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Surface texture generation using high-feed milling with spindle speed modulation Huang, Fengqing 2020

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Surface Texture Generation using High-Feed Milling with Spindle Speed Modulation by  Fengqing Huang  B.Eng., South China University of Technology, 2018  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Mechanical Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  September 2020  © Fengqing Huang, 2020    ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Surface Texture Generation using High-feed Milling with Spindle Speed Modulation  submitted by Fengqing Huang in partial fulfillment of the requirements for the degree of Master of Applied Science in Mechanical Engineering  Examining Committee: Dr. Xiaoliang Jin, Assistant Professor, Department of Mechanical Engineering, UBC Supervisor  Dr. Yusuf Altintas, Professor, Department of Mechanical Engineering, UBC Supervisory Committee Member  Dr. Minkyun Noh, Assistant Professor, Department of Mechanical Engineering, UBC Supervisory Committee Member iii  Abstract Surface texturing is a manufacturing process to generate periodic geometric patterns on component surfaces in order to achieve certain functions, such as tribological property, adhesion, and wettability. This thesis presents a surface texturing technique using ball-end milling with high feed speed and spindle speed modulation. The ratio between the feedrate and the cutting tool radius is in the range of 0.2-0.4 when the spindle speed is a constant, and a certain amount of workpiece material remains after the cutting process to form the surface texture. A sinusoidal modulation signal is added to the spindle speed command, so the spindle speed becomes time-varying in order to generate different texture profiles based on the modulated frequency and amplitude. The cutting tool kinematics of the surface texturing process are modeled considering the tool tip run-out and deflection due to the cutting forces. Z-map method is used to simulate the geometry of the 3-D surface texture based on the tool tip trajectory. The effects of modulation parameters on tool tip trajectories and surface textures are analyzed. The relationship between the micro features of the surface texture and the process parameters are determined. Surface texturing experiments are conducted based on the proposed technique, and tribology tests are performed on the textured surfaces. It is shown that the textured surfaces present frictional anisotropy, which depends on the process conditions and the modulation parameters of the spindle speed. The iv  proposed technique is able to achieve fast generation of various surface textures without additional instrumentation, and the final texture geometry is controllable based on the presented kinematics model.   v  Lay Summary  Surface texturing is the process of generating repeated geometric patterns on the surface of a component. It has been widely used in various applications such as friction property modification, manufacturing optical lens, and controlling the wettability of solid surfaces. Various manufacturing methods for surface texture generation have been developed, including chemical etching, laser processing and mechanical machining. This thesis presents a mechanical surface texturing method by using milling process with fast movement of the cutting tool. The materials which remain after milling form the surface textures. The tool rotating speed varies with respect to time, so different texture geometries are achieved by controlling the tool tip trajectories. A physical model is developed to predict the surface texture generation according to the process parameters. Surface texturing experiments are performed to demonstrate the proposed method. Friction tests are conducted to prove the effect of the surface textures on the friction properties in different directions. vi  Preface This thesis is original, unpublished, independent work by the author, Fengqing Huang. He is responsible for conducting all the simulations, experiments and completing the analyses in this thesis. Dr. Xiaoliang Jin is the supervisor for this work. vii  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables ..................................................................................................................................x List of Figures ............................................................................................................................... xi List of Symbols ........................................................................................................................... xiv List of Abbreviations ................................................................................................................. xvi Acknowledgements ................................................................................................................... xvii Chapter 1: Introduction ................................................................................................................1 1.1 Surface Textures and Applications ................................................................................. 1 1.2 Manufacturing Techniques for Surface Texture Generation .......................................... 5 1.3 Thesis Objectives and Organization ............................................................................... 9 Chapter 2: Literature Review .....................................................................................................10 2.1 Micro slot milling and dimpling for Surface Texture Generation ................................ 10 2.2 Cutting Tool Design for Surface Texturing .................................................................. 12 2.3 Surface Texturing using High-feed Trajectory Generation .......................................... 14 2.4 Surface Texturing using Vibration Assisted Machining ............................................... 14 2.5 Summary ....................................................................................................................... 17 Chapter 3: Proposed Surface Texturing Process ......................................................................18 3.1 Description of the Proposed Surface Texturing Process............................................... 18 viii  3.2 Process Kinematic Model ............................................................................................. 20 3.3 Tool Run-out and Deflection ........................................................................................ 21 3.4 Simulation of 3-D Surface Texture using Z-map Method ............................................ 26 3.5 Summary ....................................................................................................................... 28 Chapter 4: Simulations and Experimental Results ..................................................................30 4.1 Simulation Results of Surface Texture Generation ...................................................... 30 4.1.1 Texture Generation with Constant Spindle Speed .................................................... 30 4.1.2 Texture Generation with Spindle Speed Modulation................................................ 33 4.1.3 Effects of Tool Run-out and Deflection on Texture Geometry ................................ 37 4.2 Experiment Results and Discussions ............................................................................ 41 4.2.1 Experimental Setup ................................................................................................... 41 4.2.2 Run-out and Tapping Test ........................................................................................ 43 4.2.3 Experimental Results and Discussions ..................................................................... 46 4.3 Analysis on Micro-features of Surface Texture ............................................................ 51 4.3.1 Micro-features with Constant Spindle Speed ........................................................... 51 4.3.2 Micro-features with Spindle Speed Modulation ....................................................... 53 4.4 Summary ....................................................................................................................... 56 Chapter 5: Tribological Property Tests on Textured Surfaces ...............................................58 5.1 Introduction ................................................................................................................... 58 5.2 Tribological Property Tests on Textured Surfaces ....................................................... 58 5.3 Results and Discussions ................................................................................................ 60 5.4 Summary ....................................................................................................................... 64 Chapter 6: Conclusion and Future Directions ..........................................................................65 ix  6.1 Conclusions ................................................................................................................... 65 6.2 Future Directions .......................................................................................................... 66 Bibliography .................................................................................................................................68  x  List of Tables  Table 4.1 Process conditions for surface texturing experiments. ................................................. 43 Table 4.2 Run-out parameters for Sample 2 to 5. ......................................................................... 44 Table 4.3 Model parameters of tool tip. ........................................................................................ 46 Table 4.4 Comparison of distances between neighboring major cusp peaks. .............................. 51 Table 5.1 Parameters used in tribology tests. ............................................................................... 60 Table 5.2 Wear factors of the samples. ......................................................................................... 63  xi  List of Figures  Figure 1.1 Surface textures on shark skins which allow sharks to be fast swimmers [1]. .............. 1 Figure 1.2 Different hierarchical structures result in various wettability and adhesion combinations: both superhydrophobic but (a) high adhesion and (b) low adhesion [2]. ................ 2 Figure 1.3 Partially textured parallel thrust pad bearing with rectangular dimples [3]. ................. 2 Figure 1.4 Adequate spacing dimensions to have the vortex lifted so that drag force reduction achieved. [4] .................................................................................................................................... 3 Figure 1.5 Textured surfaces between platform and end-effector for better actuation performance. [5] .............................................................................................................................. 4 Figure 1.6 Snake movement directions and its caudally orientated skins texture.[6] ..................... 5 Figure 1.7 Electrochemical etching method for generating circular and chevron-like holes. [9] .. 7 Figure 1.8 Laser texturing for various powers. (a)-(d) 25, 33, 40 and 48 J/cm2 respectively. [14]......................................................................................................................................................... 7 Figure 1.9 Pyramid structured arrays generated by V-shaped cutting tool [16]. ............................ 8 Figure 1.10 Simulation of surface textures generated by vibration-assisted milling with different vibration parameters [17]. ............................................................................................................... 8 Figure 2.1 Square arrays generated by high speed micro-milling with flat end-mill tool [18]. ... 11 Figure 2.2 Dimples with different distributions generated by ball-end milling [20]. ................... 11 Figure 2.3 Microbarbs textures generated by milling with V-shaped and T-shaped micro-mill tools [21]. ...................................................................................................................................... 13 Figure 2.4 Patch division milling. (a) Triangle patches. (b) Square patches [22]. ....................... 14 Figure 2.5 Ultrasonic vibration assisted face turning [24]. ........................................................... 16 xii  Figure 2.6 Vibration assisted milling [17]. ................................................................................... 16 Figure 2.7 Grooves textures without hierarchical micro-structures: (a) and (b); Grooves with hierarchical micro-structures generated by elliptical vibration: (c) and (d) [25]. ......................... 17 Figure 3.1 Schematic of tool tip kinematics. ................................................................................ 19 Figure 3.2 Texture generation using high-feed milling with spindle speed modulation. ............. 19 Figure 3.3 Identification of run-out angle. .................................................................................... 22 Figure 3.4 Identification of run-out radius. ................................................................................... 22 Figure 3.5 Determination of uncut chip thickness in surface texturing. ....................................... 25 Figure 3.6 Flow chart of iterative method for calculating final deflection with consideration of deflection feedbacks on cutting forces. ......................................................................................... 25 Figure 3.7 Flow chart of determining surface texture generation. ................................................ 27 Figure 3.8 Finding the corresponding grid at X-Y plane for each cutting point. ......................... 27 Figure 3.9 Gathering located cutting points of multiple layers. ................................................... 28 Figure 3.10 Simulated texture by finding the minimum z value in each grid. ............................. 28 Figure 4.1 Textures with constant spindle speed and 400mm/min feed speed. ............................ 31 Figure 4.2 Textures with constant spindle speed (200mm/min feed speed) ................................. 32 Figure 4.3 Textures with constant spindle speed (100mm/min feed speed) ................................. 33 Figure 4.4 3-D view of the texture generation without and with spindle speed modulation. ....... 36 Figure 4.5 2-D view of the texture generation without and with spindle speed modulation. ....... 37 Figure 4.6 Effect of run-out on surface textures (spindle speed modulation Am = 0.9, ωm= ω). 39 Figure 4.7 Cutting forces with 20 µm run-out radius and 0˚ initial angle; spindle speed modulation: Am = 0.9, ωm= ω..................................................................................................... 40 xiii  Figure 4.8 Tool tip trajectories at z = 0.1 mm with 20 µm run-out radius and 0˚ initial angle; spindle speed modulation: Am = 0.9, ωm= ω. ............................................................................. 40 Figure 4.9 Surface texture considering tool tip deflection with 20 µm run-out radius and 0° initial angle; spindle speed modulation: Am = 0.9, ωm= ω. .................................................................. 41 Figure 4.10 Experimental setup for surface texturing. ................................................................. 42 Figure 4.11 Setup of cutting tool run-out measurement. .............................................................. 44 Figure 4.12 Setup of tapping tests................................................................................................. 45 Figure 4.13 Pictures of surface textures generated in the experiments......................................... 49 Figure 4.14 Detailed geometries of surface textures for samples 2 - 5. ........................................ 50 Figure 4.15 Height to width ratio with feedrate to cutting tool radius ratio. ................................ 53 Figure 4.16 Change of cusp size at every two cutting tool revolutions when m = 1. ................. 54 Figure 4.17 Change of cusp size pattern unit when m = 1.5. ...................................................... 55 Figure 4.18 Change of cusp size pattern unit when m = 2. ......................................................... 56 Figure 5.1 Schematic of ball-on-flat reciprocating sliding tests. .................................................. 59 Figure 5.2 Friction coefficient test results from tribology test. .................................................... 62 Figure 5.3 Friction anisotropic property of the samples. .............................................................. 63  xiv  List of Symbols 𝐴𝑚 Spindle speed modulation magnitude 𝐴𝑖 Initial rotational angle of 𝑖 flute 𝑎 Ratio between the height and width of the cusp 𝑎𝑝 Depth of cut 𝐹𝑡 Cutting forces at tangential directions 𝐹𝑟 Cutting forces at normal directions 𝐹𝑥 Cutting forces along 𝑥-axis 𝐹𝑦 Cutting forces along 𝑦- axis 𝑓 Feed speed 𝑓𝑡 Feed per tooth 𝐻𝑐𝑢𝑠𝑝 Height of cusp texture protrusion peak ℎ Instantaneous uncut chip thickness 𝑖 Index of cutting flutes 𝐾𝑡𝑐, 𝐾𝑡𝑒 , 𝐾𝑟𝑒 , 𝐾𝑟𝑐 Cutting coefficients 𝑚 Ratio between the spindle speed modulation frequency and original spindle speed 𝑛1, 𝑛2 Coprime numbers describing 𝑚 as an irreducible fraction 𝑛𝑐𝑢𝑠𝑝 Number of cusps of each unit of texture pattern 𝑅 Radius of the ball-end milling tool 𝑡 Time 𝑊𝑐𝑢𝑠𝑝 Width of cusp texture xv  𝑥𝑖 Location of cutting points generated by 𝑖 flute at 𝑥-axis 𝑥𝑑 Deflections of tool center at 𝑥-axis 𝑦𝑖 Location of cutting points generated by 𝑖 flute at 𝑦-axis 𝑦𝑑 Deflections of tool center at 𝑦-axis 𝑧 Distance of the cutting points to the bottom of ball-end tool center along z-axis 𝜑 Spindle rotation angle ω Original spindle speed ω𝑚 Spindle speed modulation frequency 𝜌 Run-out radius 𝜆 Run-out angle 𝜃 Helix angle of the ball-end tool ∅ Immersion angle of cutting  xvi  List of Abbreviations CAD - Computer-aided design CNC - Computer numerical control LST - Laser surface texturing PEA - Piezoelectric-driven actuators xvii  Acknowledgements I would like to express my gratitude to my supervisor Prof. Jin for his guidance and support throughout my research. I am truly grateful for being accepted by Prof. Jin for joining Advanced Manufacturing Processes Laboratory.  To my friends and peers in the Advanced Manufacturing Processes Laboratory, thank you for your technical and moral support throughout my program. Your kind help and accompany are crucial for me to finish my thesis. I would also like to express my gratitude to Manufacturing Automation Laboratory for all the help and support.  Finally, I would like to thank my family, who have supported me throughout my years of education, both morally and financially. 1  Chapter 1: Introduction 1.1 Surface Textures and Applications Surface textures refer to periodic geometric patterns on component surfaces in order to achieve certain functions. Textured surfaces exist in living creatures in nature. For example, shark skins have the riblet structures with orientation in the same direction as the water flow [1], as shown in Figure 1.1. The riblet textures allow streamwise vortices formed in the turbulent flow and result in drag reduction. Figure 1.2 shows rose leaves with hierarchical micro and nano scale textures [2] to provide various wettability and adhesion characteristics. Textured surfaces have also been applied in a number of artificial products, such as golf balls, Fresnel lens, and bearing components. Figure 1.3 shows the textures on the parallel thrust bearing surfaces [3] for the purpose of enhancing hydrodynamic lubrication.   Figure 1.1 Surface textures on shark skins which allow sharks to be fast swimmers [1].  2   Figure 1.2 Different hierarchical structures result in various wettability and adhesion combinations: both superhydrophobic but (a) high adhesion and (b) low adhesion [2].  Figure 1.3 Partially textured parallel thrust pad bearing with rectangular dimples [3]. 3  The surface textures are generally composed of various geometric features including dimples, prisms, or grooves. The functionality of the surface texture is determined by the geometrical parameters of the textures such as feature size, aspect ratio, pitch length, and texture orientation. For example, for surfaces with riblet textures, Martin and Bharat [4] found that an optimum spacing factor exists for the maximum lifting effect by the vortex in the turbulent flow, as shown in Figure 1.4 [4]. When the spacing factor reaches 41.2, the lifting effect (vortex distance to the wall) of a riblet surface degrades to the same as a flat surface.  Figure 1.4 Adequate spacing dimensions to have the vortex lifted so that drag force reduction achieved. [4] 4  There are also applications which generate textures in order to modify the friction behavior in different directions. Piezoelectric-driven actuators (PEA) have been used for ultraprecision positioning and manipulation by repeating the expansion and contraction of PEA, as shown in Figure 1.5 - (a). The motion of platform is due to the inertia of the moveable platform and the friction between the platform and the end effector. Zhang et al. [5] fabricated saw tooth surface textures on both the platform and the end effector (Figure 1.5 – (b)). As a result, the friction with anisotropic behavior causes faster movement in the favorable direction compared to the actuator with untextured surfaces.  Figure 1.5 Textured surfaces between platform and end-effector for better actuation performance. [5] Another example of friction anisotropy using surface textures, is the body movement of the snake skin as shown in Figure 1.6 [6]. Baum et al. used smooth glass balls to slide the cushioned snake 5  skin textures through different directions, and concluded that the friction coefficient was almost twice along the caudal direction compared to the coefficients in cranial and lateral directions. Inspired by this phenomenon, textured polymer surfaces made of epoxy resin were produced by Baum et al. to mimic the friction anisotropy effect for control of surface friction property which has potentials in directional actuation and robotic applications [7].  Figure 1.6 Snake movement directions and its caudally orientated skins texture.[6] 1.2 Manufacturing Techniques for Surface Texture Generation Various manufacturing techniques have been used to generate surface textures, including chemical etching, 3-D printing, laser texturing, and mechanical machining. Chemical method is able to generate nanometer scale patterns through etching on the workpiece surface [8]. Chemical texturing method has the advantages of achieving high geometric resolution, but has limitation of workpiece material selections. Figure 1.7 shows circular and chevron-like holes generated by electrochemical etching [9]. Chemical etching is also used as a part of photolithography process, which uses light to transfer geometric pattern from a photomask to a photosensitive chemical 6  photoresist on the substrate, followed by etching on the exposure pattern into the material to produce surface patterns [10,11]. 3-D printing uses the computer-aided design (CAD) files and prints the component layer by layer. 3-D printing has been used to produce micro-pillar arrays, square thin walls, and eggbeater-shaped super-hydrophobic surfaces [12,13]. 3D printing for texturing is flexible in generating complex geometries, and is able to achieve high aspect ratios. However, this method is also material-limited, and the efficiency is low, which makes it not suitable for texture generation in large scale. Laser texturing (LST) uses a laser beam to melt or evaporate the substrate materials to generate required textures according to the trajectory command. LST has been used to create superhydrophobic surfaces [14] and various dimple profiles [15], as shown in Figure 1.8. Laser texturing is capable of fast generation of large-scale surface patterns. However, it requires high-cost equipment and is not energy-efficient. Furthermore, heat affected zone near the texture boundaries may be generated with undesired metallurgical properties.   7  Figure 1.7 Electrochemical etching method for generating circular and chevron-like holes. [9]  Figure 1.8 Laser texturing for various powers. (a)-(d) 25, 33, 40 and 48 𝑱/𝒄𝒎𝟐 respectively. [14] Mechanical surface texturing is a material removal method for texture generation using a cutting tool driven by the computer numerical controller (CNC). In mechanical surface texturing process, various texture profiles can be achieved by designing different cutting tool geometries or designing different tool trajectories. Figure 1.9 shows the pyramid structures on the surface by designing a V-shaped cutting tool [16]. Micro milling with external vibration assistance was used to generate fish-scale surfaces for enhancing surface wettability [17], as shown in Figure 1.10. Compared to other manufacturing methods, mechanical surface texturing may not achieve the highest geometric resolution, but has the advantages of low system cost, wide-range material applicability, and minimum heat affected zone on the surface. 8   Figure 1.9 Pyramid structured arrays generated by V-shaped cutting tool [16].  Figure 1.10 Simulation of surface textures generated by vibration-assisted milling with different vibration parameters [17]. 9  1.3 Thesis Objectives and Organization In this thesis, a new mechanical surface texturing method using ball-end milling with spindle speed modulation is proposed and analyzed. High value of feed speed is implemented, so that certain amount of workpiece material remains after the milling process to form the surface texture. Cusp shape textures on grooves are generated by controlling the rotating direction of the cutting tool. In order to generate different geometric features of the textures, a modulated sinusoidal signal with defined frequency and amplitude is added to the original spindle speed command. As a result, the spindle speed changes periodically rather than keeping constant, and the tool tip trajectory is modified accordingly. The proposed technique is able to achieve fast generation of various surface textures, and no external actuator is needed.  This thesis is organized as follows: Chapter 2 is the literature review of mechanical surface texturing techniques. Chapter 3 describes the details of the proposed surface texturing process and the kinematics model. Chapter 4 shows the simulations of the surface textures with experimental results, and discusses the relationship between the micro geometric features of the textures and the process parameters. Ball-on-flat reciprocating sliding tests are conducted in Chapter 5 to demonstrate the effect of different textures on the tribological behavior of the surface. Finally, Chapter 6 presents the conclusions and proposed future work. 10  Chapter 2: Literature Review This chapter presents and discusses the state of the art in mechanical machining for surface texture generation. The main techniques involve micro milling, cutting tool design for texturing, high-feed milling and vibration assisted machining. 2.1 Micro slot milling and dimpling for Surface Texture Generation Micro-milling has been used for generating surface textures including arrays, grooves and dimples with required micro geometry with slot milling and dimpling. Wan et al. [18] used micro slot milling with 0.1 mm tool diameter to generate micro square arrays on the surface, as shown in Figure 2.1. The surface textures change the wettability of aluminum alloy surfaces from hydrophilic to hydrophobic. Pratap and Patra [19] used a ball-end milling tool with 0.2 mm diameter to generate surface dimples. The distribution and overlapping ratio of the dimples can be designed for controlling the surface wettability. Graham et al. [20] produced surface dimples using titled ball-end milling to change the surface tribological behavior, shown in Figure 2.2. The change of the tool tilting angle is able to achieve different sizes and shapes of the generated dimples. Overall, surface texture generation using micro milling is capable of producing periodic texture features with micrometer-level geometry accuracy. However, the process is slow for large-scale surfaces, and the surface quality is influenced by tool wear, which is more obvious for workpiece made of high-strength metal alloys.  11   Figure 2.1 Square arrays generated by high speed micro-milling with flat end-mill tool [18].  Figure 2.2 Dimples with different distributions generated by ball-end milling [20]. 12  2.2 Cutting Tool Design for Surface Texturing One of the possible ways for surface texture generation using mechanical machining is to use cutting tools with pre-defined geometry to match with the geometry of the texture. Cai et al. [16] generated pyramid structures for controlling surface wettability using a flat V-shaped mill tool with 40 µm tip width. The texturing process involves horizontal slotting followed by vertical slotting. This method can generate pyramid arrays efficiently. However, the shape of the textures cannot be changed due to the cutting tool design. Sinan et al. [21] fabricated microbarb surface textures for medical implants by combining horizontal milling using a V-shaped cutting tool and vertical milling using a T-shaped cutting tool, as shown in Figure 2.3. Overall, designing specific cutting tool shapes is able to achieve fast generation of surface textures with accurate geometry. However, it is not flexible for different texture generations, and the fabrication of the cutting tool takes extra cost. To create more flexibilities of generating different texture geometries, researches have been trying to modify the cutting tool trajectories either by designing the tool trajectories or adding additional actuators, which are discussed in the next two sections. 13    Figure 2.3 Microbarbs textures generated by milling with V-shaped and T-shaped micro-mill tools [21]. 14  2.3 Surface Texturing using High-feed Trajectory Generation Milling with high feed speed leaves cutting marks on the surface, which form the surface textures. This can be used for surface texture generation by designing proper cutting tool trajectories. Matsuda et al. [22,23] used patch division milling to generate aligned cutter marks array, as shown in Figure 2.4. In this method, the cutter rotation angle is aligned along the trajectory of each patch with high feedrate. The whole curved surface is divided into several small patches, and each patch has surface textures with different feature distributions and shapes (triangles, squares or hexagon). Because of the high-feed motion and the trajectory design, the surface textures can be generated efficiently on a large surface. However, the relationship between the texture geometry inside each patch and the process parameters such as feed speed, tool run-out, and deflection was not analyzed.  Figure 2.4 Patch division milling. (a) Triangle patches. (b) Square patches [22]. 2.4 Surface Texturing using Vibration Assisted Machining Vibration assisted machining has also been used to generate different textures. Actuators are added to the cutting tool or workpiece to provide vibration motion in addition to the original cutting motion. As a result, the tool tip trajectory is modified for the purpose of generating various surface 15  textures. Nouri et al. [24] designed a vibration assistance setup for face turning to create periodic ripples on the machined surfaces, with the setup and the process shown in Figure 2.5 for face turning to create periodic ripples on the machined surfaces. The created surfaces can increase the wet area for a droplet and decrease the contact angle, therefore changing the surface wettability due to the vibration assistance. Chen et al. [17] implemented a 2-D vibration stage in micro milling (shown in Figure 2.6) to generate fish scale surfaces for enhancing the surface wettability. Similarly, the contact angle of the surface for a droplet is modified according to the parameters of the vibration assistance in two directions. Guo et al. [25] generated hierarchical surface textures using elliptical vibration cutting. It was demonstrated that the machined surface was composed of multiple groove textures when cutting without elliptical vibration. When the elliptical vibration was added, the second order textures were generated on the groove textures, as shown in Figure 2.7 [25]. The hierarchical textures on the surfaces result in greater difference of contact angles for a droplet on the surface along two perpendicular axes. Overall, vibration assisted machining methods can generate various surface texture features by modifying the tool tip trajectories. However, actuators are required to provide additional vibration motion, which increase the equipment cost and complexity in process implementation. 16   Figure 2.5 Ultrasonic vibration assisted face turning [24].  Figure 2.6 Vibration assisted milling [17]. 17   Figure 2.7 Grooves textures without hierarchical micro-structures: (a) and (b); Grooves with hierarchical micro-structures generated by elliptical vibration: (c) and (d) [25]. 2.5 Summary In summary, mechanical machining is able to generate surface texture for controlling surface wettability or tribological property. Various surface textures can be achieved by designing the cutting tool geometry or providing cutting tool trajectories. Mechanical surface texturing has the advantages of low energy consumption and adaptable to different workpiece materials. Moreover, it can also be combined with other texturing methods to expand the functionality of the textured surfaces.   18  Chapter 3: Proposed Surface Texturing Process In this chapter, a new surface texture generation technique is proposed using high-feed milling with spindle speed modulation. High feed speed is implemented, so that a certain amount of workpiece material remains after the cutting process to form the surface texture. In order to generate different periodic micro geometries and distributions of the texture feature, a modulated sinusoidal signal with defined frequency and amplitude is added to the original spindle speed command. As a result, the spindle speed changes periodically rather than keeping constant, and the tool tip trajectory is modified accordingly.  3.1 Description of the Proposed Surface Texturing Process The schematic of the cutting tool trajectory in the surface texturing process is shown in Figure 3.1. The ratio between the feedrate (mm/tooth) and the cutting tool radius (mm) is in the range of 0.2 to 0.4 when the spindle speed is a constant, which is larger than the feedrate/radius ratio (normally smaller than 0.1 [26]) in regular milling operation. The cutting tool trajectories are composed of multiple unidirectional paths to form the final texture in a 2-D plane. The flow chart of the surface texturing process is shown in Figure 3.2. Sinusoidal spindle speed commands are imposed to the original speed value as modulation signals, hence the actual spindle speed becomes time-varying. By changing the frequency and the amplitude of the sinusoidal signals, the modulated spindle speed and the tool tip trajectory changes, therefore the generated texture geometry is modified accordingly. The kinematics of the cutting tool tip and the surface texture geometry depend on the spindle speed, the tool runout, and the tool tip deflection due to cutting forces. 19  WorkpieceCutting TrajectoryNon-Cutting TrajectorySpindle RotationPath 1Path 2Path 3Path 4First Edge Second EdgeTexture (Materials Left)Z-AXISPath 2Path 1Y-AXIS TextureX-AXISY-AXISTop ViewSide View Figure 3.1 Schematic of tool tip kinematics.  Figure 3.2 Texture generation using high-feed milling with spindle speed modulation. 20  3.2 Process Kinematic Model In order to understand and predict how the surface texture geometry is determined by the process parameters and the cutting tool geometry, the kinematics model of the process considering the spindle speed modulation, tool run-out, and tool tip deflection is developed in this section. High-feed slot milling using a ball end milling tool with two cutting flutes is used in the model. The radius of ball-end milling tool is 𝑅, and the tool helix angle is 𝜃. The cutting tool moves with feed speed of 𝑓. The original spindle speed without modulation is 𝜔. When sinusoidal modulation signal at amplitude of 𝐴𝑚  and circular frequency of 𝜔𝑚  is applied, the rotational angle of the cutting tool is expressed as a function of time (𝑡): 𝜑(𝑡) = ω𝑡 + 𝐴𝑚 sin ω𝑚𝑡                (3.1) The schematic of the 2-D tool tip kinematics at a certain axial depth (𝑧) is presented in Figure 3.1. The remaining workpiece material due to neighboring tool path forms the surface texture, shown as the shaded area in Figure 3.1. Assume the origin of the coordinate system is fixed at the bottom of the ball-end milling tool, the coordinates of tool tip location for the 𝑖𝑡ℎ  cutting edge (𝑥𝑖(𝑧, 𝑡),  𝑦𝑖(𝑧, 𝑡)) at an axial depth (z) is expressed as: {𝑥𝑖(𝑧, 𝑡) = 𝑓𝑡 + √2𝑅𝑧 − 𝑧2 ∙ cos(𝐴𝑖 − 𝜑(𝑡)) + 𝜌 cos(−𝜑(𝑡) − 𝜆) + 𝑥𝑑(𝑡)𝑦𝑖(𝑧, 𝑡) =  √2𝑅𝑧 − 𝑧2 ∙ sin(𝐴𝑖 − 𝜑(𝑡)) + 𝜌 sin(−𝜑(𝑡) − 𝜆) + 𝑦𝑑(𝑡)          (3.2) where 𝜌 and 𝜆 are the radius and phase angle of the tool run-out respectively. Due to large feedrate in the surface texturing process, the variations of the uncut chip thickness and the cutting forces are much larger compared to those in conventional milling. Therefore, the deflections at the tool tip caused by the cutting forces in x and y directions are considered, shown as 𝑥𝑑(𝑡) and 𝑦𝑑(𝑡) in 21  Eq. (3.2). The deflection is z-direction is neglected because of relatively higher rigidity of the milling tool compared to x and y directions. 𝐴𝑖 is the initial rotational angle for the two cutting edges, expressed as: 𝐴𝑖 =  {−𝑧∙tan (𝜃)𝑅       ( 𝑖 = 1, first edge)−𝑧∙tan (𝜃)𝑅+ 𝜋    (𝑖 = 2, second edge)   (3.3) 3.3 Tool Run-out and Deflection Tool run-out is one of the main sources of the tool tip trajectories error, especially when the cutter radius is comparable with the run-out value. In this thesis, the run-out error at the tool tip is simplified as run-out radius and angle [27] as shown in Eq. (3.2). The values of run-out radius and angle are measured using a dial indicator. Once the coordinate of the tool is established, the run-out angle is measured by attaching the dial indicator at the shank of the tool and finding the rotation angle with the maximum value at the dial indicator (Figure 3.3). The sign of run-out angle is consistent with the sign in Eq. (3.2). After that, the dial indicator is attached at the tool tip to measure the run-out radius, which is calculated by the difference of the maximum and minimum value at the dial indicator, as shown in Figure 3.4. Finally, the identified run-out radius and angle (𝜌 and 𝜆) values are put into Eq. (3.2) to simulate the surface textures. 22  ToolDial IndicatorRun-outMaximum value at dial indicatorInitial position at dial indicator-λ Figure 3.3 Identification of run-out angle. Run-out diameter Figure 3.4 Identification of run-out radius. Due to large feedrate value in the surface texturing process, the variations of the uncut chip thickness and the cutting forces are larger compared to those in conventional milling. Therefore, 23  the deflections at the tool tip caused by cutting forces in x and y directions are considered. The deflection is z direction is neglected because of relatively high rigidity of the milling tool. The cutting forces in x and y axes (𝐹𝑥 and 𝐹𝑦) are simulated using mechanistic cutting force model based on the instantaneous uncut chip thickness ℎ(𝑡): {𝐹𝑡(𝑡) =  𝐾𝑡𝑐 ∙ 𝑎𝑝 ∙ ℎ(𝑡) +  𝐾𝑡𝑒 ∙ 𝑎𝑝𝐹𝑟(𝑡) =  𝐾𝑟𝑐 ∙ 𝑎𝑝 ∙ ℎ(𝑡) +  𝐾𝑟𝑒 ∙ 𝑎𝑝     (3.4) {𝐹𝑥(𝑡) =  −𝐹𝑡(𝑡) cos(∅) − 𝐹𝑟(𝑡) sin (∅)𝐹𝑦(𝑡) =  𝐹𝑡(𝑡) sin(∅) − 𝐹𝑟(𝑡) cos (∅)    (3.5) where ∅ is the immersion angle, 𝑎𝑝 is the depth of cut while 𝐾𝑡𝑐, 𝐾𝑡𝑒 𝐾𝑟𝑐 and 𝐾𝑟𝑒 are the cutting force coefficients [28] The overall cutting force is obtained by discretizing the cutting tool into a number of slices in z-axis with the depth of cut ∆𝑎𝑝 for each slice. With simulated forces in x-axis and y-axis obtained, static deflections in x-axis and y-axis are computed using the stiffness at the tool tip identified experimentally. The static deflection is then added into the tool tip trajectory to simulate the texture generated. In milling process with smaller feedrate value, the instantaneous uncut chip thickness is generally approximated as: ℎ(𝑡) =  𝑓𝑡 ∙ sin (∅)      (3.6) where 𝑓𝑡 = 𝑓/(𝑁 ∙ 𝑛) is the feedrate determined by feed speed, the number of flute (𝑁), and the spindle speed 𝑛. However, in the proposed texturing process, this approximation is not valid under high-feed status. Furthermore, the feedrate is not a constant due to the spindle speed modulation. 24  In this study, the instantaneous uncut chip thickness ℎ(𝑡) is obtained from the actual tool tip trajectory as shown in Figure 3.5. Trochoidal motion of the cutting edges during rigid or dynamics cutting [29] can be tracked and the instantaneous uncut chip thickness as can be determined. Based on the ball-end tool cutting mechanics model by Altintas and Lee [30], at each time interval, the instantaneous uncut chip thickness is simulated numerically by tracking the previous cut surface. As shown in Figure 3.5, the removed material is formed as the blue sweep area which allows the current cutting edges to track the previous cut surface in order to determine the instantaneous uncut chip thickness. To consider the effect of deflection on the instantaneous uncut chip thickness, the instantaneous uncut chip thickness without considering tool tip deflection is first determined from tool tip location at present and previous tooth passing periods. The corresponding cutting forces are then calculated based on Eqs. (3.4) and (3.5), and the tool tip deflection is determined to update the new tool tip locations. This calculation is repeated until the uncut chip thickness converges to a steady-state value, that the difference of the tool tip location values between the current and previous iterations is smaller than a threshold. This iterative method was also presented in early works [27, 28] for considering the effect of tool deflection on uncut chip thickness. Figure 3.6 shows the flow chart of this iterative method. Finally, the converged values of tool tip locations are recorded along the whole tool path for simulating the generated surface textures. 25  Uncut Chip Thickness Figure 3.5 Determination of uncut chip thickness in surface texturing.  Figure 3.6 Flow chart of iterative method for calculating final deflection with consideration of deflection feedbacks on cutting forces. 26  3.4 Simulation of 3-D Surface Texture using Z-map Method The Z-map method has been used to simulate the finished surface in milling process due to its advantages of simplicity of data structure and fast computation [33] Therefore, it is chosen for simulating the 3-D surface texture in this thesis. This method discretizes the workpiece into grids on x-y plane and locates the cutting points on the grids. The value of each grid is then represented by the smallest z axis value among all the cutting points located on this grid to represent the materials left on the surface. Figure 3.7 shows the flow chart of determining the geometry of the surface texture using Z-map method. Based on the process parameters (spindle speed modulation, run-out radius, helix angle, cutter initial angle and position references etc.), the tool tip trajectories are generated as points cloud data. A 2-D mesh is then generated in the x-y plane as shown in Figure 3.8. Each point in the points cloud is identified by a unique grid in the mesh, and the z-axis value (height) of the point is recorded by the corresponding grid data. After that, for each meshing grid, the minimum value of recorded points’ height (z-axis value) at this grid from all layers (Figure 3.9) is considered as the height of material left on the surface. Finally, all the grids are collected to form the simulated 3-D surface texture, as shown in Figure 3.10. 27   Figure 3.7 Flow chart of determining surface texture generation.  Figure 3.8 Finding the corresponding grid at X-Y plane for each cutting point. 28  Gathering all layers to the mesh at X-Y plane Figure 3.9 Gathering located cutting points of multiple layers.  Figure 3.10 Simulated texture by finding the minimum z value in each grid. 3.5 Summary In this chapter, the surface texturing using high-feed ball-end milling with spindle speed modulation is presented. A kinematics model is used to determine the tool tip trajectory considering the tool run-out and tool tip deflection due to the cutting forces. The run-out is 29  measured using a dial indicator, and the static deflection due to cutting forces is calculated. The cutting forces are simulated from the uncut chip thickness by tracking the previous cut surface. An iterative method is used to determine the effect of the deflection on the uncut chip thickness. Z-map method is used to obtain the geometry of the final 3-D surface texture. Based on the kinematics model, the geometry of the machined surface texture is simulated based on the cutting tool geometry and the surface texturing process parameters. 30  Chapter 4: Simulations and Experimental Results 4.1 Simulation Results of Surface Texture Generation A 2-flute ball-end milling tool with 1 mm radius and 30° helix angle is used to simulate the generated textures according to the kinematics model presented in Chapter 3. The original constant spindle speed is 500 rpm with 0.1 mm depth of cut. The discretized cutting points are generated with 0.25 ms time step with 10 µm grid size. The cutting tool is discretized into 364 layers along the z-axis using the z-map method. The workpiece material is aluminum 7075, and the force coefficients are obtained from [34]. The simulation starts from milling with no spindle speed modulation, run-out or deflections. Then, the spindle speed modulation is added in the simulation to determine its effect on the texture geometry. After that, the effects of tool run-out and deflection are considered and discussed.  4.1.1 Texture Generation with Constant Spindle Speed Figure 4.1 - (a) shows the 3-D view of the simulated surface texture generation in a single milling path. Due to large feed speed, the texture geometries on the up-milling and down-milling sides of the machined slots are different. The cusp shape is formed on the down-milling side, as shown in the 2-D view in Figure 4.1 - (b), while a relatively flat surface (surface of groove) is formed on the up-milling side. This is because the direction of instantaneous tool tip velocity due to spindle rotation is opposite to the feed speed direction on down milling side, while the two directions are the same on up milling side. 31   (a) Textures in the up-milling and down-milling side (3-D view).  (b) Textures in the up-milling and down-milling side (2-D view) Figure 4.1 Textures with constant spindle speed and 400mm/min feed speed. By decreasing the feed speed, the cusp texture becomes smaller and the surface is closer to a simple groove surface without obvious cusp texture. Figure 4.2 and Figure 4.3 show the simulation results 32  with the same process parameters as Figure 4.1, but the feed speeds are decreased to 200 mm/min and 100 mm/min feed speed respectively. As can be seen by comparing the textures figures with different feed speed, that when the feed speed decreases, the cusp textures become less obvious.  (a) Textures in the up-milling and down-milling side (3-D view)  (b) Textures in the up-milling and down-milling side (2-D view) Figure 4.2 Textures with constant spindle speed (200mm/min feed speed) 33   (a) Textures in the up-milling and down-milling side (3-D view)  (b) Textures in the up-milling and down-milling side (2-D view) Figure 4.3 Textures with constant spindle speed (100mm/min feed speed) 4.1.2 Texture Generation with Spindle Speed Modulation Figure 4.4 shows the 3-D view of the surface textures after multiple cutting paths. The interval distance between neighboring paths is designed to be 0.8 mm. Figure 4.4 - (a) shows the simulated 34  surface texture without spindle speed modulation. In comparison, Figure 4.4 - (b) and (c) show the simulated surface textures by imposing spindle speed modulations at 𝐴𝑚1 = 0.9, ω𝑚1= ω and 𝐴𝑚2 = 1.6, ω𝑚2= 0.5ω respectively, while keeping other process parameters the same as Figure 4.4 - (a). The tool tip deflection or run-out effect are not considered in all cases. Figure 4.5 shows the 2-D view of the micro geometry of the texture at the vertical location of z = 0.1 mm corresponding to the results shown in Figure 4.4. It is seen that the high-feed milling is able to generate periodic surface textures, which are composed of a number of cusp shape units. The distance between the peak of neighboring cusps is 0.4 mm. When the spindle speed modulation of 𝐴𝑚1 = 0.9, ω𝑚1= ω is applied, the texture is composed of two different levels of cusps. The distance between the top of the major cusps becomes 0.8 mm, and there exists a minor cusp located in the middle between the major cusps. The peak amplitude ratio between minor and major cusps is around 0.6. For the spindle speed modulation of 𝐴𝑚2  = 1.6, ω𝑚2 = 0.5ω , the distance between the major cusps increases to 1.6 mm, and there exist three minor cusps between the neighboring major cusps. The peak amplitude ratio between minor and major cusps is around 0.4. Overall, it is found that spindle speed modulation is able to modify the texture profile, and the geometry of the texture depends on the amplitude and frequency of the modulation command. 35   (a) Surface texture without spindle speed modulation.  (b) Surface texture with 𝑨𝐦 = 0.9, 𝝎𝐦= 𝝎. 36   (c) Surface texture with 𝑨𝐦 = 1.6, 𝝎𝐦= 0.5𝝎. Figure 4.4 3-D view of the texture generation without and with spindle speed modulation.  (a) 2-D view of surface texture in Figure 4.4-(a). 37   (b) 2-D view of surface texture in Figure 4.4-(b).  (c) 2-D view of surface texture in Figure 4.4-(c). Figure 4.5 2-D view of the texture generation without and with spindle speed modulation. 4.1.3 Effects of Tool Run-out and Deflection on Texture Geometry Figure 4.6 - (a) and (b) show the surface texture feature with 20 µm and 40 µm tool run-out radii (both have run-out initial angle 𝜆 = 0) respectively, and the spindle speed modulation parameters are 𝐴m = 0.9, 𝜔m= 𝜔 (corresponding to the speed modulation in Figure 4.5 - (b)). When the tool run-out effect is not considered, the bottom boundary of each cusp is located at around the same level of y = -0.4 mm, as shown in Figure 4.5 - (b). The tool run-out causes periodic variation of the cusp bottom boundary location around the original level, as well as the variation of cusp peak distance between the minor and major cusps, shown in Figure 4.6 - (a) and (b). The minor cusp 38  peak between two neighbor major cusps is closer to the left major cusp peak, while the distance is the same in Figure 4.5 - (b). The initial angle of tool run-out also plays a role on the surface texture generation. Figure 4.6 - (c) shows the texture result with initial run-out angle of 𝜆 = 90° . Compared to Figure 4.6 - (b) with 0˚ initial run-out angle, the difference between the widths of the major and minor cusps are larger in Figure 4.6 - (c). Moreover, the centers of minor and major cusps do not align at a line along the feed direction, as shown in Figure 4.6 - (c). This is due to the change of the tool center location due to the run-out effect, which in turn modifies the tool tip trajectory.  (a) 2-D view of the texture with 20 µm run-out radius, run-out initial angle 0°.  (b) 2-D view of the texture with 40 µm run-out radius, run-out initial angle 0°. 39   (c) 2-D view of the texture with 40 µm run-out radius, run-out initial angle 90°. Figure 4.6 Effect of run-out on surface textures (spindle speed modulation 𝑨𝐦 = 0.9, 𝝎𝐦= 𝝎). Cutting forces for the surface texturing processes with 20 µm run-out radius and 0˚ initial angle; and spindle speed modulation of 𝐴m = 0.9, 𝜔m= 𝜔 are shown in Figure 4.7. The effect of tool deflections due to the cutting forces on 2-D tool tip trajectories at z = 0.1 mm is shown in Figure 4.8. The static stiffness at the tool tip is assumed to be 3 µm/N in the simulations. Figure 4.9 shows the textures generated considering the tool tip deflection as well as tool run-out (20 µm radius, 0° initial angle). By comparing Figure 4.6 - (a) with Figure 4.9, it is found that tool deflections change the protrusion part of the cusp texture (near the center of the ball-end tool part at around y = 0 mm). In Figure 4.6 - (a), this part is more symmetric with respect to its center line, while in Figure 4.9, it becomes inclined towards the left side. Another noticeable difference is that the boundary location moves upwards slightly in Figure 4.9 which can be explained by the upward tool deflection in Figure 4.8. This also reduces the variation of cusp bottom boundary caused by tool run-out. 40   Figure 4.7 Cutting forces with 20 µm run-out radius and 0˚ initial angle; spindle speed modulation: 𝑨𝐦 = 0.9, 𝝎𝐦= 𝝎.  Figure 4.8 Tool tip trajectories at z = 0.1 mm with 20 µm run-out radius and 0˚ initial angle; spindle speed modulation: 𝑨𝐦 = 0.9, 𝝎𝐦= 𝝎. 41    Figure 4.9 Surface texture considering tool tip deflection with 20 µm run-out radius and 0° initial angle; spindle speed modulation: 𝐀𝐦 = 0.9, 𝛚𝐦= 𝛚. 4.2 Experiment Results and Discussions 4.2.1 Experimental Setup A series of surface texturing experiments were conducted on a 3-axis CNC micro-milling machine to demonstrate the proposed technique and verify the kinematics model. The experimental setup is shown in Figure 4.10. The spindle speed modulation signal is sent to the driver of the spindle from a dSpace controller. The references of x-axis, y-axis and z-axis are also given to the dSpace controller according to the cutting scheme in Figure 3.1. The geometry of the milling tool and the workpiece material are the same as the condition used in the simulations of Section 4.1. 42   Figure 4.10 Experimental setup for surface texturing. Five surface texturing conditions with different spindle and feed speeds, without and with spindle speed modulations were tested in the experiments, with the process parameters listed in Table 4.1. All the samples have been milled to a relatively flat surface using an end-mill tool prior to texturing with the same process parameters for sample 1. Then, the run-out identifications and tapping tests were conducted before texturing sample 2-5 using a ball-end tool.   43  Table 4.1 Process conditions for surface texturing experiments. Sample No. Spindle Speed n [rpm] Speed Modulation Parameters Feed Speed f [m/min] 1 3000 No modulation 100  2 500 No modulation 200 3 500 No modulation 400 4 500 𝐴m1 = 1.6, 𝜔m1= 250 rpm 400 5 500 𝐴m2 = 0.9, 𝜔m2= 500 rpm 400  4.2.2 Run-out and Tapping Test Tool run-out identifications have been conducted using a dial indicator, with the setup shown in Figure 4.11. The run-out parameters of the cutting tool (especially the run-out initial angle) need to be identified every time when the tool is clamped in the tool holder. This is because the value of run-out angle can change randomly when the tool is re-clamped in the tool holder. The coordinate for run-out parameters is the same as the kinematics model in Section 3.2. The identification process is described in Section 3.3. From the tests, the radius of the run-out is identified as 23 µm, and the initial angle is 87.5° for samples 2 and 3. The run-out parameters for 44  the surface texturing of samples 4 and 5 are: radius 𝜌 = 20 𝜇𝑚, initial angle 𝜆 = 0 °. The run-out parameters are summarized in Table 4.2.  Figure 4.11 Setup of cutting tool run-out measurement. Table 4.2 Run-out parameters for Sample 2 to 5. Setup No. Run-out Angle 𝜆 [°] Run-out radius [𝜇𝑚] 1 (for Sample 2 and 3) 87.5 23  2 (for Sample 4 and 5) 0 20  Dial Indicator Tool Tip45  Hammer tests were used to obtain the stiffness at the tool tip. A force hammer (Kistler 9722A200) and an accelerometer (Dytran 3225F1) were used for the test as shown in Figure 4.12. CutPro software was used to obtain the frequency response function and identify the modal parameters, which are listed in Table 4.3. The first natural frequency at the tool tip (468.56 Hz) is higher than the tooth passing frequencies in the surface texturing experiments (16.7 Hz at 500 rpm, and 100 Hz at 3000 rpm). Therefore, only static stiffness of the tool tip is considered to determine the tool tip deflection.  Figure 4.12 Setup of tapping tests.   46  Table 4.3 Model parameters of tool tip. Mode No. Frequency [Hz] Damping Ratio [%] Modal Stiffness [N/μm] Mass [kg] 1 468.56 1.87 3.16 0.37 2 597.55 1.94 5.32 0.038  4.2.3 Experimental Results and Discussions The workpiece samples with generated surface textures corresponding to the five texturing conditions are shown in Figure 4.13. For sample 1, due to the relative high spindle speed and low feedrate, all the material was removed, therefore, a relatively flat surface was produced without obvious textures. For samples 2-5, surface textures were generated due to the high feed speed. Different process parameters and modulation signals result in different layouts of the surface textures. The comparison between Figure 4.14 shows the detailed texture geometries examined by an optical microscope. Figure 4.14 - (a) and (b) shows that with a larger feed rate, the cusp texture is more obvious. It is observed from Figure 4.14 - (c) and (d) that minor cusps exist between the major cusps, which demonstrates the effect of the spindle speed modulation on the surface texture generation. Moreover, three and one minor cusp(s) are observed between the major cusps in Figure 4.14 - (c) and (d) respectively, as predicted from the simulations shown in Figure 4.5 - (c) and (b). Figure 4.14 - (a) and (b) show that the shapes of the neighboring cusps are not identical, but changing periodically due to the tool run-out effect. In addition, with the tool run-out, the centers 47  of the minor and major cusp peaks (neighboring cusps) do not align at a line along feed direction. As a result of tool deflection, the cusp shapes around the tool center are not symmetrical. This is particularly noticeable in Figure 4.14 - (c). The measured distances of neighboring major cusp peaks in comparison with the simulated results are shown in Table 4.4, and the differences are within 4%. 1 mm (1) Sample 1 1 mm (2) Sample 2 48  1 mm (3) Sample 3 1 mm (4) Sample 4 1 mm (5) Sample 5 49  Figure 4.13 Pictures of surface textures generated in the experiments.   (a) n = 500 rpm, f = 200 mm/min; No spindle speed modulation.  (b) n = 500 rpm, f = 400 mm/min; No spindle speed modulation. 50μmCusp CuspCusp Distance50μmCuspCusp Distance50  50μmMajor Cusp Major CuspMajor Cusp DistanceMinor Cusps         (c) n = 500 rpm, f = 400 mm/min; spindle speed modulation: 𝑨𝒎 = 1.6, 𝝎𝒎= 250 rpm.     (d) n = 500 rpm, f = 400 mm/min; spindle speed modulation: 𝑨𝒎 = 0.9, 𝝎𝒎= 500 rpm.                  Figure 4.14 Detailed geometries of surface textures for samples 2 - 5.  50μmMajor Cusp Major CuspMajor Cusp DistanceMinor Cusp51  Table 4.4 Comparison of distances between neighboring major cusp peaks. Sample Number Simulations (µm) Experiments (μm) Difference (%) 2 200 207 3.5 3 400 393 1.8 4 1600 1654 3.4 5 800 805 0.6  4.3 Analysis on Micro-features of Surface Texture This section presents the analysis on the micro geometric features of the generated surface without and with spindle speed modulation. The relationship between the process parameters (the ratio between the cutter’s radius and the feedrate value) and the cusp textures features (aspect ratio and periodicity of the pitch) are discussed. 4.3.1 Micro-features with Constant Spindle Speed The cusp shape in general ball end milling was studied by Jung et al. [35]. According to their analysis, with a constant spindle speed, the width of each cusp 𝑊𝑐𝑢𝑠𝑝  and height 𝐻𝑐𝑢𝑠𝑝  are obtained as: 𝑊𝑐𝑢𝑠𝑝 = 𝑓𝑡     (4.1) 52  𝐻𝑐𝑢𝑠𝑝 = 𝑅 − √𝑅2 − 𝑓𝑡2     (4.2) It is worth mentioning that 𝑓𝑡 should be smaller than 𝑅 in the equation, so 𝐻𝑐𝑢𝑠𝑝 should be a real number in their study. From the surface texture perspective, the ratio between the height and width of the cusp (aspect ratio) is an important geometric property for functionality of the texture [1,36,37]. The aspect ratio is obtained as: 𝑎 =𝐻𝑐𝑢𝑠𝑝𝑊𝑐𝑢𝑠𝑝=1𝑘− √(1𝑘)2 − 1    (4.3) where 𝑘 =𝑓𝑡𝑅 is the ratio between the feedrate and the cutting tool radius. In conventional milling, the feedrate is generally smaller than 0.1 of the cutter radius in order to produce a smooth surface. Figure 4.15 shows the relationship between the aspect ratio (𝑎) of the cusp texture and the process parameter (𝑘) according to Eq. (4.3). It is shown that when the ratio between the feedrate and the tool radius is smaller than 0.2, the aspect ratio of the cusp is smaller than 0.1, indicating that the machined surface almost becomes a groove without obvious cusp shape; when k is in the range of 0.2 – 0.4, the aspect ratio of the cusp is in the range of 0.1 – 0.2, with relatively obvious surface texture generated on the down milling side. However, larger k value may lead to the risk of breaking the cutting tool due to large chip load, which is also dependent on the workpiece material.  53   Figure 4.15 Height to width ratio with feedrate to cutting tool radius ratio. 4.3.2 Micro-features with Spindle Speed Modulation When spindle speed modulation is added into the surface texturing process, the aspect ratio of the cusp shown in Eq. (4.3) varies along the tool path. The frequency of the modulation signal is expressed as a linear function of the original spindle rotational frequency, expressed as: ω𝑚 = 𝑚 ∙ ω     (4.4) The value 𝑚 can control the periodic variations of the aspect ratio and size of the cusps in the surface texture. For example, in sample 5 in Table 4.1, the spindle speed modulation frequency is the same as the original spindle rotational frequency. Because the cutting tool has two flutes, the cusp size changes periodically per two cutting tool revolutions, with the aspect ratio of the cusp changes between 0.25 (major cusps) and 0.13 (minor cusps). The simulated cusp shapes are shown 54  in Figure 4.16. Therefore, it is shown that the modulation signal is able to control the cusp aspect ratio and size in the generated surface texture.  Figure 4.16 Change of cusp size at every two cutting tool revolutions when 𝐦 = 𝟏. The variation of cusp numbers in each texture unit can be studied in more general situations given the spindle speed modulation parameters. Let 𝑚 =𝑛1𝑛2     (4.5) as an irreducible fraction where 𝑛1 and 𝑛2 are coprime. Without the spindle speed modulation, the tool returns to its starting angle position (180° or 0°) every 𝜋ω seconds because the tool has two edges, which corresponds to m/2 period of the spindle speed modulation. Because of the spindle speed modulation, the cutting tool edge does not arrive at the starting angle position every 𝜋ω seconds. However, after certain integer number of m/2 period, the cutting tool edge return to its starting status (180° or 0°) and the spindle speed also returns to its starting value at the same time. 55  This means that the number of cusp in each texture unit should be the least integer which multiples m/2 with an integer result: for each cutting path, the texture unit consists of 𝑛𝑐𝑢𝑠𝑝 cusps, where 𝑛𝑐𝑢𝑠𝑝 should be: 𝑛𝑐𝑢𝑠𝑝 = {2𝑛2 (𝑛1 𝑖𝑠 𝑜𝑑𝑑)𝑛2  (𝑛1 𝑖𝑠 𝑒𝑣𝑒𝑛)     (4.6) This explains the reason that Figure 4.5 - (c) (𝑚 = 0.5) and Figure 4.5 - (b) (𝑚 = 1) have 4 cusps (1 major one with 3 minor ones) and 2 cusps (1 major one and 1 minor one) as the unit of their patterns respectively. It is also seen from Figure 4.17 (𝑚 = 1.5) and Figure 4.18 (𝑚 = 2) that each dash block is a unit with 4 cusps and 1 cusp as their patterns’ unit. It can be further concluded from the equation that if 𝑚 is an even number, then each pattern unit contains only one type of cusp; if 𝑚 is an odd number, then each pattern unit contains one major cusp and one minor cusp. This is because when 𝑚 is an integer, 𝑛2 = 1 which means 𝑛𝑐𝑢𝑠𝑝 can only be 1 or 2 depending on 𝑛1 which has the same value as 𝑚.  Figure 4.17 Change of cusp size pattern unit when 𝐦 = 𝟏. 𝟓.  56    Figure 4.18 Change of cusp size pattern unit when 𝐦 = 𝟐.  4.4 Summary In this Chapter, simulations and experiments are conducted according to the models presented in Chapter 3. The generation mechanism for cusp texture is due to the direction difference between the feedrate and the instantaneous tool tip velocity on the down-milling side. It is demonstrated both from the simulations and experimental results that the proposed texturing process is able to generate surface textures with different distributions of cusp shapes using high feed speed and spindle speed modulation. The texture geometries are predicted by the simulations from the proposed model, and therefore can be controlled by the process and modulation frequencies and magnitude. The difference between the simulation and experimental results is due to the fact that the model does not consider ploughing effect, which involves contact between the tool flank face with the workpiece material in high-feed motion. Other possible reasons may be caused by tracking errors of the spindle speed caused by the speed controller, and the material related deformation of the workpiece. It is shown from the simulations that run-out influences the size difference of neighboring cusp and cause misalignment of peak centers, while tool tip deflection makes the protrusion part of cusp to be non-symmetric, which is also observed in the experiment results. The 57  aspect ratio of cusp texture and the number of cusps in the pattern unit are discussed. The relationship between the texture’s aspect ratio and the cutting parameters (the ratio between the feedrate and the cutting tool radius) are discussed. It is concluded that the spindle speed modulation frequency can control the texture pattern unit (the number of cusps in each unit) while the magnitude of the spindle speed modulation signal can control the magnitude of variations in aspect ratio and size.  58  Chapter 5: Tribological Property Tests on Textured Surfaces 5.1 Introduction It is shown in the literature [11,38] that with different geometric features, surface texture can achieve different degrees of hydrodynamic lubrication due to the lift mechanism of the lubricant films in the surface cavities. Moreover, the shape and size of the surface texture play significant roles in the directionality of tribology behavior (friction anisotropy) [11], which has been shown in bio-inspired surface textures examples [39]. In this chapter, tribology tests were conducted to quantitatively determine the effect of the generated texture on the tribology property of the machined surface. A series of ball-on-flat reciprocating sliding tests were performed, with the setup and the test procedure described in Section 5.2, Section 5.3 discusses the test results of the textured surfaces. 5.2 Tribological Property Tests on Textured Surfaces To evaluate the friction behavior of the texture generated in the proposed process, a series of standard ball-on-flat reciprocating sliding tribology tests are performed (ASTM G133A standard), with the schematic of the experiment shown in Figure 5.1. ASTM G133A is a standard method for linear reciprocating ball-on-flat sliding wear test. The tribology test uses a spherically ended specimen (ball specimen) to slide on the test sample back and forth (reciprocating sliding). The test sample is clamped under the lubricant level and constant loading force is applied vertically through the ball specimen on the touching surface. Friction forces are measured using a force transducer equipped with the tribometer stage at the bottom. The measurement data is analyzed using “InstrumX” software version 2.7. The tribology tests are designed according to ASTM G133A standard procedure, with three modifications due to the equipment capability: 1. Lubricant 59  is used to immerse the sample; 2. The sliding frequency is 1.6 Hz instead of 5 Hz in the standard to prevent the lubricant from splashing out; 3. The test duration for each test is 52 min instead of 16 min 40 seconds because a slower speed is applied.  Figure 5.1 Schematic of ball-on-flat reciprocating sliding tests. The dimension of the samples (materials Al7075) in the tribology tests is 50mm * 28mm * 5mm. An Anton Paar tribometer with a linear module of S/N 1-120 is used. For each sample, reciprocating sliding is performed along two perpendicular axes (x-axis and y-axis are consistent with Figure 3.1) to investigate the friction anisotropy effect. The samples are cleaned using IPA prior to the tribology tests using an ultrasonic bath for 2 minutes and are immersed in the lubricant during the tests.  Table 5.1 shows the detailed parameters of the tribology tests. Lubricant LevelBall SpecimenSliding MovementLoadingTest SampleFriction Force Transducer 60  Table 5.1 Parameters used in tribology tests. Applied normal force 25.063 N Stroke length 10 mm Sliding distance 100 m Test duration 52 min 20 second Max linear speed 0.05 m/s Frequency oscillation 1.6 Hz Number of cycles 5,000 Ball diameter 9.525 mm Ball material SS440C grade 25 Lubricant Mineral oil Cleaning static partner IPA Cleaning sample Acetone Atmosphere Air Relative Humidity 45~48% Temperature 21~23° 5.3 Results and Discussions The measured friction coefficients in two orthogonal directions from the tribology tests are shown Figure 5.2. It is observed that compared to sample one (surface without texture), all friction 61  coefficients in the surface textured samples (sample 2 to 5) increase which indicates that the cusps on groove textures result in higher friction. This may due to the roughness of texture and hydrodynamics lifting effect did not occur given the texture parameters of the samples. Moreover, compared to sample 1 in which the friction coefficients in two directions are almost the same (the difference is 0.001), the other four samples (textured surfaces) all show friction anisotropic property, with the difference of the friction coefficients between the two axes ranging from 0.007 to 0.027. The friction coefficient difference between the two sliding directions is shown in Figure 5.3. The comparison between sample 3 with samples 4 and 5 shows that the spindle speed modulation decreases the friction coefficient in the y-axis by 6% and 7.2% respectively, while the increases of the friction coefficient in the x-axis are 7.2% and 2.2% respectively. As a result, the differences of friction coefficients between two directions for sample 4 and 5 decrease by 74.1% and 55.5% respectively when the spindle speed modulation is added. The friction anisotropy difference between sample 3 and sample 5 is smaller and one of the main reasons is that their patterns are more similar. Because of run-out angle effect, cusp size difference between neighboring cusps can be observed on sample 3 which is similar to the modulation effect on sample 5 pattern (one major cusp and one minor cusp). Table 5.2 shows the wear factors of the surfaces of the five samples. The wear factor describes the rate of materials volume loss (𝑚𝑚3) due to wear per unit of exposure parameter (𝑁𝑚). As shown from the table, compared to the flat surface (sample 1), the wear factors increase in all the textured samples which may be caused by larger coefficients of friction in the textured samples. The wear factors are the largest for sample 3, with approximately 3.6 and 11.7 times of the wear factors for sample 1 in x and y axes respectively. The wear factors for sample 4 (with spindle speed 62  modulation of 𝐴m1 = 1.6, 𝜔m1= 250 rpm) are 42% and 81% of the wear factors for sample 3 (without spindle speed modulation) in x and y axes respectively, and the wear factors of sample 5 (with spindle speed modulation of 𝐴m1 = 0.9, 𝜔m1= 500 rpm) are 67.5% and 72.5% of the factors of sample 3. This shows that the spindle speed modulation decreases the wear factors of the textured samples, which may be caused by different geometric distributions of the cusps textures.  Figure 5.2 Friction coefficient test results from tribology test. 0.10.110.120.130.140.150.160.17Sample 1 Sample 2 Sample 3 Sample 4 Sample 5Friction coefficientX-axis Y-axis63   Figure 5.3 Friction anisotropic property of the samples. Table 5.2 Wear factors of the samples. Sample Number Axis Wear Factor (10−5𝑚𝑚3/𝑁𝑚) 1 X 1.56 Y 0.57 2 X 2.7 Y 1.7 3 X 5.67 Y 6.66 4 X 2.41 Y 5.39 00.0050.010.0150.020.0250.03Sample 1 Sample 2 Sample 3 Sample 4 Sample 5Friction coefficient difference between two sliding directions64  5 X 3.83 Y 4.83 5.4 Summary In summary, the results from tribology tests show that the surface textures generated by the proposed technique are able to change the friction coefficient of the surface. More importantly, it is shown that the surface textures increase the friction anisotropy, and the spindle speed modulation influences the friction anisotropy behavior and the wear factor of the surface. However, the mechanism of how the geometric feature of the surface textures change the friction property still needs further investigation, which is out of the scope of this thesis.  65  Chapter 6: Conclusion and Future Directions 6.1 Conclusions Mechanical machining for texturing has the advantages of geometric control, low cost and easy implementation. In this thesis, a texture generation method using high-feed milling with spindle speed modulation is proposed. The kinematics model considering the tool tip trajectory modification with spindle speed variation, tool run-out, and tool tip deflection due to cutting forces is presented to predict the surface texture geometries. Surface texturing experiments are conducted to investigate the profile of the generated surface textures, and tribology tests are performed to determine the friction property of different textures. The main conclusions are listed below: 1. The milling process with high-feed and time varying spindle speed is able to generate obvious textures on the machined surface. The textures are composed of periodic cusp shapes on down milling side and grooves on up milling side. The proposed process does not need additional instrument, and it is able to achieve fast generation of the surface textures due to the high feed motion of the milling tool. 2. The tool tip trajectories and the texture geometries are influenced by the spindle speed variation, cutting tool run-out and deflection, which are predicted by the presented kinematics model. Spindle speed variation results in different distributions of major and minor cusps. Run-out causes the misalignment of the neighboring cusp peaks, and tool deflection results in asymmetry of the cusps with respect to its center line.  66  3. The relationship between the process parameters and the micro-features of texture has been analyzed. The aspect ratio of the cusps is determined by the ratio between the feedrate and the tool radius when the spindle speed is a constant. The spindle speed modulation changes the aspect ratio and size of the cusps along the feed direction. The periodicity of the cusps (number of cusps in each pattern unit) is determined by the modulation frequency. 4. The tribology tests show that the generated surface textures increase the friction coefficient and cause friction anisotropy on the surface. The friction property of the two orthogonal axes are influenced by the amplitude and frequency of the spindle speed modulation. 6.2 Future Directions Recommendations for future research in this area are summarized below: 1. Inclined ball-end milling can be integrated in the surface texture generation. This is due to the consideration that the rotation speed at the bottom of ball-end tool is low, and using the inclined ball-end milling can not only increase the cutting speed and but also reduce the damage at the bottom of the cutting tool. 2. A general kinematics model for texturing on a curved surface using spindle speed modulation can be developed. This thesis only considers surface texture generation on a flat surface, while in practice, a number of applications involve texturing on curved surfaces. 67  3. The relationship between the cusps texture design and friction behavior is still unclear. More cusp texture designs can be used for friction behavior study. The size of the geometric features plays an important role in the functionality of texture [1]. A smaller scale of cutting tool can be used for achieving smaller features using the kinematics model in this work, and the size effect of the texture on the functionality of the surface should be investigated. 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