Effectiveness of minimum quantity lubrication in hard milling of AISI h13

Quang-Cherng Hsu Institute of Mechanical Engineering National Kaohsiung University of Applied Sciences ABSTRACT As a successful alternative to flood coolant processing and dry cutting,

國 立 高 雄 應 用 科 技 大 學

機械工程系

博士論文

AISI H13 硬銑削最少量潤滑有效性之研究 Effectiveness of Minimum Quantity Lubrication in Hard Milling of

AISI H13

研究生: 杜 勢 榮 指導教授: 許光城 教授

國立高雄應用科技大學

機械工程系 博士論文

A Dissertation Submitted to Institute of Mechanical Engineering

National Kaohsiung University of Applied Sciences

In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy

In Mechanical Engineering

June 2017 Kaohsiung, Taiwan, Republic of China

中華民國 106 年 6 月

中文摘要

最小量潤滑(MQL)可有效取代濕切及乾切製程，其應用於硬銑削可改善表面光度、降低刀具磨耗、增加刀具壽命及降低切削溫度等優點。

本研究分為兩部分：第一部分以降低表面粗度值為品質目標利用田口方法找出 AISI H13 於硬銑削下之最佳切削參數。本研究以槽銑加工進行

分析最小量潤滑參數(切消液種類、壓力及流量)對表面光度的影響。結果

之水溶性切削液，其流量與壓力貢獻度經變異數分析後依序為 68.13%及 30.19%。

在第二部分之研究主要基於表面粗糙度及切削力來驗證最小量潤滑之

進行實驗規劃，運用反應曲面法及變異數分析來分析切削參數對切削力及表面粗糙度的影響。結果顯示在乾切與最小潤滑的條件下進給率及切深皆對表面粗糙度影響最大。切削力分量主要受切削深度影響其次為進給速率。當切削條件為高切速、低進給與低切深且低硬度之材料即可獲得較良好的表面粗糙度和最小的切削力。而最小量潤滑切削可提供較好的表面粗糙度及降低刀具磨耗。以統計模型建立出預測模型用以預測乾切與最小量潤滑條件下之切削力和表面粗糙度，其結果顯示最小量潤滑相較於乾切條件下更具有顯著的效果。

關鍵字：最小量潤滑、優化、切削力、表面粗糙度、刀具磨耗、硬銑削、

田口方法、反應曲面法

Effectiveness of Minimum Quantity Lubrication in Hard Milling of

AISI H13

Student：Do The Vinh Advisors: Prof Quang-Cherng Hsu

Institute of Mechanical Engineering National Kaohsiung University of Applied Sciences

ABSTRACT

As a successful alternative to flood coolant processing and dry cutting, the minimum quantity lubricant (MQL) has already been applied to hard milling for improvement of surface finish, reduction of tool wear, an increase of tool life, reduction of cutting temperature, etc

This research was divided into two parts In the first part, Taguchi method was used to find the optimal values of MQL condition in the hard milling of AISI H13 with consideration of improved surface roughness Slot milling was selected for the investigation as an operation that is commonly applied for machining of the closed slots or pockets and grooves, etc Taguchi’s L9 array was used to design the experiments The signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were utilized to analyze the influence of the performance characteristics of MQL parameters (i.e., cutting fluid type, pressure, and fluid flow) on surface finish In the results section, the water-soluble oil lubricant, the

50 ml/h fluid flow and the 3 kg/cm2 pressures provided the best results for surface roughness in hard-milling of AISI H13 Lubricant and pressure of MQL condition are determined to be the most influential factors giving a statistically significant effect on machined surfaces The pressure factor contributed 68.13 % and the lubricant factor contributed 30.19 % of the total effect The effect of them

carried statistical significance The three parameters of MQL conditions explained 99.76 % of the variability in surface roughness

In the second part, the research objective is to demonstrate the efficiency of MQL based on certain process parameters such as surface roughness and cutting force A comparative analysis was done to prove the effectiveness of MQL versus dry cutting The characteristics of the cutting force and the surface roughness obtained under dry cutting and MQL condition were experimentally investigated The experiments were conducted using the L27 orthogonal array of Taguchi’s experimental design technique The response surface methodology (RSM) and analysis of variance (ANOVA) were employed for analysis the influence of cutting parameters (i.e., cutting speed, feed rate, depth-of-cut and hardness of work-piece) on the cutting force and the surface roughness As the result, under both cutting conditions (MQL and dry), feed rate and depth of cut are the most influential variables regarding surface roughness The cutting force components get affected mostly by depth of cut followed by feed rate Higher cutting speed, lower feed rate, lower depth of cut and lower work-piece hardness applied lead to good surface roughness and minimum cutting force MQL cutting provided better surface roughness and reduced tool wear The difference of values of cutting force components under two cutting conditions (MQL and dry)

is negligible in short machining time The statistical models to predict cutting force and surface roughness under dry cutting and MQL condition were established The results of the research showed the outstanding effectiveness of MQL compared to dry cutting

Keywords: Minimum quantity lubricant, optimization, cutting force, surface

roughness, tool wear, hard milling, Taguchi method, response surface methodology

ACKNOWLEDGMENTS

The fulfillment of over four years of study at National Kaohsiung University

of Applied Sciences (KUAS) has brought me into closer relations with many enthusiastic people who wholeheartedly devoted their time, energy, and support

to help me during my studies Therefore, this is my opportunity to acknowledge

my great debt of thanks to them

I wish to express my thanks and gratitude to my academic supervisor, Prof

Dr Quang-Cherng Hsu, for his continuous guidance, valuable advice, and helpful supports during my studies He has always been supportive of my research work and gave me the freedom to fully explore the different research areas related with MQL hard milling

I would also like to thank Prof Yung-Chou Kao, my first supervisor, for his help and advice during my first study time at KUAS

I wish to acknowledge my deepest thanks to President of KUAS and Office

of International Affairs for giving me a great opportunity, necessary scholarships

to study at KUAS and many enthusiastic helps during my time in KUAS I am also particularly grateful to Thai Nguyen University provided me unflagging encouragement, continuous helps and support to complete this course

My gratitude also goes to all of the teachers, Dean and staffs of Department

of Mechanical Engineering for their devoted teaching, great helping and thoughtful serving during my study in ME

I would also like to express my sincere gratitude to all of my colleagues at the Precision and Nano Engineering Laboratory, Department of Mechanical Engineering, KUAS I would specially like to thank Mr Ye Jhan Hong, Mr Li Wen Hsiung and Mr Wei Lin for their great helps in my experimental process

I want to express my sincere thanks to all my Vietnamese friends in KUAS for their helpful sharing and precious helping me over the past time

I also wish to express my gratitude to all those who directly or indirectly helped me during my study in KUAS

Finally, my special thanks to my dad Đỗ Văn Kiểu and my mom Nguyễn Thị

Hà, to my brother Đỗ Minh Khoa, to my adorable wife Nguyễn Thị Nguyên, to lovely little daughter Đỗ Khánh Linh, who is the most motivation for me over years in Taiwan

CONTENTS

中文摘要 iv

ABSTRACT v

ACKNOWLEDGMENTS vii

CONTENTS ix

LIST OF FIGURES xiii

LIST OF TABLES xvi

NOMENCLATURE xvii

Chapter 1 INTRODUCTION 1

1.1 Motivation of the research 1

1.2 Objective of the research 4

1.3 Scopes of the research 5

1.4 Organization of the Dissertation 6

Chapter 2 BACKGROUND 8

2.1 Hard machining 8

2.1.1 Overview 8

2.1.1.1 Concepts of hard machining 8

2.1.1.2 Advantages and disadvantages 8

2.1.2 Basic operations in hard machining 9

2.1.2.1 Hard turning 9

2.1.2.2 Hard milling 10

2.1.2.3 Other operations 11

2.1.3 The characterization of hard machining 13

2.1.3.1 Cutting temperature 13

2.1.3.2 Surface roughness 14

2.1.3.3 Cutting force 15

2.1.3.4 Tool wear 17

2.2 Cooling and lubrication in metal cutting 19

2.2.1 Functions of cutting fluid 19

2.2.1.1 Cooling 20

2.2.1.2 Lubrication 22

2.2.2 Types of cutting fluid 22

2.2.2.1 Neat cutting oil 23

2.2.2.2 Soluble oil 24

2.2.2.3 Semisynthetic 25

2.2.2.4 Synthetic 26

2.2.3 Cooling/lubrication methods 26

2.2.3.1 Wet machining method 26

2.2.3.2 Dry machining method 28

2.2.3.3 Minimum quantity lubrication method 28

2.3 Minimum quantity lubrication 29

2.3.1 Introduction 29

2.3.2 Principles of MQL system 30

2.3.3 The MQL systems 32

2.3.4 The lubricant feeding forms in MQL 33

2.3.4.1 Internal feeding form 33

2.3.4.2 External feeding form 34

2.3.5 Cutting fluids and parameters for MQL 34

2.3.5.1 Fluid types for MQL 34

2.3.5.2 Wetting of fluids 35

2.3.5.3 Viscosity of fluids 35

2.3.5.4 Position of external nozzle 35

2.3.5.5 Fluid flow and air pressure 37

2.3.6 Benefits and limitations of MQL 37

2.3.6.1 Benefits 37

2.3.6.2 Limitations 38

2.4 Literature review 38

Chapter 3 RESEARCH METHODS 46

3.1 Taguchi method 46

3.2 Response surface methodology 50

3.3 Analysis of variance 52

Chapter 4 OPTIMIZATION OF MQL PARAMETERS 53

4.1 Design of experiment 53

4.2 Experimental procedure 56

4.3 Results and Discussions 59

4.4 Summary 62

Chapter 5 EFFECTIVENESS OF MQL IN HARD MILLING 63

5.1 Design of experiment 63

5.2 Experimental procedure 64

5.3 Results and Discussions 66

5.3.1 The analysis of variance and the mathematical model of surface roughness and cutting force under MQL conditions 68

5.3.2 The analysis of variance and the mathematical model of surface

roughness and cutting force under dry conditions 74

5.3.3 The comparative analysis 81

5.4 User interface of calculation of response characteristics in hard milling of AISI H13 99

5.5 Summary 100

Chapter 6 CONCLUSION AND FUTURE WORKS 102

6.1 Conclusion 102

6.1.1 Optimization of MQL parameters 102

6.1.2 Comparison between MQL and dry condition 103

6.2 Future works 104

LIST OF PUBLICATIONS 106

SCIE papers 106

EI papers 106

International conferences 106

APPENDICES 108

I Technical Drawing 108

II Settings for machining program 108

III NC program 109

IV Code for the calculation of response characteristics 111

REFERENCE 119

LIST OF FIGURES

Figure 1- 1 Research procedure 6

Figure 2- 1 Hard turning process [33] 9

Figure 2- 2 Hard milling process [35] 10

Figure 2- 3 Hard boring operation [39] 11

Figure 2- 4 Hard hobbing operation [41] 12

Figure 2- 5 Dissipation of heat through chips during cutting process [43] 13

Figure 2-6 Influences of cutting speed and work material on cutting temperature[44] 14

Figure 2- 7 Influence of cutting parameters on surface roughness[43] 15

Figure 2- 8 The relationship between cutting force and hardness[4] 16

Figure 2- 9 Relationship of cutting speed and cutting force components[50] 16

Figure 2- 10 The influence of feed-rate and depth-of-cut on cutting force[43] 17

Figure 2- 11 Two basic wear types (a) crater wear, (b) flank wear [53] 18

Figure 2- 12 Wear of end milling tool 19

Figure 2- 13 The distribution of the heat sources in cutting [57] 20

Figure 2- 14 Built-up edge [58] 22

Figure 2- 15 Classification of cutting fluids in metal cutting [58] 23

Figure 2- 16 Machining with wet cutting method [67] 27

Figure 2- 17 Benefits of dry cutting [15] 28

Figure 2- 18 Metal-working fluid costs in metal machining [12] 30

Figure 2- 19 The ideal concept of MQL[72] 31

Figure 2- 20 Model of a simple MQL atomizer [70] 31

Figure 2- 21 External and internal MQL system 32

Figure 2- 22 External and internal lubrication feeding forms [73] 33

Figure 2- 23 The special drill tool with internal feed[71] 34

Figure 2- 24 Levels of wetting of fluids[71] 35

Figure 2- 25 Optimal position of external MQL nozzle for end milling [71] 36

Figure 2- 26 The dead zone in feeding by external MQL nozzle [71] 36

Figure 2- 27 The optimal angle of MQL nozzle [71] 37

Figure 3- 1 The general Design Of Experiments process [87] 47

Figure 4- 1 Peanut oil 55

Figure 4- 2 Test - tube 55

Figure 4- 3 TiAlN coated-carbide end mill tool 55

Figure 4- 4 Experimental set-up 57

Figure 4- 5 Measurement of surface roughness by SJ-400 Mitutoyo surf-test instrument 58

Figure 4- 6 Noga MC 1700 cooling system 58

Figure 4- 7 Statistical analysis by Minitab software 59

Figure 4- 8 Effect of MQL parameters on surface roughness 60

Figure 5- 1 Experimental setup 65

Figure 5- 2 Using Minitab for statistical analysis 66

Figure 5- 3 Optimization plot of surface roughness: a) Optimization for Ra-mql; and, b) Optimization for Ra-dry 82

Figure 5- 4 Plot of response surface for Ra-mql (other factors are held at middle value) 83

Figure 5- 5 Plot of response surface for Ra-dry (other factors are held at middle value) 84

Figure 5- 6 Optimization plot of cutting-force components: a) Optimization for cutting force components of MQL conditions; and, b) Optimization for cutting force components of dry conditions 86

Figure 5- 7 Plot of response surface for Fx-mql (other factors are held at middle value) 87

Figure 5- 8 Plot of response surface for Fy-mql (other factors are held at middle value) 88

Figure 5- 9 Plot of response surface for Fz-mql (other factors are held at middle value) 89

Figure 5- 10 Plot of response surface for Fx-dry (other factors are held at middle

value) 90

Figure 5- 11 Plot of response surface for Fy-dry (other factors are held at middle value) 91

Figure 5- 12 Plot of response surface for Fz-dry (other factors are held at middle value) 92

Figure 5- 13 Comparison of Ra under dry- and MQL-cutting conditions 93

Figure 5- 14 Comparison of cutting-force components under dry and MQL cutting conditions 95

Figure 5- 15 Comparison of experimental and predicted values of cutting force 96 Figure 5- 16 Comparison of dry and MQL condition in longer machining time (cutting speed of 55m/min, feed rate of 0.02 mm/teeth, depth-of-cut of 0.6mm, and hardness of 50HRC) 98

Figure 5- 17 User interface of calculation of response characteristics 99

Figure 5- 18 Resulting file of the program 99

Figure I Technical drawing for experimental process 108

Figure II Settings for tool parameters 108

Figure III Settings for contour parameters 109

LIST OF TABLES

Table 2 1 Comparison of different cutting fluids[56] 26

Table 2 2 Some of the remarkable involved studies 42

Table 3 1 L9 orthogonal array 48

Table 3 2 L27 orthogonal array 50

Table 4 1 Chemical compositions of the AISI H13 steel (weight %) 53

Table 4 2 Material properties of AISI H13 steel 53

Table 4 3 Parameters and levels 56

Table 4 4 Milling process information 56

Table 4 5 Technical information of milling tool 56

Table 4 6 The surface roughness result and S/N ratio[93] 59

Table 4 7 Mean of S/N response for surface roughness 60

Table 4 8 Analysis of variance for surface roughness (Ra) 61

Table 5 1 Cutting parameters with levels 63

Table 5 2 Hard-milling process information 65

Table 5 3 Information about the MQL process 65

Table 5 4 Experimental results for surface roughness and cutting force components 67

Table 5 5 Analysis of Variance for Ra-mql 69

Table 5 6 Analysis of Variance for Fx-mql 70

Table 5 7 Analysis of Variance for Fy-mql 71

Table 5 8 Analysis of Variance for Fz-mql 72

Table 5 9 Analysis of Variance for Ra-dry 75

Table 5 10 Analysis of Variance for Fx-dry 76

Table 5 11 Analysis of Variance for Fy-dry 77

Table 5 12 Analysis of Variance for Fz-dry 78

NOMENCLATURE

ANOVA Analysis of Variance

Adj SS Adjusted sums of squares

Adj MS Adjusted mean of squares

CAD/CAM Computer Aided Design/Computer Aided Manufacturing

DOE Design of Experiments

EDM Electrical Discharge Machining

PC Percentage contribution

R-Sq Coefficient of determination

Ra Arithmetic average roughness (μm)

Rz Mean roughness depth (μm)

Rq Root mean square roughness (μm)

RSM Response surface methodology

VB1 Uniform flank wear (μm)

VB2 Non-uniform flank wear (μm)

VB3 Localized flank wear (μm)

Chapter 1 INTRODUCTION

1.1 Motivation of the research

Containing the valuable particularity such as the great resistance to thermal softening, good toughness, high hardenability and high resistance to abrasion, AISI H13 is used widely in manufacturing, especially in high pressure die casting and extrusion molding, cutting blades, and forging [1] According to many manufacturers, this steel is the most popular grade for various industries The hardness of AISI H13 recommended is at 40-50 HRC with its application in mold and die manufacture A traditional manufacture process of AISI H13 often includes rough machining, heat treatment, and then finish machining such as grinding operation This process has many limitations in which costly and time consuming are typical

In order to improve production and quality of product in mold and die manufacture, the traditional machining processes were gradually replaced by a new machining process having many advantages The new machining process is hard machining that is a term used for machining process of steels with 40-60 HRC hardness with many improvements [2, 3] The quality of finished products

in hard machining is significantly improved [4] The surface roughness obtained

by hard machining is equivalent to grinding process when suitable cutting parameters are applied [3] Further, reducing manufacturing cost, having high material removal rates and decreasing machining time are features of hard milling Flexible process design is also an advantage of hard machining [5-8]

In actual hard machining, dry cutting condition is often applied In their research, Sreejith, et al [9] remarked that dry cutting is the machining method of the future The appearance of CAD/CAM (Computer-aided design/Computer-aided manufacturing) systems, the significant advances in manufacturing, coating

techniques for cutting tools, and the industrial development of cutting machines have all changed the metal-cutting process entirely The application of fluid cutting (or wet cutting) in traditional metal machining has decreased as of late due to the environmental and human health concerns it causes In fluid cutting, a lubricant or cutting fluid is applied in order to improve the tribological processes that occur between the surfaces of the cutting tool and the work-piece

The immediate advantages of using a cutting fluid during the machining process are an improvement in tool life, a reduction of tool wear, and a lowered cutting temperature [10-13] However, the fluid cutting method has many negative effects, especially with the significant adverse environmental effects it causes and possible damage to health of operators [9-14] Any reduction, or even elimination, of the use of cutting fluids involved during machining process would

be seen as a major incentive to switch to a non-cutting-fluid method Therefore, dry cutting is presented as both an efficient and a desirable alternative to fluid cutting The advantages of dry cutting include non-polluting of the environment,

a reduction in manufacturing cost through saving in coolant-related cost and lowered cleaning costs, and no further danger to the health of operators due to prohibitive contact with toxic cutting fluid substances [9, 10, 12, 15] In cases of interrupted cutting, dry cutting improves tool life due to an avoidance of thermal shock to the tool [9] In some of the cases, the required cutting forces using dry cutting is lower than what occurs under wet cutting due to the effect of increased cutting temperature and a thermal softening of the materials [16] Nevertheless, hard machining under dry conditions also has disadvantages such as increased tool wear and reduced tool life because of the influence of contact with ultra-hard materials [6, 17] The application of flood coolant in hard-milling is not recommended due to the reasons mentioned above

Many studies have addressed the question "Why perform MQL cutting?" According to Diniz, et al [11] MQL is an acronym used to describe a procedure

in which a very small volume of lubricant (<50 ml/h) is pulverized in a flow of

air directed at the cutting zone during milling MQL has been widely applied in the machining processes (i.e., milling, turning and drilling) due to efficiency and environmental issues The effectiveness of MQL has already been demonstrated with the improvement of surface roughness [15, 18-21], reduction of tool wear, enhancement of tool life, a decrease in cutting temperature, and a reduction in lubricant-related costs[15, 18, 20-25] Many studies proved for the benefits of using MQL in machining in comparison to dry cutting and wet cutting methods

In the research of Dhar, et al [18], the effect of MQL on tool wear and surface roughness in turning AISI-4340 steel was significant There was a noticeable reduction in tool wear and surface roughness by MQL due to a reduction of temperature in the cutting zone and a favorable change in the chip–tool and workpiece–tool interaction In comparison to wet and dry cutting [20], MQL effects using vegetable oil-based cutting fluid were presented The significant contributions of MQL in turning AISI 9310 alloy steel were a reduction of cutting temperature, a decrease of tool wear, and an improvement of surface roughness Similarly, in research of Dhar, et al [22], reduction of cutting temperature was presented in turning AISI-1040 steel by employing MQL In the milling process, the effectiveness of MQL was also demonstrated in many other studies The tool life improved by an application of MQL was expressed in high-speed end milling of AISI D2 cold-worked die steel with 62 HRC in a study by Kang, et al.[23], and in the research by Iqbal, et al [24] In a study by Inconel

718 steel milling [26], Thamizhmanii, et al concluded that surface roughness obtained by using MQL is lower than that obtained by dry cutting The tool life was improved by 43.75 % by MQL rather than by dry cutting Rahman, et al [27] concluded that the surface roughness obtained by MQL is equivalent to what was obtained through wet cooling means The difference in cutting force between that of flood cooling and MQL was considered to be insignificant

Many researchers have taken H13 steel being the object of study Ding, et al [2] performed an experimental investigation into the establishment of two

prediction models used for determining cutting force and surface roughness in the hard milling of AISI H13 steel with carbide-coated tools The effect of cutting parameters on cutting forces and surface roughness was analyzed by using Taguchi method and ANOVA The results of research expressed that depth-of-cut is the main factor affecting surface roughness and cutting force In a study by Ozel, et al [28], an investigation into the influences of cutting edge geometry, cutting-speed, feed-rate, and workpiece-hardness on surface roughness and cutting force in the finished hard-turning of AISI H13 steel was carried out The conclusions showed that honed-edge geometry and lower work-piece surface hardness lead to good surface roughness The lower work-piece surface-hardness and honed-edge geometry lead to lower tangential and radial forces Ghani, et al [29] used Taguchi method to investigate the effects of cutting parameters on surface roughness and cutting force in the milling process of H13 steel The result showed that higher cutting-speed, lower feed-rate and lower depth-of-cut lead to a better surface roughness and lower cutting force Similarly, in hard-milling of AISI H13 alloy steel (JIS SKD61), Nguyen, and Hsu, [30] concluded that high cutting-speed, low feed-rate, lower depth-of-cut and lower hardness resulted in good surface roughness

However, the application of MQL in hard-milling of AISI H13 steel has not been adequately studied to date Consequently, the author continued to respond

to the question of “Why use MQL cutting?” In this research, an attempt has been made to demonstrate the efficiency of MQL compared with dry cutting in the hard-milling of AISI H13 steel based on the combination of the Taguchi method and RSM

1.2 Objective of the research

The objective of the research is the suitability of MQL in hard milling of

AISI H13 In order to achieve the objective of the topic, there are five main

research works undertaken as follows:

1 Optimization of MQL parameters in hard milling of H13 steel

2 Study on the influence of cutting parameters on surface roughness, cutting force under dry and MQL condition

3 Optimization of cutting parameters in hard milling under MQL and dry cutting

4 Establishing the second-order models for prediction of surface roughness and cutting force under MQL and dry cutting

5 Performance a comparative analysis to prove the effectiveness of MQL versus dry cutting

1.3 Scopes of the research

In this research, the hard milling process of AISI H13 was conducted under two cutting conditions such as dry and MQL Wet cutting (or fluid cutting) was not investigated due to its environmental matter as mentioned in the first section The milling operation using end milling tool consists of side milling, face milling, slotting milling, plunge milling and ramping The slotting milling is an operation being commonly applied for machining of the closed slots or pockets and grooves, etc Thus, the author only concentrated on the slotting milling in the research

In hard milling, there are many independent input factors that have significant influences on quality features, such as cutting parameters, work-piece material, cutting tool, and system parameters The research only considers the effect of cutting parameters and cooling/lubrication condition The other input factors were fixed which is seen as a case study of the research This consideration is due to the economy and efficiency which provides on the basis

of available equipment conditions Furthermore, independent output factor of hard milling studied in the research is surface roughness, cutting force components and tool wear These are important factors reflecting quality features

of hard milling process

1.4 Organization of the Dissertation

The dissertation was divided into six chapters The organization of the dissertation can be expressed as Figure 1-1

Chapter 1 presents the motivation, objective, scopes of the research and organization of the dissertation

Chapter 2 shows a brief background of the research In this chapter, an overview of hard machining, cooling and lubrication in metal cutting, and overview of minimum quantity lubrication were described

Figure 1- 1 Research procedure

Chapter 3 presents research methods that were used in this study such as Taguchi and response surface methodology

Chapter 4 gives the optimization of MQL parameters The Taguchi method was utilized to design the experiment and find the optimal condition in MQL

ANOVA is also used to perform an analysis of the influence of MQL parameters

on surface roughness

Chapter 5 describes the outstanding effectiveness of MQL when compared with dry cutting In this chapter, a series of meticulous experiments related to the hard-milling of AISI H13 steel were conducted under two different conditions given as dry machining and MQL Its result shows the influence of cutting parameters on surface roughness and cutting force under both dry and MQL- cutting conditions Simultaneously, the result expresses the effectiveness of MQL when compared with dry cutting

The last section is chapter 6 This chapter show conclusion of the research and recommendation for the future work

Chapter 2 BACKGROUND

This chapter shows a brief background of the research In this chapter, an overview of hard machining, cooling and lubrication in metal cutting, and overview of minimum quantity lubrication were described

2.1 Hard machining

2.1.1 Overview

2.1.1.1 Concepts of hard machining

The hard machining is a term used for machining process of steels with

40-60 HRC hardness [2, 3] It is a good solution that can replace the traditional machining process in the mold and die manufacturing industry There are many kinds of work-piece material in hard machining such as hardened alloy-steels, tool-steels, case-hardened steels, super-alloys, nitrite irons, etc [4] Generally, hard machining is applied as a finishing or semi-finishing process that requires high dimensional, form, and surface finish accuracy [4, 31]

2.1.1.2 Advantages and disadvantages

Hard machining has many advantages when compared with traditional machining The benefits are listed as following[4, 32]:

- Flexibility in design and high adaptability to complex machined contours

- Reducing the product cycle time, increasing productivity, and improving the quality of finished products

- Having high metal removal rates

- Environmental friendliness when used with dry or MQL condition

- Improving the fatigue life of the machined parts and reducing the residual stress in the moldable parts

- Low cost for machine tool compared to traditional machining process

However, the hard machining also contains limitations and drawbacks as following [4, 32]:

- The high cost of tooling per unit when compared to grinding process

- High rigidity of machine tool is required due to the accuracy of machined part depend on the rigidity of machining systems

- Increasing tool wear, and reducing tool life because of the influence of contact with ultra-hard materials

2.1.2 Basic operations in hard machining

Hard machining can be applied in many machining operations such as hard turning, hard milling, hard boring/reaming, hard broaching and hard gear-manufacturing-process The brief overview of these most common operations in hard machining is introduced in this section

2.1.2.1 Hard turning

Hard turning is an operation defined as the turning of a part or bar-stock with more than 45HRC in hardness on a lathe or turning center Figure 2-1 shows a hard turning operation

Figure 2- 1 Hard turning process [33]

Since high quality of finish machined surface can be obtained, hard turning process is a replacement considered for grinding operations Compared to grinding, the hard turning has benefits such as high complex machining contours, the high performance, low cost, and short setup times [8, 34] The successful key

in hard turning is high system rigidity [4] The hard turning operation can be applied for machining of many materials containing high-speed steels (HSS), die-steels, alloy-steels, bearing-steels, case hardened steels, etc

2.1.2.2 Hard milling

Hard milling is typically defined as the milling of parts having a range of hardness from 45 HRC to 64 HRC The hard milling process is an operation to improve the delivery, accuracy, surface finish, and overall quality The fundamental hard milling operation can be illustrated in Figure 2-2

Figure 2- 2 Hard milling process [35]

The application of hard milling have been concentrated mainly in the mold and die manufacturing industry in which materials such as P20, H13, W5, S7, and some others are used commonly Advantages of hard milling are reducing or even eliminating EDM, reducing or even eliminating secondary operations, improving surface roughness, having short production time and low cost, improving accuracy when compared to the traditional process Successful key in hard milling is the result of performance a complete system that includes the machine, cutting tools and tool-holders, and CAD/CAM system [36]

The cutting tool has an important role in hard milling A great amount of stress existing on the tool is generated by elevated heat and high abrasive wear Therefore, coating need be applied to cutting tool as a protective layer Coating characteristics are hardness and maintaining hardness, wear resistance, surface lubricity, anti-seizure With coatings, the tool life is substantially increased The most common coatings are titanium-nitride (TiN), titanium-carbon-nitride (TiCN), titanium-aluminum-nitride (TiAlN) and titanium-aluminum-carbon-nitride (TiAlCN) are the most popular coatings applying to the cutting tool in hard milling [4, 8, 32, 36] Each coating has its own advantages Titanium-nitride (TiN) has benefits such as high hardness, excellent adhesion, high ductility, excellent lubricity, high chemical stability and good wear resistance [37] Titanium-carbon-nitride (TiCN) have significant benefits such as more hardness and better surface lubricity compared to TiN due to the addition of carbon In comparison with TiCN or TiN, the life of cutting tool with coating titanium-aluminum-nitride (TiAlN) is better due to a layer of aluminum oxide is formed at high temperature On the other hand, the TiAlN coating also has higher hardness

in high temperature (above 7500C) compared to TiCN or TiN coatings [38]

2.1.2.3 Other operations

Figure 2- 3 Hard boring operation [39]

- Hard boring and hard reaming: They are utilized to enlarge an existing

hole The hard boring operation is shown in Figure 2-3 Hard boring and reaming

can be used to replace grinding when proper cutting parameters are applied In actual production, multi-insert tools are utilized to obtain high productivity [4]

- Hard broaching: Hard broaching applied as finishing process is a

machining process of hardened parts (above 60 HRC) with any inner profile[40]

In mass production of products having complex profiles, broaching operation will be considered due to its high economics and high production rates In hard broaching, carbides, PCBN and diamond used for tool material to raise metal cutting effective

- Hard gear manufacturing process: Hard gear manufacturing process

containing hard hobbing, shaving, and broaching are required as a substitute for gear finishing operations These operations can be implemented before and after heat treatment Advantages of hard gear manufacturing operations compared to the traditional process are low cost, high productivity, and centralization of equipment [4] Figure 2-4 shows a hard hobbing operation

Figure 2- 4 Hard hobbing operation [41]

2.1.3 The characterization of hard machining

2.1.3.1 Cutting temperature

Heat generated in machining process is a factor which affects wear mechanisms, formation of white layer and distribution of residual stress Thus, consideration for cutting temperature is important in cutting operation In hard machining, the cutting temperature depends not only on cutting conditions but also on hardness of the work-piece Most of the heat generated during cutting process is dissipated by the chips as reported in research of O’Sullivan and Cottrell [42] and research of Yallese, M.A [43] Heat dissipation by chip can be observed in Figure 2-5

Figure 2- 5 Dissipation of heat through chips during cutting process [43]

The relationship between the tool temperature and the hardness of piece is close as shown in Figure 2-6 In hard machining, an increase of hardness

work-of work-piece leads to an increase work-of the cutting temperature It is explained that increase of hardness causes an increase of cutting force Thus, the temperature is elevated by the higher cutting energy On the other hand, higher cutting speed results in an increase of cutting temperature [7, 44, 45] Feed-rate and depth-of-cut have insignificant affect cutting temperature when compared to the influence

of cutting speed [45, 46]

Figure 2- 6 Influences of cutting speed and work material on cutting

temperature[44]

2.1.3.2 Surface roughness

Surface roughness (briefly called as roughness) is an important feature to evaluate surface integrity of machined parts It is used as a principal index in most of technical requirement of mechanical products Specifically, roughness affects fatigue strength, friction coefficient, wear-rate, and corrosion resistance of machined parts In hard machining, formation of built-up-edge (BUE) is hard due

to its low ductility and high temperature[47] Thus, the negative effect of BUE on roughness is mitigated Surface roughness depends on many factors including cutting parameters, parameters of cutting tool and parameters of work-piece The cutting parameters include feed-rate, depth-of-cut and cutting speed The parameters of cutting tool consist of material of tool, nose radius, rake angle, the geometry of cutting edge, etc The work-piece parameters include material of work-piece, hardness, etc In many types of research, the feed rate and tool nose radius recognized are principal parameters affecting surface roughness [4]

In research of Yallese, M.A., et al [43], hard machining of bearing steel was carried out by use of cubic boron nitride tool The results indicate that the roughness is improved when cutting speed is up to 120m/min Moreover, increase of depth of cut and feed rate result in an increase of roughness The effect of feed-rate on roughness is superior to that of depth-of-cut and cutting speed The result is shown in Figure 2-7

Figure 2- 7 Influence of cutting parameters on surface roughness[43]

The similar results are also recorded in study work of Lima, J.G., et al [48]

In their study, hard turning of AISI 4340 steel and AISI D2 steel was performed The results indicate that roughness is improved when elevated cutting speed and deteriorated feed rate are used The effect of depth of cut on roughness is insignificant

When performing hard machining of 100Cr6 bearing steel using ceramic and CBN tools, Benga, G C concluded that the feed rate has the most influence on surface finish while the influence of cutting speed on surface finish is very little [49]

Performing hard milling process of AISI H13 steel, Ding, T presented an inclusion that surface roughness (Ra) obtained can be less than 0.25 μm It proves that hard milling is a substitute for the grinding as a semi-finish process[2]

2.1.3.3 Cutting force

Cutting force is a principal criticism in metal cutting process because of its strong interaction with performance of cutting operation such as surface accuracy, cutting temperature, tool wear, tool breakage, and vibrations, etc.[2] Hard machining is carried out under unique conditions about technology and thermo mechanics Therefore, the mechanisms of the cutting process including chip formation, heat generation, and tool wear have different behaviors It is

proposed that value of cutting force in hard machining and its relationships have the substantial difference to those obtained in “soft” machining The relationship between cutting forces and hardness is observed that cutting force increase distinctly when hardness increase more than 45 HRC as shown in Figure 2-8 [4,

7, 44]

Figure 2- 8 The relationship between cutting force and hardness[4]

The relationship between cutting velocity and cutting force is shown in Figure 2-9 At cutting velocity below 50 m/min, an increase of cutting velocity results in an increase in cutting force components including both cutting force and thrust force However, beyond about 50m/min, increase of cutting velocity results in a decrease in cutting force components [50] It is explained that the decrease in cutting force components due to the softening of work-piece material

by high temperature generated and decrease of tool-chip contact[43, 51]

Figure 2- 9 Relationship of cutting speed and cutting force components[50]

Figure 2-10 shows the influence of feed-rate and depth-of-cut on cutting force The rise of the amount of feed-rate and depth-of-cut produce an increase in cutting force components The reason is due to an increase in chip load; thus, there is a bigger amount of energy required to generate a chip in the shear zone

In this way, materials are removed much more difficult during the cutting process

Figure 2- 10 The influence of feed-rate and depth-of-cut on cutting force[43]

2.1.3.4 Tool wear

In order to understand the advantages and disadvantages of each material of cutting tool, knowledge of the different wear mechanisms of each material is very important Tool wear appearing on the contact area of the cutting tool with the work-piece and the chips is occurred by the chip removal process [4, 52] It is the inevitable origination causing tool failure during the cutting operation The scale of tool wear influences strongly on surface finish and dimensional accuracy obtained Tool wear is classified into two basic wear types as rake face wear (or crater wear) and flank wear as depicted in Figure 2-11 [4]

Figure 2- 11 Two basic wear types (a) crater wear, (b) flank wear [53]

Rake face wear (crater wear) occurs at a short distance from cutting edge on

the cutting tool face by the continuous sliding of chip flow over the face of cutting tool at high temperature[54] Diffusion and abrasion are the main cause generating crater wear In the cutting of the ductile material, diffusion and abrasion commonly happen where the continuous chip is formed In cutting of the brittle material, the chip is formed in the shape of a small segment lead to abrasive on the tool face is lower than that caused by the continuous chip The crater wear is measured by the depth of crater

Flank wear is caused by work hardening It happens at the tool flanks, where

contact between tool flank and finished surface take place Flank wear is a result

of abrasion and adhesion wear It influences on the great extend the mechanics of cutting Flank wear causes the significant increases of the cutting force Flank wear is measured by the width of wear land If the width of wear land exceeds the critical value (0.5-0.6mm), tool failure will occur by the excessive increase of cutting forces [54]

According to the standard ISO 8688 [55], the end milling tool wear is shown

in Figure 2-12

Figure 2- 12 Wear of end milling tool

2.2 Cooling and lubrication in metal cutting

2.2.1 Functions of cutting fluid

In metal cutting, cutting fluids typically perform for a number of objectives simultaneously such as cooling and lubricating the tool-workpiece and tool-chip interfaces, mitigating the negative effect of built-up edge (BUE), protecting the machined surface from corrosion, and clear the chips from the cutting area [56] However, the cutting fluids perform two main functions that are lubrication and cooling The lubrication function is normally at low cutting speeds and the cooling function is at high cutting speeds [57] At low cutting speed, cooling function of cutting fluid is relatively unimportant and at high cutting speed, time

is not enough to take place penetration of fluid to the chip–tool interface, the wear zone, or micro-cracks on the back of the chip to accommodate lubrication The typical machining processes with high-cutting-speed where cooling plays an important role are turning and milling The selection of cutting fluids also

contains some other secondary considerations such as chip disposal, corrosion, health, safety, aesthetic considerations, and cost

2.2.1.1 Cooling

The cooling function of cutting fluids is the most important function It is necessary to decrease the influences of cutting temperature on cutting tool and machined work-piece Therefore, tool wear is decreased lead to the improvement

of tool life and the dimensional accuracy of machined work-piece [58-60] The cutting fluid performs the cooling function via removing heat and thus reducing the temperature in the cutting zone by flowing over the tool, work-piece, and chip The shearing mechanism in cutting process is to transform the work-piece

to the desired shape In the fundamental mechanism, high friction load is created between the cutting tool and the work-piece It causes a significant increase of the cutting temperature in cutting zone If the increase of temperature is not properly controlled, it might have adverse effects on the cutting tool and machined surface In metal cutting, the sources of heat are generated and distributed in three areas [59] Three main heat sources are distributed as shown

in Figure 2-13

Figure 2- 13 The distribution of the heat sources in cutting [57]

As shown in Figure 2-13, the main sources of heat are (1) a primary shear zone, (2) secondary shear zone and (3) tool flank zone In the primary shear zone,

the major heat source is generated due to plastic deformation Most of this heat concentrates in the chip In the secondary shear zone or tool-chip interface, sliding friction results in considerable heat In the third zone of heat or the tool flank region, rubbing between the cutting tool and the finished surface occurs Thus, heat is generated by friction

In metal cutting, the effect of cutting temperature include both positive and negative The excessive cutting temperature produced affects tool life, cutting force, quality of the machined surface Control of heat sources by reducing friction and cutting temperature is important

Generally, a reduction in cutting temperature results in a decrease in wear rate of cutting tool and an increase in tool life That is explained that the cutting tool is more resistant to abrasive wear and harder at lower cutting temperature Moreover, the diffusion rate of constituents in the material of cutting tool is lower at lower temperature [58] However, a reduction in the temperature of the work-piece results in an increase of its shear flow stress, thus, the cutting force and the power consumption may be increased These effects lead to a decrease in tool life[58]

At medium or lower cutting speed, cooling effect of cutting fluid is not very significant with the surface finish produced It can, however, prevents or reduces the formation of BUE on the tool surface Shaw [57] and Korkut I., and Donertas M.A [61] found that BUE will be formed at cutting speeds ranging from low to moderate as displayed in Figure 2-14 But the BUE formation can be retarded by employing cutting fluid[57]

Figure 2- 14 Built-up edge [58]

2.2.1.2 Lubrication

The lubrication function of cutting fluid gives a significant contribution to friction reduction In the cutting process, heat is primarily generated at the interface of chip and tool The cutting fluid particles penetrate into the chip-tool interface It will reduce the friction, cutting force, heat generated, temperature and tool wear Further, lubrication effect of cutting fluid will cause less built-up edge when machining some materials Thus, applying cutting fluid in machining results in improvement of surface roughness [58-60]

2.2.2 Types of cutting fluid

In order to perform effectively the cooling function, the cutting fluid must obtain an approach to the heat sources and must be able to remove heat A cutting fluid with satisfactory cooling function must have high thermal conductivity and high specific heat [58] Water can meet this requirement Also, water is inexpensive However, lubrication characteristic of water is poor lead on reducing friction between chip and face of the tool is not effective Additionally, water is a damaging agent which is corrosive to ferrous metals Therefore, it causes great damage to the workpiece, fixture, and machine tool Furthermore, the water caused the loss of lubricating oil from the sliding and rotating the