MODELING OF DUCTILE-MODE MACHINING OF BRITTLE MATERIALS FOR END-MILLING MUHAMMAD ARIF NATIONAL UNIVERSITY OF SINGAPORE 2011 MODELING OF DUCTILE-MODE MACHINING OF BRITTLE MATERIALS FOR END-MILLING MUHAMMAD ARIF (B. Sc. Industrial and Manufacturing Engineering) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgements Acknowledgements First of all, I express my heartiest gratitude and humbleness to the almighty ALLAH (S.W.T.) who is the most merciful and the most gracious for He blessed me with the strength and the ability to complete my doctoral studies and subsequently write this thesis. I would like to express my deepest appreciation and respect to my supervisors Prof Mustafizur Rahman and Prof Wong Yoke San for their exceptional guidance, continuous support and encouragement throughout the course of my graduate study. Their valuable recommendations, ideas and advice on technical issues have contributed immensely to the successful completion of this research work. I want to extend my appreciation to the staff in the Advanced Manufacturing Laboratory (AML) especially Mr Tan Choon Huat, Mr Wong Chian Loong, Mr Lim Soon Cheong, Mr Yeo Nelson and others for their support during the experimentation, use of SEM and other lab matters etc. I would also like to thank Mr Silva Kumar for his support during the preparation of experimental setups in Microfabrication Lab. I am also thankful to Dr Tanveer Saleh and Mr Vijay from Mikrotools Pte Ltd for their assistance to prepare experimental setup for some of my experiments performed at their facility. Thanks to SIMTech for allowing partial financial support to purchase tools for experiments. I also take the pleasure to express my cordial appreciation, for the support and the encouragement at various stages during my research and stay at NUS, to my lab mates i Acknowledgements and friends. In this regard, I would like to say special thanks to Abu Bakar Muhammad Ali Asad, Mohammad Ahsan Habib, Muhammad Tarik Arafat, Indraneel Biswas, Asma Perveen, Huynh Kim Tho, Zhang Xinquan, Wang Jingjing, Zhong Xin, Nguyen Minh Dang, Wang Xue and many others. I am indebted to my friend, Amir Khurram Rashid from NTU, for his selfless support and encouragement throughout my graduate studies. I would also like to mention here the respect for my old roommate Ahmed Badawi Mustapha and my friend Muhammad Jawad Majeed for their support and encouragement during the early stage of my graduate study. I am also thankful to admin staff especially Ms Sharen and Ms Salmiah in Mechanical Engineering department for showing courtesy in the administration work throughout my studies. At the end, it will be injustice not to mention here the support, prayers and encouragement of my mother and sisters. I wish to pay special respect and thank to my mother for her sacrifices and prayers for my well-being and success throughout my life. I am really indebted to her for such precious support. I also wish to pay tribute to my late father, Mahr Mir Muhammad, who encouraged and inspired me for higher studies. I am also thankful to my loving wife Fariha Rahman for her continuous support and hospitality. Finally, I dedicate this thesis to my doll, my loving daughter, Aiza Arif. ii Table of Contents Table of Contents Acknowledgements ……………………………………………………………i Table of Contents …………………………………….……………….…… iii Summary ………………………………………………………… ix List of Tables ……………………………………………………………xi List of Figures ……………………………………………………………xii List of Symbols ……………………………………………………………xix Chapter Introduction ……………………………………………… ……1 1.1. Background of micro/nanomachining 1.2. Why micro/nanomachining of brittle materials? 1.3. The challenge and novelty of the research …………………… ………5 1.4. Goals of the Research 1.5. Significance of the research …………………………………… ………8 1.6. Organization of the thesis …………………………………………….10 Chapter Literature review ……………………… ……2 …………… ………4 ………………………………… …………7 ……………………………………… … 12 2.1. Ductility and plastic deformation of brittle material 2.2. Physics of micro-cutting 2.2.1. Size effect ………………………………………… 15 ………………………………………………… 16 2.2.2. Minimum chip thickness concept 2.2.3. Effective rake angle 2.3. ……….… .12 Ductile mode machining ……………………………17 ……………………………………………19 ……………………………………………20 2.3.1. Mechanism of material removal in ductile-mode machining ……22 iii Table of Contents 2.3.2. Phase transformation ……………………………………………24 2.3.3. Effect of machining parameters ……………………………………28 2.3.4. Surface characteristics ……………………………………………29 2.3.5. Tool wear characteristics ……………………………………31 2.3.6. Ductile machining by multipoint cutting process Chapter ……………33 Analytical model to determine the critical chip thickness for ductile- brittle transition in milling process of tungsten carbide ……………………35 3.1. Theoretical analysis ……………………………………………………36 3.2. Mechanics of machining in milling process of brittle material 3.3. Griffith’s energy-balance principle ……………………………………40 3.4. Modeling of machining process 3.5. Modeling of milling forces ……………………………………………44 3.6. Modeling of average rake and shear angles ……………………………45 3.7. Scope of proposed model 3.8. Experimental setup and procedure ……………………………………48 3.9. Results and discussion ……38 ……………………………………42 ……………………………………………48 ……………………………………………51 3.9.1. Determination of empirical constants ……………………………51 3.9.2. Predicted value of critical undeformed chip thickness ……………54 3.9.3. Experimental verification of model and discussion ……………55 3.9.4. Validity of the model by results reported in the past literature ……58 3.9.5. Further discussion on results 3.10. ……………………………………58 Conclusions ……………………………………………………………59 iv Table of Contents Chapter Analytical model to determine the critical feed per edge for ductile- brittle transition in milling process of brittle materials 4.1. Mechanism of ductile machining for endmilling 4.2. Development of an analytical model ………………… .60 ………………… .61 ………………………… .63 4.2.1. Indentation of brittle material ………………………………… .63 4.2.2. Analogous machining process ………………………………… .64 4.2.3. Tool deflection ………………………………………………… .72 4.3. Experimental apparatus and procedure ………………………… .74 4.3.1. Test apparatus ………………………………………………… .74 4.3.2. Data acquisition ………………………………………………… .75 4.4. Determination of empirical constants ………………………… .76 4.4.1. Determination of critical chip thickness ………………… .76 4.4.2. Determination of constants Ks and Kr ………………………… .77 4.4.3. Determination of constant χ ………………………………… .80 4.4.4. Predicted value of feed per edge 4.5. Results and discussion ………………………… .80 ………………………………………… .80 4.5.1. Experimental value of feedrate ………………………………… .80 4.5.2. Characterization of machined surface ………………………… .83 4.6. Equivalent value of constant χ for machining 4.7. Conclusions ………………………………………………………… .89 Chapter ………………… .87 Modeling of critical conditions for the modes of material removal in milling process of brittle material ………………………………………… 91 v Table of Contents 5.1. Development of model 5.1.1. Case I ………………………………………… 91 ………………………………………………………… 93 5.1.2. Case II ………………………………………………………… 96 5.2. Zones of machining ………………………………………………… 98 5.2.1. Zone A ………………………………………………………… 98 5.2.2. Zone B ………………………………………………………… 99 5.2.3. Zone C ………………………………………………………… 99 5.2.4. Zone D ………………………………………………………….100 5.3. Experimental procedure ………………………………………….100 5.4. Results and discussion ………………………………… .101 5.4.1. Determination of empirical constant ………………………….101 5.4.2. Validation of case I ………………………………………….103 5.4.3. Validation of case II ………………………………………….105 5.4.4. Surface roughness ………………………………………….107 5.4.5. Machining force study ………………………………………….108 5.5. Conclusions ………………………………………………………….109 Chapter Analytical model to determine the effect of tool diameter on critical feed rate for ductile-brittle transition in milling process of brittle material ….110 6.1. Development of model ………………………………………….110 6.1.1. Modification due to new crack orientation because of change in cutting edge trajectory………………………………………………….115 6.2. Experimental setup and procedure ………………………………….118 6.3. Results and discussion 6.3.1. Case ………………………………………….119 ………………………………………………………….119 vi Table of Contents 6.3.2. Case 6.4. ………………………………………………………….123 Conclusions ………………………………………………………….125 Chapter Ultra-precision slot-milling of glass ……………………….…126 7.1. Slot-milling …………………………………………………….……126 7.2. Plowing effect 7.3. Experimental setup and design …………………………………………….……128 …………………………….……129 7.3.1. Surface characterization ……………………………………….…132 7.3.2. Cutting strategy …………………………………………….……133 7.4. Results and discussion …………………………………….……133 7.4.1. Cutting process …………………………………………….……133 7.4.2. Cutting force analysis ………………………………….………136 7.4.3. Effect of feedrate 7.4.4. Tool wear 7.5. ………………………………….………138 ………………………………………….………140 Conclusions ………………………………………………….………142 Chapter An experimental investigation into micro ball-end -milling of silicon ………………………………………………………………… ….….143 8.1. Mechanism of ball-end milling of brittle material ………… …… 144 8.1.1. Cutting-speed gradient ………………………………… …… 145 8.1.2. Cutting edge engagement length ……………… ……… 146 8.2. Experimental setup and procedure ………………………… …… 147 8.3. Results and discussion ……………………………… ……… 149 8.3.1. Effect of inclination direction ……………………… ……… 149 8.3.2. Effect of inclination angle ……………………… ……… 153 vii Table of Contents 8.3.3. Effect of feed rate on surface roughness …… ………… 156 …………………………………… ………… 157 8.3.5. Tool wear ……………………………………… ……… 158 8.4. 8.3.4. Cutting forces Conclusions ……………………………………………… ……… 161 Chapter Conclusions and future work ………………………… …… 162 9.1. Conclusions …………………………………………………………162 9.2. Future work …………………………………………………………163 Bibliography ………………………………………………………… …… 165 List of Publications …………………………………………………………176 viii Conclusions and future work Chapter 9.1. Conclusions and Future Work Conclusions The work presented in this thesis investigates the ductile-mode machining of brittle materials by endmilling. Endmilling is a transient cutting process and is different from the single-point cutting process. Due to the typical scope of the process, the endmilling has been used to produce crack-free machined surfaces on brittle workmaterial in this study. Feed per edge has been identified as the dominant parameter to achieve crack-free machined surface on brittle material by endmilling. a) The first analytical model presented determines the critical undeformed chip thickness for ductile-brittle transition based on the Griffith energy balance theorem. It determines the critical chip thickness as a function of cutting edge geometry and material properties. The cutting edge geometry is mainly responsible for producing the cutting forces and consequently distribution of cutting forces whereas material properties govern the extent of plastic deformation and onset of brittle fracture. The proposed model has been validated experimentally. b) The second and most important contribution is made by determination of the critical feed per edge for ductile-brittle transition as a function of material properties and cutting edge geometry. The model is unique in a sense that most of the previous analytical models presented with single-edge cutting tool were focused on determination of ductile-brittle transition point in terms of critical undeformed chip thickness whilst this model determines the processing parameter based on the indentation test results. It provides a way to achieve crack-free surface on brittle material without too much experimental trial. 162 Conclusions and future work c) The third contribution is the determination of the possible zones or regimes in endmilling of brittle materials. It has been established that the subsurface damage depth is an important consideration to achieve the maximum permissible material removal in ductile-mode. Based on the mutual relationship of radial depth of cut and subsurface damage depth, there are four possible regimes of machining i.e. pure ductile-mode, ductile-mode with plowing, ductile-brittle mode with crack-free final machined surface and brittle mode machining. d) The fourth contribution is made to determine the influence of tool diameter on the ductile-brittle transition in endmilling of brittle material. Previously, there was the usual misconception that only the geometry of cutting edge can influence the ductilebrittle transition, but this model has clearly indicated that the diameter of endmill can have a significant impact on the critical conditions and a cutter of larger diameter gives higher value of critical feed per edge. e) The experimental contribution has focused on two important approaches i.e. machining ultra-precision slots by flat endmill and machining deep slots by ball-end mill. In slot-milling, brittle-fracture occurred for very small critical feed per edge due to different mechanism of cutting involved from peripheral milling process. The slotmilling, especially those cutting deeper slots by ball-milling is a contribution to the fabrication of microfluidic devices and biomedical slides containing various patterns of slots/channels on brittle material such as glass and silicon. 9.2. Future work Ductile-mode milling process is a relatively less explored area of research and still there are number of aspects that need to be researched such as detailed mechanism of cutting in ball-end milling, determination of critical chip thickness in ball-end milling, 163 Conclusions and future work influence of coolant on ductile-brittle transition, the chip formation mechanism in ball-end milling etc. Another important consideration in ductile-mode milling process is the influence of temperature on ductile-brittle transition mechanism. In ductilemode machining, the energy consumed in plastically deforming and removing the material is eventually converted into heat energy. This heat is supplemented by the heat from two other sources i.e. heat generated due to friction at the tool-chip interface and heat generated at the flank face of the tool due to friction between the elastic recovery of newly machined surface and flank face of the tool. This total heat content can be significant and may influence the material properties in the cutting zone. The future work may focus on numerical method to estimate the total temperature in the cutting zone due to these three phenomenons and its effect especially on glass that has lower softening point compared to the softening point of some brittle ceramics. The heat generation at very high cutting speed can adversely affect the texture and integrity of the machined surface. 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XPS analysis of the groove wearing marks on flank face of diamond tool in nanometric cutting of silicon wafer. International Journal of Machine Tools and Manufacture, 48, pp. 1678-87. 175 List of Publications List of publications International peer-reviewed journals 1. Arif M, Rahman M, Wong YS. “Analytical model to determine the critical feed per edge for ductile-brittle transition in milling process of brittle materials”. International journal of machine tools and manufacture, Volume 51, Issue 3, March 2011, Pages 170-181. 2. Arif M, Rahman M, Wong YS. “Analytical modeling of ductile-regime machining of tungsten carbide by endmilling”. The international journal of advanced manufacturing technology (Accepted 2010, DOI: 10.1007/s00170010-3027-2). 3. Arif M, Rahman M, Wong YS. “Ultraprecision ductile mode machining of glass by micromilling process”. Journal of manufacturing processes, Volume 13, Issue 1, January 2011, Pages 50-59. 4. Arif M, Rahman M. Wong YS., Neha D. “An experimental approach to study the capability of end-milling for microcutting of glass”. The international journal of advanced manufacturing technology (Accepted 2010, DOI: 10.1007/s00170-010-2893-y). 5. Arif M, Rahman M. Wong YS. “An experimental study on microcutting of soda-lime glass by milling process”. Key engineering material, vol. 47-48, July 2010, Pages 116-120. 6. Arif M, Rahman M. Wong YS. “Analytical model to determine the effect of tool diameter on critical feed rate for ductile-brittle transition in milling process of brittle material”. Transactions of ASME – Journal of Manufacturing Science and Engineering (under review) 7. Arif M, Rahman M. Wong YS. “An experimental investigation into micro ball-end -milling of silicon”. Journal of Manufacturing Processes (under review) 8. Arif M, Rahman M. Wong YS. “A model to determine the critical conditions for the modes of material removal in milling process of brittle material”. Precision Engineering (under review). International conferences 9. Arif M, Rahman M. Wong YS. “An experimental study on microcutting of soda-lime glass by milling process”. International Conferences on Precision Engineering (ICoPE 2010 & 13ICPE), 28-30 July 2010, Singapore. 176 List of Publications 10. Arif M, Rahman M. Wong YS. Kui, L. “Microcutting of glass by milling process”. Machine Tool Technologies Research Foundation (MTTRF), 7-9 July 2010, San Francisco, USA. 11. Arif M, Rahman M. Wong YS. “Fracture-free precision machining of sintered tungsten carbide by endmilling”. Accepted and to be presented at 11th International Conference of the European Society for Precision Engineering and Nanotechnology, Como Ital in May 2011. 12. Arif M, Rahman M. Wong YS. “Micromilling of silicon using ball-end mill”. Machine Tool Technologies Research Foundation (MTTRF), July 2011, Chicago, USA. (submitted). 177 [...]... influence ductile- brittle transition in endmilling of brittle material The analytical work is focused on the determination and prediction of critical conditions for ductile- brittle transition in milling process of brittle material in terms of process parameters such as undeformed chip thickness and feed per edge The analytical modeling has been performed by bringing together modeling of machining process... List of Figures Figure 7.7 AFM image of surface machined in (a) ductile mode (b) brittle mode (c) ductile mode with plowing effect …………………………………………… 136 Figure 7.8 Sampled cutting force (in cross feed direction) signal and corresponding machined surfaces for (a) ductile mode (b) brittle mode …… 137 Figure 7.9.Variation of machining force with rotation angle of cutter (cutting conditions: axial depth of. .. interdisciplinary effort has rendered significant contribution to the field of micro/nano -machining of brittle material to achieve fracture-free machined surfaces This research work is an effort to drive the technology of ductile- mode machining by milling process forward towards the established state 1.4 Goals of the research This project aims at performing the ductile- mode machining of brittle materials by... state of ductile- mode machining The specific goals of this research are summarized below:  Study and establish the fundamental mechanism involved in ductile- mode machining of brittle materials by endmilling, especially its difference from the singleedge machining process  Identify the key machining parameters governing the ductile- brittle transition mechanism in milling process of brittle materials. .. several machining parameters such as radial depth of cut, axial depth of cut, feederate etc This study will investigate the influence of these parameters on the ductile- brittle transition machining by theoretical and experimental work on various brittle materials  Develop an analytical model to determine the critical undeformed chip thickness for ductile- brittle transition in milling process of brittle. .. machined in ductile- mode without brittle fracture This success has led to the new era of machining brittle material with optical surface finish by traditional machining process like diamond turning Ductile- mode machining eliminates the requirement for secondary finishing processes giving a mirror-like machined surface with nanometric accuracy 1.3 The challenge and novelty of the research Machining of metals... is on ductile- brittle transition phenomenon and on the factors governing such transition, an analytical approach based on the modeling of machining process and linear elastic fracture mechanic was inevitable This approach had to be executed such that the ductile- brittle transition phenomenon could be explained on the basis of established scientific knowledge of 6 Introduction both modeling of machining. .. (a) ductile mode (b) brittle mode ….……57 Figure 4.1 Side cutting of brittle material with upmilling technique at (a) low feed per edge (b) high feed per edge ………………………………………….……62 Figure 4.2 Indentation process of brittle materials with sharp point indenter (a) Loading and formation of plastic zone (b) Further loading and onset of median cracks (c) Unloading, closing of median cracks and onset of lateral...Summary Summary Brittle materials such as glass and ceramics are considered as difficult-to-machine materials because of their high tendency towards brittle fracture during machining The most important challenge in machining these brittle materials is to achieve the material removal by plastic deformation rather than characteristic brittle fracture Ductile- mode machining is a promising technology... smaller endmill D2 Diameter of larger endmill Y1 Height of first brittle fracture point from the plane of final machined surface for small diameter cutter Y2 Height of first brittle fracture point from the plane of final machined surface for larger endmill I Moment of inertia Cr Length of radial crack ap Axial depth of cut in ball-end mill hp Scalp height Lt Length of contact of cutting edge on ball type . MODELING OF DUCTILE-MODE MACHINING OF BRITTLE MATERIALS FOR END-MILLING MUHAMMAD ARIF NATIONAL UNIVERSITY OF SINGAPORE 2011 MODELING OF DUCTILE-MODE MACHINING. comprehensive study on ductile-mode machining of brittle material by milling process. The underlying mechanism of material removal in endmilling of brittle materials and influence of machining parameters. value of constant χ for machining ………………… 87 4.7. Conclusions ………………………………………………………… 89 Chapter 5 Modeling of critical conditions for the modes of material removal in milling process of brittle