Study of Sharp Corner Cutting in Wire EDM - Muhammad Iswan Ismail - TJ1191.M58 2008

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

Study of Sharp Corner Cutting In Wire EDM

Thesis submitted in accordance with the requirements of the Universiti Teknikal Malaysia Melaka for the Bachelor Degree of Manufacturing Engineering in

Manufacturing Process

By

Muhammad Iswan bin Ismail

Faculty of Manufacturing EngineeringApril 2008

ABSTRACT

EDM or Electrical Discharge Machining is a process by which metal is removed by

electrical energy. A spark is discharged from an electrode vaporizing the metal. In really

EDM is a very precise method of machining and is used when a metal is too hard or

tough to machine conventionally. The experimental study presented in this paper aims to

select the most suitable cutting and offset parameter combination for the wire electrical

discharge machining process in order to get the desired surface roughness value,

dimensional and corner cutting accuracy for the machined workpieces. A series of

experiments have been performed on two types of steel material with same thicknesses.

The test specimens have been cut by using different cutting and offset parameter

combinations of the Mitsubishi RA90 wire electrical discharge machine in the

University Technical Malaysia Melaka CNC center lab. The surface roughness of the test

pieces has been measured by using a surface roughness measuring device. The dimension

accuracy will be measured by optical comparator and digital caliper. For accuracy of

sharp corner cutting image analyzer will be used. The related tables and charts have been

prepared for material type, wire diameter and wire type. The tables and charts can be

practically used for EDM parameter selection for the desired workpiece.

i (a)

ABSTRACT

EDM atau Mesin Discas Electrik adalah satu proses di mana pemotongan logam oleh

tenaga elektrik. Satu percikan api dihasilkan daripada satu elektrod menjadi wap logam.

Dengan maksud lain EDM adalah satu kaedah pemesinan yang sangat tepat dan sesuai

digunakan bagi logam-logam yang sangat keras atau sukar untuk pemesinanan secara

konvensional. Kajian yang disampaikan di dalam laporan ini bermatlamat memilih

pemotongan yang paling sesuai dan gabungan parameter bagi mengimbangi proses

nyahcas elektrik untuk mendapatkan permukaan pemotonagan yang licin, dimensi dan

ketepatan darjah untuk sudut yang tajam bagi sesuatu hasil kerja. Satu siri ujian telah

dilakukan ke atas dua jenis bahan keluli dengan ketebalan yang sama. Spesimen-

spesimen ujian telah dipotong dengan menggunakan gabungan-gabungan parameter yang

berbeza bagi memotong dan mengimbangi nyahcas elektrik dari mesin Mitsubishi

RA90 di dalam makmal CNC, Universiti Teknikal Malaysia Melaka. Profile permukaan

kepingan-kepingan telah diukur dengan menggunakan satu penyukat profile permukaan

bahan. Ketepatan dimensi pula akan diukur oleh pembanding optik dan angkup venier.

Untuk ketepatan pemotongan sudut tajam, mikroskop penganalisis akan digunakan.

Jadual-jadual berhubungan dan carta-carta yang akan disediakan bagi jenis bahan, jejari

elektrod dan jenis elektrod. Jadual-jadual dan carta-carta yang dihasilkan boleh dijadikan

panduan bagi pemilihan parameter untuk pemesinan EDM.

i (b)

CHAPTER 1INTRODUCTION

1.1 Background

Machining is an importance process in manufacturing engineering. It is dividing by two

methods to say conventional machining and unconventional machining. For example of

the conventional machining are milling, drilling and lathe. According this method, the

quality of production are depend of efficiency operator, type of process is choosing and

type of material is used. Also get in certain case where the conventional machining is not

suitable to applicable. Such as an example where the shape we want to produced are

complicated. It is because to producing the complicated partial are need high cost and a

long of producing time. Furthermore in a modern technology, get a various new material

from other sources. It is shown indeed we need a new method for improvement and

upgrading the quality of metal machining. So that, to solve this problem we need

unconventional machining for exceed the weakness by unconventional machining is

better if the parameter choosing is better. It is because this method is limited for certain

parameter only. Electrical discharge machining EDM is one of the most accurate

manufacturing methods of working exceptionally hard metals and other materials that are

difficult to machine cleanly with more conventional methods. EDM is a process of

elimination that erodes or removes metal and material in the path of electrical discharges

that form an arc between an electrode tool and the workpiece until the desired part is

attained. Using this process is extremely accurate, reliable and affordable, so it is

becoming an increasingly popular choice for many companies. But refers to the demand

of the industries that need very accuracy products so the EDM process must be have an

improvement process and research time to time so it can fulfill the demand of the

industries.

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1.2 Problem Statement

In machining process, it is very important to get optimum cutting result where it can

reduce cutting time, reduce the wear of electrode, save the operation cost, save the

material cost, perfect sharp corner cutting with high accuracy dimensional and finishing

with a good surface texture. But then, the problem will be occurs is finishing surface

texture are not good and unsatisfactory. Also for sharp corner cutting the have many

errors will occurs. In industry actually sharp corner must be avoided because of it

difficult to produce with high accuracy but the problem will occurs when the product

need the sharp corner machining. The problem also included how to find the best

machining parameter for sharp corner cutting that will be reach to the best result of sharp

corner cutting.

1.3 Objective of Study

The purposes of this study are:

(a) To study the cutting process of sharp corner

(b) To study the relationship of sharp corner angle, surface roughness and

dimensional accuracy and workpiece material.

(c) To propose the optimum condition for machining sharp corner.

1.4 Scope and Limitation

For this study, EDM wire cut (Model Mitsubishi RA 90) will be used in CNC laboratory in

UTeM. The result of this study just suitable for this EDM wire cut (Model Mitsubishi RA 90) and

not applicable to other types of EDM wire cut or other project. Overall the project is taken as long as

5 month which is started on December 2007 until April 2008. The early choice of wire electrode is

brass wire with diameter 0.25mm. The material that will be used is mild steel and aluminum with the

thickness is 10mm.

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1.5 Important of Study

The important of this study is as follows:

(a) To fine the best method and parameters to produce the high accuracy of sharp

corner cutting using EDM wire cut machine (Model Mitsubishi RA 90).

(b) To fine the improvement method for produce good surface finish and high accuracy of

dimensional using EDM wire cut machine (Model Mitsubishi RA 90).

(c) Can reduce the damage of the material and the machine when the machining

process.

(d) Will be a reference for academic studies which is related to EDM wire cut

machining (Model Mitsubishi RA 90).

1.6 Outline of Study

Overall this report is divided into 5 chapters. For Chapter 1 is mainly describe about the

introduction which is highlight to background, problem statement, objective of study,

scope and limitation, important of study and study outline. Chapter 2 is the literature

review which in this chapter will discuss the definition and the introduction of EDM wire cut

generally includes the operation of the machine. This chapter also will discuss the characteristic

and the basic concept of EDM machining. Then, Chapter 3 is describing about methodology of

this study. This chapter will discuss the process planning, parameter of machining and types of

material. This chapter also will describe about the specification of EDM wire cut (Model

Mitsubishi RA 90) that will be uses for the study and the measurement apparatus. For Chapter 4

is result and discussion of the study observation. This chapter will compile all the result and the

result will be form into graphs to be analyzed. The result will be discussed also in this chapter.

Chapter 6 will shown the conclusion and the recommendation of the whole this project that has

been done.

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CHAPTER 2LITERATURE REVIEW

2.1 Introduction

Material (Ismail M. A, 2005) removal is happening during the work piece machinery with wire-cut EDM based on the effect of erosion sparks. Multifarious of theory have already

shown to explain the phenomenon of sparks. Also found a three theory to demonstrated are:

(a) Electro Mechanical Theory

(b) Thermo Mechanical Theory

(c) Thermo Electric Theory

2.1.1 Electro Mechanical Theory

This theory are designate that the material atomic going to be crude caused a concentrated

of electric field. The electric field isolated the atomic at work piece after that exceeding to

resistant force in material cleft. The theory decline any effect of heated and the EDM process

are not given any effect to material characteristic at surface of work piece. Although that,

the experimental running are shown weak prove to support this theory.

2.1.2 Thermo Mechanical Theory

This theory is designate that the material throw away during electric discharge of machinery

caused by molten material are produced by reason of sparks. The sparks outcome from

various electric effects is produced during discharge. Even though, this theory are not

support from experimental data and failure to given explanation a reasonable about erosion

of sparks.

4

2.1.3 Thermo Electric Theory

Verification according the experimental is successful to support this theory. The theory

suggestion indeed the throw away metals during electric discharge machinery caused by

resultant of over temperature from generated discharge current. Even though, the theory not

accepted with sure and completed because the interpretation are complicated. In this time,

the theory are better depend the other theory.

2.2 Introduction of EDM

Electrical Discharge Machining abbreviated as EDM machining is the process of machining

electrically conductive materials by using precisely controlled sparks that occur between an

electrode and a workpiece in the presence of a dielectric fluid. The electrode may be

considered the cutting tool. EDM is also a thermal process: material is removed by heat. Heat

is introduced by the flow of electricity between the electrode and workpiece in the form of a

spark. Material at the closest points between the electrode and workpiece, where the spark

originates and terminates, are heated to the point where the material vaporizes. While the

electrode and workpiece should never feel more than warm to the touch during EDM, the

area where each spark occurs is very hot. The area heated by each spark is very small so the

dielectric fluid quickly cools the vaporized material and the electrode and workpiece

surfaces. However, it is possible for metallurgical changes to occur from the spark heating

the workpiece surface. (Richard.B, 2006).

It is also called, removes material with repetitive spark discharges from a pulsating dc power

supply, with a dielectric flowing between the workpiece and the tool. (Booothroyd.G &

Khight.W.A, 2005). The principle of EDM is based on the erosion of metals by spark

discharges. (Kalpakjian. S & Schmid.S.R, 2000). If we refer to (IQS Directory, 2007), EDM

is a method by which electrical energy is used to shape and form metal parts. This

process uses an electrical discharge that forms an arc between an electrode tool and a

work piece to erode or remove metal in its path. It is a reliable, affordable and accurate

method of machining otherwise difficult materials, and is thus becoming more widely

used.

5

The electric discharge process is thermo electric process wear away material from work

piece through sparks production which continuous between workpiece and cutting tool.

During the operation, electrode and work piece to be submerged in dielectric fluid.

(Yankee H.W., 1979). Level of fluid must be not less of 50 mm on surface of work piece

and this depth must be permanent for shunt to be happen a burn case. The material reducing

by electric discharge will be terminated by dielectric fluid. The energy source for each

discharge is from capacitance which discharging with alternating current or direct current.

Normally, a continuation to power sourced is allotted for given the negative polarities to

electrode and positive polarities to work piece. (Yankee H.W., 1979).

2.3 General History of EDM

Electric discharge machining (EDM) is one of the most popular non-traditional material

removal processes and has became a basic machining method for the manufacturing

industries of aerospace, automotive, nuclear, medical and die-mold production. The

theory of the process was established by two Soviet scientists B.R. and N.I. Lazarenko in

the middle of 1940s. They invented the relaxation circuit and a simple servo controller

tool that helped to maintain the gap width between the tool and the workpiece. This

reduced arcing and made EDM machining more profitable and produced first EDM

machine in 1950s. Major development of EDM was observed when computer numerical

control systems were applied for the machine tool industry. Thus, the EDM process

became automatic and unattended machining method. (Ho & Newman, 2003)

According to (Damm Company, 2007), EDM become a commercial machining process

during World War II. A United States company, The Ex-Cell-O Corporation, acquired

the research findings from the Soviet Union and started a secret wartime program called

"Method X" in the 1940's. Through further research, refinement and development, Ex-

Cell-O was the first company to offer a commercially available and effective machine

tool in the 1950's. It was called the XLO EDM Machine Tool and it turned EDM into the

viable and profitable machining process we know today.

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2.4 Working Principle of EDM

Electric discharge machining (EDM) differs from most chip-making machining operations

in that the electrode does not make physical contact with the workpiece for material

removal. Since the electrode does not contact the workpiece, EDM has no tool force. The

electrode must always be spaced away from the workpiece by the distance required for

sparking, known as the sparking gap. Should the electrode contact the workpiece, sparking

will cease and no material will be removed. There are some EDM machines that do allow

the electrode to contact the workpiece. These machines are used primarily for removing

broken taps and drills and are not considered die-sinker or wire-cut types of EDM

machines. (Richard.B, 2006). Figure 2.1 below is show the EDM sparking gap.

Figure 2.1: Electric discharge machining sparking gap

The electrode with negative charge is carried nearer metal work piece with positive

charge. Electrode distance from surface of work piece is 0.025 mm until 0.050 mm and will

be controlled by servo motor (Boothroyd et. al., 1979). The gap between two parts is fully

with dielectric fluid. Become more nearer of two conductors, become more high potential

differential that the produce between electrode and work piece. Till the potential differential

become more high level, the electron produced will be ionizing dielectric fluid causing

increasing the electron flowing. Next, the situation causing sparks is generating. Figure 2.2

illustrates the basic components of the EDM process.

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Figure 2.2: Basic components of EDM.

Another basic fundamental of the process is that only one spark occurs at any instant.

Sparking occurs in a frequency range from 2,000 to 500,000 sparks per second causing it to

appear that many sparks are occurring simultaneously. (Richard. B, 2006). The wire

electro-discharge machining (WEDM) process is widely used in the manufacturing of

high-hardness steel precision tooling. Even though the process is characterized by its

high accuracy level (sufficient even for micromachining applications), the development

of enhanced generators that produce more energetic discharges yielding cutting speeds as

high as 500mm2/min has resulted in stronger forces acting on the wire. These forces,

together with the low rigidity of the wire, especially in the cutting of parts of high

thickness, are responsible for wire deformation that has a direct influence on the

accuracy of the part, mainly on wall-flatness and corners. (Sanchez et. al.,2006).

In normal EDM, the sparks move from one point on the electrode to another as sparking

takes place. Figure 2.3 illustrates that each spark occurs between the closest points of the elec-

trode and the workpiece. The spark removes material from both the electrode and

workpiece. This increases the distance between the electrode and the workpiece at that

point. This causes the next spark to occur at the next-closest points between the electrode

and workpiece. Figure 2.4 illustrates how this works.

8

Figure 2.3: Sparking occurs at closest points between the electrode and workpiece.

Figure 2.4: Next spark occurs of closest points between electrode one workpiece.

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The dielectric fluid used must remain nonconducting until breakdown occurs; when the

critical voltage is reached, it must break down rapidly and then deionize rapidly as each spark

is discharged. The latent heat of vaporization of the dielectric fluid must be high so that only

a small quantity vaporizes and the spark channel is confined to a small area. The dielectric

fluid must have sufficiently low viscosity to flow easily and remove the metal globules

effectively from the working zone. The dielectric fluid also acts as a coolant to carry away

the heat generated by each spark. Figure 2.5 illustrates the EDM spark occurring within an

ionized column of the dielectric fluid.

Figure 2.5: Spark occurs within a column of ionized dielectric fluid.

As each spark occurs, a small amount of the electrode and workpiece material is vaporized.

The vaporized material is positioned in the sparking gap between the electrode and workpiece

in what can be described as a cloud. When the spark is turned off, the vaporized cloud

solidifies. Each spark then produces an EDM chip or a very tiny hollow sphere of

material made up of the electrode and workpiece material. Figures 2.6, 2.7, and 2.8

illustrate the spark producing the vapor cloud, the cloud in suspension, and the vaporized

cloud being cooled and forming into an EDM chip. For efficient machining, the EDM chip

must be removed from the sparking area. Removal of this chip is accomplished by flowing

dielectric fluid through the sparking gap.

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Figure 2.6: Spark ON: electrode and workpiece material vaporized

Figure 2.7: Spark OFF: vaporized cloud suspended in dielectric fluid.

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Figure 2.8: Spark-OFF: vaporized cloud solidifies to form EDM chip.

The material removal rate, electrode wear, surface finish, dimensional accuracy, surface

hardness and texture and cracking depend on the size and morphology of the craters

formed. The applied current, voltage and pulse duration, thermal conductivity, electrical

resistivity, specific heat, melting temperature of the electrode and workpiece, size and

composition of the debris in dielectric liquid can be considered as the main physical

parameters effecting to the process. Among them, applied current, voltage and pulse duration

are the parameters which can be controlled easily. Every EDM machine has the following

basic elements as shown in Figure 2.9:

(a) Mechanical structure

(b) Spark generator

(c) Servo system

(d) Dielectric circuit

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Figure 2.9: Basic Elements of an EDM system

2.4.1 Mechanical Structure

EDM machine has similar construction with conventional drilling and milling machine

frames with vertical tool feeding and horizontal worktable movements. Since there is not a

real contact between electrodes, the frame elements not taking much force as in conventional

machining so simpler design is possible. This consideration needs a little bit attention,

because gas bubbles collapses at the end of discharge and cause high frontal shock waves,

therefore; the frame should be strong enough to keep its dimensional stability.

2.4.2 Spark Generator

The required energy is in the form of pulses usually in rectangular form. Recent studies have

been shown that application of pulses in the form of trapezoids resulted with a marked

improvement in cutting efficiency. The optimum pulse form is not exactly a trapezoid, but

similar. Electrical energy in the form of short duration impulses with a desired shape

should be supplied to the machining gap. For this purpose, spark generators are used as the

source of electrical pulses in EDM.

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The generators can be distinguished according to the way in which the voltage is transformed

and the pulse is controlled. The discharge may be produced in a controlled manner by natural

ignition and relaxation, or by means of a controllable semiconductor switching elements.

Nowadays, sophisticated computer aided spark generators are in use as a result of fast

development in electronics industry. These types of generators give us a better manner in

controlling physical parameters.

2.4.3 Servo system

Both electrode and workpiece are eroded during the process, after a certain time dimensions

of the electrodes will be changed considerably. The result is increase in interelectrode

gap. This will increase the voltage required for sparking. This problem can be solved by

increasing the pulse voltage or decreasing the gap distance. The former is not feasible since

most of the electrical energy is used for overcoming breaking strength and producing

plasma in dielectric liquid rather than machining, in addition to that, the required voltage can

increase to the levels that spark generator cannot supply, therefore; the interelectrode gap

should be maintained constant during the process. This can be achieved by a servo system

which maintains a movement of the electrode towards the workpiece at such a speed that

the working gap, and hence, the sparking voltage remains unaltered.

2.4.4 Dielectric circuit

High cooling rates during the resolidification process changes the chemical composition of

the both electrodes and dielectric liquid machining particles called debris are formed.

Formation of such particles effects on machining performance, therefore, dielectric liquid

should be circulated to prevent contamination in working gap. This circulation is done by a

dielectric circuit which is composed of a pump, filter, tank and gages.

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2.5 Types of EDM

According to (Anonymous, 2007), electric discharge machine enables the machining

operation in several ways. Some of these operations are similar to conventional operations

such as milling and die sinking others have its own characteristic. Different classifications are

possible and also it should be kept in mind that, current developments in its technology

add different types of operations. But a simple and general classification can do by

considering famous applications such as:

(a) Die Sinking EDM

(b) Wire Cut EDM

(c) Electric Discharge Grinding

(d) Electrical discharge machining small hole

2.5.1 Die-Sinking EDM

Die-Sinking EDM removes metal with rapid electrical discharges. Sinker EDM, also

called Ram or Conventional EDM, uses an electrically charged electrode to burn a

specific shape into a metal component. It sinks shape from the electrode part into the oil

immersed workpiece, not cutting all the way through the piece. The electrode discharges

plus electrical sparks that jump to the work piece and tear out small particles. The

materials most commonly used for the electrode are graphite, brass or copper tungsten.

Graphite is used because of its machining capabilities and wear ability, and copper for its

fine finish requirements. Figure 2.10 show the basic of die-sinker machining.

Figure 2.10: Die Sinking or penetration

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Through sinker EDM, parts can be formed out of even the most rigid materials and

formed into very complex shapes. However, there are also some materials that cannot be

cut with sinker EDM because they are not electrically conductive. These materials

include hard and soft ferrite materials and epoxy-rich bonded magnetic materials. Sinker

EDM is used when parts need tight tolerances or when a tight corner radius is required.

Sinker EDM is a versatile process, allowing for a variety of sized parts from those that

can fit in the palm of a hand to parts that weigh over 1,000 pounds, and everything in

between. Production dies and molds are often made through the sinker EDM process for

these reasons as well.

2.5.2 Wire Cut EDM

Wire electrical discharge machining (WEDM) is a common EDM process that removes

material with a wire electrode moving longitudinally through the workpiece. A CNC

machine with special software maintains the movement of the wire electrode relative to

the workpiece. Figure 2.11 show one of the process in WEDM

Figure 2.11: Wire electrical discharge machining (WEDM)

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2.5.3 Electrical discharge grinding (EDG)

Electrical discharge grinding (EDG) uses a revolving electrically conductive wheel as the

electrode tool for electrical discharge erosion. EDG is an alternative method for

sharpening diamond and carbide tipped cutting tools, reducing the extreme cost of

diamond grinding wheels. Figure 2.12 show us how the process of electrical discharge

grinding (EDG).

Figure 2.12: Electrical discharge grinding (EDG)

2.5.4 Electrical discharge machining small hole

Electrical discharge machining small hole processes use a tool electrode to gradually

impress a mirror image of the electrode onto a workpiece. Small Hole EDM is a process

similar to a drill spinning with a drill bit that uses a small hollow electrode that spins

around a spindle. During the process, a spark is produced by the electrically charged

electrode, which is charged by a servo-controlled generator. The spark then erodes the

surface of the work piece through the dielectric fluid, creating small pockets. These

pockets are microscopic and after millions of them are formed, a small hole is created.

Figure 2.13 show us the process EDM for small hole.

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Figure 2.13: Small Hole EDM Drilling

The diameter of the electrode determines the size of the hole, and the depth and location

of the holes are often programmed by CNC ISO codes. Conventional drilling is very

similar to small hole EDM, but small hole EDM has the advantages of having the ability

to machine hard work pieces, drill more quickly and accurately, and drill holes in any

conductive material. Small hole EDM also has the capability of drilling holes through

hard alloys from all angles. Small hole EDM also automatically creates burr-free pieces,

so it cuts down on time and money spent on secondary operations. Some of the

applications for small hole EDM include removing broken drill bits, adding holes after

heat treatment, and creating holes that are too small or too difficult for conventional

drilling. Small hole EDM is used on parts for industrial applications, production dies,

parts to be used in wire EDM and many more.

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2.6 Wire electrical discharge machining (WEDM)

Wire electrical discharge machining (WEDM) is used to cut shapes through a selected

conductive part or assembly. After the part has a hole drilled into it, the thin wire, which

is used as the electrode, is fed through the hole in order to complete the machining.

Because there is no physical contact between the machined part and the wire, the

hardness of the work piece has no effect on cutting speed. The wire is surrounded by de-

ionized water and is charged rapidly to a voltage. Once it is at the right level, a spark will

jump from the wire to the work piece, melting a small part of it. The water then flushes

away the particles in the gap and cools the piece.

2.6.1 Introduction of WEDM wire cut

Compared to die sinking electrical discharge machining, (WEDM) is a technology with

increasing economic importance in terms of units sold per year. This fact motivates the

present work that consists of a survey on wire modeling, implications for control and a

catalogue of solutions. (Altpeter & Perez, 2003). According to (Mustafa & Ozanozg,

2000) in WEDM, the cost of machining is rather high due to a high initial investment for

the machine and the cost of the wire which is used as a tool in this process. The WEDM

process is economical if it is used to cut complex workpieces and difficult to machine

materials. In manufacturing die and mold components like sheet metal press dies,

extrusion dies, etc., prototype and special form inserts manufacturing, WEDM is

commonly used. Figure 2.14 below are the examples of WEDM wire cut products.

Figure 2.14: Wire electrical discharge machining (WEDM) Products

2.6.2 History of WEDM19

It is difficult to establish a time when wire-cut EDM came into being. According to

(Richard. B, 2006) the development of the process took place over a period of

approximately ten years ranging from the early 60s to the early 70s. In all probability, the

developers and users of the die-sinker EDM machines started imagining how the machined

electrodes could be replaced with something less labor-intensive and costly. In trying to

solve the problem, they may have reasoned that a stationary wire could serve as an

electrode, but spark erosion on the electrode surface would weaken the wire to the

breaking point. However, a wire that continuously traveled past the surface being machined

would solve the wire breakage problem.

According to (Booothroyd.G & Khight.W.A, 2005), the first major event in the evolution of

wire-cut EDM was numerical control (NC). Accurate axis positioning was achieved by having the

EDM machines read perforated tape to control operational movements. These tapes became very

long and most of the smaller machine shops did not have programming capabilities. In the 1960s,

some of the larger die-making machine shops that had punched-tape programming facilities

converted their conventional vertical milling machines into wire-cut EDM machines. These

machines used power supplies from the die-sinker type of EDM machines. The EDM rate on

these machines was reported to be approximately .750 in. (19.05 mm) per hour when machining

.250-in. (6.35-mm) thick, hardened die steel. This machining rate would be unacceptably slow if

compared to later wire-cut machines.

Many things happened in a fairly short period of time, starting in the early 1970s, which

made wire-cut EDM a practical machining system. Numerical control was replaced by

computer-numerical control, eliminating the need for punched tape. Ball screws became

available, which allowed for an anti-backlash, anti-friction means of table-axis

movement. Anti-friction, pre-loaded table ways became available, which reduced stick

friction in the table movement. Servo motors with encoder and tachometer-feedback

capability made table-axis feed and position control practical.

These items were developed for the more conventional chip-making machines, but their

availability was perfectly timed for computer-numerical-controlled, wire-cut EDM. With

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all of the hardware available for the wire-cut EDM system in place, the final, and

possibly most important, item was developedwire-cut, process-computer software. In

its original form, the software was difficult to use. But with time, the programming

software was refined and simplified to the point that it was acceptable even to shops

without CNC programming specialists. Simplification of the programming brought about

nearly universal acceptance of the wire-cut EDM process. As a result, wire-cut EDM has

become the EDM process of first choice for through-hole applications, such as stamping

and extrusion dies. (Richard. B, 2006).

2.6.3 Wire-cut Machine

The basic components that make up a wire-cut machine are quite different than those of

the die-sinker machine. Figure 2.15 illustrates one style of wire-cut machine with a fixed-

position worktable. It is also possible to have the workpiece supported on a movable X-Y-

positioning table, with the electrode wire held in a stationary position. (Richard. B, 2006).

Figure 2.15: Basic wire-cut EDM machine.

Wire electrical-discharge machining (wire-EDM) is an adaptation of the basic EDM

process, which can be used for cutting complex two- and three-dimensional shapes through 21

electrically conducting materials. Wire-EDM utilizes a thin, continuously moving wire as an

electrode as shown in figure 2.16. It is a relatively new process and applications have grown

rapidly, particularly in the tool making field. The wire electrode is drawn from a supply

reel and collected on a take-up reel. This continuously delivers fresh wire to the work

area. The wire is guided by sapphire or diamond guides and kept straight by high tension,

which is important to avoid tapering of the cut surface. (Booothroyd. G & Khight. W. A,

2005).

Figure 2.16: Basic features of wire-EDM

The wire-cut machine's moves are controlled by servomotors, commanded by computer

numerical control (CNC). There must always be an opening for the passage of the electrode

wire. Precision machining with a wire-cut machine requires very close attention to the

travelling-wire-feed system. This includes the top-and-bottom wire guides, the wire-

tensioning mechanism, and the condition of the wire on the supply spool. Electrode wire is

only used once, since the material removed from the wire surface during the sparking

process weakens it. Upon travelling through the sparking area, the used wire is collected

for disposal on a spool, or cut into short lengths and dropped into a container.

Wire guides are provided in different styles, designs, and materials. The potential machine

user should evaluate the wire-guide design to make sure that it will provide the required

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machining accuracy necessary over an extended time period. Two items of importance are

not shown in Figure 2.16. These are the electrical-sparking-power contacts to the electrode

wire and the elevating mechanism for adjusting vertical distance between the wire guides to

accommodate different workpiece heights. For efficient wire-cut-machining operations, the

electrical contacts must be clean. A dirty contact will cause machining problems. Contacts

should be easily accessible for cleaning and they should be maintained to the machine

manufacturer's specification.

The wire-guide elevating mechanism is set to allow proper machining-workpiece heights,

within the required range of applications performed. The top wire guide is normally adjustable

for workpiece height. The bottom wire guide is fixed in close proximity to the bottom

surface of the workpiece. Wire guides should be inspected periodically for wear. They

should also be inspected for cleanliness. Worn or dirty electrode guides can cause

inaccurate machining and erratic machine operation. The top guide of the elevating

mechanism must be set properly so that the mechanical structure will not come into contact

with surfaces that project from the workpiece during operation.

2.6.4 Wire Cut EDM Electrodes

As the wire-cut description indicates, electrodes for wire-cut machine are always round

wire, purchase on a spool or reel. There is no need to consider how the electrode will be

machined. Only one-half of electrode diameters are subject to wear as the wire passes

through the sparking area. Figure 2.17 illustrates the wire-cut-wear pattern on the electrode

wire. It may appear that the wire-wear pattern would allow the electrode wire to be reused.

But re-using the electrode reduces the tensile strength and could result in breakage when the

required tension is applied for the machining operation during reuse. The normal practice is

to dispose of the electrode wire after it makes one pass through the sparking area.

23

Figure 2.17: Wire-cut electrode wear.

A wire electrode for wire-cut electrical discharge machining includes a wire composed of

a core made of one of an amorphous pure metal and an amorphous alloy and a thin

crystalline layer serving as a surface of the core. The wire may be coated on its surface

with a layer of a material of high electrical conductivity. The amorphous alloy may be

Cu-based, Fe-based, or Co-based. The use of an amorphous pure metal or alloy yields a

wire electrode of much greater tensile strength than previous wire electrodes, enabling a

higher machining speed. (Patent Storm, 1989).

According to (Richard. B, 2006), wire-cut wire suppliers normally provide the electrode

material in sealed packages to prevent deterioration of the surface by oxidation. Surface

oxidation reduces the electrical conductivity of the wire at the point when sparking electricity is

applied from the power supply. This detrimentally affects the machining operation. It is

recommended that any spool or reel removed from the machine for future use be repackaged

and sealed to prevent further oxidation of the electrode surface. Most wire-cut machines use

electrode wire in a diameter range of.004-.014 in (0.1-0.35 mm).

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KUTKM Library (Pind.2/2007)Karung Berkunci 1200, Ayer Keroh, 75450 MelakaAbstrak.pdfABSTRACT

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Chapter2.pdfLITERATURE REVIEW