Development of an Ultra Precision Machine Tool for Micro Machining

Quang Huy Pham School of Mechanical and Manufacturing Engineering The University of New South Wales

A dissertation submitted in fulﬁlment

of the requirements for the degree of

Master of Engineering

2010 January Abstract

The development of micro-fabrication has gone through a long period of evolution. The application of micro scale components has been enormous growth over recent years in not only micro-electro mechanical system (MEMS), micro-optical applications and micro-chemical applications but also biotechnology, industry and daily life applications. However, an effective mechanical micro machining method has not yet been in market or in industry. This thesis presents the development of a micro-machining research set-up through the design of the mechanical system, electrical and electronic hardware and the control system. The developed system also has an integrated machining visualization system. The micro machining carried out on the developed system is also presented. Special attention has been paid to the cut quality including the burr formation, chip formation, finished surface. Experiments have been done using different materials such as copper, aluminum, silica, titanium and platinum. Acknowledgements

This thesis is the result of my two years working as a research student in L-219 ground vehicle lab. It gave me a lot of valuable experiences that are relevant to my future professional life. On the academic side, I would like to give thanks to my supervisor Associate professor Jayantha Katupitiya. I really appreciate your help and guide. From the first time and contacted you for this research to now, you gave hints, and guide to find my own way in research, to overcome the difficulties. I am eternally grateful for this, and all of the time you have invested in helping me to achieve my goal. I appreciate the kindly help and advices from Dr Ray Eaton, Mr Jim Sander and Mr Alfried Hu. My Initial design work would not have been possible without your helps. For my postgraduate lab-mate James, Blair and Kim, to you I owe much. I did learned a lot from you. We did had a great time working together in L-219. To my house-mates and friends, Vicky, Ken, Louis, Van and Duc. You have helped me a lot and gave me supports when I had difficulties in my daily life and my research. The times I lived and hang out with you are the happiest time I have had in Sydney. Thanks for giving me another family when I live far away from Vietnam. To my dear family in Vietnam, dad, mum and my little brother, I love you all. Thanks for always interested in what I was doing without ever becoming tiresome. You have all been so supportive and I know it has dragged on for quite some time. I hope you take pride in the fact that a great deal of what I have achieved is due to your efforts and love. Contents

List of Figures v

1 Introduction 1

2 Literature Review 4 2.1 Current Implementations of Micro Devices ...... 5 2.1.1 Semiconductor Industry ...... 5 2.1.2 MicroDevicesinAutomotiveIndustry...... 5 2.1.3 Medical and Biomedical Applications ...... 6 2.1.4 Environmental Monitoring Applications ...... 6 2.1.5 Automation Applications ...... 7 2.1.6 IT and Telecommunications Applications ...... 7 2.2 Micro-machining Methodology ...... 8 2.2.1 Lithography and Other Silicon Micro-machining Technologies 8 2.2.2 LIGA & High Aspect Ratio Machining ...... 9 2.2.3 Bulk & Surface Machining ...... 10 2.2.4 Micro Electro Discharge Machining (EDM) ...... 11 2.2.5 Laser Machining ...... 13 2.2.6 Micro Ultrasonic Machining ...... 13 2.3 Mechanical Micro-machining ...... 15 2.3.1 Micro-machining Mechanics ...... 15 2.3.1.1 ChipFormation...... 16 2.3.1.2 Cutting Force in Micro-machining ...... 18 2.3.1.3 Material Properties and Size Eﬀect in Micro-machining 19 2.3.1.4 SurfaceQuality...... 20

ii CONTENTS

2.3.2 Micro-machining Machine Tools ...... 20 2.3.2.1 Machine Structure ...... 21 2.3.2.2 ActuatorsandControl...... 22 2.3.2.3 Spindle Technology ...... 23 2.3.2.4 Handling Issues ...... 23 2.3.2.5 MicroTools...... 25 2.4Conclusion...... 26

3 Micro Milling Process 27 3.1 Mechanics of Micro Milling Processes ...... 27 3.2 Chip Formation in Micro Milling ...... 34 3.3 Burr formation and Surface Finish, Material and Machinability in Micro Milling ...... 37 3.4 Micro Milling Tools and Issues ...... 40 3.5Conclusion...... 43

4 Design of the Ultra Precision Micro Milling Machine 45 4.1OverviewandConceptualDesign...... 45 4.2 Workpiece Manipulation System ...... 47 4.2.1 TheMicrostage...... 48 4.2.2 TheNanostage...... 49 4.2.3 Conﬁguration Workpiece Manipulation ...... 52 4.3 Tooling System ...... 53 4.4MicroTools...... 55 4.5VisionSystem...... 56 4.6 Cooling System ...... 57 4.7 Summary and Conclusions ...... 60

5 System Calibration and Control 62 5.1 System Calibration ...... 64 5.1.1 Calibration of the Machine ...... 64 5.1.2 Workpiece Surface Inclination Measurement ...... 66 5.2ControlSystem...... 68 5.2.1 Host Computer System ...... 68

iii CONTENTS

5.2.2 Realtime Application Interface ...... 71 5.2.3 ControlofMicrostages...... 73 5.2.4 ControlofNanostage...... 74 5.2.5 Control of Spindle System ...... 77 5.2.6 Control of The Cooling System ...... 79 5.3ToolPathInterpolation...... 80 5.3.1 CuttingParameterSet-up...... 80 5.3.2 ToolPathCreationandInterpolation...... 81 5.4 Summary and Conclusion ...... 84

6 Micro Milling Result 87 6.1ChipandBurrFormationExperiments...... 87 6.2ExperimentswithDiﬀerentMaterials...... 89 6.3 Experiments with Other Machining Techniques ...... 92 6.4 Summary and Conclusion ...... 93

7 Discussion 95 7.1 Summary and Achievement ...... 95 7.2Discussion...... 96 7.3FurtherWork...... 97

References 99

iv List of Figures

2.1 A typical photolithography process (www.coe.drexel.edu) ..... 9 2.2 The LIGA process (http://ankaweb.fzk.de) ...... 10 2.3 Wet etching of silicon wafer (www.parallel-synthesis.com) ..... 11 2.4 Two diﬀerent wet etching types (www.memsnet.org) ...... 11 2.5 A wire EDM process (www.new.manufacturinget.com) ...... 12 2.6 A typical USM process taken from [1] ...... 14 2.7 Comparison of micro-machining techniques [2] ...... 15 2.8 Instability during the formation of a chip during micro-machining: (a) segmented, continuous chip; (b) chip forming instability due to the built-up edge; (c) movement of a built-up edge to form a chip; (d)serrated,continuouschipcurl[3]...... 17 2.9 Two axis piezo actuated test bed conﬁguration taken from [4] . . 21 2.10 The ”two-way” SMA gripper grasping a lens (the lens has a diam- eterof250micronandalengthof500micron)[5]...... 24 2.11 Microgripper grasping the interface feature of a micropart [6] . . . 24

3.1Twoclassicalcuttingmodel[7]...... 28 3.2 Orthogonal cutting model ...... 29 3.3Twobasiccuttingforcemodel...... 30 3.4Twobasiccuttingforcemodel...... 30 3.5DeformationZone...... 31 3.6Generalcuttingforcemodel...... 32 3.7Cuttingratio...... 32 3.8Cuttingvelocitymodel...... 33 3.9Typesofmodelsforestimationofcuttingforce[8]...... 33

v LIST OF FIGURES

3.10 Chip formation with respect to the minimum chip thickness with

tool edge radius Re[9]...... 36 3.11 Burr deﬁnition and its location in slot milling (a) and machined slots(b)[10]...... 38 3.12 Milling cutter types (www.efunda.com) ...... 41

3.13 Wear in micro milling (1) working at Vc=130m/min (2)working at

Vc = 240m/min [11]...... 43

4.1Analysismodelofsystem...... 46 4.2 The first model of the vertical micro milling system ...... 47 4.3Newportvibrationisolationtable...... 48 4.4 The micro milling machine and work station ...... 49 4.5Recirculatingballscrews(www.physikinstrumente.com)...... 50 4.6 M-605.1DD translation stage (from PI catalogue) ...... 50 4.7 Working principle of nanostage (www.physikinstrumente.com) . . 51 4.8Thenanostage(fromPIcatalogue)...... 52 4.9 The Precise SC40 Spindle (www.fischerprecise.com) ...... 53 4.10 Graph of the change of SC40 spindle torque and power with respect totheRPM(www.fischerprecise.com)...... 53 4.11 The collet and the spindle mount (www.fischerprecise.com) .... 54 4.12 The micro end mill geometry parameters (www.pmtnow.com) . . 55 4.13TheDFW-V500camerafromSony...... 56 4.14Navitar12xzoommicrolense...... 57 4.15 Vision system with the milling machine ...... 58 4.16 Function diagram of PHK 525 HZ cooling system (from PHK 525 HZusermanual)...... 59 4.17FPHK525HZ...... 59 4.18 VIP4Tool cooling system(Dropsa) ...... 60 4.19 Spray tube(Dropsa) ...... 60

5.1 Schematic of the control system developed for micro milling machine 63 5.2 Calibration setup for checking perpendicular between two axes . . 64 5.3 Calibration setup for checking the spindle run-out ...... 65

vi LIST OF FIGURES

5.4 The micro coordinate measuring system used for checking the surface inclination ...... 66 5.5Cicuitdevelopedforinclinationchecking...... 67 5.6 Surface proﬁle of a copper surface in one dimension (1 count = 0.1 μm...... 68 5.7 The host computer with PCM-6893 and PC-104 modules ..... 69 5.8Therealtimemicrokernel[12]...... 72 5.9 (1) C-862 Mercury Controller (www.pi.ws) and (2) Three con- trollersconnectedinnetwork...... 73 5.10 Velocity and position response of the vertical stage at K = 150, I =0,D=0 ...... 75 5.11 A E-500 PZT controller produced by PI (www.pi.ws) ...... 75 5.12 Signal path diagram of an E-500 series PZT controller (www.pi.ws) 76 5.13PCS410solidstatefrequencyconverter...... 78 5.14 Speed response of spindle control by PCS 410 (www.ﬁscherprecise.com) 78 5.15PCS410solidstatefrequencyconverterconnection...... 79 5.16 Diagram of workpiece cooling system control ...... 80 5.17 Working of the linear algorithm with generalized periodicity[13]. 85 5.18 Example of line interpolation algorithm[13]...... 86 5.19CircleinterpolationalgorithmMatlabsimulationresult...... 86

6.1 Experiment set up and machining progress ...... 88 6.2Aseriesofslots...... 88 6.3Apartofaslotthatwaspartlyremovedburrs...... 89 6.4 A part of a slot with 20 μm depthofcut...... 89 6.5 A micro slot, a micro hole and a micro square on copper created by 100 μm micro endmill, depth of cut is 70μm...... 90 6.6 SEM images of micro hole on copper. Depth of cut is 70 μm, tool diameter 100 μm...... 91 6.7 SEM images of micro square on copper. Depth of cut is 20 μm, tool diameter 50 μm...... 91 6.8 SEM images of micro slot on copper. Depth of cut is 10 μm, tool diameter 50 μm...... 92

vii LIST OF FIGURES

6.9 SEM images of micro hole on silica. Depth of cut is 50 μm, tool diameter 50 μm...... 92 6.10 Optical microscopic image of slots on platinum workpiece. Depth of cut is 30 μm, tool diameter 50 μm...... 93 6.11 An 100 μm micro endmill after cutting by FIB (front surface) and laser(rightsurface)...... 94

viii Chapter 1

Introduction

The use of micro-scale components has seen an exponential increase in the recent period. Given the availability of micro tools, especially the micro end-mills now reaching diameters as small as 20μm, it is worthwhile re-considering the use of conventional milling methods of micro-machining. Among the applications of micro-scale components are ink-jet nozzle arrays, accelerometers, electro- mechanical switches, cantilever light modulators and many parts of medical equipments. Conventional milling methods may provide cost eﬀective ways of achieving the desired precision and productivity required by the micro component examples listed above. This thesis describes in detail the design and manufacture of a complete micro- machining system that employs conventional milling methods. This thesis also presents the details of the control system of the machine and its integrated microscopic machining visualization system. The mechanical system uses a set of ultra high precision micro stages, a nano stage capable of minimum incremental motion as low as 2 nm and a high precision spindle that can reach speeds as high as 90000 rpm. This spindle has a runout less than 3 μm and it is able to ac- commodate a variety of tools with high precision. In this study tungsten carbide end-mills of diameters ranging from 100 μmdownto20μmwithsquaredand ball nosed ends have been used. To Verify the successful operation of the designed machine, a variety materials have been machined and photographed using scanning electron microscopy. These images enable the quantiﬁcation of surface quality and the burr formation.

1 There are many techniques available for the micro machining of components. In this study, mechanical micro machining has been chosen due to cost effectiveness and the straight forward way of controlling the precision and accuracy of machining. The system presented in this work has a high precision spindle fixed on the machine. The work-piece is moved via a set of high precision micro and nano stages. A method is also provided to ensure parallelism between the work-piece surface and the direction of motion. The machine is capable of machining 3D profile, especially with the use of ball nosed end-mills. The control system is based on the real-time application interface (RTAI) embed- ded into a Linux variant. A digital image processing library was also incorporated into the Linux system thereby enabling the image capture, processing and dis- playing on the control computer, to enable the visualization of the machining process. The visualization system is also used for the coarse positioning of the work-piece relative to the tool. This system uses a camera with a microscopic lens, interfaced through a firewire port. As part of the machining, a variety of materials generally used in fabricating micro scale components have been machined using the entire variety of micro end-mills with square ends and ball nosed ends. In addition, a large variety of spindle speed/feedrate combination have also been used to study the cut quality. The aim of this project is to design and build an effective mechanical micro milling system which can be used to manufacture biomedical components such as the medical implants (microbarbs) and microgrippers which can be used in surgery and in assembly tasks. A machine configuration was designed and a control system was developed. Some experiments have been done to check the capability of the system before using for manufacturing. The development of this system will be presented through this thesis in the following fashion. Chapter 2 contains a detail review of the current state of the art of microfabrication techniques as well as their potential applications. The mechanical micro- machine and applications is discussed in more details. The background information of micro milling process is discussed in Chapter 3. Micro milling mechanism, parameters and machinability are discussed in this

2 chapter. Chapter 4 presents the system conﬁguration and design. All the devices and components used in this system and how they are connected to each other to make the machine are described in this chapter. Chapter 5 discusses the calibration of the system to ensure the precision alignment of the workpiece surface, perpendicularity of axes, and the control system. A method to calculate the machining parameters and to carry out the tool path interpolation are also described. Chapter 6 presents the micro component production procedures and the results of such production methods.

3 Chapter 2

Literature Review

Recently, micro-machining and its product market has shown tremendous development. Micro-machining (micro-fabrication, miniaturization, micron manufacturing) can be defined as the technique that is used to fabricate structures, devices and profiles in micron or sub-micron scale or devices that require micro or nano accuracy. Components produced through micro-fabrication has been used in a wide variety of industries ranging from advanced medical industries to automotive industry. The success of using the micro-fabricated components in these industries have motivated further research into micro-scale components production. The same has significantly strengthened the micro-component market. Some advantages of using micro components instead of their macro or full scale components are:

• Low energy consumption during production due to smaller size of the components.

• Reduced material wastage due to micro and nano scale chip formation (which generally amounts to the direct waste) in contrast to full size chip formation in conventional machining.

• Facilitates the integration of electronics at the time of micro machining (e.g. machining of sensor elements on silicon wafers)

• The components manufactured can be tightly compacted within the product itself, thereby miniaturizing the complete product.

4 2.1 Current Implementations of Micro Devices

• In the case of components such as micro-machined sensing elements, their sensitivity and accuracy can be signiﬁcantly improved, leading up a to a wide selection of operational parameters.

One of the main disadvantages of micro machined components is their handling. This requires high accuracy and high precision machinery that can apply or sense micro level forces.

2.1 Current Implementations of Micro Devices

This section gives examples of currently deployed micro devices in a number of dominating industry applications.

2.1.1 Semiconductor Industry

With intense research taking place in the electronics industry, with a focus on miniaturizing and compacting the maximum number of electronics devices per foot-print, micro machining on silicon wafers has been in high demand. Silicon wafers has been the material of choice due to excellent electron mobility and hole mobility aided by the very acceptable wafer oxidation that takes place during micro machining. It is not diﬃcult to imagine the thirst in these industries to develop more advanced and innovative machining methods to achieve this feat of compaction. [14]

2.1.2 Micro Devices in Automotive Industry

Automotive industry has placed the greatest demand on micro machined components by emphasizing the need for accurate, reliable and cost eﬀective sensors. The number of sensors required by the automotive industry increases every year by 7-10% and the market was $12.8 billion in 2008 as reported in [15]. Speed and position sensors account for 38% of the total market value of total automotive sensors, followed by oxygen sensors (20%), mass air ﬂow sensors (13%), acceleration sensors (11%), pressure sensors (10%), thermometers (5%), and others (3%) [16]. Sensors that dominate the current Si automotive market are pressure

5 2.1 Current Implementations of Micro Devices sensors. It is said that Si has been used in the construction of pressure sensors making them the most mature Si micro-machined devices commercially available today [16]. Piezoresistive and capacitive are the two main varieties of Si pressure sensors and manifold absolute pressure (MAP) sensor which is used to control the air-fuel ratio in the engine is a prime example of Si pressure sensor. Another type of sensor is the accelerometer that is used to activate the air bags systems by detecting the impact. Accelerometers are speciﬁed by their operational g values, and hence need only a limited variety. In contrast, pressure sensors has a much wider selection range and hence wider fabrication methods. Hence, one of the ﬁrst sensors to come out through micro-fabrication was the accelerometer.

2.1.3 Medical and Biomedical Applications

Unlike the mechanical sensors that has been in the market for many many yeas, the miniaturization of sensors and their manufacture using medically acceptable material has shown a dramatic increase in the number of micro-machined components used in the medical industry. Within a total market of $2000 billion, about $100 million is used in medical instrumentation [17]. This clearly indicates the strength of the medical instrumentation market and the very high potential, the micro components has within that market. Among the micro-machined components are; life saving disposable diagnostic sensors, systems that speed up drug delivery and smart pills by way of MEMS that can eﬀectively deliver drugs to dif- ﬁcult to reach locations. In addition, micro-channels, micro-valves, micro-pumps, micro-mixers and micro-reactors are in use, albeit most of it disposable. Further, mechanical micro machined components such as micro grippers and micro surgery tools also has a substantial demands for tasks such as cell separation and eye surgery.

2.1.4 Environmental Monitoring Applications

The implementation of micro-fabrication in environmental monitoring are the sensors for air detection, water pollution testing such as micro-spectrometers,

6 2.1 Current Implementations of Micro Devices micro gas chromatography systems, micro ion mobility spectrometers, and infrared detectors [16]. The Nexus (Grace/Nexus, July 2000) predicted that the total market value of micro-fabricated component usage in environmental monitoring in 2004 to be $1.75 billion, in comparison to $520 million in 2000. The majority of sensor usage is in sensors for air quality detection, water pollution testing using micro-spectrometers, micro-gas chromatography systems, micro ion- mobility spectrometers and infrared detectors - all of which has to play important roles in environmental monitoring.

2.1.5 Automation Applications

Micro-machined devices play an important role in Automation Industry. The total market value of these devices is $1.85 billion in 2004 as reported by Nexus (Grace/Nexus, July 2000). Some popular micro-machined devices that are used in other application areas such as pressure sensors, flow rate sensors, accelerometers, IR imaginer and gyroscopes. besides ISFETs (the operation of ion-sensitive field effect transistors which are used to measure ion concentrations in solution), micro-hydrometers (the micro scale version of the hydrometers which are used for measuring relative humidity), and valves for industrial control and automation are also very attractive.

2.1.6 IT and Telecommunications Applications

The information technology (IT), peripherals and communications market is also one of the largest markets for micro-fabrication. In IT and peripherals, micro- fabrication is used to manufacture memory read/write devices such as hard disk drive heads, magneto-optical heads for optical disks, projection displays, gyros and electronic paper. Some sensors used in memory read/write devices such as magnetoresistive (MR) sensor are made from permalloy (Ni-Fe) or other materials that are different from Si. Another application of micro-fabrication in IT is the ink-jet cartridges. The heads of printers are very complicated with the combination of high temperature fluidics, integrated electronics, sensors and actuators. The bubble-jets which are inexpensive and disposable and the piezos which are expensive are two different types of print heads. The micro-fabrication

7 2.2 Micro-machining Methodology techniques used for this are mechanical drilling, etching and electro-forming. Telecommunication applications are also a large and promising market for micro- manufacturing. Fiber optics, miniature laser diodes, light ampliﬁer, ﬁlters, multi- plexers and optical, micro switches are prominent examples of these applications. There are still many telecommunication applications that can be developed and optimized using micro-fabrication. In conclusion, there is a high demand for micro-manufacturing and micro systems. Many researches on micro-fabrication have been done to improve the machining process and its quality. Some of them are still only at research stage and in laboratories while some have already been widely used in industry and commer- cialization.

2.2 Micro-machining Methodology

2.2.1 Lithography and Other Silicon Micro-machining Tech- nologies

Lithography is a top-down miniaturization method which is done by building down from bigger chunks of material. This technique which is shown in ﬁgure 2.1, is used to transfer copies of master patterns onto the surfaces of a solid material such as a silicon wafer [16]. It is a traditional silicon micro-machining technology that is developed directly from etching and deposing processes used in microelectronics in which silicon wafers are machined with chemicals or physical etch and elements are realized layer by layer from a silicon substrate [2]. The most common form of lithography used is photolithography which is used mostly in IC industry. This is the optical lithography which selectively remove parts of a thin ﬁlm or the bulk of a substrate by using light to transfer a geometric pattern from a photo mask to a light sensitive chemical photo resist on the substrate. Photolithography has a limit in resolution which is a function of the wave length and the numerical aperture as described by Harriot [18]. Another form of lithography is electron beam lithography (or e-beam lithography) which is the progress of scanning a beam of electrons in a patterned fashion

8 2.2 Micro-machining Methodology

Figure 2.1: A typical photolithography process (www.coe.drexel.edu) across a surface covered with resist (a ﬁlm) [19]. This technique has a higher resolution compared to photolithography. Vieu reported that the resolution of this method can be pushed below 10nm for isolated features using the conventional polymethylmethacrylate (PMMA) organic resist [20]. However, the type of materials that can be machined using this method is very limited.

2.2.2 LIGA & High Aspect Ratio Machining

LIGA (an abbreviation from German phrase: Litographie Galvanoformung Ab- formung), which is based on lithography with X-ray source, is the most popular method primarily used for manufacturing a single mould for plastic component replication[2]. By using small wavelength, this technique can machine deep in the resist. As a result of this, obtainable components has a high aspect ratio with the thickness of thousands micrometers with sub-micron pattern precisions [2]. A typical LIGA progress is shown in ﬁgure 2.2.

9 2.2 Micro-machining Methodology

E.Genili stated that this the availability of synchrotron accelerator and pro-

Figure 2.2: The LIGA process (http://ankaweb.fzk.de) duction costs of the single mould or master element is the main limit of this technique [2]. Most of the research about LIGA have been done focused on the synchrotron sources. Some focus on the material found that the limitation in the range of materials that can be electro-formed preclude many application areas [14]. However, there have been just few applications in the industry.

2.2.3 Bulk & Surface Machining

Bulk machining is a subtractive technique for Si based on dry or wet etching techniques, used for manufacturing silicon devices. A wet etching progress and its’ two different types are shown in figure 2.3 and figure 2.4 respectively. In wet etching, immersion in a chemical solution results in the dissolution of materials. This is a simple and inexpensive technique that can give high etching rate with good selectivity for most materials. However, the requirement of the mask is complicated. Besides, it is inadequate for defining feature size that is less than 1 micron. Dry etching uses reactive ions or a vapor phase etchant or sputter instead of a chemical solution. This technique is capable of defining small features of size which is smaller than 100 nm. This technique is expensive and hard to implement but it can be adopted for various materials such as metals, ceramics and plastics while wet etching is mainly used for machining silicon and glass. The main limits of these techniques are the difficulty to obtain high thickness,

10 2.2 Micro-machining Methodology

Figure 2.3: Wet etching of silicon wafer (www.parallel-synthesis.com)

Figure 2.4: Two diﬀerent wet etching types (www.memsnet.org) masks alignment in layer by layer processes. Besides, it is very limited in machining complex three-dimensional features. However, these can be used for mass production of microchip and electronic devices.

2.2.4 Micro Electro Discharge Machining (EDM)

Electro discharge machining (EDM) is a most eﬀective non-conventional machining technique that uses electrical energy to generate electrical spark. Material removal mainly occurs due to thermal energy of the spark. When two electrons are enclosed proximally in a dielectric liquid, a voltage pulse can produce a spark discharge between them, resulting in a small amount of material removal from both electrodes. A pulse at discharge energies in the range of micro-Joules results in the continuous material removal process [21]. EDM can be used for not only hard metals or carbides but also semiconductors and conductive ceramics. Generally the material has to be electrically conductive. There are two primary EDM techniques. The ﬁrst technique is sinker EDM where

11 2.2 Micro-machining Methodology an electrode and work-piece are submerged in an insulating liquid such as oil. The power supply which connects electrode and work-piece generates an electrical potential between them. Dielectric breakdown occurs in the fluid forms a plasma channel and a small spark when the electrode approaches the workpiece. The fluid used in this technique helps to flush chips away, and serve as a coolant to reduce the effect of heat. This technique can be called cavity type EDM, volume EDM or rams EDM. The second technique is wire EDM shown in figure 2.5. In this technique a very thin wire which is made from special brass serves as electrode. The electron discharges cut the work-piece when the wire is slowly fed through the material. EDM is mainly used for drilling holes, milling, making dies, and manufacturing

Figure 2.5: A wire EDM process (www.new.manufacturinget.com) prototypes. Although EDM has some limitations such as the slow rate of material removal, time consuming manufacturing process which most of the researches on EDM focus on, it is still an eﬀective micro machining technique. This is because EDM can be used for complex shapes that are diﬃcult to machine using standard cutting tool. Moreover EDM can be used for hard material and very small workpiece which may be damaged by the excess cutting tool pressure from cutting tool. Another technique that has a similar material removal mechanism with EDM is focus ion beam machining (FIB). This technique consists of an ion beam which is accelerated and focused in an area of few nano meters in diameter. Similar to SEM microscope, in order to form images electrostatic lenses are used to scan

12 2.2 Micro-machining Methodology the beam over the samples surface. Because of the eﬀect of the beams’ energy and current, the material of the scanned areas will be sputtered away, allowing micro and nano-machining without the use of masks [22]. The main limits of this technique are the time consumption when machining and it is diﬃcult to machine three dimensional features and high aspect ratio (the depth is bigger comparing with other dimensions) features.

2.2.5 Laser Machining

Laser machining is a technique that uses high density light radiation as a machine tool for micromachining a wide range of materials without any mechanical interference or chemical reaction with the workpiece [2]. The material removal process starts when electromagnetic waves interact with the particles in the material. Afterwards, the electrons will reradiate or be constrained by the lattice. The lattice breaks down and material begins to melt when enough energy is put into it [14]. Pyrolytic and photolytic progress are two different types of laser machining pro- gresses. In pyrolytic processes, laser energy is absorbed by heating the material resulting in a temperature rise, melting or evaporation of material. Whereas in photolytic processes, photon induces chemical reactions [23] . The limitation of laser machining, especially pyrolytic processes, is the difficulty in controlling the area affected by heating. The appropriate energy of laser beam is different for machining different materials.

2.2.6 Micro Ultrasonic Machining

Ultrasonic machining (USM) technique which uses the vibration of the abrasive grains between the tool and work-piece by a high-frequency vibrator system has effectively been used for fabricating various patterns and drilling holes on brittle materials[1; 24]. A typical USM process is shown in figure 2.6. The vibrated abrasive grains that impact on the workpiece surface, cause cracks, and finally remove the material from the workpiece and to a certain extent from the tool which has the same shape and size as that of the designed part [1]. An assembly of transducer, concentrator, and other related parts that transmit

13 2.2 Micro-machining Methodology

Figure 2.6: A typical USM process taken from [1] the ultrasonic vibration to the abrasive grain causes a large eccentricity of tool rotation[1]. This problem can be solved by using the workpiece vibration. Other problems and also the limitations of USM are the surfaces generated by USM are normally rough and covered by deep penetrated cracks and the tool wear that affects machining accuracy[1; 24]. Two other non-conventional micro-machining techniques are Abrasive water-jet micro-machining, pulsed water drop micro-machining. Those techniques are not as widely used as the techniques above. Figure 2.7 compares the properties of different techniques. In this figure, + stands for high quality, − stands for low quality, ± stands for average and ++ stands for very high quality. As shown in this figure and discussed above, Lithography techniques are limited by machinable materials, bi-dimensional approach and real high aspect ratios are difficult to archieve without expensive subsequent masking operations and bonding methods. the LIGA and S-LIGA methods appear still too expensive and the limits related with mask-based process [2]. A technique that is very promising and still under research and development is mechanical micro-machining. This technique has some advantages comparing with other techniques such as machining time, wider range of machinable materials and ability of machining 3D profiles. Mechanical micro milling technique used in this thesis is a part of mechanical micro machining which is described in more detail in the next section.

14 2.3 Mechanical Micro-machining

Figure 2.7: Comparison of micro-machining techniques [2]

2.3 Mechanical Micro-machining

Mechanical micro-machining techniques developed from traditional machining techniques that have been used from a long time ago to produce large scale components. These techniques such as turning, milling and grinding are still widely used in mechanical and manufacturing engineering. They can be developed and improved for micro machining requirement. The two most widely used techniques are micro-milling and micro-grinding. Some aspects and properties of mechanical micro machining will be discussed next.

2.3.1 Micro-machining Mechanics

The first thing that need to be considered in micro-machining is the mechanics of the cutting process. Because of the differences in scaling and the size of tools and workpiece, there are differences between micro and conventional cutting processes. The chip formation and cutting force, the effects of material properties such as micro-structure, size effects and surface generation are the points that most of the researchers in this area focus on.

15 2.3 Mechanical Micro-machining

2.3.1.1 Chip Formation

The material removal processes in micro-machining have significantly different behavior and characteristics compared to conventional macro scale machining. There are some characteristics of chip formation process that are of interest. Among them are chip curvature, minimum chip thickness, uncut chip thickness and the effects of rake angle to the cutting force. Understanding chip formation is the first step to good chip control, an essential requirement for automated machining. The lack of this often leads to coarse surface finish, poor machining accuracy, and problem with chip removal from the machining zone [25]. Therefore, the chip formation need to be addressed. Chip curvature is a special parameter in machining operations from which a continuous chip is produced. Jackson stated that there is a great deal of uncertainty regarding the mechanism of curly chip formation and the factors determining the chip radius. Observations are made in initial chip curl in the simplified case of orthogonal cutting at the micro scale [14]. A simple primary shear plane and frictional sliding of the chip along the rake face are used to model the cutting process. When the region of the chip and tool interaction at the rake face is treated as a secondary shear zone and the shear zones are analyzed by means of slip-line field theory, it is predicted that the chip will curl. As a result of this, chip curvature may be interpreted as the consequence of secondary shear. Tight chip curl is usually associated with conditions of good rake face lubrication [14; 26]. It is suggested that tight curl is an integral part of the initial deformation and the process of continuous chip formation is not uniquely defined by the boundary conditions in the steady state and that the radius of curl may depend on the build-up of deformation at the beginning of the cut [26]. The instability during the formation of a chip during micro-machining is shown in figure 2.8. Jackson [3] stated that previous treatments of chip curl analysis have focused on chip formation with perfectly stiff cutting tool. Whereas, during micro-machining the cutting bends the chip as it machines the workpiece material. Therefore it is encouraged in [3] that primary chip curl models must account for deflection of the cutting tool by bending during an orthogonal micro-machining operation. From this, Jackson [3] expressed an expression that shows the relationship between chip

16 2.3 Mechanical Micro-machining

Figure 2.8: Instability during the formation of a chip during micro-machining: (a) segmented, continuous chip; (b) chip forming instability due to the built-up edge; (c) movement of a built-up edge to form a chip; (d) serrated, continuous chip curl [3] radius and rake angle.

d2 r = (2.1) s cos αb sin φ Where r is the chip radius (mm), s is the lamellar (striations) spacing (mm), tool rake angle during bending of the cutting tool (deg) and shear plane angle (deg) are αb,φ. Another important factor to determine the machining accuracy attainable for a specific set of cutting conditions is the minimum thickness of cut [27]. This is defined as the minimum uncut chip thickness of chip removed from the work surface at a cutting edge under perfect performance of cutting system. Ikawa [28] has experimentally produced a very thin chip nominally of the order of 1 nm under the combination of a specially prepared diamond cutting edge and free machining of electroplated copper as a work material. There is a significant difference in chip formation when machining in low and high speed. In low speed machining, many of the removed material are formed as chunks of material rather than nicely formed chips. The chunks are possi-

17 2.3 Mechanical Micro-machining bly formerly parts of larger chips that have broken down so the chip thickness values should be recalculated based on the larger chip size. In contrast, at high speed machining the chips are more consistent in terms of their length, width and depth. Moreover, their lamellar spacing is regular in period. That means cutting condition in high speed machining is more stable (minimization of any sudden changes in the cutting direction) [14]. In [14], molecular dynamics simulation has been used to simulate the chip formation. There, it was assumed that none of the built-up edge passed underneath the cutting tool. This means that the material that sticks to the tool edge was eventually deposited on to the chip that slides over the cutting tool. With the use of this method, it has been found in [3] that primary chip curl is initiated by the amount of material deposited on to the cutting tool, which manifests itself as a wedge angle that controls the amount of material pushed into the base of segment of the chip between oscillations of the primary shear plane.

2.3.1.2 Cutting Force in Micro-machining

Tool life, machining accuracy and surface finish quality are significantly effected by the cutting forces. There are three basic forces in cutting process. The fist and second forces are tangential (Ft) and feed forces (Ff ) which exerted only in the directions of the cutting velocity and the uncut chip thickness respectively. These two forces are the only two forces in orthogonal cutting in which the cutting progress is assumed to be uniform along the cutting edge. Consequently, it is a two-dimensional plane strain deformation process without lateral spreading of the material. However, in oblique cutting, the cutting edge is oriented with an inclination angle and there is another force that acts in the radial direction (Fr) [29]. The resultant force on the chip is formed by the tangential force and feed force. These forces are affected by machining parameters such as feedrate, depth of cut, spindle speed, run-out error and other effects such as vibration. Finite element method is commonly used to model and simulate the effects of these parameters.

18 2.3 Mechanical Micro-machining

2.3.1.3 Material Properties and Size Eﬀect in Micro-machining

The machining accuracy, surface finish, chip formation and cutting forces are also affected by material properties. There are some factors that need to be considered when looking at the effect of material properties. They relate to micro-structure and material grain orientation. The cutting process in micro-machining is performed within a grain because the depth of cut is usually less than the average grain size of a polycrystalline aggre- gate [30]. Yuan et al. [30] investigated the effect of crystallographic orientation of the substrate material on cutting forces and surface quality in diamond cutting of single crystal copper and aluminum. It has been found in this study that the crystallographic orientations of the substrate material being cut has a great influence on the cutting force and surface roughness. This study also found that the shear strength of a crystallite is not a constant but varies with the orientation of crystallography. Lee et al. [31] stated that if the change in the crystallographic orientation of substrate material with respect to the cutting direction is known the pattern of variation in the micro-cutting force can be predicted. In addition, the chip formation mechanism is also influenced by the crystallographic orientation [32]. However, none of the researches above state the mathematical relationship between the orientation and the cutting force, surface roughness and chip formation mechanism. In addition to the effects of material properties, the size effect is also investigated. There is a substantial increase in the energy required with decrease in chip size in material removal process. This is called size effect. If the material being cut is very brittle or the compressive stress on the shear plane is relatively low, micro- cracks will grow into gross cracks, giving rise to discontinuous chip formation [14]. The origin of the size effect in metal cutting is believed to be primarily due to short-range inhomogeneities present in all commercial engineering materials [33]. Another study by Liu et al. [34] stated that material strengthening at small uncut chip thickness values is an important contributor to the size effect in specific cutting energy. The effects of two main strengthening factors such as the con- tribution of the decrease in the secondary deformation zone cutting temperature

19 2.3 Mechanical Micro-machining with decrease in uncut chip thickness and strain gradient strengthening on the size eﬀect are investigated in this study.

2.3.1.4 Surface Quality

When looking at the surface quality, the factors that affect the surface roughness, surface finish and the mechanisms that cause them are investigated. Zhang et al. [35] professed the view that the non-homogeneous distribution of micro-hardness present in the material has been a major random excitation source which causes the surface irregularity. Besides, the effect of tool vibration on this is quite significant at low values of feed due to the presence of a powerful random excitation system [36]. It is also stated in this study that the decaying of the profile pattern in the form of an arc-chain when a low value of feed is used suggests that the prediction of surface roughness indices based on geometrical-based theoretical formulas alone may not be accurate. For example, the surface roughness values produced in single phase ferrite and pearlite do not monotonically increase as feedrate is increased due to the trade off between traditional effect of feedmarks and the effect of minimum cutting thickness [37]. By using molecular dynamics simulation, Shimada et al. [38] estimated that the ultimate surface roughness in cutting monocrystalline copper is less than 1 nm. In micro and nano scale machining, cutting temperature has a significant effect on surface finish, surface roughness and tool life. This temperature can be calculated in molecula dynamics simulation by assuming that cutting energy totally transfers into cutting heat and results in the rising of cutting temperature and kinetic energy of the system. The extend of research in this area is still very limited and most of the reported result have been generate using simulation models and are not verified through reliable experimentation.

2.3.2 Micro-machining Machine Tools

In contrast to machining macro scale components, the machines used in micro- machining need to satisfy new requirements such as the need of micro-tools and

20 2.3 Mechanical Micro-machining the expectation of submicron accuracy. Consequently, the high precision actuators with minimum incremental motions down to tens of nanometers are required.

2.3.2.1 Machine Structure

Two conventional machining prototype that are widely used for micro-machining are milling and grinding. Different prototypes with different number of axes, different handling methods have been built and tested. Most of the studies in this area tried to approach the high accuracy, efficiency, and low cost. Kussel et al. [4] developed an machine tool of 100-200 mm in overall size using inexpensive components as shown in Figure 2.9. This machine allows the manufacture of micro-mechanical components having sizes from 50 micron to 5 mm. In another study, a meso-scale machine tool for producing meso-scale components (100 to 10,000 micron in size) is built to achieve precision and productivity [39]. Vogler et al. [40] built a meso-scale machining system with more compactness. This system used voice-coil actuated and piezoelectric feed drive technologies. Many endeavors on building accurate, low cost and effective micro-machining

Figure 2.9: Two axis piezo actuated test bed conﬁguration taken from [4] systems have been made. However, there is still a gap between research and industry. A very limited number of researches can be applied in the industry.

21 2.3 Mechanical Micro-machining

2.3.2.2 Actuators and Control

The excellent choice of actuation and the implementation of a good control methodology is essential to fulfil the requirements of a good micro-machining system. There have been many investigations into the development of actuation and as a result, a large number of high quality actuator are currently available in the market. In Holmes et al. [41], the magnetic-bearing motion control stage (the Angstrom Stage) that can achieve very high positioning accuracies with an ultimate goal of 0.1 nm positioning resolution is investigated. Although the description of stage design and components and the development of control algo- rithms are presented, a practical stage is not proposed in this study. Other high accuracy precision actuators which are based on piezoelectric actuation have been developed. These are either piezoelectric motors with bearing systems or flexure stages. These actuators are now widely used in micro-machining systems where precision is critical. Together with the use of air bearings, these systems have high resolution (subnano), fast response, low friction, stiffness, and low or no backlash. The measuring instruments used in these systems are high resolution encoders or especially capacitive sensors. In order to effectively control those systems, many control systems have been developed. Yagyu et al. [42] designed and developed an adaptive control system for multiple micro motion systems based on fuzzy control. The system developed in this study had two parts. The reasoning part of the system is the part in which the system accepts the present x-y position, speed, direction and the rest of energy of the micro- machines as inputs, which are non deterministic. These data are interpreted by probability distribution and combined into macrostates to represent group behaviors of the micro-machines. A BAM ( Bidirectional Associative Memory) is used to avoid increasing fuzziness. The planning part of the system is the part in which the system generates a rough control plan for micro-machine groups and a detailed plan for individual micro-machines using the genetic algorithm. The control plans predict the future working states and directional signals for the micro-machines. Although the effectiveness of the method has been theo- retically demonstrated, the practical implementation of this systems has not yet been achieved.

22 2.3 Mechanical Micro-machining

Generally, PID controllers and advanced PID controllers with auto-turning parameters are still widely implemented in this area because they appear to be the controllers that can easily be applied in most of the systems with reliability and robustness.

2.3.2.3 Spindle Technology

Another aspect which is also very important and interesting in micro-machining is the spindle technology. The spindle systems may be the work-piece spindle system with the work-piece centering system which hold the work-pieces in grinding machine or the systems that hold the tools in milling machine. Many attempts to accurate control and investigate the vibration, torque, and the speed of high speed spindle systems have been made. Barney et al. [43] implemented an adaptive LMS (Least Mean Square) digital control algorithm to maintain concentricity of an intentionally unbalanced spindle. This wrapped over an existing PID rigid-body controller and concentricity is improved by two orders of magnitude. Another study, Du et al. [44], attempted to minimize track min-registration in ﬂuid bearing and ball bearing spindles using H2 controller with the better performance of ﬂuid bearing comparing drive. While Huang et al. [45] investigated the dynamic instability of a gas bearing spindle.

2.3.2.4 Handling Issues

Another issue when design and construct a micro-machine are material handling. Comparing to conventional machining, material handling in micro-machining is more difficult because of the effects of the size of the components. Qiao et al. [46] suggested using suction of vacuum pumps for holding micro-components and described a new formula to estimate suction force which is believed to be linear with the size of micro-components and in direct proportion to the pressure difference. the aperture dimension of suction plate was believed to be approximately two-thirds of the size of the micro-components to obtain the required suction force with as low pressure difference as possible [46]. another handling method is the use of shape memory alloy (SMA). Bellouard et al. [5] developed an SMA based

23 2.3 Mechanical Micro-machining micro-gripper, shown in Figure 2.10, using the integration and monolithic structure concept. This method could help to avoid assembly of devices with small dimensions but it is hard to control the gripping force and there is a limitation in the gripping force in this method. Anther study, Dechev et al. [6] fabricated a compliant, passive gripper and soldered it to a robotic arm as shown in Figure 2.11. This gripper can grasp a micro-part, remove it from the chip, reorient it about two independent axes, translate it along the axes to a secondary location, and join it to another micro-part.

Figure 2.10: The ”two-way” SMA gripper grasping a lens (the lens has a diameter of 250 micron and a length of 500 micron) [5]

Figure 2.11: Microgripper grasping the interface feature of a micropart [6]

24 2.3 Mechanical Micro-machining

2.3.2.5 Micro Tools

Another important part of micro-machine is micro tool. The importance of solv- ing micro tools’ issues such as tool wear, tool life, tool geometry and design, and tool material cannot be under-emphasized. The majority of micro tools are still being produced by conventional methods based on grinding operations. A study, [47], developed an efficient and accurate method for fabricating micro cutting tools. In this study, ultrasonic vibration grinding was used to produce cylindrical tools and micro flat drills of ultra fine grain cemented carbides. Lee et al. [48] suggested using cylindrical grinding using electrolytic in-process dressing (ELID) technique to manufacture micro tool. This method is said to reduce the grinding force and surface roughness. The surface roughness was found in this study was 55-86 nm using atomic-force microscopy. Another method, which is different from grinding, is focused ion beam introduced in Picard et al.[49]. In this study, focused ion beam sputtering is used to shape a variety of cutting tools with dimensions in the 15-100 micron range and cutting edge radii of curvature of 40 nm. The shape of each micro-tool is controlled to a pre-specified geometry that includes rake and relief features [49]. This method is capable of manufacturing tools with various type of material. Moreover, it allows observation of the tool during fabrication, and therefore reproducible features are generated with sub-micron precision. Langford, 2002 [50] and Adams, 2000 [51] also investigated the use of focus ion beam on producing micro tool with 3D shape or curvilinear features. Another technique that is widely used in both research and industry to produce micro tools is electro-discharge machining (EDM). This technique is better in machining time compared to focused ion beam technique and capability of producing more complicated shaped tool with better accuracy comparing with conventional grinding technique. It is capable of producing tools with diameters down to 3 micron and lengths up to fifty times of their diameter [51]. Understanding tools characterization is very important to improve surface quality, tool life and to design micro tools. It is shown that unpredictable tool life and premature tool failure present a serious concern in micro-machining [52]. Gong at al. [53], when modeled the micro-drill as a pre-twisted, rotating beam, sub- jected to a compressive axial force and radial forces at the drill tip and found that

25 2.4 Conclusion the critical speeds and the buckling loads will improve with an increase in the cross-sectional area and of the helix angle and markedly improve by decreasing the flute length. This study also stated that the buckling loads decrease almost linearly with an increase in the rotational speed of the drill. A suitable tool material is critical in design and produce micro tools. Convention- ally, fine grade carbides are the principal tool material. Another material that is widely used for producing micro tools especially micro grinding tools is diamond. While tungsten carbide is used for fabricating micro tools such as micro end-mill and micro drill [30]. Tool wear is another factor that need to be considered in order to achieve longer tool life. Bao et al. [54] believed that the typical tool wear increase cutting forces and cutting edges wear out very slowly when soft materials are cut. Moreover, uneven cutting force on both cutting edges exacerbated by deflection decreasing tool life significantly [55]. However, imbalance of tool wear on both cutting edges is believed to be suppressed to extend tool life by reducing tool stiffness to the lowest possible value beyond breakage of the ball end mill [54].

2.4 Conclusion

Micro-machines and micro-machining techniques have been developed for a few decades. However mechanical micro-machine techniques are still new areas in which more researches are required. Because of their advantages compared to other techniques, these techniques are gradually widely used in both research and industry. In mechanical micro-machining techniques, micro milling technique is used to fabricate complicated 3D proﬁles especially surface machining. This technique can be applied for various type of materials. Therefore, micro milling is a promising technique for manufacturing micro components used in everyday living applications, researches and industry.

26 Chapter 3

Micro Milling Process

Milling is widely known as a very versatile progress capable of creating three- dimensional features and structure and it is likely the most popular machining technique used in manufacturing and industry. The use of this process to create micro-scale features is deﬁned as micro-milling. Milling machine is deﬁned as a machine tool in which a rotating cutter is moved against the work-piece (or vice versa) to cut it into a design shape. Two basic forms of milling machine are vertical and horizontal milling machine. Micro Milling process and its’ properties is discussed in details in this chapter.

3.1 Mechanics of Micro Milling Processes

Many researchers have attempted to have better understanding in cutting phenomena and the cutting forces in both conventional and micro machining. Conse- quently many techniques to measure and deﬁne cutting forces and many cutting forces modeling techniques have been developed with considerable precision and accuracy. Although, the general cutting operation and cutting forces are in three-dimensional space and geometrical complexion, two-dimensional orthogonal cutting model is mainly used to explain the general mechanics of cutting processes. In this model, the cutting edge of the tool is assumed to be set in a position that is perpendicular to the direction of relative motion. This allows forces acting only in one plane to be considered. The two classic cutting model and orthogonal cutting model

27 3.1 Mechanics of Micro Milling Processes are shown in Figures 3.1 and 3.2, respectively. It is assumed that there are two perpendicular cutting forces acting as shown in

Figure 3.1: Two classical cutting model [7]

Figure 3.3: These forces can be resolved into two forces, one is tangential to the cutting edge and another is perpendicular to the cutting edge as shown in Figure 3.4. There are three deformation zones in the cutting process: primary, secondary and tertiary shear zones which are shown in Figure 3.5. When the edge of the tool penetrates into the workpiece, it shears the material ahead over the primary shear zone to form a chip. This chip partially deforms and move along the rake face of the tool which is called the secondary shear zone. The tertiary zone is the friction area where the flank of the tool rubs the newly machined surface. Different forces act on different shear planes. The shear force acts on the primary shear zone and the normal force acts on the secondary shear zone. The general model of cutting forces is shown in Figure 3.6. We have the friction coefficient

F μ =tan(τ)= (3.1) N The term cutting ratio shown in Figure 3.7, is deﬁned as the ratio of deformed and undeformed chip thicknesses.

t1 rc = (3.2) t2

28 3.1 Mechanics of Micro Milling Processes

Figure 3.2: Orthogonal cutting model

We have

t1 = h sin φt2 = h cos(φ − α) (3.3)

t1 h sin φ sin φ rc = = = (3.4) t2 h cos(φ − α) cos φ cos α +sinφ sin α

rc cos φ cos α rc sin φ sin α + =1 (3.5) sin φ sin φ rc cos α tan φ = (3.6) 1 − rc sin α And by Force resolution we have:

F = Ft cos α + Fc sin α (3.7)

Fs = Ft cos φ − Fc sin φ (3.8)

N = Fc cos α − Ft sin α (3.9)

Fn = Ft sin φ + Fc cos φ (3.10) Equations 3.7 to 3.10 can be used to build the cutting force models can play an important role in setting cutting conditions that are safe, eﬃcient and produce

29 3.1 Mechanics of Micro Milling Processes

Figure 3.3: Two basic cutting force model

Figure 3.4: Two basic cutting force model parts of the desired quality. Velocities are also important cutting parameters that can aﬀect the quality of surface ﬁnish and the tool life. They can be calculated as shown in Figure 3.8:

π − Vc sin( 2 α) Vc cos α Vs = = (3.11) π − − sin( 2 + α φ) cos(φ α) Also, Vc sin φ Vf = (3.12) cos(φ − α) There are various force models for micro-machining to calculate cutting forces. These models are based on shear plan (as described above), slip line ﬁelds, spe- ciﬁc cutting force and numerical methods as shown in Figure 3.9. Perez [8] also stated another model for cutting force estimation in which the proportion between cutting force and cutting are counted in. This model considered the errors in the radial position of the cutting edges of the tool. This is tested and believed to be more precise than those developed previously for helical

30 3.1 Mechanics of Micro Milling Processes

Figure 3.5: Deformation Zone mills under any cutting conditions and its implementation is believed to be more effective than others in automation of the micro milling process since it provides an easier identification system due to its simplicity [8]. Besides, when building a model for force estimation of a cutting progress, understanding the factors that influence the cutting force is necessary. Lai et al. [56] described factors that influence the cutting forces in endmilling operations. Those factors are flute engagement, feedrate, and rake angle. The flutes on a endmill that are engaged at any instant in time as a milling operation is performed is called flute engagement. It plays an important role in milling operations because it affects not only the cutting forces, but also the surface finish. On the other hand, flute engagement is affected by the radial and axial depth of cut in the way that the deeper the radial and axial depths of cut, the more flutes will be engaged, and therefore the length of the engaged flutes is increased. This is because the width and length of the contact area of the cutting tool and workpiece are influenced by the radial and axial depth of cut in the axial feed and rotational direction, respectively. Another factor is feedrate. The increasing of the feedrate will simultaneously creates an increase in the chip thickness and thus increases the tangential forces due to the forces being proportional to the chip area [8]. The final factor is the rake angle which is defined as the angle between the leading edge of a cutting tool and a perpendicular to the surface being cut. The rake angle can be negative when the leading edge of the cutting tool is ahead of the perpendicular or positive when the leading edge of the cutting tool is lagging

31 3.1 Mechanics of Micro Milling Processes

Figure 3.6: General cutting force model

Figure 3.7: Cutting ratio behind the perpendicular. Material is cut by applying downward pressure when using a negative rake angle endmill. As a result, a compression wave is created ahead of the cutting tools. In this case, a lot of pressure is required to keep the cutting tools in contact with the surface being cut. Whereas, when using a positive rake angle, the material is cut by separating one molecule of material from the workpiece and creating a chip that curls away from the edge of the cutting tools. In this case, we have to make an effort to keep the cutting tool from digging into the workpiece. Consequently, this is an efficient way of cutting in comparison to using the negative rake angle tools.[8] There is some differences in the cutting force in macro-machining and micro- machining. In macro model, shear takes place along a shear plane. Conversely,

32 3.1 Mechanics of Micro Milling Processes

Figure 3.8: Cutting velocity model

Figure 3.9: Types of models for estimation of cutting force [8] in micro-machining, the shear stress rises continuously around the cutting edge [57]. Another issue that need to be investigated in mechanics of milling process especially micro milling is vibration phenomena. Many studies has attempted to investigate the effects of vibration on micro cutting process. It is said that the stability of micro cutting process, different from macro cutting process, is very sensitive to the feed rate. The low feed rate will cause an instability in micro milling process [58; 59]. The vibration influence directly the surface roughness and the quality of surface finish. The vibration is affected by the chip thickness, feed rate, nonlinear cutting force, cutting depth, microstructure, tool geometry and the machine structure.

33 3.2 Chip Formation in Micro Milling

3.2 Chip Formation in Micro Milling

When the tool contacts the workpiece, it directs a force into the workpiece material in a direction that is perpendicular to the shear plane. This force increases as the workpiece continues to transverse until the material shears in the direction of this force. When the tool moves forward, the material ahead of the tool passes through this shear plane. Fracture will not occur and the chip will be in the form of continuous ribbon if the material is ductile. While, with brittle materials, pe- riodical fracture of the chips occur and separate chips will be formed. The gross deformation of the material that takes place in the shear zone allows the chip to be removed as showed in Figure 3.4. As discussed above, there are two principle chip types, discontinuous chips and continuous chips. Discontinuous chips are created when cutting at low speed or cutting materials that contains points of stress concentration such as the graphite flakes in cast ion, the manganese sulfide inclusions in a free machining steel or brittle materials. The result of this is the fracture into a series discrete chip segments at stress concentrations in the workpiece. The orthogonal model and subsequent calculations are not accurate for cutting with discrete chips. Whereas, the continuous chips are formed when cutting soft or ductile material such as aluminium or copper. Those chips can become very long and become entangled with the machine or pose a safety hazard. This can be overcome by applying chip breaker which is a device that is clamped to the top of the tool surface that encourages the chip to curl more sharply, thereby hitting the workpiece and breaking off. However, this method is very problematic in micro-machining. The use of chip breaker seems to be impractical. There have been a considerable effort in research and experimentation to explain the phenomena of chip formation and find a qualitative relationship between the chip formation parameters and dependent factors especially the chip thickness. It is said that chip thickness depends on the trajectory and run-out of the tool [60]. The general chip thickness for an active cutting point on a cutter without runout is dependent upon the trajectory of the tool and this relationship was approximated by Martellotti [60; 61].

34 3.2 Chip Formation in Micro Milling

tc(θ)=tx sin θ (3.13)

Where the instantaneous chip thickness, the feed per tooth, and the angular position of the tooth are tc,tx,θ. However, the chip formation in micro end mill operation and conventional endmill operation is diﬀerent. The conventional cutting force model is based on three as- sumptions: the tangential cutting force (F(c)) is proportional to the cutting area, the radial cutting force (F(r)) is proportional to the tangential cutting force and the chip thickness (t(c)) is a function of feed and the angle of the cutting point (θ). Whereas, in micro endmill operation, the feeds are high so the third assumption cannot be applied. Consequently, the chip thickness in micro endmill operation can not be calculated using equation 3.13. Newby [60] stated another formula for estimation of chip thickness in micro endmill operation.

N 2 1 2 2 tc ≈ tx sin φ − t sin θ cos θ + t cos θ. (3.14) 2πR x 2R x Where N is the number of cutter ﬂutes and R is the cutter radius. A formula for estimating the average chip thickness was stated by Newby [60; 61]. For conventional endmill operation:

2txηr t¯c = (3.15) θrr And for micro endmill operation:

2 2 2txηr Ntx tx t¯c = + (cos 2θ2r −cos 2θ1r)+ (2θrr +sin2θ2r −sin 2θ1r). (3.16) θrr θrr8πR 8Rθrr

dr Where ηr = D with D is the cutter diameter and dr is the radial depth of cut, θrr is the radial cutting range from cutting conﬁguration, θ1r is the entry angle and θ2r is the exit angle. The minimum chip thickness is the value that many researches have been focusing on. The chip formation with respect to the chip thickness is showed in Figure 3.10. Elastic deformation occurs and no workpiece material is removed when the uncut chip thickness is less than a critical minimum chip thickness (a). While, chips

35 3.2 Chip Formation in Micro Milling

Figure 3.10: Chip formation with respect to the minimum chip thickness with tool edge radius Re[9] are formed by shearing of the workpiece with some elastic deformation occurs when the uncut chip thickness approaches to minimum chip thickness (b). When the uncut chip thickness is higher than the minimum chip thickness, the elastic deformation decrease strongly and the entire depth of cut is removed as a chip (c). There are some factors that influence the minimum chip thickness. Weuler et al. [62] found that minimum chip thickness depends primarily on sharpness of the tool and secondarily on material on material properties. It is found by Chae [63] that the relationship between the tool radius and minimum chip thickness depends on the cutting edge radius and the material of the workpiece. This study also stated that it is difficult to directly measure the minimum chip thickness so the minimum chip thickness is estimated using finite element or experimental prediction [63; 64]. It is said by Ducobu et al. [64] that the minimum chip thickness phenomenon causes an increase in slipping forces and ploughing of the machined surface, con- tributing to the rising of the cutting forces, burr formation and surface roughness. Therefore, it is crucial to estimate this value to choose the correct cutting condition. Son et al. [9] stated an expression of the minimum chip thickness depending on the tool edge radius and the friction angle between the tool and an uncut or continuous chip(β). π β tcmin = Re[1 − cos( − )] (3.17) 4 2

36 3.3 Burr formation and Surface Finish, Material and Machinability in Micro Milling

3.3 Burr formation and Surface Finish, Mate- rial and Machinability in Micro Milling

A phenomenon similar to the chips formation is the burrs formation at the end of the cut. Burrs can be defined as as small alterations related to the cutting mechanisms, resulting in protruding material out of the workpiece, and causing geometric and dimensional variation. They are , unlike chips, undesirable because they present a hazard to handling, is a risk to machine operator, interfere with subsequent assembly operation and accelerate tool wear. Therefore, burrs need to be removed. The deburring progress in micro machining is much more difficult than this in macro-machining. Gillespie et al [65] proposed simple analytic model to predict some burr properties and geometry such as height, thickness and hardness. This study stated that burr geometry depends on the properties of the workpiece material partic- ularly the module of elasticity and the geometry of the tool, including cutting edge radius. This study also described four basic mechanism of burr formation: material deformation on the direction of cutting edge, chip curling on the cutting speed direction, chips and workpiece separation during chip formation. The burrs are divided into four type based on this model: poison burr, roll-over burr, tear burr and cutting off burr. It is believed in this study that elimination of the burr during the operation by changing parameters like cutting speed, feed rate or tool geometry is impossible, but it can be possible to minimize burr geometry. Lee et al. [10] investigated the burr formation in micro milling and stated a new definition and classification of burr as shown in figure 3.11. Lee [66] stated in another study that the influence of tool run-out on burr formation was significant in micro-slots milling. Bissaco et al.[67] found that top burrs are relatively large in micromilling due to the size effect. When ratio of the depth of cut to the cutting edge radius is small, high biaxial compressive stress pushes material toward the free surface and generates large top burrs. Donfeld [68] suggested some strategies to minimize burr in milling such as avoiding exits of inserts (or always machining on to the part edge), sequencing process steps to create any burr on the last, less significant edge, control of exit order sequence by tool geometry and path variation, maintaining uniform tool chip

37 3.3 Burr formation and Surface Finish, Material and Machinability in Micro Milling

Figure 3.11: Burr definition and its location in slot milling (a) and machined slots (b) [10] loads over critical feature, lift and re-contact of milling cutter for some features where maneuverability is limited, and avoiding push exit (those with long cutter path/edge contact length). Next discussion is about the surface finish or surface texture in micro milling. These terms are used to describe the general quality of a workpiece surface. This consists of four factors. The first factor is roughness which includes all irregularities which generally result from the production process. These include transverse feed marks and other irregularities within the limit of sampling length. The second factor is the waviness which includes all irregularities having spacing that is greater than the roughness sampling length and may be the result of machine or work deflection, chatter, vibration, heat, treatment, or cutting tool runout. This can be considered to be superimposed on wavy surface. Thirdly, lay, which is the direction of predominant surface pattern (relative to a reference edge), ordinar- ily determined by production method used. Finally, unintentional irregularities which occur at one place or at relatively infrequent or widely varying intervals on the workpiece surface are termed flaws. They can occur either during the manufacture of the material or during the machining of workpiece. Flaws include cracks, bow holes, inclusions, checks, ridges, and scratches. Surface finish is effected by many machining parameters such as machine tool, workpiece con- siderations, cutting tool, cutting fluid and workpiece material. Vogler et al. [69] studied the relationship between surface roughness and mi-

38 3.3 Burr formation and Surface Finish, Material and Machinability in Micro Milling crostructure of the ferrous materials and developed a model to predict surface generation in micro end milling. Sun et al. [70] found that the surface roughness values is significantly affected by the feed. This study also found that the surface roughness values increases when the feed per tooth is reduced to a certain value and the existence of an optimal feed that will produce smallest surface roughness values. The minimum chip thickness is also found having a significant influence on the achievable surface roughness value. The existence of the optimal feed is due to the combined effects of the geometry of the cutter and the feed, and the minimum chip thickness effect. This study assured that the surface generation in micro milling process is strongly effected by the combined effects. Takacs et al. [71] found that the harder the material the better the surface quality is the tool life is better if the material is more ductile. Wang et al. [72] investigated the surface generation considering the influence of grains in metallic materials. In micro end milling, the cutting parameters shrink to micrometer order which are less than or equal to the grain size. Consequently, the effects of material grain partly an important role in micro end milling. The chip in micro milling was believed in this study that always discontinuous as a consequence of the grain boundary influence. As discussed above, many of the micro milling parameters are influence by the material properties especially microstructure. The material properties of workpiece influences the chip thickness formation, burr formation, and surface quality. The softer the workpiece material, the more continuous the chips created and the more brittle the material the more discontinuous the chips created. The microstructure also has a great influence in the surface quality. Popov [70] established that the roughness of microfeatures produced by micromilling is highly dependent on the material grain size. In Popov’s experiment, the surface roughness of thin features in microcomponents was found improved more than three times as a result of reduction in grain size and material anisotropy of the Al alloy, from 100-200 micron to 0.6 micron. Moreover, a crystalline texture of the material with regard to the machine direction and the narrow grain size distribution could lead to surface roughness improvements. However, the improvements achievable though a refinement of the material gain structure are of the magnitude bigger than that. It is also found in this study that there is an important correlation between the subtle structural features of

39 3.4 Micro Milling Tools and Issues the material and the post-process surface quality. In general, machinability is a term that is used for testing and designing machining progress. It is defined as the ease with which a given material may be worked with a cutting tool changes with the machine variables. Those variables are classified into three categories. The first is the common machine variables which are cutting speed, dimension of the cut, tool form, tool material, cutting fluid, rigidity and freedom from chatter of machine tool and workhold device, and nature of engagement of tool with work. The second category is the common work material variable such as hardness, tensile properties, chemical composition, microstructure, degree fo cold work, strain shareability, shape and dimensions of work, and rigidity of workpiece. The third category is the common criteria for judging ease of cutting. This category is divided into two subcategories. The general criteria including life of the cutting tool between resharpening, expressed in various term, magnitude of the tool forces, machining work or energy, or power consumption, and quality of the surface finish on the work is the first subcategory. The second subcategory is the specific criteria including cutting torque or thrust, cutting time or rate of penetration, energy absorbed in pendulum-type milling cut, temperature of cutting tool or chip, amount that chip is hardened during removal, cutting ratio of chip, combined values of the mechanical variables that control forces and chip geometry, feed rate under constant feed pressure at a constant speed, and the ease of chip disposal.

3.4 Micro Milling Tools and Issues

The conventional milling cutters are categorized into four basic types: square cutters, T-slot cutters, ball nose cutters, shell cutters as in Figure 3.12. There are some parameters with respect to the geometry of the cutter. The shape of the cutter determines the type of cutting operation it can perform. There are many standard shapes used in various industries. The deep helical grooves running up the cutter are called the ﬂutes along which sharp blades are located. These blades are referred to as the teeth and are the components that cut through the material. The next parameter is centered Cutting which is the orientation of the cutting tool which determines whether the cutting tool can

40 3.4 Micro Milling Tools and Issues

Figure 3.12: Milling cutter types (www.efunda.com) plunge through a material or not. The helix angle sets the gradual entry of the tooth on the material and reduces vibration of the cutting tool. Next parameter is the shank which is the cylindrical part of the cutting tool that is attached to the milling machine and is responsible for holding the tool in place. Roughing is a cutter configuration which is composed of serrated teeth effective for breaking the material into smaller pieces. And finally, Coatings are responsible for improving the surface finish, increasing the speed, and increasing the tool life. The majority of micro milling tools are ball nose endmills and square end endmills with the different number of flute (two and four flutes are mostly used) and different flute length (standard length with the length of the flute is approximately three times the cutter diaster and the stub length with the flute length is approximately 1.5 times the cutter diameter). The micro tools and their properties are very important because they affect the surface quality and feature size of the microstructure. Tungsten carbide cutting tool are generally used due to their hardness over a broad range of temperatures. Diamond tools have a limited in their ability to machine ferrous materials because of the high chemical affinity between diamond and ferrous material. Focus ion beam and EDM are two techniques that are commonly used in fabrication of micro tool. The accuracy of those technique directly effects the accuracy of micro tool and micro tool geometry. These will effect the tolerance and surface quality. There are some issues in micro-machining with respect to the micro tools that

41 3.4 Micro Milling Tools and Issues need to be investigated in order to achieve the better surface quality [9]. The first problem are the tool wear and tool life. When investigated the micro end milling of graphite electro, Zhou [73] found that because Graphite is so brittle so the chips produced are very fine and tend to disperse, accumulate and adhere to the surfaces of the cutting tools ad the workpieces. This reduces the working rake angle and clearance angle and even cause premature failure. Consequently, wear and breakage of the cutting tools for graphite are so remarkable that not only the cost of cutting tools and machine tim, but also the surface finish of workpiece are influenced seriously. From this experiment, Zhou divided wear into two major wear forms: flank wear and rake wear. The flank wear which caused by abrasive particles friction is the primary wear pattern in high speed micro milling of graphite, which defines the tool life decisively. It is stated by zhou that part of edges of the carbide tool are often eroded easily than those of the coated tool, and become erosion rather than straight. Another wear form is rake wear. In high speed micro milling, a narrow zone on the rake face within the depth of cut suffers crush and rubs by graphite workpiece and chips with a great material loss of tool tips. As a result of this wear occurs in zone. Zhou suggested using an efficient dust collector and an air jet nozzle with the proper air pressure, the nozzle diameter and the spray direction to reduce the tool wear and breakage. zhou also stated that different from the conventional milling, the wear mechanism of micro milling has not been deeply studied and it is difficult to estimate. The unpredictable tool life and premature tool failure are major problems in micro- machining. Rodriguez [11] described a technique that observed the mills wear by using pho- tographs taken with digital camera which includes a length reference for each zoom used and divided the wear into tip wear and flank wear. As in Figure 3.13, the wear observed in the first case is caused by built-up edge while in the second case, a clear flank wear can be seen along the end of the edge. By comparing the wear with the reference length, a value for the wear related to the machining time can be obtained. Another problem of micro tools is run-out. Although conventional precision machining has been developed toward higher accuracy, run-out is still one of the big challenges in micro-machining. Because of the scale effect, run-out to tool

42 3.5 Conclusion

Figure 3.13: Wear in micro milling (1) working at Vc=130m/min (2)working at Vc = 240m/min [11] diameter increases with the smaller tool diameters. Tool run-out is caused by a misalignment of the axis of symmetry between the tool and the tool holder or spindle. Tool run-out mainly depends on the characteristics of the spindle, tool holder (collet) and tool itself. With run-out, the tool wears out much more quickly, and tool are easy to break. This causes the interruption of the cut at the edge of the tool due to lack of fixation. The next problem is built up edge which is defined as an accumulation of material against the rake face in single point cutting of metal. This seizes the tool tip which, as a result will be separated from the chip. In conventional machining, built-up edge is usually associated with the sudden change of surface roughness and this can also be true in micro-machining [74]. It is said that a stable built-up edge forms is on the cutting edge and significantly increases the plowing force when the feed rate is smaller than the cutting edge radius [74; 75].

3.5 Conclusion

In this chapter, the background of the micro milling process are discussed. The cutting parameters and the factors that inﬂuence the surface quality are also described. Those need to be considered when design a micro milling machine, choosing a suitable cutting parameters and workpiece materials. These will help to get better surface quality, improve the machining progress, get a longer tool life. Some of the issues and solutions are only in theoretical and expressions to get

43 3.5 Conclusion the approximate value of parameters has not been ﬁgured out. However if these factors are considered in the conceptual design progress, Their eﬀects on the surface quality can be eliminated. The design, building and testing a micro milling machine will be described in the next chapters.

44 Chapter 4

Design of the Ultra Precision Micro Milling Machine

The research platform used in this project is a micro milling machine which is capable of producing two or three dimensional micro scale components with high accuracy. This was done using the background knowledge of the micro milling process with considering the milling parameters and other factors as discussed in Chapter 3. This chapter presents the design and building of the system.

4.1 Overview and Conceptual Design

The micro milling machine to be designed is required to machine micro scale components. As tools for machining, the wide variety of micro micro endmills available in the market may be used. Regardless of the type and size of the endmills used, a proper conﬁguration for the machine needs to be decided. Taking into consideration the suﬃciency of a work volume of 15625 mm3,anx,y,ztravel of 25 mm was chosen. This system is conceptually separated into six subsystems.

• High precision workpiece manipulation system.

• Tooling system consisting of a high speed spindle.

• Integrated axis control system.

45 4.1 Overview and Conceptual Design

• Calibration and measuring system.

• Vision system for process monitoring.

• Cooling systems for the spindle and workpiece-tool interface.

From among the above listed subsystems, this chapter will present the workpiece manipulation system, tooling system, cooling systems and the vision system. The calibration and measuring system and the control system will be discussed in the next chapter. After the conceptual system was chosen, The ﬁrst model was built as in Figure

Figure 4.1: Analysis model of system

4.2 based on the analysis of the model shown in Figure 4.1 with the chosen position, stages and mounting position. The entire system is on a Newport Scientiﬁc pneumatic vibration isolation table which can be seen in Figure 4.3. This table is pneumatically isolated from its base so that all excitation that exist at its legs are not transmitted to the table’s surface. The entire machine is covered by perspex sheets which not only help to protect

46 4.2 Workpiece Manipulation System

Figure 4.2: The ﬁrst model of the vertical micro milling system the device form dust but also protect the operator from the metal or chip from the machining process. These perspex sheets are transparent so the operator can easily see the machining process. An aluminum frame attached to the legs of the vibration isolation table helps support the perspex cubic that encloses the vibration isolation table surface. The entire machine is shown in Figure 4.4. The next section will describe in detail, the positioning stages, the spindle and the assemble system, together with their integration to form the complete machine.

4.2 Workpiece Manipulation System

The workpiece manipulation system is designed to hold and move the workpiece to machine. The system is designed in such a manner that all complex motion are realized by a sequence of linear or circular motions. The 15625 mm3 workspace is realized using 3 micro stages with minimum incremental motion of 0.1 μmanda travel of 25 mm. To cover for situation requiring a minimum incremental motion less than 0.1 μm, a two axis nanostage with minimum incremental motion of 2

47 4.2 Workpiece Manipulation System

Figure 4.3: Newport vibration isolation table nm and a travel of 200 μm has been incorporated. In reference to the machine’s coordinate frame, the nanostage can only provide motion in x-y plane. Due to hardware limitation as dictated by the electronics, the minimum incremental motion currently possible on the nanostage is only 48.8 nm.

4.2.1 The Microstage

Three linear microstages used are the products of Physik Instrument (PI) GmbH. They are M-605.1DD Linear Positioning Stages, shown in Figure 4.6, in each of which functional ﬂat design is combined to allow multi axis combination. A precision-machined base is featured from high-density, stress relieved aluminium for exceptional stability and minimum weight. The stages’ size is 156x113.5 mm with the weight just 1.5kg. In these stages, precision-ground recirculating ballscrews, shown in Figure 4.5, are used. These ballscrews provide low-friction, backlash-free positioning. Uni-directional and bi-directional repeatability of these stages are 0.1 and 0.2 μm respectively with maximum pitch and yaw errors are 50 μrad. Non-contact, Hall-eﬀect sensors and limit switches with direction sensing at the origin are equipped to provide full control with high accuracy. The resolution of this stage is 0.1 μm which is far better than most other indirect metrology stages. The direct metrology sensing directly measures the controlled quantity, that is, the stage’s displacement.

48 4.2 Workpiece Manipulation System

Figure 4.4: The micro milling machine and work station

4.2.2 The Nanostage

The nanostage used is the P-527.2CL multi-Axis, single-module piezo flexure nanopositioner and scanner which is the highest precision stage used in this machine. It is a PI’s product in which low-profile, high-resolution, piezo electric driven 2 axis flexure stage. This stage employs Low-voltage PZT (plumbum (lead) zirconate titanate, which is the poly crystalline ceramic material with piezoelectric properties) (0 to 100 V), and flexures as the drive and guiding system. The flexures provide zero stiction/friction, ultra-high resolution and exceptional guiding precision. Integrated capacitive position feedback sensors provide sub-nano resolution and stability in closed-loop operation. The piezo electric actuators can perform sub-nanometer motions at high frequen- cies because their motion are derived from solid state crystalline effects. There is no friction because no sliding or rotating parts. Moreover, they can move high load up to several tons requiring no power in static condition. Piezo actuators

49 4.2 Workpiece Manipulation System

Figure 4.5: Recirculating ballscrews (www.physikinstrumente.com)

Figure 4.6: M-605.1DD translation stage (from PI catalogue) especially require no maintenance and are not subject to wear because because there are no moving parts. The displacement of piezo actuators can be calculated using the following expression:

ΔL = S.L0 ≈±E.dij.L0 (4.1)

Where ΔL is the change in length of an unloaded single layer piezo actuator, S is the strain (relative length changeΔL/L, dimensionless), L0 is the ceramic length in m, E is the electric ﬁeld strength in V/m, and dij is the piezo electric coeﬃcient of the material in m/V with d33 describing the strain parallel to the polarization vector of the ceramics (thickness) and is used when calculating the displacement

50 4.2 Workpiece Manipulation System

Figure 4.7: Working principle of nanostage (www.physikinstrumente.com)

of stack actuators, d31 is the strain orthogonal to the polarization vector (width) and is used for calculating tube and strip actuators. d33 and d31 are sometimes referred to as piezo gain [76]. The working principle of a nanostage is shown in Figure 4.7. The sensor used in this stage to measure the displacement used in control loop is the high resolution capacitive sensor which perform noncontact measurements of linear geometric quantities representing distance, displacement, separation, position and/or length with sub nanometer accuracy. Two basic types of capacitive sensors are one and two plates sensors. They have the same measuring principle. Two conductive surfaces set up a homogenous electric field. The change in displacement of two plates is proportional to the signal conditioner output. the distance between two well defined sensor plates is measured by the dual-plate sensors with carefully aligned surfaces by which the most accurate electric field is generated and therefore optimal results are provided. The capacitance against electrically conductive reference such as metallic plates is measured by single plate capacitive sensors which are very convenient to install and connect. The capacitive sensor nanostages use LEMO connectors (fibre optic push-pull connectors) to connect to the controller. This project used the nanostage, shown in Figure 4.8, which has the size of 150x150x40 mm and the weight of 5kg with a series of mounting holes that help easily mount the stage on the base or mount other devices on it.

51 4.2 Workpiece Manipulation System

Figure 4.8: The nano stage (from PI catalogue)

4.2.3 Conﬁguration Workpiece Manipulation

This system used three linear stages. The first linear stage was mounted onto the isolation table. It was aligned in a way that its axis of travel was parallel to the frame that hold the tooling spindle. The next linear stage was mounted on the first linear stage in the way that its axis of travel was perpendicular to the traveling direction of the first linear stage and on the plane that is parallel to the surface of the table. Those two stages create two horizontal axes that mainly used for creating the tool paths. The third linear stage was mounted on the top of the second linear stage. The axis of travel of this stage is perpendicular with that of the second linear stage and on the plane that is perpendicular to the surface of the table. this stage decides the distance between the workpiece surface and the cutting tools, hence decides the depth of profile and the depth of cut. The nanostage was mounted on the top of the third linear stage and on the plane that parallel to the plane of the first and second linear stage and perpendicular to the plane of the vertical stage. This stage has two axis of travel but the travel range are small compared to the travel range of the linear stage. However, the nanostage has a high resolution that can offer very precise motions. Therefore, this stage was used for the small steps or fine machining. The workpiece is mounted on the manual microstages, produced by Mitutoyo,

52 4.3 Tooling System which is mounted on the top of the nanostage. This stage is used to help ease the mounting of the workpiece and it is more convenient for the operator, with the use of microscope, to ﬁnd the machining area before the machining starts.

4.3 Tooling System

The tooling system is the system which hold the micro endmills. A SC 40 high speed spindle system, shown in Figure 4.9, which is produced by Precise GmbH is used in this system. The maximum speed this spindle can approach is 90000rpm with the radial run-out in spindle taper is less than 2μm. With the increasing of the spindle speed, there is a slight increase in torque but it causes a signiﬁcant change in power as can be seen in Figure 4.10.

Figure 4.9: The Precise SC40 Spindle (www.ﬁscherprecise.com)

Figure 4.10: Graph of the change of SC40 spindle torque and power with respect to the RPM (www.ﬁscherprecise.com)

53 4.3 Tooling System

The spindle then will be mounted in a plane mount, shown in Figure 4.11, which will then be mounted onto a frame or a stage. In order to hold the micro tools, a suitable collet, shown in Figure 4.11, is used. These collets insert the tool shank into the loosely screwed-in collet which is held by the ring spanner supplied. They are made with different sizes that fit with different shank diameters and they can tightly grip the tools. The design and fabrication of the tool mounting parts and collets are very important because it affects directly the run-out and hence affects the tool life and surface quality. The spindle rotation direction is indicated by the red arrow marked on the

Figure 4.11: The collet and the spindle mount (www.fischerprecise.com) spindle. Together with the cooling systems, a thermsitor was used to protect the spindle from exceeding temperature. This thermsitor is wound into the motor stator coils which reacts to increases in resistance by increasing temperature cor- respondingly. If the maximum permitted operating temperature is exceed, the resistance rises significantly and this triggers an automatic shut-down of the frequency converter that control the spindle via the thermistor switching device. The ball bearing of SC 40 spindle are provided with a special grease filling for the entire service life so it does not need to be lubricated. This spindle is mounted on a U-shaped frame built from Maytec aluminium tubes. this frame is mounted on the vibration isolation table. By loosening the screw, the spindle can manually slide along the frame in the plane that is parallel to the table. This helps the operator adjust the position of the spindle and the milling machine easily.

54 4.4 Micro Tools

4.4 Micro Tools

The micro tools carry out the material removal. The tools and their geometry and material properties influence the tolerance of the profile and the quality of the finished surface. There are three types of micro endmills that can be used in this system. They are T-shaped micro endmills, square ended micro endmills and ball nose micro endmills. They have the different number of flutes (two or four flute) with the different flute length (standard length or stub length) and different flute diameters. However, the shank diameters are the same for all endmills. The square ended and ball nose endmill are widely provided by some companies such as PMT Tool, Kyocera and Zecha but the T-shaped micro endmills are not currently available in stock. However, these endmill can be made from squared end endmills using EDM or FIB technique. The micro endmills used in this project are 2 flute square ended micro endmills, shown in Figure 4.12, with the flute diameters of 100, 50 and 20μm respectively with both stub and standard lengths. The shank diameter is 3mm. They are produced by Performance Micro Tool (PMT). They are solid carbide micro tools that have nominal plus/minus diameter tolerances, primary relief angles, TIR controlled to 1/3 industry standard. They can be applied for graphite electrodes, plastics for optical and medical, electronic components, circuit board prototyping, engraving and sign making and small aluminium parts.

Figure 4.12: The micro end mill geometry parameters (www.pmtnow.com)

55 4.5 Vision System

With the dimensional tolerances: of these tools are as follows: D0 + / − 12.70,

L1 + / − 177.80 − 0.00, d0 + / − 0.00 − 5.08, and L + / − 127.00.

4.5 Vision System

A vision system was implemented to fulﬁl the need of in process vision and control as well as checking the machining process. This vision system employs a Sony DFW-V500 digital camera, shown in Figure 4.13, coupled to a variable zoom microscope lens, shown in Figure 4.14. This system is mounted on a frame which is mounted on the top of two manual linear stages. This camera is a fully digital camera which adopts the IEEE1394-1995 standard. This camera incorporate the 4000Mbps chip set and feature Sony’s ﬁrewire

CCDTM in which a primary color ﬁlter is integrated to ensuring high color accuracy square pixels and progress scan technology which provides sharp, high resolution images, even of fast motion. An external trigger mode is also included for asynchronous trigger operation that provides jitter-free pictures because the camera acquisition can be synchronized to full random events. Moreover, this camera presents 30fps in VGA (640x480 pixels) resolution format and non-compressed YUV(4:2:2) digital output (luminance and chrominance, typical of PAL European formats, as opposed to RGB (red,green,blue) factors used in computer monitors). All the functions of this camera can be controlled and powered by a computer. For acquiring images in micro scale, an microlense system from Navitar is em-

Figure 4.13: The DFW-V500 camera from Sony ployed. This system gives a 12X(0.58-7X) magnification for inspection of a wider range of parts with field coverage up to 50mm. The resolution was increased with 0.018-0.1 N.A (Numerical Aperture). It have a depth of field from 32 to 341 mm.

56 4.6 Cooling System

This lens is coupled to a 2x magnification adapter with fine focus adjustment. It provides a total 24x maximum magnification. The magnification and the focus lenses can be manually adjusted. At the maximum zoom, the camera and lens system can sense a field of view

Figure 4.14: Navitar 12x zoom microlense of approximately 176μm-132μm and a maximum-zoom per-pixel resolution of 215nm/pixel. The camera and the lenses are mounted on a frame which is mounted onto two manual microstage. This help to manually adjust the work-piece within the visi- ble range of the lenses. The illumination of the visual ﬁeld of vision is provided by a 150W halogen lamp from which microlight is piped into the ﬁeld of vision and directed parallel to the camera axis. A machine system with the co-operation of the vision system is shown in Figure 4.15.

4.6 Cooling System

The cooling systems are used to cool the spindle and the tools, hence keep the systems work longer, reduce the failures of components and prevent system from exceeding temperature. They are very important in manufacturing systems. There are two cooling systems. The spindle coolant system is used to cool the

57 4.6 Cooling System

Figure 4.15: Vision system with the milling machine spindle. This helps to prevent the spindle from over heating and therefore, ensure the process of machining is not interrupted due to spindle shut down, therefore improving the spindle life. The tool and workpiece are also need to be cooled. They are heated when machining because of the friction. If the temperature is exceeded the cutting edge may be burned, tool material properties may be changed and tool wear speed may be increased. Consequently, the tool life can decreased significantly and leading to the machining cost increased. Moreover, the material properties of the workpiece may undergo substantial changes due to workpiece becoming excessively hot. The changing material properties may lead to softer workpiece material, which in turn may develop longer chips which are sticker. As a result, the burrs may become larger and surface finish and surface quality may be adversely affected. The spindle cooling system is the PHK 525 HZ cooling system from Precise GmbH as shown in Figure 4.17. The cooling liquid used is the mix of distilled water and N43-73 anti-corrosion agent from Precise with the ratio of 100:1. This system is described in figure 4.16. The temperature of the cooling liquid is set to be in a predefined range. The default range is from 19 to 250C.

58 4.6 Cooling System

Figure 4.16: Function diagram of PHK 525 HZ cooling system (from PHK 525 HZ user manual)

The other cooling system is for micro tool and workpiece as shown in Figure 4.18.

Figure 4.17: FPHK 525 HZ

This system is the VIP4Tools oil mist cooling and lubrication system produced by Dropsa. It uses the normal cooling liquid for CNC and metal cutting machine. The system comprises a pneumatically controlled mini-pump and the mixer base. The mini-pump can be manually regulated to cover a wide range of need (0-30 mm3).

59 4.7 Summary and Conclusions

The oil mist formed is directed at the area where machining is taking place by

Figure 4.18: VIP4Tool cooling system(Dropsa) using the spray tube which is mouted on the spindle frame using magnetic base shown in Figure 4.19.

Figure 4.19: Spray tube(Dropsa)

4.7 Summary and Conclusions

In summary, this chapter described a micro milling system and how it was con- structed. This system consists of two subsystems, the workpiece manipulation

60 4.7 Summary and Conclusions system and the tooling spindle system. The workpiece manipulation system has 4 axes that can securely hold the workpiece and when operated in unison, can make a variety of tool paths based on lines and arcs. The motion can be very precise due to the high resolution which is up to 2nm. The tooling spindle system used a high speed spindle that hold a micro endmill and this spindle can be manually moved parallel to the table surface. The vision system and the cooling system are used to aid the control and machining process to improve the surface quality. This system can be seen as a stiﬀ and reliable system which can be improved and together with development of a eﬀective and reliable control, calibration and interpolation system, this machine has the promise to be a system that can be employed for further research and development.

61 Chapter 5

System Calibration and Control

This chapter describes the calibration procedures and the control system that have been developed to effectively impart micro motions in a coordinated manner to carry out micro-machining. Given that the machine is a mechanical assembly requiring high precision alignment, a calibration method has been developed to identify the misalignment. Methods have also been developed to take into account the misalignment during the machining process to eliminate the effects of misalignment. The misalignment and other imperfection that needs to be detected are the perpendicularity of axes, the parallelism of motion between axes, the near zero but not exactly zero inclination of the workpiece surface generally arisen from the mounting of the workpiece and the run-out of the spindle that carries the tool. The parallelism, perpendicularity and the tool run-out are inherent to the machine as built and occasional testing if these quantities is sufficient. For this purpose, a high precision dial gauge with minimum incremental resolution of 100 nm was used and through repeatable tests all stages were aligned. The spindle run-out however could not be eliminated and as such contributed to increase the minimum machinable width although in comparison to endmill size of 50-100 μm, a 2 μm maximum run-out is still less than 5 percents of the groove size. The workpiece surface inclination was measured using a special sensing system that logs the microstage reading based on contact during vertical motion. These

62 measurements were carried out everytime a new workpiece is mounted. The results obtained using these tests are presented later in the chapter. The control system developed conforms to the block diagram shown in Figure 5.1. This system has the ability to control all microstages, nanostages, the spindle and the tool paths. The control system receives the commands that needs to be sent across the stages from a central control computer. All feed rate, depth of cut and tool paths are determined by the central control computer.

Figure 5.1: Schematic of the control system developed for micro milling machine

63 5.1 System Calibration

5.1 System Calibration

5.1.1 Calibration of the Machine

It is critical to ensure the correct geometric relationship between axes of workpiece manipulation system in order to improve the accuracy of the machine. Besides, the run-out is also need to be deﬁned because it eﬀects the surface quality and the tool life as discussed in Chapter 3.

As can be seen in Figure 4.1, the travel axis of the ﬁrst horizontal linear stage x6 and the travel axis of the second horizontal linear stage x5 should be perpendicular to each other and they both have to be parallel to the surface of the vibration isolation table. Moreover, the travel axis of the vertical stage should be perpendicular to the table surface and the surface of the nanostage should be parallel to the table surface. This could be done by using a dial gauge indicator with an 100 nm accuracy produced by Mitutoyo. The surface of the isolation table was assumed to be ﬂat and was chosen as the standard surface for all measurements. A measurement setup for calibration is shown in Figure 5.2. It is assumed that the travel axis of each stage is perpendicular to one edge

Figure 5.2: Calibration setup for checking perpendicular between two axes of the stage cover and parallel to the other edge. The stage covers and the surfaces of the mounting plates are assumed to be perfectly ﬂat. The calibration of the perpendicular between x5 and x6 was checked by mounting the dial gauge

64 5.1 System Calibration on the table in the way that the point of the dial gauge touch the edge of the second microstage. The second stage was mounted on the top of the first stage in the way that the two travel axes are perpendicular to each other. The first microstage moved slowly and the indicated value from the dial gauge was checked and recorded. If the value displayed kept unchanged or the differences between values displayed are less then one step (0.1 μm), these two axes can be considered as being perfectly perpendicular to each other. A similar process was carried out to check the perpendicularity and parallelism between other axes. The errors from this calibration were used to adjust the mounting plates. The dial gauge was also employed to measure the spindle run-out. The ball tip of a dial gauge indicator was placed as close as possible to the center line of the the shank and as far down on the shank as possible. The position of the indicator and the dial gauge stand was adjusted until there is an acceptable value of pre-load indicated on the dial. Afterwards, the collet was rotated using index finger to determine the orientation of the spindle that corresponds to minimum indicated deflection. The pressure of finger against the collet nut can give a false indicated deflection so the finger must be removed when reading the dial. The values indicated on the indicator were read and recorded. The measurement setup is shown in Figure 5.3.

Figure 5.3: Calibration setup for checking the spindle run-out

65 5.1 System Calibration

5.1.2 Workpiece Surface Inclination Measurement

The inclination measurement system consisted of a probe that was attach to the body of the machine. The probe comes in contact with the surface under investigation when vertical height is changed through computer control. A contact detection circuit is interfered to the control computer so that as soon as contact has been detected the vertical position of the stage could be logged. It is also important to note that the vertical movement of the stage must be stopped immediately after contact has been detected. The rate of vertical movement has been reduced to a minimum at 100 nm per step. To improve the efficiency of this test, the first sample taken is used to establish the approximate vertical position of the surface. the test origin for vertical movement is then set 15 μm, the largest expected inclination error, below the initial contact. This vertical position is considered as the home position for inclination testing. It must also be noted that the probe is designed in such a manner that at no time its deflection results in a plastic deformation of the probe. The calibration process was programmed to be conducted automatically with

Figure 5.4: The micro coordinate measuring system used for checking the surface inclination the following step:

• Choose the are on the work-piece surface that need to check the inclination and set the home position for vertical and two horizontal stages.

• By controlling the vertical stage, move the work-piece surface up step by step (one step is equivalent to 100 nm). The current position of the stage is

66 5.1 System Calibration

Figure 5.5: Cicuit developed for inclination checking

checked after each step and the stage will stop immediately if it reach the probe’s tip.

• Read the current position from three stages.

• Move the vertical stage to home position.

• Move the two horizontal stages to the next point and conduct the measurement again.

These steps were carried out until all the necessary points on the work-piece surface were checked. This system was designed for conductive workpiece material only. With the insulated material, It needs to be gold coated ﬁrst so that the surface will be covered by a very thin conductive layer. This data was used to adjust the level of the workpiece surface and hence the distance between the workpiece surface and tool tip, the depth of cut can be changed. The electronic circuit of the sensor system is shown in Figure 5.5 and the results from the measurement of a copper surface is shown in Figure 5.6.

67 5.2 Control System

Figure 5.6: Surface proﬁle of a copper surface in one dimension (1 count = 0.1 μm

5.2 Control System

5.2.1 Host Computer System

The host computer is an compact board PCM-6893 with Intel Pentium II CPU running at 733 MHz. This compact board has PC-104 Connector so it can be extended with I/O devices which are used for machine control as shown in Figure 5.7.

• A PC104-DAC06 which is a six-channel analogue output board with the output resolution of 12 bits (1-4096). This board provides 1 independent D/A per channel. Channels can work independently or simultaneously in a coupled mode. The output ranges are ±10V, ±5V, 0-10V, 0-5V which are selectable. This board with the 0-10V is chosen to ouput the analogue signal to control the ﬂexure stage (nanostage).

• A PCM-3718HG 12 bit DAS board with programmable gain. This board offers 16 single-ended or 8 diﬀerential analogue input with jumpers selectable.

68 5.2 Control System

The A/D converter is up to 100kHz sampling rate with DMA transfer. The gain value for analogue input and input range are software selectable. These ports can be used to read the position sensors of the flexure stage. More- over, this board offers two 8-bit digital input/output channels with TTL compatibility. These digital I/Os are used for controlling the frequency converter, cooling system and the micro coordinate measuring system for surface inclination measurement. some other features of this board are flexible trigger option, data transfer by program control, interrupt handler routine and DMA and 16 bit programmable counter/timer.

• A generic IEEE-1394 PCI card for image capture.

• On-board RS232 serial ports.

Figure 5.7: The host computer with PCM-6893 and PC-104 modules

This computer runs the Linux kernel patched with Real Time Application Inter- face (RTAI) extensions named RTKnoppix. This operating system was chosen because of the requirement of a low cost realtime system. This operating system is developed from Knoppix 5.1 which is a free Debian based system that includes Linux 2.6 Kernel, automatic hardware detection, KDE desktop and hundreds of other software packages. A version of Knoppix 5.1 can be downloaded from www.knoppix.net. Because Knoppix works as a liveCD so it needs to be remastered to change set- tings, kernel and install software as well as customize Knoppix. The first step in remastering progress is preparing a storage device that has at least 4GB Linux file-system (ext2, ext3) partition for source, at least 2GB Linux file-system (ext2,

69 5.2 Control System ext3) partition for master and at least 1GB FAT ﬁle-system for Linux and win- dows sharing ﬁles. Afterwards, the computer is booted from Knoopix. The next steps are:

• Copy all in /KNOPPIX/* to KNOPPIX folder that was created in source partition.

• Copy all from the CD to the master partition except the 700MB KNOPPIX ﬁle.

• Change the name server in /etc/dhcpc/resolv.conf so that the internet can be connected in chrooted environment.

• Start chrooted environment where changes are made and kept permanent.

• Remove irrelevant softwares such as KDE games, kde-i18n-*, to save disk space (in order to make a live CD).

• Update source list with apt-get update.

• Install emacs (free text editor), gcc (C compiler), g++ (C++ compiler) and Coriander (free Linux software for ﬁrewire camera ).

• Download the RTAI tarball and the linux kernel tarball from www.rtai.org and www.kernel.org. This is the ﬁrst step to install RTAI (real-time application interface) which provides deterministic and preemptive performance in addition to allowing the use of all standard Linux drivers, applications and functions. RTAI was initially developed by The Dipartimento di In- generia Aerospaziale Politecnico di Milano (DIAPM) as a variant of the New Mexico Institute of Technology’s (NMT) RTLinux, at a time when neither ﬂoating point support nor periodic mode scheduling were provided by RTLinux. Since then, RTAI has added many new features without com- promising performance.

• Unpack the tarballs and apply RTAI patch to the kernel source.

• Conﬁguring the kernel with appropriate options then compiling the kernel.

70 5.2 Control System

• Conﬁguring RTAI and install RTAI.

• Unmount /proc and /dev and exit chrooted environment.

• In boot folder in master partition, substitute boot/isolinux/linux with the new kernel, then decompress minirt.gz, mount the ramdisk imagine (minirt), substitute the kernel modules contained into it with the corresponding new kernel modules (in minirt: scsi, cloop.ko, unionfs.ko). The module extension must be .o instead of .ko to accomplish the module loader requirements, then unmount and compress the ramdisk image. Finally, replace the old ramdisk with the new one that has just been created.

• Make the ISO9660 compressed ﬁle-system and then updating the md5 hashes of the ﬁles included in the ISO.

• Make the RTKnoppix.iso file, test iso file by a virtual machine and then burn that file to the CD.

After RTKnoppix liveCD was built, it was installed in a compact ﬂash for a better reading speed and a stable operating system. In this operating system, Coriander was installed as discussed above. Coriander is the Linux graphical user interface (GUI) for controlling a digital camera through IEEE1394 bus which is developed by Damien Douxchamps. Coriander is full featured and in addition to changing the parameters of the camera it also can record video, send images to an FTP site, convert the video to a V4L stream and a live display. Coriander works with any camera that is compatible with the IIDC speciﬁcation including DFX-V500. A simple program that grabs images from the camera using raw1394, libraw1394 and especially libdc1394 library was developed. These images, then can be displayed by using GTK+- library.

5.2.2 Realtime Application Interface

Realtime operating system is deﬁned as a system that all of its tasks can be executed without violating speciﬁed timing constraints. It means that tasks will

71 5.2 Control System execute can be predicted deterministically on the basic knowledge of system hardware and softwares. RTAI is a realtime extension that allows writing programs with strict timing constraints for Linux (www.rtai.org). RTAI implements micro kernel, shown in Figure 5.8, which provides a second kernel that is an interface layer between the standard kernel and the hardware layer. The realtime tasks are executed and the standard Linux kernel is run as a background task by this compact code module. The micro kernel intercepts the hardware interrupts and ensures that the standard kernel cannot pre-empt any interrupt processing in the micro kernel. The realtime tasks are also scheduled with the highest possible priority to minimize the task latency [12]. RTAI consists of ﬁve complementary parts: the hardware abstract layer (HAL)

Figure 5.8: The realtime micro kernel [12] which provides an interface to the hardware on top of which both Linux and the hard real-time core can run, Linux compatiblility layer with which RTAI tasks can be integrated into the linux task management without Linux noticing, RTOS core, LX/RT which makes soft and hard realtime features available to user space tasks in Linux, and ﬁnally extended functionality packages with POSIX interface, third-party toolboxs (Labview, Comedi, Real-time Workshop, software

72 5.2 Control System watch dog, etc.). RTAI used in this system allowed accurate timing of control loop and allowed parallel control of two separate stages for tool path interpolation. A realtime driver was developed for PCM-3718, PC104-DAC06 and serial port which allowed access from realtime threads. The user space commands communicate with the realtime controller threads via shared memory.

5.2.3 Control of Microstages

M-605.1DD linear microstages were control using PI C-862 Mercury controllers through serial port. Three linear stages were used so three Mercury controllers were needed. However, only one serial port was needed because the controllers are connected in a daisy chain (an interconnection of computer devices, peripherals, or network nodes in series, one after another). The Mercury controllers and their connection are shown in Figure 5.9. The network feature allows addressing each controller individually. This can

Figure 5.9: (1) C-862 Mercury Controller (www.pi.ws) and (2) Three controllers connected in network be done by adjusting the switch bank on the front of each controller. When a

73 5.2 Control System controller communicates with a stage, all other controllers in the network will stop communication. A set of commands were developed to assist the control using ASCII characters output to serial port. Switching between controllers requires sending ASCII character 0x01 followed by an address-number character. Any motion sequence or operation begun prior to receiving a disabling address selection command will continue to be executed. A PID speed, acceleration and position control was integrated in the controller. The speed of each stage is set depending on the feed rate in counts per second (1 count is equivalent to 0.1μm) and in the range from 1 to 250000 counts per second. When under control a stage can move in a relative or absolute movement with the resolution of 0.1μm.The current position of the stage can be read. This is used in determining the PID parameters and in the micro coordinate measuring system used for checking the surface incline. The PID parameter can be manually turned. P term (from 50 to 150), I term (from 0 to 40), D term (from 0 to 3500) can be determined as follows:

• Increase P until there is an overshoot and oscillate in the step response. D and I are set to 0.

• Reduce P to the half of oscillation gain.

• Set I to two times the period of the oscillation

Another method to turn PID parameters is using ziegler-Nichols. The character- ized constant, delay time and time constant can be found from the step response of the system. Afterwards, P, I and D can be determined from those constant. The velocity and position responses of the microstage with P = 150 and I = D = 0 are shown in Figure 5.10.

5.2.4 Control of Nanostage

The P-527.2CL ﬂexure stage is controlled by using E-500 series PZT controller produced by PI, shown in Figure 5.11. PC104-DAC06 analogue ouput ports were used to set the design values of controller and PCM-3718 analogue input ports

74 5.2 Control System

Figure 5.10: Velocity and position response of the vertical stage at K = 150, I = 0, D = 0 were used to read feedback from sensors. This controller includes a E-503 amplifier module with LVPZT (low voltage PZT), 6 Watts, three channels and a E-509.C2 PZT sensor/controller module with 2 channels for capacitive sensors. The signal path diagram of an E-500 series PZT controller is shown in Figure 5.12. E-503 LVPZT amplifier module is a three channels amplifier for low voltage

Figure 5.11: A E-500 PZT controller produced by PI (www.pi.ws)

75 5.2 Control System

Figure 5.12: Signal path diagram of an E-500 series PZT controller (www.pi.ws)

PZTs. It contain three independent amplifiers that can each output and sink a peak current of 140 mA and an average current of 60 mA. this module can be used for static and dynamic operations providing a peak current of 140mA for some ms allowing fast PZT expansion changes. The output voltages can be controlled either by the manual 10-turn offset potentionmeters or by analogue input signals from PC104-DAC06. These signals will be multiplied by the gain factor of 10 to create output voltages range between -20 and +120 volts. The DC offset potentionmeter is active at the same time and produces an internal offset voltage of 0 to 10 voltages added to the input signal [77]. E-509 is a displacement sensor module with an integrated position servo controller

76 5.2 Control System for PZT positioning systems. A E-802.50 analogue P-I controller submodule is integrated. P and I gain can be set internally by trimmers. Sensor bandwidth and control bandwidth can also be set. A notch filter was installed to allow operation of the piezo positioning system closer to it mechanical resonant frequency. This module generate the input signal for the amplifier module according to the difference of target and actual position. Drift and hysteresis of the stage are com- pensated. It also increases the apparent stiffness of stage by quickly adjusting the operating voltage on the PZT as soon as a change in force or load occurs [77]. To achieve optimum performance E-509 must be calibrated with the stage. This calibration is done at the PI factory. However, the zero-point adjustment need to be done before controlling the stages. This is to ensure that the full output voltage swing of the amplifier can be used without reaching the output voltage limits of the amplifier and causing overflow conditions, both in open-loop and closed loop operation. Each channel of E-509 has one sensor monitor output which is the signal reading from the capacitive sensor. This analogue signal was read by PCM-3718 analogue input to feedback the current position of the stage. This signal is read to get current position feedback. Each axis of the stage is controlled by one analogue output port. Therefore, P- 527.2CL is controlled separately by two analogue output ports of PC104-DAC06. The sensor monitor signal of each axis read using an analogue input port of PCM-3718. Because PC104-DAC06 resolution is 12 bits, the 200 μm travel of nanostage may be controlled in 4096 quantised steps of 48.8 nm. The analogue output signal can only control the position of the nanostage. Hence, a velocity control software was developed as a realtime thread. The change of the number of steps per control cycle can effectively change the velocity of the stage.

5.2.5 Control of Spindle System

The SC 40 spindle system was controlled by the PCS 410 solid state frequency converter produced by Precise as shown in Figure 5.13. This frequency converter is for driving spindles, speed of which can be adjusted continuously, also for simultaneously high torque constancy. Besides, it uses

77 5.2 Control System

Figure 5.13: PCS 410 solid state frequency converter special pulse techniques to approach the phase current to sine. The spindle can quickly be decelerated due to the integrated brake circuit. Moreover, this system integrated protective circuit to protect against pulse shape overload and short circuit, protect the internal low voltage supply, mains voltage and protect the spindle. Furthermore, the potentials are separated in this frequency converter. The speed response of the spindle which is controlled by PCS 410 is shown in Figure 5.14. This system can easily be manually controlled using the front panel controls

Figure 5.14: Speed response of spindle control by PCS 410 (www.ﬁscherprecise.com) or automatically controlled using the connector at the back connected to digital inputs and outputs of PCM-3718 as shown in Figure 5.15.

78 5.2 Control System

Figure 5.15: PCS 410 solid state frequency converter connection

5.2.6 Control of The Cooling System

The cooling system of the spindle is always active when the spindle is running and this system should run before turning the frequency converter on. Therefore, it is not necessary to computer control this system. However, the cooling system for the tool needs to be controlled. This is due to the high humidity in the working area which may damage the electronic circuits inside the stages and the actuator and the ball bearing system of the spindle. Therefore this system can only be activated in a speciﬁc period of time. This system was controlled using a pneumatic solenoid valve. The PCM-3718 digital outputs, then, was used to control this valve as shown in Figure 5.16.

79 5.3 Tool Path Interpolation

Figure 5.16: Diagram of workpiece cooling system control

5.3 Tool Path Interpolation

5.3.1 Cutting Parameter Set-up

A specific method which is used to optimize cutting parameters in micro-machining has not yet been developed. A method which based on the technical properties and requirements of micro endmills from PMT (the micro endmills manufacturer) and the calculation of cutting parameters in conventional machining was used for defining cutting parameters in this project. The two main cutting parameters that need to be defined are the feed rate and the spindle speed (RPM). Basically, these parameters can be defined using equation 5.1, 5.2 and 5.3: SFM × 4 RPM = (5.1) D Where RPM is the spindle speed in revolutions per minute, SFM is the cutting speed of material in surface feet per minute (SFM = 500 for plastic, 300 for aluminium, 200 for brass, 1000 for mild steel and 50 for stainless steel) and D is the tool diameter in inches. If the RPM is far higher than the spindle can archieve, the highest RPM is used with the reduction of the feed rate accordingly. The feed rate (IPM) in inches per minute is calculated as below:

IPM =2%× n × RPM (5.2)

80 5.3 Tool Path Interpolation

Where the chip load is the amount of material that each flute will remove on each revolution in inches (chip load is equal to 0.005 inch for roughing, 0.001 to 0.002 inch for finishing) and n is the number of flutes on the tool. The micro endmills from PMT are designed for chip load of approximately 3% so the expression becomes:

IPM =(chipload) × n × RPM (5.3)

As discussion in Chapter 3, a chip load/feed rate too high will snap the tool immediately, a chip load/feed rate too low will wear the tool out prematurely. The depth of cut is also an important parameter. Depending on the type of workpiece material, multiple passes a little deeper per pass may be used. Some materials can be milled at full depth of tool, harder materials may require 1x diameter depth per pass. After deﬁning the cutting parameters, the speeds and steps of stages can be deﬁned.

5.3.2 Tool Path Creation and Interpolation

The machine coordinate has axes that are perpendicular or parallel to the travel axes of the stages. The algorithm stated in Zhang et al. [13] was employed to create linear tool paths. This is the base of all kinds of curve interpolation in CNC system. The method helps to reduce calculation time and improve working eﬃciency. This algorithm was described in this study as below: Two axis of the coordinate system are divided into a number of steps based on the resolution of the system. These axes were named long axis and short axis. Long axis is the axis along which the number moving steps is more than another and L is its total number of steps. The other axis was called short axis with S the total number of steps. Some other terms used in this algorithm are: the length of section (P) is the L L integer part of S , E is the awl of section which is the decimal part of S and value of calculating bias e which is equal to 0 at the initial state. If S =0,the line created is parallel to the long axis and the interpolation algorithm is not necessary.

81 5.3 Tool Path Interpolation

≤ L L It is obvious that P S < P+1.WhenE=0thereisP= S ,thatiscontinue moving P-1 steps along the long axis then move each one step each along the two L axes; When E is different from 0, P = (int) S , that is continue moving P-1 or P steps along the long axis then move each one step along the two axes. To deal with this, e and E are used. e is increased by E after moving each step along the two axes. If e < 1, the next number of steps is P-1. Otherwise, the next number of steps is P and e is decreased by 1. For improving the precision of interpolation, length of first section is (int)P/2, and initial value of bias e=E/2. To approach the realization of the algorithm, the generalized periodicity of section interpolation algorithm, which is formed from the changing rule of the bias, is calculated. At initial status the numbers of period (T) can be estimated by L formula: T = (n+2)P +(n+2) . An realization of the algorithm, which is developed from the use of T, is shown in Figure 5.17. This algorithm helps to prevent from using floating point calculation in realtime threads. However, its’ quality depends on the resolution of the system. An example of this algorithm is shown in Figure 5.18. As shown in this figure, linear interpolation algorithm is used to interpolate a line whose start points coordinate is (0, 1) and end points coordinate is (14,4). An arc is a part of a the circle and it can be used to create curves such as splines. The arc interpolation was developed based on the linear interpolation described above. The arc interpolation algorithm is described below: An arc is divided in to multiple lines. The higher the resolution of the stages the smoother the arc is. In this algorithm, first the total displacement of each axis is calculated. This value can be negative or positive because the direction can be clockwise (CW) or counter clockwise (CCW). The axis which has a bigger displacement will then be chosen to be counted up and the other will be calculated with respect to this axis using the equation of the circle. A tool path created is an arc which has a starting point A(a,0) and an end point B(x,y). The value d is defined as the direction flag (d=1 if the tool path is CW and d=-1 if the tool path is CCW). The center of this arc is the center of the coordinate frame. The radius of this arc is: R = |a|. The total displacement of each axis (dx and dy is the total displacement along x and y, respectively) is

82 5.3 Tool Path Interpolation calculated as described in the pseudo code below: If a > 0:

• If ((y < 0) and (x ≥ 0) and (d = 1)) or ((y > 0) and (x > 0) and (d = -1)) then dx = R -xanddy=-d×y

• If ((y ≤ 0) and (x < 0) and (d = 1)) or ((y > 0) and (x ≤ 0) and (d = -1)) then dx = R -xanddy=2×R +d×y

• If ((y > 0) and (x ≤ 0) and (d = 1)) or ((y > 0) and (x ≤ 0) and (d = -1)) then dx = 3×R +xanddy=2×R +d×y

• If ((y ≥ 0) and (x > 0) and (d = 1)) or ((y > 0) and (x ≥ 0) and (d = -1)) then dx = 3×R +xanddy=4×R -d×y

If a < 0:

• If ((y < 0) and (x ≥ 0) and (d = 1)) or ((y > 0) and (x ≥ 0) and (d = -1)) then dx = 3×R -xanddy=2×R-d×y

• If ((y ≤ 0) and (x < 0) and (d = 1)) or ((y ≥ 0) and (x < 0) and (d = -1)) then dx = 3×R -xanddy=4×R +d×y

• If ((y > 0) and (x ≤ 0) and (d = 1)) or ((y < 0) and (x ≤ 0) and (d = -1)) then dx = R +xanddy=d×y

• If ((y ≥ 0) and (x > 0) and (d = 1)) or ((y ≤ 0) and (x > 0) and (d = -1)) then dx = R +xanddy=2×R -d×y

After the total displacement along each coordinate axis is deﬁned, the interpolation loop starts to deﬁne coordinate points that the tool path will go through. The interpolation loop is described as follows.

• (x(i), y(i)) is the current position with initial values are a and 0, respectively and L is the resolution, i.e. the minimum possible incremental motion.

• Set another direction ﬂag b as: if a > 0 then b = -d, else b = d.

83 5.4 Summary and Conclusion

• ≥ If dx dy then: for i runs from 1 to (dx+1) with a step of 1 we have: x(i+1) = x(i)+b×d×Landy(i +1)= b× | (R2 − x2(i +1))|.If|x(i +1)| >R then b = - b, y(i +1)=0,x(i +1)=-b×d×R.

• For the case dx < dy, same logic applies with dx and dy interchanged.

With this algorithm, an arc section is divided into many lines. An arc tool path now becomes multiple linear tool paths and the linear interpolation algorithm can be used. The testing and simulation of this algorithm using Matlab is shown in Figure 5.19. In this simulation, an arc whose start points coordinate is (1300, 0) and end points coordinate is (500, 1200) (the unit is the minimum possible incremental motion (100 nanometers)) and the direction is CCW. The designed curve and the tool path from interpolation algorithm can be classiﬁed as shown in Figure 5.18.

5.4 Summary and Conclusion

In summary, this chapter described the calibration, the control systems and al- gorithms that were developed for micro milling machine designed and built as part of this project. This is the combination of control system for spindle system, workpiece manipulation system and cooling system. The realtime operating system using RTKnoppix was used to implement the control software. The entire system was calibrated to ensure the geometric fidelity. Furthermore, a method for calculating the cutting parameters was also presented. The tool path interpolation is also described. A completed micro milling machine was successfully developed. The configuration is a the vertical milling machine in which the clamped workpiece is moved against a fixed, rotating milling tool. Milling experiments using this machine were carried out and the results of these experiments are presented in Chapter 6.

84 5.4 Summary and Conclusion

Figure 5.17: Working of the linear algorithm with generalized periodicity[13].

85 5.4 Summary and Conclusion

Figure 5.18: Example of line interpolation algorithm[13].

Figure 5.19: Circle interpolation algorithm Matlab simulation result.

86 Chapter 6

Micro Milling Result

The milling processes was conducted and the results were photographed. Some images were taken using optical microscope. Other samples were imaged using scanning electron microscope (SEM). The SEM images used in this chapter were taken using Hitachi SEM400-I. However, SEM can only be used for objects made from conductive materials. Therefore, insulated surface needs to be gold coated before scanning.

6.1 Chip and Burr Formation Experiments

In the first experiment, the micro cutting process was carried out on an aluminium work-piece as shown in Figure 6.1. The two flute square ended micro endmills used have diameters of 100 μm and 50 μm. Both tools are stub length tools with the flute lengths of 150 μm and 75 μm, respectively. The spindle speed chosen was the maximum speed of 90000rpm and the feed rate was 20 μm/s. The feed rate and depth of cut were increased gradually in the machining process. The entire machining process was visualized using the microscope. The finished surface was imaged using the SEM. As can be seen from Figure 6.2, the chips formed were long and stuck on to the workpiece surface and the tool edges and caused built-up-edges. Some of the broken chips stuck on the machined surface (the bottom of the slots). Most of the burrs formed are top burr up-milling and top burr down-milling as per the burr definition in [10]. These burrs and chips could be partly removed by moving the cutting tool along the

87 6.1 Chip and Burr Formation Experiments

Figure 6.1: Experiment set up and machining progress work-piece surface when the tool tip just reached this surface and the spindle was still running. Figure 6.2 is the SEM image of a series of slots with the length of 500 μm, 50 μm wide ad 30 μm deep. Figure 6.3 shows one slot after partly removing burrs and Figure 6.4 shows part that slot before the burrs were removed. They have the same depth of cut of 20 μm and were machined using 50 μm micro endmill.

Figure 6.2: A series of slots

88 6.2 Experiments with Diﬀerent Materials

Figure 6.3: A part of a slot that was partly removed burrs

Figure 6.4: A part of a slot with 20 μm depth of cut

6.2 Experiments with Diﬀerent Materials

In these experiments, different materials are machined to test the machinability, chip and burr formation. Different work-pieces with different materials such as copper, aluminium, silica, titanium and platinum were used. A hole, a square and a slot were created on aluminium, copper and silica. There are two different ways to acquire images of copper workpieces. The first method was using microscope. The images were taken right after the machining process. The second method was with SEM. Because the copper layer is 100 μm thick and covers on the top of a plastic sheet and the depth of cut can be bigger than the copper layer thickness, this sample needs to be gold coated. Silica is an insulated material so it needs to be gold coated before taking image by SEM.

89 6.2 Experiments with Diﬀerent Materials

Figure 6.5: A micro slot, a micro hole and a micro square on copper created by 100 μm micro endmill, depth of cut is 70μm.

As can be seen from the Figure 6.5, 6.6, 6.7 and 6.8, the chips formed from aluminium is more sticky than copper and the size of burrs from copper is less than aluminium. Besides, when machining copper, the temperature is higher than when machining aluminium. The temperatures developed during cutting as a result of heat are mainly dependent on the contact between the tool and chip, the amount of cutting forces and the friction between the tool and chip. Consequently, the increase of the cutting speed will result in an increase in the temperature. This shows the need to employ a cooling system. The burr formation and chip formation in cutting silica work-piece are quite different from those formed on aluminium and copper. Chips were broken into very small parts (discrete chips) and they did not stick onto the workpiece surface because silica is a brittle material. Burrs created from cutting silica is significantly less than those resulted from machining copper and aluminium. However, there were some breakages around the edge of the profile. This may be because of the high speed of the vertical stage and the pressure from cutting process. As shown in Figure 6.11, an experiment was carried out on a platinum wafer and chips created were continuous chips. Burrs created from cutting platinum

90 6.2 Experiments with Diﬀerent Materials

Figure 6.6: SEM images of micro hole on copper. Depth of cut is 70 μm, tool diameter 100 μm.

Figure 6.7: SEM images of micro square on copper. Depth of cut is 20 μm, tool diameter 50 μm. are top burr up-milling, top burr down-milling, exit side burr up-milling, exit-side burr down milling and exit burr bottom. they were bigger than burrs created from copper, aluminum and silica. Consequently, a deburring technique such as microblasting with diamond powder is needed to ensure the quality ﬁnished surface requirement.

91 6.3 Experiments with Other Machining Techniques

Figure 6.8: SEM images of micro slot on copper. Depth of cut is 10 μm, tool diameter 50 μm.

Figure 6.9: SEM images of micro hole on silica. Depth of cut is 50 μm, tool diameter 50 μm.

6.3 Experiments with Other Machining Tech- niques

An attempt to use laser machining and FIB (Focus Ion Beam) to cut the solid carbide material to make a T-shaped endmill was made as shown in Figure 6.11. With the laser machining, a laser beam with the beam width is approximately 34 μm was employed. There was a difficulty in defining the appropriate energy for laser to cut this material. It is also hard to define the area on the workpiece that effected by the heat of laser beam. As a result, the surface created did not meet the requirements about the size, shape and accuracy. FIB could produce superior quality surface. However, it took nearly 34 hours to cut an area of 30x80 μm down to a depth of 25 μm. Therefore, this is not an effective method that can be employed in manufacturing especially for cutting hard materials.

92 6.4 Summary and Conclusion

Figure 6.10: Optical microscopic image of slots on platinum workpiece. Depth of cut is 30 μm, tool diameter 50 μm.

6.4 Summary and Conclusion

In summary, the results shown in this chapter illustrate the potential of the micro milling system. Experiments were carried out with different materials and different feed rates and depths of cuts. They showed the different chip formation and burr formation behaviors with different materials which help to choose appropriate cutting parameters. These experiments also illustrate that the micro milling system has a potential to produced a wide range of profiles. However, there is still room for developing this system. A microblasting system can be developed to used with this system as the post-processing system to removed burrs, and improve surface quality when machining soft material such as aluminium, copper.

93 6.4 Summary and Conclusion

Figure 6.11: An 100 μm micro endmill after cutting by FIB (front surface) and laser (right surface)

94 Chapter 7

Discussion

7.1 Summary and Achievement

This thesis presented new equipments that can be used to fabricate micro devices and profiles in micro scale using mechanical micro milling technique. The micro milling system developed is capable of machining micro scale components from different materials. A background of micro milling machines was also presented. This is applied to build the machine system described in this work. A 4 axis vertical micro milling machine was successfully built. A combination of high accuracy linear microstages and ultra precision nanostage allows ultra precision movements. Besides, a high speed spindle was used to securely hold micro tools with different diameters from 100 μm, 50 μmand20μm. A vision system was used to aid the in process visualization and control of the machining process. A cooling systems was employed for both spindle cooling and tool, workpiece cooling. Those systems was used to protect the spindle, improve the tool life, reduce the cutting temperature and protect the workpiece materials from burning and melting. A control system and interpolation system were developed especially for this machine. A real-time operating system based on RTAI and Knoppix was developed to ensure the real-time implementation of the control system. This helps to control the machine in realtime and concurrently control stages for tool path interpolation. A speed and position control system for each microstage and nanostage was developed. In addition, the cooling system was also computer controlled to

95 7.2 Discussion ensure it works effective operation. A method to define the machining parameters for this machine was also described. Finally, a tool path interpolation algorithm was developed for line and arc sections. These are the two basic profile segments that can be used to form various different profiles. A number of experiments were conducted. The results show different burr and chip formation with different depth of cuts, feed rates and tool diameters. Ex- periments were also carried out on different materials such as silica, copper, aluminium and platinum and the difference in burr and chip formation were stated. A few profiles were machined on those materials such as slots, squares and holes with different depths.

7.2 Discussion

As stated, this machine is capable of producing good surface quality and high accuracy components. This system has several advantages:

• Time consumption: the machining time of this machine is considerably less than the machining times of FIB and laser machining. The machine setup is uncomplicated.

• Easy to operate and maintain: This machine was divided into modules. Each module was built and thoroughly tested at the factory. The controller was designed in a way that is easy to operate. All the machining steps are quite straightforward.

• High accuracy: With the high resolution, high accuracy stages and the realtime control system, this system can machine micro scale proﬁles with precision within hundreds of nanometers.

• Reconfigurable: stages and mounting plates were produced separately and can be easily assembled and dissembled so it is a reconfigurable machine system. However, it requires re-calibration after reconfiguration. The micro tools can be easily changed.

• Has the capability to produce three dimension proﬁles.

96 7.3 Further Work

• Relatively inexpensive compared to the FIB and μEDM systems.

• Low level of hazards because this system does not rely on dangerous and toxic chemicals, and hazardous laser or other beams. Moreover, a cover case help to prevent dust and chip escaping to the environment.

As discussed above, this system has many advantages compared to other machining techniques. With further developments, this system has the potential to be used in industry for mass production manufacturing.

7.3 Further Work

Due to lengthy delays in ordering components such as stage from Germany and to repair the old stages, some experiments has not yet been finished. The machined surfaces were affected by the burrs and chips. However, this can be overcome by using microblasting system. Another limitation of this system is the vision system. With each change of the spindle position, it needed to change the position of the microscope. Moreover, the microlight was not really coaxial and the angle of view was not straight this make it is difficult to fully see the in progress machining. This can be overcome by using a coaxial microscope. The improvements of this system can be defined as follows: • Improve the load capacity of the vertical stage. The stage used is not designed to hold an appropriate push/pull load.

• One rotation axis can be added to the spindle by using a rotation micro stage. This will allow milling in diﬀerent angles and therefore increase the number of three dimension proﬁles that can be machined.

• A higher speed spindle can be used to improve the cutting quality.

• Computer system can be upgraded to a modern processor to increase the speed of the vision system and control system. Together with a coaxial lens, a better camera can also improve the vision system. This, then can be applied for image processing control. The tool level and tool workpiece contact can be deﬁned automatically.

97 7.3 Further Work

• The sensors to measure the error motions and run-out of tool shaft during high speed rotation can be incorporated.

• The use of chip breaker needs to be considered when machining soft materials such as aluminum and platinum.

• Develop a method to speciﬁcally deﬁne and optimize cutting parameters for this micro milling machine.

• In addition to deburring, a method to remove broken chips that stick on the machined surface such as the bottom of the machined slots needs to be considered.

More experiments can be carried out to thoroughly check the capability of the system. The more difficult profiles with complicated curves can be tested. Dif- ferent material such as biocompatible material (PMMA), steel can be tested. Furthermore, the smaller endmill diameter can be used with the current available endmill diameter up to 2 μm. This can improve the accuracy. However, the spindle run-out needs to be considerably reduced. Another type of endmill that can be used is the T-shaped endmill. Although it is not available in the market but it can be fabricated from a square ended endmill using FIB. This can be used to make micro T slots which are used in fabricating medical implants such as microbarbs. This machine can also be used to machine different profiles of microgrippers on either steel or shape memory alloys.

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105 Appendix A

Machine Operating Procedure There are several step for operating the machine.

1. Mount the workpiece on the surface of the manual microstage on top on nanostage. 2. Check the level of the cooling liquid. The cooling liquid should be change annually. Turn the spindle cooling system on. 3. After the cooling system work about 2 minutes, turn the frequency converter on and set the speed at one fourth of the maximum speed if the spindle has not been working for a long time or half of the maximum speed if the spindle has not been working regularly and leave the spindle run unload for half an hour. 4. Log on the RTKnoppix PC. 5. Open console window and cd to the control directory. 6. Load the RTAI module. 7. Run the control program. 8. Move the vertical surface up gradually. Check on the live image of coriander to see whether the tool tip contact the workpiece surface. 9. Start machining progress by insert the proﬁle, start and target point (and the direction of the tool path is a curve). Appendix B

Manufacturing Drawings The manufacturing drawings includes:

• Adapting plate 1 and 2 for mounting the nanostage with the vertical micro stage.

• Adapting plate 3 and 5 for mounting the vertical stage on top of the horizontal stage 2 and mounting 2 horizontal microstages together.

• Adapting plate 6 for mounting the spindle frame to the table.

• Adapting plate 7 and 8 for mounting the camera on camera frame and the camera frame on the table.

• The camera and the frame to hold and position the camera.

• Assembly the assembly of the milling machine system.

• Adapting plate 9 and 10 for mounting the case on the table.