Applications Of Electro Discharge Machine Biology Essay

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CHAPTER 1

INTRODUCTION

Wire-cut electro discharge machining (WEDM) is one of the applications of Electro Discharge Machine (EDM). WEDM process, with a thin wire as the electrode transforms electrical energy to thermal energy for cutting materials. WEDM is capable of producing complex shapes such as tapers, involutes, parabolas and ellipses. This non-traditional is widely used in manufacturing of dies, moulds, precision manufacturing and contour cutting. Furthermore, WEDM is capable of produce a fine cutting, precise, corrosion resistance and wear resistance surface (Liao et al., 2004).

The WEDM is a precision machining process for micro machining of micro-structures such as high aspect ratio micro holes, slots and moulds. The basic characteristics of the WEDM process is similar to that of the EDM process with the main different being in size of the tools used, the power supply of discharge energy and the resolution of the X, Y, and Z axes movement (Rao and Sarcar, 2009). Therefore, to improve the process control and quality surface integrity, it is very important to understand the machining parameters involved in the material removal mechanism.

Wire cut electro discharge machine (WEDM) is an adaptation of the basic EDM process, which can be used for cutting complex two and three dimensional shapes through electrically conducting materials. WEDM is a widespread technique used in industry for high precision machining of all types of conductive materials such as metals, metallic alloys, graphite, or even some nonconductive materials of any hardness (Singh and Garg, 2009). WEDM applied a thin wire continuously moving as an electrode. The electrode is a thin wires that diameter ranging between 0.05 to 0.3 mm (Zakaria et al., 2005).

The wire electrode is drawn from a supply reel and collected on a take-up reel at top of machine. The wire is guided by sapphire or diamonds guides and kept straight by high tension. The wire tension sensor always kept touching the wire during the cutting process. Tension is important to avoid tapering or unsmooth of the cut surface. During the cutting process, high frequency DC (direct current) pulse delivered to wire and workpiece. The WEDM process used electrical sparks between a thin, travelling wire electrode and the workpiece to erode the work material and generate the desired shape (Miller et al., 2004). The WEDM cutting process is shown in Figure 1.1.

Figure 1.1 : Schematic view of Wire Cut EDM Cutting Process

The pulse delivered will causing a spark discharges in the narrow gap between the two. The wire workpiece gap usually ranges from 0.025 to 0.05 mm and is constantly maintained by a computer controlled positioning system. The power supply for WEDM basically same as for conventional EDM, except the current carrying capacity of the wire limits currents to less than 20A, with 10A or less being most normal. The spark frequencies are higher, up to 1MHz to give a fine surface on the workpiece (Hassan et al., 2009).

Many WEDM machines have adopted the pulse generating circuit using low power for ignition and high power for machining. But, it is not suitable for finishing process since the energy generated by the high voltage sub circuit is too high to obtain a desired fine surface (Singh and Garg, 2009). WEDM is capable of producing any complex shape simply generated with high grade of accuracy and surface finish using computer numerical control (CNC).

1.1 Problem Statement

WEDM is a non-traditional machining method that widely used to pattern tools for die manufacturing industry, but the problem occurs on the surface integrity after machining because in manufacturing process industries, the variety, precision and accuracy has become the essential part (Parashar et al., 2009). The problem happened when the machining parameters tables provided by machine manufacturers often do not meet the operator requirements and sometimes do not provide efficient guidelines to manufacturing engineers. From that situation, they make some adjustments to meet their requirements, which may effected in poor quality and not reach the standard of the final product.

WEDM process is used to achieve high accuracy, fine surface finish, high removal rate and increased productivity. However there are some problems that might occur when machining the work-piece in WEDM process such as bad surface finish, micro-cracks, undercut and others. The problem still happened even the skilled operator is used. It is difficult to achieve the optimal performance machining. These problems made the product have bad surface finish, low mechanical strength and other problems.

(Rao and Sarcar, 2009) stated that in the field of dies and moulds making, precision manufacturing and contour cutting has a many challenges due to achieve a high accuracy and precision finish. This complex's shape can be generated easily with higher accuracy and surface finish using the WEDM. Besides, (Zakaria et al., 2004) stated that there are several problems on the production of die-making, precision machining, cutting of sheet materials and for the manufacturing of prototypes. The problems also occur to produce the complex two and three dimensional shapes.

Additionally, to produce a complex shapes such as tapers, involutes, parabolas and ellipses especially in the tool making field has a several challengings (Hassan et al., 2009). Furthermore, the increasing applications of WEDM need the higher demands of precision and accurately machining processes. Some parts need accurately machining process with varying hardness or complex shapes that are very difficult to be machined by conventional machine (Singh and Garg, 2009).

The demand for high performance materials machining of complex shapes is another challenge. Accompanying the development of mechanical industry, the demands for alloy materials having high hardness, toughness and impact resistance are increasing (Liao and Huang, 2003). The rough cutting operation is also treated as a challenging issue in WEDM manufacturing process (Mahapatra and Patnaik, 2006). Therefore, using the full factorial method as a tool to investigate the machining parameters due to achieve the optimal performance characteristic for aluminium alloy (Al) 5052 material in WEDM process.

1.2 Research Question

The research questions are:

What is the most optimization parameter influenced on material removal rate and surface roughness for machining of Al 5052 alloy in WEDM process?

How the parameters affect the machining process in WEDM?

What are the optimum machining parameter that can increase the material removal rate and surface roughness in WEDM process using the mathematical model?

Research Objective

The research objectives are as follows:

To investigate the most optimal parameter influenced on material removal rate and surface roughness for machining of Al 5052 alloy in WEDM process.

To identify the effects of parameters to machining process in WEDM.

To investigate the optimum machining parameter that can increase the material removal rate and surface roughness in WEDM process using the mathematical model.

1.4 Research Significance

The research significance such as:

To help the operator to improve the machining performance in WEDM manufacturing process for producing the high quality finish, accurate and precise work.

To set the optimization of machining parameters for machining process in WEDM.

1.5 Research Scope

The scope of the research is:

To investigated the performance characteristics of WEDM machining process for material removal rate (MRR) and surface roughness (SR).

The machining parameters to be investigated include pulse off time (TOFF), peak current (IP), wire feed (WF), and servo voltage (SV).

The material of the study is using the Al 5052 alloy as a specimen of experiment.

To apply the full factorial method in the designation of experiment.

CHAPTER 2

LITERATURE REVIEW

2.1 Wire-Electro Discharge Machine (WEDM)

Wire-electro discharge machining is a process of material removal of electrically conductive materials by the thermo-electric source of energy. The material removal by controlled erosion through a series of repetitive sparks between work-piece and electrode. (Rhoney, 2001) states that the WEDM, as shown in Figure 1.2 (a), uses a traveling brass wire, ranging from 0.02 to 0.40 mm in diameter, as the electrode. Continuous electrical sparks, Figure 1.2 (b), are generated between the wire and work-piece for material removal. By using computer numerical control, the thin wire is guided in the X and Y directions to cut a precise shape in the work-piece.

Figure 2.1 : (a) WEDM Cutting Operation Process,

(b) Enlarged view of the Wire and Work-piece

In the WEDM process there is no relative contact between the tool and work material, therefore the work material hardness is not a limiting factor for machining materials by this process. In this operation the material removal occurs from any electrically conductive material by the initiation of rapid and repetitive spark discharges between the gap of the work and tool electrode connected in an electrical circuit and the liquid dielectric medium is continuously supplied to deliver the eroded particles and to provide the cooling effect.

A small diameter wire ranging from 0.05 to 0.25 mm is applied as the tool electrode. The wire is continuously supplied from the supply spool through the work-piece, which is clamped on the table by the wire traction rollers. A gap of 0.025 to 0.05 mm is maintained constantly between the wire and work-piece. The wires once used cannot be reused again due to the variation in dimensional accuracy. The dielectric fluid is continuously flashed through the gap along the wire, to the sparking area to remove the by products formed during the erosion. Brass wire is most commonly used for wire cutting, and zinc or aluminum coatings are employed for high-speed cuts (Abdul, 2006).

The WEDM has become an important nontraditional machining process, widely used in the aerospace, nuclear and automotive industries. This is because the WEDM process provides an effective solution for machining hard materials with intricate shapes, which are not possible by conventional machining methods. In WEDM the cost of machining is rather high due to high initial investment for the machine and cost of the wire electrode tool. The WEDM process is more economical, if it is used to cut difficult to machine materials with complex, precise and accurate contours in low volume and greater variety (Abdul, 2006).

There are many research studies by previous researcher about the WEDM machining process for different material and parameter setting. (Kern, 2007) stated that during the cutting process the high on wire tension is necessary to maximize accuracy. For the numerous jobs with more relaxed tolerance such as +/- .001" or larger, a lower level of wire tension will allow the machine to cut faster. Wire tension combined with heat and the attacks of the spark upon the wire cross-sectional area, is what ultimately will break the wire. If we lower the wire tension significantly for thinner, less accurate jobs, we can apply more power to the wire without breaking it.

Besides, there are a several myths in WEDM machining process like a larger wire always gives more economical performance. That is not really correct for all cutting jobs because it is just a prediction. So the key to successful performance optimization is methodical and small adjustment of parameters. It is also important to allow the machine to stabilize after each change is made and always monitor the cutting rate and stability during the straight line cut (Kern, 2007).

The selection of optimum machine setting parameters plays an important role for obtaining higher cutting speed or good surface finish. Improperly selected parameters may also result in serious consequences like short-circuiting of wire and wire breakage. Wire breakage imposes certain limits on the cutting speed that in turn reduces productivity. As surface finish, material removal rate and cutting speed are most important parameters in manufacturing, various investigations have been carried out by several researchers for improving the surface finish, material removal rate, cutting speed and width of kerfs of WEDM process (Kern, 2007).

However the problem of selection of cutting parameters in WEDM process is not fully solved, even though the latest version of Computer Numerical Control (CNC) WEDM machine are presently available. WEDM process involves a number of machine setting parameters such as peak current (IP), pulse on-time (TON), pulse off time (TOFF), servo voltage (SV), wire feed (WF), wire tension (WT), and dielectric pressure flushing (DFP). The material of work-piece and its height also influence the process. All these parameters influence the surface finish and material removal rate of WEDM machining process. (Parashar et al., 2009) stated that the strength and hardness of the work materials are not significant factors in WEDM machining process.

2.2 Performance Characteristics

2.2.1 Surface Roughness Characteristics

According to the development of mechanical industry, the demands for alloy materials having high hardness, toughness and impact resistance are increasing, so a good quality finish for the workpiece have to improve (Liao et al., 2004). Therefore, good quality surface improves the fatigue strength, corrosion and wear resistance of the workpiece (Singh and Garg, 2009). Usually, the settings of machining parameters always based on the operators working experience and sometimes they refer to the machining parameters tables provided by machine tool builders. An analysis of effects of various process parameters for achieving improved machining characteristics is required for successful utilization of process with high productivity (Rao and Sarcar, 2009).

Therefore, the surface finish is controlled by number of discharge per second, more often referred to as the frequency sparks. The greater amount of energy applied the greater amount of material removed. However, when greater amount of current are used, larger craters are eroded from the work, causing a rougher surface finish. To maintain increased metal-removal rates and at the same time improve the surface finish, it is necessary to increase the frequency of the discharge (Abdul, 2006).

Several researches have been made in the past to study the influence of different process parameters on the important performance measures of the WEDM process by using various problem solving tools. (Nihat et al., 2003) investigated that the effect of the pulse duration, open circuit voltage, wire speed, and dielectric flushing pressure on workpiece surface roughness, it was found that the increasing pulse duration, open circuit voltage, and wire speed increases the surface roughness whereas the increasing dielectric fluid pressure decreases the surface roughness. The material removal rate (MRR) directly increases with increase in pulse on time (TON) and peak current (IP) while decreases with increase in pulse off time (TOFF) and servo voltage (SV) (Miller et al., 2004).

(Singh and Garg, 2009) stated that the pulse on time is the most influencing machining parameter for surface roughness, compare to the gap voltage, pulse off time and flushing pressure are little effects while wire feed lowest effect to the surface roughness. (Hassan et al., 2009) investigated that to produce the good surface finish such as the WEDM machining parameters should be set at low pulse current and small pulse on duration. It is because the pulse on duration has major influence in defining the WEDM surface texture as compared to the pulse current.

2.2.2 Material Removal Rate (MRR)

The amount of material removed in a given unit of time during WEDM process is called material removal rate (MRR). The MRR for WEDM are slower than conventional machining methods. The rate of material removal is dependent on the following factors:

Amount of current of each discharge.

Frequency of the discharge.

Material of wire.

Work-piece material.

Cutting speed.

Feed rate.

Dielectric flushing conditions.

The common concept for WEDM machining process is when the current increases, MRR also will increase with a spark of 1 ampere (A) erodes a certain amount of metal. When the current is doubled, the energy in the discharge is also doubled and approximately twice the amount of metal is removed. Modern WEDM machine have power supplies capable generating more than 100 A to material per hour for every 20 A of machining current. However material removal rates will up to 245 cm3/h are possible for roughing cuts with special power supply (Abdul, 2006).

Factors like discharge current, pulse duration and dielectric flow rate and their interactions have been found a significant role in rough cutting operations for maximizations of MRR, minimization of surface roughness and maximization of cutting width (kerf) (Mahapatra and Patnaik, 2006). The effects of parameters (discharge current, job thickness, and material composition) were studied on machining criteria such as cutting speed, spark gap and MRR (Rao and Sarcar, 2009). The study has been done by different researcher on different material still shows the similar results as well.

The servo voltages have signifant influence on the MRR while lower value of servo voltage can increased MRR and machining production rate will be increased simultaneously (Ahmad et. al, 2010). Using an artificial neural network modelling to performed the optimal process parameter settings to achieve the characteristics for WEDM workpiece surfaces. They obtained the optimum combination of the parameters, namely pulse width, time between two pulses, wire mechanical tension, and wire feed space for maximum cutting speed, keeping the surface roughness and waviness within the required limits (Spedding and Wang, 1997). The material removal is increased but the efficiency of material removal (volume of material removal per unit energy input) is decreased with the increase of discharge on time. Under the same machining conditions, the surface roughness will become better when there is greater specific discharge energy (SDE), and vice versa (Liao and Yu, 2004).

The investigated on the effect and optimization of machining parameters on the kerf (cutting width) and material removal rate (MRR) in wire electrical discharge machining (WEDM) operations. The experimental studies were conducted under varying pulse duration, open circuit voltage, wire speed and dielectric flushing pressure. The settings of machining parameters were determined by using Taguchi experimental design method. The simulated annealing algorithm was then applied to select optimal values of machining parameters for a multi-objective problem considering minimization of kerf and maximization of MRR (Nihat et al., 2004). The cutting performance outputs considered in this study were surface roughness and cutting speed. It is found experimentally that increasing pulse time, open circuit voltage, wire speed and dielectric fluid pressure increase the surface roughness and cutting speed (Nihat, 2003).

The relational and signal to-noise (S/N) ratio analysis to demonstrate the influence of table feed and pulse-on time on the material removal rate. It was found that the table feed rate had a significant influence on the metal removal rate, while the gap width and surface roughness were mainly influenced by pulse-on time. The investigation on the optimization and the effect of machining parameters on kerf and the MRR in WEDM operations has been studied (Huang and Liao, 2003).

Some researcher has developed mathematical models correlating the various WEDM machining parameters (peak current, duty factor, wire tension and water pressure) with metal removal rate, wear ratio and surface roughness based on the response surface methodology (Hewidy et al., 2005). Another researcher has presented a multiple regression model to represent relationship between input variables and two conflicting objectives, such as cutting velocity and surface finish. A multi-objective optimization method based on a non-dominated sorting genetic algorithm (NSGA) was then used to optimize the WEDM process (Kuriakose and Shunmugam, 2005). The genetic algorithm, a popular revolutionary approach, is employed to optimize the wire electrical discharge machining process with multiple objectives.

2.3 Effected of Machining Parameter

2.3.1 Effect of Peak Current

As illustrated in Figure 2.2, increased discharge frequency can improve the surface finish. Within limits, by doubling the amperage and frequency, the MRR will double without changing the finish (Abdul, 2006).

Figure 2.2 : Effect of current and frequency on surface finish and MRR.

At high frequencies, the amperage is reduced due to inductance, thereby reducing the MRR. The economics involved, therefore, set a practical limit on surface finish. The relationship between current and frequency on surface finish is shown in Figure 2.3 (Abdul, 2006). The value of peak current should be high to obtain higher MRR (Singh and Garg, 2009).

Figure 2.3 : Surface Finish as Related to Frequency and Current

2.3.2 Effect of Pulse Duration

As illustrated in Figure 2.4, the value of pulse off time can be selected in such a way to get the desired of MRR in machining process. So, when the pulse off time is increased the MRR will decrease. The effect of pulse duration in machining process has a signifant to the MRR (Singh and Garg, 2009). (Rao et al., 2010) investigated that the MRR increase with decrease in TOFF, this is because when TOFF increase there will be an undesirable heat loss which does not contribute to MRR. This will lead to drop in the temperature of the work-piece before the next spark starts and therefore MRR decreases as shows in Figure 2.4.

Figure 2.4 : Pulse off time and MRR

During the off time, TON, the capacitors are charged up and melted material is flushed from the gap between the wire electrode and work-piece. After circuit is completed, the spark is discharged and the energy and the energy are delivered during the on time, TON, which is the spark on time. This total time for charging and discharging is the spark cycle as shows in Figure 2.5 (Hassan, 2009).

Figure 2.5 : Voltage vs. Duration during the WEDM cutting process

2.3.3 Effect of Wire Feed

The Figure 2.6 shows that the MRR remains neraly constant with variation in the wire feed. Therefore, the wire feed should be selected in a way that there is no wastage of the wire in the machining process (Singh and Garg, 2009). (Rao et al., 2010) evaluated that the wire feed is signifant to MRR, when the wire feed increases the mean (dB) for MRR also increases as shows in Figure 2.7.

Figure 2.6 : Material Removal Rate (MRR) and Wire Feed (WF)

Figure 2.7 : Average S/N Ratios (dB) to Wire Feed Rate

2.3.4 Effect of Servo Voltage

The MRR is maximum at low servo voltage and minimum at high voltage. Figure 2.6 show that the MRR decreases regurlarly with increase in the servo voltage (Singh and Garg, 2009).

Figure 2.8 : Material Removal Rate (MRR) and Servo Voltage (SV)

2.4 Material

The material will be use in the experiment is aluminium 5052 alloy. The Al 5052 alloy contents with several elements such as copper, zinc, manganese, silicon, magnesium and aluminium. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions (Degarmo et al., 2003).

Aluminum 5052 alloy is one of the higher strength non-heat treatable alloys (annealed it is stronger than 1100 and 3003).  Alloy 5052 has excellent characteristics with a high fatigue strength it is used for structures which are subject to excessive vibrations.  The Al 5052 alloy also has excellent corrosion resistance, especially in marine atmospheres and are therefore commonly used in boats, marine components, fuel and oil tubing (Degarmo et al., 2003).

The aluminium 5xxx series alloys are based on magnesium and are strain hardenable and not heat treatable. The major characteristics of the 5xxx series are:

â- Excellent corrosion resistance, toughness, weldability, moderate strength.

â- Building and construction, automotive, cryogenic, and marine applications.

â- Typical ultimate tensile strength range: 18 - 51 ksi.

The aluminium magnesium (Al-Mg) alloys of the 5xxx series are strain hardenable and have moderately high strength, excellent corrosion resistance even in saltwater, and very high toughness even at cryogenic temperatures to near absolute zero. As a result, 5xxx alloys find wide application in building and construction, highways structures including bridges, storage tanks and pressure vessels, cryogenic tankage, and systems for temperatures as low as -4590F (-2700C, near absolute zero), transportation, and marine applications, including offshore drilling rigs. Alloys 5052, 5086, and 5083 are the workhorses from the structural standpoint, with increasingly higher strength associated with the increasingly higher Mg content (Kutz, 2002).

Summary

Therefore, from the review for the previous research, to run the experiment for machining process in WEDM have to understand all the parameters involved that influence to the performance characteristics. Besides, the output responses have to decide in order to achieve the optimization performance machining process in WEDM. For this research the parameters will be investigated are pulse off time (TOFF), peak current (IP), wire feed (WF), and servo voltage (SV) while the output response are material removal rate (MRR) and surface roughness (SR). From the review mention that the strength and hardness of the material not are signifant rather than the melting point of the material more signifant to the performance machining characteristics in WEDM process.

The Design of Experiment (DOE) has to decide in order to run the experiments also to setting the number of samples needed. So, in this research the full factorial design will be used to run the experiments. The number of parameters and level for the machining process will be used to obtain the number of samples that will be used in the experiment. Additionally, the analysis of the each samples from the experiments have to analyse using the software as Design Expert Software to obtain all the value involved in the machining process. The results will be obtained after the data analyse in order to pursue the optimal parameters that most influence to the performance machining characteristics in WEDM. From that the conclusion will be obtained to achieve the research objectives.

The review shows that the optimal parameters setting are important due to produce the high precision, accuracy and good surface finish of the material. The high precision and accuracy of cutting process can pursue the high strength, high yield and high quality product. So, the parameters controlled are very important due to achieve the objective.

CHAPTER 3

METHODOLOGY

3.1 Experimental Procedure

The complete procedures in conducting the experiment diagram are shows in Figure 3.1.

3.2 Machine

The WEDM machine will be used in this research was developed by FANUC, Japan and the model is ROBOCUT α-0iD 5 axis. Various input parameters varying during the experiments that will give effects for the work-piece as well. The brass wire is use for this machine which is suitable to use and also recommendation from the machine tools manufacturer. The size of the wire is 0.25 mm which is hard brass wire will be used for the experiments. The manufactured manual provided will be referring for conduct the experiments. This machine will allow the operator to choose the input parameters according to the material and height of the work-piece. The WEDM will be used is shows in Figure 3.2.

Start

Recognition of problem statements

Material removal rate (MRR) and surface roughness (SR).

Performance characteristic

Identify the parameters

TOFF, IP, WF, and SV.

Design of Experiments (DOE)

Full factorial, 24.

Weights of Samples (before machining)

Wire - Brass (0.25mm) & Work-piece size (100 X 25 X 10 mm).

Run the experiments (WEDM)

Effect on the MRR and SR.

Weights of Samples (after machining)

Signal to noise (S/N).

Analyse Results

ANOVA

Conformation Test

Obtain actual result.

Discussion and Conclusion

End

No

Figure 3.1: Flow Chart for the Experiment Procedures

Figure 3.2 : FANUC ROBOCUT α-0iD WEDM Machine

3.3 Performance Characteristic

The main purpose of this research is to analyze the effects of machining parameters on performance characteristics such as material removal rate (MRR) and surface roughness (SR) Al 5052 alloy in WEDM. The machining parameters will be observed such as pulse-off time (TOFF), peak current (IP), wire feed (WF), and servo voltage (SV). There are the important controllable machining parameters of the WEDM process.

(Mahapatra and Patnaik, 2006) investigated experimentally the optimization on MRR, surface finish and cutting width (kerf) of machining parameters on the discharge current, pulse duration, pulse frequency, wire speed, wire tension and dielectric fluid pressure on the machined work-piece surface. (Nihat et al., 2004) investigated on the effect and optimization of machining parameters of pulse duration, open circuit voltage, wire speed and dielectric fluid pressure on kerf and MRR.

(Parashar, 2009) study on optimization of surface roughness of machining parameters of gap voltage, pulse on time, pulse off time, wire speed and dielectric fluid pressure. (Rao and Pawar, 2009) study on modelling and optimization of MRR on process parameters of pulse on time, pulse off time, peak current, and servo feed setting. (Singh and Garg, 2009) investigated on the effect on MRR of process parameters such as pulse on time, pulse off time, peak current, servo voltage, wire feed, and wire tension.

3.4 Workpiece Preparation

The specimen will be cut from a rectangular bar of Al 5052 alloy with dimension of 100 X 25 X 10 mm size, will be cut 10 mm in depth along the longer length using WEDM cutting process. A thin brass wire of 0.25 mm in diameter will be use as tool for cutting the workpiece. The experiments will be conducted in water by sinking the workpiece to a thickness of 10 mm. The chemical composition and the properties of selected work-piece material are shows in Table 3.1 and Table 3.2. The workpiece material preparation is shows in Figure 3.3.

100 mm

10 mm

25 mm

Figure 3.3 : Work-piece Material Dimension

Table 3.1 : Chemical composition of Al 5052 alloy (Kutz, 2002)

No.

Chemical

Percentage (%)

1

Si

0.45

2

Fe

0.45

3

Cu

0.10

4

Mn

0.10

5

Mg

2.8

6

Cr

0.35

7

Zn

0.10

8

Al

Bal.

Table 3.2 : Mechanical properties of Aluminium 5052 alloy (Kutz, 2002)

Properties

Al 5052 Alloy

Yield Strength (ksi)

13

Ultimate Tensile Strength (ksi)

28

Shear Ultimate Strength (ksi)

18

Fatigue Strength (ksi)

16

Elongation in 2 in (%)

25

Brinell Hardness (Hb)

47

Modulus of Elasticity (103 ksi)

10.2

3.5 Design of Experiment

Therefore, each the parameters have to investigate properly to achieve the optimum performance of cutting process. The experiment will be done by the specific design of experiments (DOE) where is conducted to perform more accurate, less costly and more efficient experiments. In this research, the experimental strategy in this experiment is applying a full factorial design (2k) where k is the number of controlled variables in the experiment. There are four controlled variables investigated including pulse-off time (TOFF), peak current (IP), wire feed (WF), and servo voltage (SV). Through dry run step, the levels have been selected such the wide range of value are covered. The input parameters and their level will be investigates as shows in Table 3.3.

Table 3.3 : Input parameters of the setting and the level (Abdul, 2006)

Control factors

Parameter

Level 1

Level 2

Factor A

Pulse off time ,TOFF (µs)

10

30

Factor B

Peak current, IP (A)

10

15

Factor C

Wire feed, WF (mm/min)

10

20

Factor D

Servo voltage, SV (V)

10

50

The number of level is 2 levels of each factor were selected for the 24 experiment such as pulse-off times (TOFF), peak current (IP), wire feed (WF), and servo voltage (SV). For the DOE, there are 16 samples will be conducted for run the experiments. A common experimental design is one with all input factors set at two levels each. These levels are called `high' and `low' or `+1' and `-1', respectively. A design with all possible high or low combinations of all the input factors is called a full factorial design in two levels.

3.5.1 Selection of Response Variable

The performance characteristics measures for this experiment are material removal rate (MRR) in g/min and surface roughness (SR) in micron.

3.5.1.1 Material Removal Rate (MRR)

After the experiments completed, each samples will be evaluate by testing machine and the specific formula to achieve the values involved. The MRR can be expressed as the volume of work piece loss divided by machining time. The unit for MRR is in g/min. To evaluate the value of MRR is measured by using a digital weighing machine such as Precisa Balances series XT that only measured the weight loss of the work-piece before and after machining process. The result of MRR will be calculated using this formula:

MRR = Xb - Xa / t

Where, MRR = material removal rate (g/min)

Xb = Weight of specimen before cutting (g)

Xa = Weight of specimen after cutting (g)

t = machining time (min)

Figure 3.4 : Precisa Balances series XT

3.5.1.2 Surface Roughness (SR)

The surface roughness response will be evaluated through the Taylor Hobson surface roughness tester to achieve the value. It is direct transform as surface roughness of the work-piece after machining and the unit of SR is μm. The SR is measured by using a Portable Surface Roughness Tester. The SR values are measured three times for each sample and get the average. Result of SR can get direct from the machined process so no calculation needed.

Figure 3.5 : Taylor Hobson Surface Roughness Tester

3.6 Data Analysis

To find the optimal settings and factors for the machining of Al 5052 alloy can be determine using signal to noise (S/N) ratio and analysis of variance (ANOVA) by using the Design Expert software analyse. The parameters in the machining performance of the WEDM process are identified based on experience, discussion with expert and analysis from literature. Experiments will performed by varying each input parameter keeping others constant. The main reason for trial experiments was to find a range of input parameters so that it would be safe for setting parameters at levels where the test would not end in a failure half way like wire breakage, no spark condition or gap short.

3.6.1 Signal-to-Noise Ratio (S/N Ratio)

The S/N will be use to determine the effects each parameters has on the output for each experiments. The average of S/N value will be calculated for each factors and levels to find the range (R) of the S/N for each parameter. The larger the R values for parameter, the larger the effect of the parameter on the machining process. (Mahapatra and Patnaik, 2006) state that the characteristic that higher value represents better machining performance, such as MRR, is known as 'higher is better' (HB). Inversely, the characteristics that lower value represents better machining performance, such as SR, are known as 'lower is better' (LB). Therefore, 'HB' for the MRR and 'LB' for SR was selected for obtaining the optimum machining performance characteristics.

3.6.2 Analysis of Variance (ANOVA)

The ANOVA analysis is able to identify the active and inactive factors effect to the performance characteristics. The purpose of ANOVA experimentation is to reduce and control the variation of a process subsequently, decisions can be made concerning which parameters affect the performance of the process. ANOVA is the statistical method used to interpret experimental data to make the necessary decisions. Through ANOVA, the parameters can be categorized into significant and insignificant machining parameters.

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