM) is based on the electrochemical

dissolution of a metal. In conventional ECM, the machining shape is specified by

the shape of the electrode, similar to the case of EDM or USM. However, even

when an electrolyte with very small throwing power is used, dissolution occurs in

an area wider than that facing the electrode. This characteristic is not suitable for

micromachining. However, ECM has an invaluable advantage, that is, the

machined surface is very smooth and there are no layers affected by machining.

This makes micro-ECM suitable for smoothing micro-metallic products. In order

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to adjust the unit removal suitable for micro removal, a short pulse and a low

current are required. The low current can also be realized by using a high-

resistance electrolyte. Some trials are conducted using an electrolyte jet as the

micro tool instead of a metal tool. A high speed jet can localize electrochemical

dissolution and this enables the machining of micro indentations with controlled

dimensions by switching the current synchronously to the movement of the

workpiece. When the Scanning Tunneling Microscope (STM) technique is used

in combination with ECM, microgrooves with submicron width can be realized.

This is due to the fact that the removal in ECM is basically an atom-by-atom

phenomenon.

1.3.5 Need for Electrical Discharge Machining

Among the various competent processes EDM is considered for its

following advantages.

Used to machine any electrically conductive material regardless

of its hardness and strength.

Complex 3D micro shapes can be machined.

Short machining time.

Cost effective.

High precision can be achieved.

Versatility to machine any kind of material.

The quality of good surface finish makes this process more

attractive for drilling holes on the components exposed to high

temperature.

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1.4 ELECTRICAL DISCHARGE MACHINING

In 1970, the English scientist, Priestley, first detected the erosive

effect of electrical discharges on metals. More recently, during research (to

eliminate erosive effects on electrical contacts) the soviet scientists, Lazarenko

and Lazarenko, decided to exploit the destructive effect of an electrical discharge

and develop a controlled method of metal machining. In 1943, they announced

the construction of the first spark erosion machining. The spark generator used in

1943, known as the Lazarenko circuit, has been employed over many years in

power supplies for EDM machines and an improved form is being used in many

current applications (Pandey et al. 2003). The EDM process can be compared

with the conventional cutting process, except that 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.

The EDM has found ready application in the machining of hard metals

or alloys (necessarily electrically conductive) which cannot be machined easily

by conventional methods. It has proved valuable and effective in machining of

super tough, hard, high strength and temperature resistance of conductive

material. These metals would have been difficult to machine by conventional

methods. It thus plays a major role in the machining of dies and tools made of

tungsten carbides, stellites or hard steels. Alloys used in the aeronautics industry,

for example, hastalloy, nimonic could also be machined conveniently by this

process.The EDM is also used to machining of exotic materials, refractory metals

and hard enable steels. This process has an added advantage of being capable of

machining complicated components and making intricate shapes. Most of the

surgical components are being machined by this process since the EDM is one of

the unconventional processes which can produce better surface quality.

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1.4.1 Principle of Electric Discharge Machining

Figure 1.7 shows the concept of EDM. Pulsed arc discharges occur in

the “gap” filled with an insulating medium, preferably a dielectric liquid like

hydrocarbon oil or de-ionized (de-mineralized) water between tool electrode and

workpiece. The insulating effect of the dielectric medium has some importance in

avoiding electrolysis effects on the electrodes during an EDM process. As the

electrode shape is copied with an offset equal to the gap-size, the liquid should be

selected to minimize the gap (10-100 m) to obtain precise machining. On the

other hand, a certain gap width is needed to avoid short circuiting, especially

when electrodes that are sensitive to vibration (like wire-electrodes) or

deformation are used. The ignition of the discharge is initiated by a high voltage,

overcoming the dielectric breakdown strength of the small gap.

A channel of plasma (ionized, electrically conductive gas with high

temperature) is formed between the electrodes and develops further with

discharge duration. As the metal removal per discharge is very small, discharges

should occur at high frequencies (103 -106 Hz). For every pulse, discharge

occurs at a single location where the electrode materials are evaporated and/or

ejected in the molten phase. As a result, a small crater is generated both on the

tool electrode and workpiece surfaces. Removed materials are cooled and

resolidified in the dielectric liquid forming several hundreds of spherical debris

particles, which are then flushed away from the gap by the dielectric flow. After

the end of the discharge duration, the temperature of the plasma and the electrode

surfaces contacting the plasma rapidly drops, resulting in a recombination of ions

and electrons and a recovery of the dielectric breakdown strength.

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To obtain stable conditions in EDM, it is essential for the next pulse

discharge to occur at a spot distanced sufficiently far from the previous discharge

location. Such a spot may be the place where the gap is small or contaminated

with debris particles which may weaken the dielectric breakdown strength of the

liquid. Accordingly, the interval time between pulse discharges must be

sufficiently long so that the plasma generated by the previous discharge can be

deionized and the dielectric breakdown strength around the previous discharge

location can be recovered by the time the next pulse voltage is applied. Otherwise

discharges occur at the same location for every pulse, resulting in thermal

overheating and a non uniform erosion of the workpiece.

Figure 1.7 Concept of EDM (Kunieda et al. (2004))

The schematic of an EDM machine tool is shown in Figure 1.8. The

tool and the work piece form the two conductive electrodes in the electric circuit.

Pulsed power is supplied to the electrodes from a separate power supply unit. The

appropriate feed motion of the tool towards the work piece is generally provided

for maintaining a constant gap distance between the tool and the work piece

during machining. This is performed by either a servo motor control or stepper

motor control of the tool holder. As material gets removed from the work piece,

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the tool is moved downward towards the work piece to maintain a constant Inter

Electrode Gap (IEG). The tool and the work piece are plunged in a dielectric tank

and flushing arrangements are made for the proper flow of dielectric in the IEG.

Typically in oil die-sinking EDM, pulsed DC power supply is used where the

tool is connected to the negative terminal and the work piece is connected to the

positive terminal. The pulse frequency may vary from a few kHz to several MHz.

The IEG is in the range of a few tens of micro meter to a few hundred micro

meter. Material removal rates of up to 300 cubic mm/min can be achieved during

EDM. The surface finish (Ra value) can be as high as 50 µm during rough

machining and even less than 1 µm during finish machining.

Figure 1.8 Schematic of an Electric Discharge Machining (EDM) machinetool

1.4.2. EDM Gap Phenomena

EDM gap phenomena are usually depicted as shown in Figure 1.9.

The arc column is depicted with a diameter smaller than the gap width. Debris

particles are very fine like dust and minute bubbles are suspended in the gap.

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However, this sketch is not entirely correct. As described later, the gap is mostly

occupied with gas and the diameter of debris particles is in the same order as the

gap width and surface roughness of the electrode surfaces as schematically

shown in Figure 1.10. Dielectric breakdown in consecutive pulse discharges

therefore occurs at the boundary between bubbles, where debris particles are

condensed, or inside the bubble.

Figure 1.9 Model of EDM gap phenomena (Kunieda et al. (2005))

Figure 1.11 shows a schematic view of the discharge spot. The arc

column diameter is considered to increase with the passage of time and equal to

the diameter of the generated discharge crater. If this is true, the diameter of the

arc column measures several times larger than the gap width. The electrode

materials and dielectric liquid are evaporated, molecules are dissociated, and

atoms are ionized, resulting in a rapid expansion of the bubble. Since the

expansion is restricted by the inertia and viscosity of the dielectric liquid, the

pressure inside the bubble becomes extremely high and the boundary between the

bubble and liquid expands with the velocity of several tens m/s. It is still believed

that the dielectric liquid plays a significant role in material removal because the

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high pressure and velocity field in the bubble may serve as the dynamics of the

material removal in EDM.

Figure 1.10 Schematic view of discharge gap (Kunieda et al. (2005))

After the end of the discharge duration, ions and electrons are

recombined and the dielectric breakdown strength is recovered. The evaporated

atoms and molecules are solidified or condensed to form debris particles or

dielectric liquid, but gases such as hydrogen and methane which are generated by

the dissociation of the working oil are left to form a bubble. The diameter of the

bubble reaches several millimeters, several tens of times larger than the gap

width as shown in Figure 1.12.

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Figure 1.11 Discharge phenomena in EDM gap (Kunieda et al. (2005))

Since pulse discharge occurs several thousand times or more per

second, obviously the gap becomes filled with gas in typical EDM processes.

Hundreds of debris particles are generated per single pulse discharge, blown off

through the bubble generated by the discharge and stopped at the boundary of the

bubble due to the viscosity of the dielectric liquid [Yoshida and Kunieda (1998)].

The melted and evaporated materials are cooled by the dielectric liquid and

solidified to form spherical debris particles. Thus debris particles are removed

from the gap with the dielectric liquid and not reattached on the electrode

surfaces. Furthermore, heat convection in the boundary layers of the dielectric

liquid cools the electrode surfaces, resulting in machining stability. Thus, the

dielectric liquid plays another important role in flushing debris particles and

cooling of the gap.

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Figure 1.12 Bubble and debris particles generated by single pulse discharge(Kunieda et al. (2005))

1.5 STAINLESS STEEL

Stainless steel is widely used in cookware, cutlery, and kitchen

utensils. These are along with hardware supplies, industrial equipments,

structural buildings, automotive, and aerospace industries. Because of strong

resistance to corrosion, it is largely employed for production of storage tanks and

other food containers of fruit drinks. Likewise, medical equipment production

also makes use of SS because it can be steamed and sterilized over and over. It is

also a prevalent material applied in the manufacturing of jewelries and watches.

The Rolex is one best example of watch brand that uses SS for its numerous

watch models. SS comes out with different grades. The 304 and 316 are the most

common grades of SS chosen in the present work. It is non-magnetic, corrosion

resistant material having nickel no more than 35% and chromium content on

average ranges from 16 to 26%.

1.6 OBJECTIVES OF THE RESEARCH

The need for products containing micro-holes has shown a steady

growth in recent years. In all micromachining technologies, the gap between the

workpiece and tool electrode is usually of the order of few micro meters and

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removal of debris particles from such a small gap has been a challenge. This

research aims to alleviate the problem of flushing of debris by machining micro

holes using conventional EDM. In conventional EDM machine rotary brass

hollow tubular electrode is fed downward into the workpiece under servo control

in which the removal of debris is easier. The quality of the hole produced by this

rotary EDM is superior owing to the improvement in flushing, and reduction in

tool wear.

In this research, two commercially available machines have been

considered for conducting the experiments in order to reveal the machining

characteristics and predict the machined shape of holes. The optimization of

process parameters is essential to achieve of high responsiveness in production,

and is crucial to dynamically changing market conditions. Taguchi Technique

and Grey relational theory have been considered to get optimization of process

parameters. Since they provide a significant reduction in the number of

experiments thereby speeding up the experimental process. Experiments are

conducted using EDM to drill micro-holes on SS 304 and SS 316 in the view of

maximizing the Material Removal Rate (MRR), and minimizing Tool Wear Rate

(TWR), Overcut (OC) and Taper (T) individually and simultaneously.

In order to achieve this aim, the following objectives have been

framed.

To compare the performance of a micro hole produced in EDM

with the micro-EDM.

To optimize the process parameters using Taguchi technique for

single objective optimization.

To optimize the process parameters using Grey Relational

Analysis for Multiple objective optimization.

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1.7 ORGANIZATION OF THE THESIS

Chapter1 presents the Introduction and objectives of the research.

Chapter 2 presents a literature review of EDM process describing the

machining performance, micro-hole machining by varying EDM specifications

and process optimization.

Chapter 3 presents the experimental investigation of micro-electrode

preparation and micro-hole drilling using micro-EDM machine followed by EDX

analysis of machined micro-holes.

Chapter 4 includes the details of experimental setups,specifications

and planning of experiments.

Chapter 5 explains the experiments conducted on EDM machine and

optimization by using Taguchi method for SS 304.

Chapter 6 describes the optimization of machining parameters of SS

304 by using Grey Relational Analysis followed by the SEM analysis of micro-

holes.

Chapter 7 describes the optimization of machining parameters of SS

316 by using Grey Relational Analysis followed by the confirmation test and

Analysis based on SEM Micrographs.

Chapter 8 presents a summary of the major contributions from this

research and recommendations for future research.