Novel EDM Screw Threading Technique Using Threaded Cavity Electrode.

NOVEL EDM SCREW THREADING TECHNIQUE USING

THREADED CAVITY ELECTRODE

AHMAD HISYAMUDDIIN BIN HASSAN

B050910008

UNIVERSITI TEKNIKAL MALAYSIA MELAKA 2013

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

NOVEL EDM SCREW THREADING TECHNIQUE USING

THREADED CAVITY ELECTRODE

This report submitted in accordance with the requirement of the UniversitiTeknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Manufacturing Process) (Hons.)

by

AHMAD HISYAMUDDIIN BIN HASSAN B050910008

871015-26-5145

FACULTY OF MANUFACTURING ENGINEERING 2013

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: Novel EDM Screw Threading Technique using Threaded Cavity ELectrode

SESI PENGAJIAN: 2012/13 Semester 2

Saya AHMAD HISYAMUDDIIN BIN HASSAN

mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut: 1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan

untuk tujuan pengajian sahaja dengan izin penulis.

3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan pertukaran antara institusi pengajian tinggi.

4. **Sila tandakan (√)

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysiasebagaimana yang termaktub dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)

Alamat Tetap:

105,Taman Sri Gunung 05150, Alor Setar, Kedah Darul Aman

Tarikh: _________________________

Disahkan oleh:

Cop Rasmi:

Tarikh: _______________________ ** Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh laporan PSM ini perlu dikelaskan sebagai SULIT atau TERHAD.

DECLARATION

I hereby, declared this report entitled “Novel EDM Screw Threading Technique Using Threaded Cavity Electrode” is the results of my own research except as cited in the

references.

Signature : ... Author's Name : Ahmad Hisyamuddiin Bin Hassan

APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the degree of Bachelor of Manufacturing Engineering (Manufacturing Process) (Hons.). The member of the supervisory is as follows:

...

Dr Raja Izamshah Bin Raja Abdullah Project Supervisor

i

ABSTRACT

The complicated geometrical design and tight tolerance of orthopedic screw such as titanium alloys were fabricated using single-point threading and thread whirling technique. However, there are few problem encounters with these techniques such as rapid tool wear, limited design flexibility, low accuracy and high capital cost. The suitable candidate to overcome this problem is using Electrical Discharge Machining (EDM) process in which the material is removed by spark erosion. Since there is no physical contact between tool electrode and workpiece, the tool wear rate is low and the material hardness is not dependent. In addition, the main advantages of using EDM technique are the accuracy and fine surface integrity which is suitable for medical application. However, to ensure the success of EDM as an alternative manufacturing technique for orthopedic screw required a scientific investigation especially on the effect of EDM machining parameter. This study aims to investigate the effect of EDM machining parameter namely current, pulse on time and pulse off time on machining performances. A statistical response surface methodology are use to evaluate the effect of the machining parameter on the machining performances namely surface roughness, machining time and electrode wear. Lastly, the optimal machining condition base on the multi-objective performance criteria will be proposed.

ii

ABSTRAK

Reka bentuk geometri yang rumit dan ketepatan yang tinggi skru ortopedik seperti aloi titanium telah direka menggunakan satu titik ulir dan teknik “thread whirling”. Walau bagaimanapun, terdapat beberapa masalah yang timbul dengan mengunakan teknik-teknik ini, seperti kehausan mata alat, reka bentuk terhad, ketepatan yang rendah dan kos modal yang tinggi. Calon yang sesuai untuk mengatasi masalah ini adalah dengan menggunakan “Electrical Discharge Machining (EDM) proses di mana bahan kerja dia hakis melalui teknik percikan api. Kerana tidak ada hubungan fizikal antara elektrod alat dan bahan kerja, kadar kehausan mata alat adalah rendah dan kekerasan bahan tidak bergantung. Di samping itu, kelebihan utama menggunakan teknik EDM adalah ketepatan dan integriti permukaan halus yang sesuai untuk aplikasi perubatan. Walau bagaimanapun, untuk memastikan kejayaan EDM sebagai teknik pembuatan alternatif untuk skru ortopedik memerlukan penyiasatan saintifik terutamanya mengenai kesan parameter pemesinan. Kajian ini bertujuan untuk mengkaji kesan parameter pemesinan EDM iaitu arus elektrik,

“pulse on time” dan “pulse off time” di persembahan pemesinan. “Response surface

methodology” digunakan untuk menilai kesan parameter pemesinan pada

persembahan pemesinan iaitu kekasaran permukaan, masa pemesinan dan kadar kehausan mata alat. Akhir sekali, keadaan pemesinan asas optimum pada kriteria prestasi pelbagai objektif akan dicadangkan

iii

DEDICATION

Dedicate to my beloved parents Hassan Bin Salleh and Hasnah Binti Hamid. Dedicated to my beloved brothers,

Dedicated to all my family and my friends.

Thank you for your support and encouragement. You all are everything for me.

iv

ACKNOWLEDGEMENTS

First and foremost, I would like to express my highest appreciation to my supportive academic supervisor, Dr. Raja Izamshah B. Raja Abdullah for his supervision and support in completing this thesis.

Also with the greatest thanks to my beloved parents and family who always pray and give the encouragement while pursuing my research and project. Their sacrifices are never being forgotten.

I would like to acknowledge to (EDM) laboratory technicians, who has been so warmth and kind to provide sincere assistance and good cooperation during the training period. Their co-operation is much appreciated. Last but not least, I would like to convey my appreciation to all the staff of Faculty of Manufacturing Engineering, FKP, my friend and colleagues for their support and their help in the project. Thank you.

v

TABLE OF CONTENT

Abstract i

Abstrak ii

Dedication iii

Acknowlegment iv

Table of Content v

List of Tables viii

List of Figures ix

List of Abbreviations, Symbols & Nomenclature xi

CHAPTER 1: INTRODUCTION 1

1.1 Background 1

1.2 Current Method Produced Screw thread 2

1.2.1 Thread Turning 2

1.2.2 Thread Rolling 3

1.2.3 Thread Whirling 4

1.3 Problem Happen Making Titanium Screw 6

1.5 Objective Of Research 6

CHAPTER 2: LITERATURE REVIEW 7

2.1 Introduction to Electrical Discharge Machining (EDM) 7

2.2 EDM Machining System 7

2.3 Process Parameter 8

2.3.1 Discharge Voltage 8

2.3.2 Peak Current 9

2.3.3 Pulse Duration 9

2.3.4 Pulse Interval 9

2.3.5 Polarity 10

2.3.6 Type of Dielectric Flushing 10

vi

2.4.1 Duty Factor 10

2.4.2 Pulse Frequency 11

2.4.3 Average Current 11

2.5 Thread Specification 12

2.6 Electrode 13

2.6.1 Electrode Selection 13

2.6.2 Electrode Material 14

2.7 Related Work on EDMing Titanium 14

CHAPTER 3: METHODOLOGY 16

3.1 Introduction 16

3.2 Project Overview 16

3.3 Identify the Problem 18

3.4 Literature Review 18

3.5 Research Methodology 19

3.5.1 Control Factor and Level 19

3.5.2 Response Surface Methodology 20

3.6 Preparation of the Experiment 22

3.6.1 Specimen Preparation 22

3.6.2 Conduct the Experiment 23

3.6.3 Testing 25

3.7 Summary of This Chapter 26

CHAPTER 4: RESULT AND DISCUSSION 27

4.1 Statistical Analysis 27

4.1.1 Design Summary 28

4.2 Modelling of Machining Performance 29

4.3 Machining Time Analysis 31

4.4 Suface Roughness Analysis 32

4.5 Electrode Wear Analysis 34

4.6 Multi Objective Optimization 36

4.7 Dimension of Screw 36

vii

CHAPTER 4: CONCLUSION AND RECOMMENDATIONS 40

5.1 Conclusion 40

5.2 Recommendations 41

REFERENCES 42

APPENDICES

A Gantt Chart PSM 1 B Gantt Chart PSM 2

viii

LIST OF TABLES

1.1 Qualitative comparison between screw threading fabrication methods 6

2.1 M10 screw specification 13

2.2 Physical properties of copper 14

2.3 Material properties Ti-6Al-4V 15

3.1 The level of process parameter for copper electrode 20

3.2 The setting of EDM parameters 20

3.3 Table 3.3: Table for experiment data 21

4.1 Data of machining performance 27

ix

LIST OF FIGURES

1.1 Thread turning 3

1.2 Thread rolling 3

1.3 Thread whirling 5

2.1 Basic EDM die sinker component 8

2.2 ISO unified thread profile 12

3.1 Methodology flow chart 17

3.2 Experiment layout preparation 22

3.3a Copper electrode 23

3.3b Titanium workpiece 23

3.4a Workpiece 23

3.4b Electrode 23

3.4c Electrode and workpiece submerged in dielectric fluid 24

3.5 EDM die sinker machine 24

3.6a Portable surface roughness tester (SJ 301) 25

3.6b Stereo digital imaging (EM2 – 13TR) 25

4.1 Design summary 28

4.2 ANOVA response surface linear model for machining time 29 4.3 ANOVA response surface linear model for surface roughness 29 4.4 ANOVA response surface linear model for electrode wear 30

4.5 Current vs machining time 31

4.6 Pulse on vs machining time 31

4.7 Pulse off vs machining time 32

4.8 Current vs surface roughness 32

4.9 Pulse on vs surface roughness 33

4.10 Pulse off vs Surface roughness 32

4.11 Current vs electrode wear 35

x

4.13 Pulse off vs electrode wear 35

4.14 Exp 1 36

4.15 Exp 2 36

4.16 Exp 3 37

4.17 Exp 4 37

4.18 Exp 5 37

4.19 Exp 6 37

4.20 Exp 7 37

4.21 Exp 8 37

4.22 Surface defect 38

xi

LIST OF ABBREVIATIONS, SYMBOLS AND

NOMENCLATURE

CNC - Computer Numerical Control EDM - Electric Discharge Machine MRR - Material Removal Rate EWR - Electrode Wear Rate

SR - Surface Roughness

TWR - Tool Wear Rate

CAD - Computer Aided Design

CAM - Computer Aided Manufacturing

% - Percentages

kHz - Kilohertz

A - Ampere

µs - Micro Second

mm - Milimeter

1

1.1 Background

Orthopedic medical is one of path in medical. Orthopedic is related with bone and joint human body anatomy. Orthopedic pain mostly causes by accident or injury on work, sport and others. Treatment on orthopedic pain taken by doctor is applying the implant device between the fracture bone and orthopedic disease area. Medical device is one of important requirement in orthopedic surgery. The device needs to fulfill the criteria state on medical standard. In the criteria, material selected is important. Recent trends in the medical industry increase the use of titanium alloys because of its outstanding mechanical properties such as lightweight material with greater strength and thougness, and non-corrosive.

The complicated geometrical design and tight tolerance of orthopedic screw such as titanium alloys were fabricated using single-point threading and thread whirling technique. However, there are few problem encounters with these techniques such as rapid tool wear, limited design flexibility, low accuracy and high capital cost. The suitable candidate to overcome this problem is using (EDM) process in which the material is removed by spark erosion. Since there is no physical contact between tool electrode and workpiece, the tool wear rate is low and the material hardness is not dependent. In addition, the main advantages of using EDM technique are the accuracy and fine surface integrity which is suitable for medical application.

INTRODUCTION

2

1.2 Current Method Produced Screw Thread

There are many methods of generating thread including subtractive methods, deformative methods and additive method. However for complicated geometrical design and tight tolerance of orthopedic screw, the thread can only be produced by method thread turning, thread rolling and thread whirling. The method chosen for any one application is chosen based on constraint- time, cost and degree of precision.[1]

1.2.1 Thread Turning

Thread turning is the oldest and the most widely used threading operations. It is a process for producing external or internal threads that usually using a single point tool. The disavantages of this method are the tool wear and costs to fabricate orthopedic screw are high.

3 Figure1.1: Thread Turning

1.2.2 Thread Rolling

Thread rolling is a cold forming process for producing threads on a cylindrical or conical blank. It is similar to internal roll form tapping. The tool or die displaces or extrudes the metal from the part surface to form the thread. Thread rolling machines are designed with two flat die rollers in a side-by-side position. The threads on the blank, which is inserted between the rollers, are usually completed after eight revolutions. However this method is having limitation about highly rate of tool wear.

4 Figure 1.2: Thread rolling

1.2.3 Thread Whirling

Thread whirling is a process used for internal and external threads. Whirling removes material in a manner similar to thread milling. The process uses a special head supporting the threading tool. The whirling head is mounted in machine spindles that rotates eccentrically at high speed around the slowly rotating workpiece, or perform a planetary rotation along a stationary workpiece. The cutter uses insert arranged along the inside or the outside circumference of a ring for external and internal thread, respectively. The most critical issues are about the cost for fabricating orthopedic screw.

5 Figure1.3: Thread whirling

6

1.3Problem Happen Making Titanium Screw

Table 1.1: Qualitative comparison between screw threading fabrication methods.

Turning Rolling Whirling EDM

Tool Wear High High Medium Low

Accuracy Medium Low High High

Time Medium Medium High Low

Cost High Low High Low

Shape Hard Material Medium Low High High

Table 1.1 shows the comparison between screws threading technique which depicted EDM is the most suitable as compared to other technique. Based on this justification, EDM are chosen as an alternative screw fabrication method.

1.4Objective of Research

Both the difficulties and disadvantages for fabricating titanium screw were causing to initiate this project:

1. To determine the capability of EDM Die Sinking on screw threading fabrication using threaded cavity electrode.

2. To investigate EDM machining parameter namely current, pulse on time and pulse off time on machining performances (surface roughness, electrode wear and machining time).

3. To propose an optimal machining condition based on multi-objective performance criteria.

7

2.1 Introduction to Electrical Discharge Machining (EDM)

Electrical Discharge Machining (EDM) 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 one type of advance machining that can be milled, drilling grinding and other machining operation. The commonly operation of this machine are to producing mould and die component. Die-sinking or also known as RAM type EDM machine require the electrode to be machined in the exact opposite shape as the one in the workpiece. 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 for sparking, known as the sparking gap. [2]

2.2 EDM Machining System

Its three major assemblies are the machine tool, power supply and dielectric unit. The assemblies are dependent upon one another to such an extent that the EDM system will not function unless all assemblies are operating properly.

LITERATURE REVIEW

8 The power supply must provide each individual spark to the sparking gap for material removal. It must also monitor the electrical conditions at the sparking gap and direct the machine servo in advancing, retracting or maintaining the position of the electrode, in reference to the workpiece. The dielectric unit must provide the dielectric fluid to the machine for submersing the workpiece. In addition, the dielectric unit must send fluid to the sparking gap for cooling purpose and to remove the EDM chip. The dielectric unit includes a filtration system for cleaning the dielectric fluid. [2]

Figure 2.1 Basic EDM die sinker component

2.3 Process Parameter

The process parameters that affected the properties of EDM parts need to be identified in the first step of the experiment [3]. The parameters are listed as below:

2.3.1 Discharge Voltage

Discharge voltage in EDM is related to the spark gap and breakdown strength of the dielectric. The preset voltage determines the width of the spark gap between the leading edge of the electrode and workpiece. Higher voltage settings increase the gap, which improves the flushing conditions and helps to stabilize the cut. MRR, tool wear rate (TWR) and surface roughness increases, by increasing open circuit voltage.

3

Figure1.1: Thread Turning

1.2.2 Thread Rolling

Thread rolling is a cold forming process for producing threads on a cylindrical or conical blank. It is similar to internal roll form tapping. The tool or die displaces or extrudes the metal from the part surface to form the thread. Thread rolling machines are designed with two flat die rollers in a side-by-side position. The threads on the blank, which is inserted between the rollers, are usually completed after eight revolutions. However this method is having limitation about highly rate of tool wear.

4

Figure 1.2: Thread rolling

1.2.3 Thread Whirling

Thread whirling is a process used for internal and external threads. Whirling removes material in a manner similar to thread milling. The process uses a special head supporting the threading tool. The whirling head is mounted in machine spindles that rotates eccentrically at high speed around the slowly rotating workpiece, or perform a planetary rotation along a stationary workpiece. The cutter uses insert arranged along the inside or the outside circumference of a ring for external and internal thread, respectively. The most critical issues are about the cost for fabricating orthopedic screw.

5

6 1.3Problem Happen Making Titanium Screw

Table 1.1: Qualitative comparison between screw threading fabrication methods.

Turning Rolling Whirling EDM Tool Wear High High Medium Low

Accuracy Medium Low High High

Time Medium Medium High Low

Cost High Low High Low

Shape Hard Material Medium Low High High

Table 1.1 shows the comparison between screws threading technique which depicted EDM is the most suitable as compared to other technique. Based on this justification, EDM are chosen as an alternative screw fabrication method.

1.4Objective of Research

Both the difficulties and disadvantages for fabricating titanium screw were causing to initiate this project:

1. To determine the capability of EDM Die Sinking on screw threading fabrication using threaded cavity electrode.

2. To investigate EDM machining parameter namely current, pulse on time and pulse off time on machining performances (surface roughness, electrode wear and machining time).

3. To propose an optimal machining condition based on multi-objective performance criteria.

7 2.1 Introduction to Electrical Discharge Machining (EDM)

Electrical Discharge Machining (EDM) 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 one type of advance machining that can be milled, drilling grinding and other machining operation. The commonly operation of this machine are to producing mould and die component. Die-sinking or also known as RAM type EDM machine require the electrode to be machined in the exact opposite shape as the one in the workpiece. 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 for sparking, known as the sparking gap. [2]

2.2 EDM Machining System

Its three major assemblies are the machine tool, power supply and dielectric unit. The assemblies are dependent upon one another to such an extent that the EDM system will not function unless all assemblies are operating properly.

LITERATURE REVIEW

8 The power supply must provide each individual spark to the sparking gap for material removal. It must also monitor the electrical conditions at the sparking gap and direct the machine servo in advancing, retracting or maintaining the position of the electrode, in reference to the workpiece. The dielectric unit must provide the dielectric fluid to the machine for submersing the workpiece. In addition, the dielectric unit must send fluid to the sparking gap for cooling purpose and to remove the EDM chip. The dielectric unit includes a filtration system for cleaning the dielectric fluid. [2]

Figure 2.1 Basic EDM die sinker component

2.3 Process Parameter

The process parameters that affected the properties of EDM parts need to be identified in the first step of the experiment [3]. The parameters are listed as below:

2.3.1 Discharge Voltage

Discharge voltage in EDM is related to the spark gap and breakdown strength of the dielectric. The preset voltage determines the width of the spark gap between the leading edge of the electrode and workpiece. Higher voltage settings increase the gap, which improves the flushing conditions and helps to stabilize the cut. MRR, tool wear rate (TWR) and surface roughness increases, by increasing open circuit voltage.