At this time, unfortunately, a convergence in terminology has not been forthcoming and, hence, a definition of the term Micro-Machining to be implied in the context of this assessment is in order. In principle, one may take two viewpoints:(1) The first viewpoint may define Micro-Machining as the collection of all cutting operations that are performed on micro/meso-scale components and products that fall into the 100 μm to 10,000 μm size range as shown in the figure below. The Micro-Machining regime is characterized by the requirement of producing high accuracy complex geometric features in a wide variety of materials in the above-defined size range. These requirements impose the use of considerably downsized tooling (micro-tools, e.g. endmills in the 50 to 500 micron diameter range), small undeformed chip thicknesses and feedrates (submicron to a few microns) and speed settings (50K to 200K RPM might not be uncommon) that would be considered technologically infeasible at the conventional macro-scale. As a consequence, the principal distinction between Macro and Micro-Machining operations emerges and manifests itself as the dominance of ploughing and rubbing phenomena at the cutting edge over shearing and the necessity to take micro-structural effects into consideration. (2) The second viewpoint approaches the definition of the Micro-Machining regime from the standpoint of the magnitude of the undeformed chip thickness being removed in the cutting process. It is difficult to define a clear-cut value of the undeformed chip thickness that would differentiate the macro-, micro/meso- and nano-scale cutting regimes since other factors such as grain-structure, cutting edge radius, etc., also come into play. The authors of this report would suggest the following classification: Miniaturization is the order of the day. Until recently a decade ago traditionally watch parts were considered to be the micro components one can think off. Recent changes in society’s demand have forced us to manufacture variety of micro components used in different fields starting from entertainment electronics to bio medical implants.
We are all familiar with the phrase “the world is getting smaller.” However, it is not just that the world is getting smaller, practically everything we use is getting smaller. Manufacturing technology has advanced to higher levels of precision to satisfy the increasing demand to reduce the size of parts and products in the electronics, computer, and biomedical industrial sectors. New processing concepts, procedures and machines are thus needed to fulfill the increasingly stringent requirements and expectations. Micromachining refers to techniques for fabrication of 3D structures on the micrometer scale. Until recently watch parts were considered to be the micro components produced for the purpose of making watches. Recent demands for micro parts have required us to manufacture variety of micro components used in different fields from entertainment electronics to biomedical implants. The convenience and value of many products can be substantially increased with reduced size and weight. With the trend toward miniaturization, micromachining becomes increasingly important in fabricating micro parts1. In the medical field, diagnosis and surgery without pain is possible through miniaturization of medical tools. The convenience and value of many products can be substantially increased with reduced size and weight. With the trend towards miniaturization, micromachining becomes increasingly important in fabricating micro parts. Micro parts may have overall size of few millimeters but it has many features that falls in micro range from 1 μm to 500 μm features size of 100 μm is common in micromachining. This means small as hair size, the average hair diameter is about 100 μm1-5. The design and construction of tools, tool holders, cutting tools, and electrodes need to evolve as greater demands are placed on them for machining these miniature parts. A study of micromachining process proves that micro cutting processes are not just a miniaturization of the conventional cutting technology, and requires an adjustment of the entire machining setup and processes. Miniaturization technologies are perceived as potentially key technologies of the future that will bring about completely different ways people and machines interact with the physical world. In the industrial world the interest in microscopic scale manufacturing is exponentially increasing in relation to the rapid growth of Micro Electro Mechanical Systems (MEMS) research. Thus a greater attention is given to improve traditional techniques and developing nonconventional methods, in order to obtain more precision. Micromachining Techniques Micromachining is a specific technique applied to micro scale parts. Micro Electro Mechanical Systems (MEMS) are microscopic devices processed, designed, and used to interact with or modify the local environment. They can be referred as microstructures, microsystems, mechatronics and microstructure technology. MEMS can also be referred to devices with moving parts (smaller than human hair) containing both mechanical and electrical components on silicon. With rapid growth of MEMS a greater attention is given to traditional methods and developing non-conventional machining methods. The most important techniques are used for micromachining are photolithography, laser, micro-EDM and micromechanical machining (micro-cutting and micro-milling) which is the focus of this paper. Lithographic Process: It is a traditional technique of micromachining on silicon based on lithographic approach, by etching and deposing process used in microelectronics. Silicon wafers are machined with chemical or physical etch and parts are realized layer by layer from silicon wafer. This noncontact method is based on masking and light exposure2. Laser Micromachining: Laser uses light radiation with high energy as a machine tool. High precision can be achieved and material removal is obtained by ablation. Ceramics and metal layers can be machined with higher laser densities. Focused beam could allow real 3D shaping by correct motion control3. Micro Electronic Discharge (EDM) Machining: The erosive action of an electric discharge between conductive tool and work piece is used to remove material. Electro-thermal erosion creates small craters in the piece during machining process. The tool shape is copied in the work piece with a no contact system. EDM machining process is able to machine both hard materials like steels and carbides and semi-conductors and conductive ceramics4. Micro Ultrasonic Machining: Micro ultrasonic machining is a process that uses micro tool ultrasonic vibration to create accurate holes in brittle materials like silicon, glass and ceramics. Abrasive slurry is interposed between tool and work piece and the tool is used as a micro-mill to obtain drills or pattern on the work piece surface. The vibrating tool impacts abrasive grains into the work piece producing a mechanical removal of the material5. Mechanical Micromachining Technology: Mechanical micromachining technology is a new field in micromachining that is achieved by optimization of cutting process for micro-milling, turning and grinding process for a wide range of materials. In this process the unwanted part of the work piece is removed by mechanical force through brittle breakage. A high stress that causes breakage of material is applied to a very small area or volume of the work piece. Extremely precise cutting machines with high level of positioning accuracy are designed to perform milling, turning and grinding in micro scale. Tool dimensions and cutting edge sharpness represents challenges in developing this technology in micrometer scale. Coated micro mills down to 30 μm in diameter are commercially available for micro machining purposes. Mono crystalline diamond cutters of no less than 50μm in diameter is also used in the mechanical micro machining process and the diameter cannot be any smaller due to part stability reasons during manufacturing of the part. Mechanical machining because of removing the material by contact and chip generation is linked with heat generation. The cutting fluid is used to prevent the heat and keep the cutter sharp. The cutting fluid is applied as mist; a few milliliters of oil per hour are atomized by pressurized air. The right amount of oil in the air stream is essential in mechanical micro machining to prevent the sticking of chips to the tool6. The shape of the micro cutting tool also has been modified to reduce breakage and chatter. The fluted length has been reduced to increase stiffness. The conical part of the cutting tool, where bending occurs has been designed to a round shape. The shape prevents it from cracking and breakage due to machining stress. The CAD/CAM process and 3D modeling can be used to create G-code program for a mechanical micro machine using Computer Numerical Control (CNC) system. An effective CNC support is required to machine curvilinear features and sculptured 3D surfaces. The emerging miniaturization technologies are perceived as potentially key technologies of the future that will bring about completely different ways people and machines interact with the physical world. A study of micro machining processes proves that micro cutting processes are not just a miniaturization of the conventional cutting technology, and requires an adjustment of the entire machining set-up and process to implement this technology. Conclusion With increased demand for miniaturized functional equipment, micro machining is becoming an important industry. Micromachining is the technology for manufacturing micro sized structures. This technology has many applications, and has driven innovation in many areas such as the automotive and biomedical engineering fields. The potential of micromaching has been noticed by the research community, inspiring the creation of many academic works. Since its beginning, micromachining has evolved greatly to include more techniques and methods, and the array of materials being processed under these techniques also has expanded. With the prediction of nanotech and micromachining market expansion by 2015 to 1 trillion dollars and creation of new industries related to this technology the demand to technical workers in this fields will increase. This will provide a great opportunity for technical programs at two year colleges to offer programs related to micromaching. This will be possible by getting involve in the trends and acquiring technology and tools used in this new field. Some conventional methods like laser cutting, Electrical Discharge Machining (EDM) and conventional machining at the existing programs can be updated by purchasing micro scale machines to provide training in micromachining technology. Over the past several years there has been an increased interest in micro machining technology that has captured the imagination of every manufacturing and industry segment; from aerospace, medical appliance and the automotive world, the potential for product miniaturization continues to grow and while posing numerous technical challenges. In response to this continued miniaturization, companies are developing new technologies to meet the unique challenges posed by micro manufacturing and must develop appropriate machining systems to support this growth. The manufacture of miniature parts is not new. Many companies have used various machining technologies such as EDM and laser to produce micro details for many years. The difference today is the shear volume of products that require micro machining. The accelerated rate of change is unbelievable. New miniature products are changing how we view the world. Many manufacturers’ are developing micro machining technologies and techniques to support this growth. Companies are looking for parts having feature sizes of less than 100 microns, or somewhat larger than a human hair. At this scale, the slightest variation in the manufacturing process caused by material or cutting tool characteristics, thermal variations in the machine, vibration and any number of minute changes will have a direct impact on the ability to produce features of this type on a production scale. In response to this continued miniaturization, companies are developing new products and technologies to meet the unique challenges posed by micro manufacturing. These types of tolerances are mind boggling and would have been unthinkable just a few years ago. Talk of using end mills of 0.002" (50 µm) diameter and EDM wire diameters of 0.00078" (20 µm), or electrodes smaller than a few tenths is becoming more commonplace. The application micro-meso machining technologies are being employed in the manufacture of a wide variety of products and devices. Medical Components Micro Molds Electronic Tooling MEMS (Micro-Electrical- mechanical-System) Fluidic Circuits Micro-Valves Particle Filters Subminiature Actuators & Motors. The trends in the ultra high accuracy and micro-miniature manufacturing fields require a fresh look at new machine technology and process techniques. DEFINING MICRO~MESO MACHINING TECHNOLOGY The term micro machining has a variety of definitions, depending upon whom you are speaking too. Micro machining simply means small or miniature to many of us in manufacturing. Those in academia and research define Micro in a very literal way, or 10-6 , in other words, as one-millionth of a meter. (A micron n: a metric unit of length equal to one millionth of a meter [syn. – micrometer. Origin - Greek m kro-, from m kros, small] quite literally as 106 (mm)], We need to further define, for purposes of clarification, what all of this means to us in manufacturing based on the part or feature size. Figure 2 illustrates the range of part and feature size machining capability. Parts with machined features below <0.004" (100 µm) fit into the middle (e.g. - meso) range of manufacturing. The chart below provides a basic reference of the micro machining range of various machining technologies. CHALLENGES OF MICRO MACHINING SYSTEM DEVELOPMENT Maintaining control of all of the machining variables, including the machine tool, work and tool-holding, the environment, cutting tools or electrodes will all have a huge cumulative effect on the end result. With such small parts and feature sizes, accuracy takes on a completely new meaning. For example, ±0.0002 tolerance is very different if the feature being machined is 0.200" vs. only 0.002" in size. For this reason, it becomes necessary to re-think the meaning of precision. There are several key areas of concern when machining details this small. 1. Environmental changes that impact Accuracy; process predictability and repeatability 2. Vibration (Internal and External) 3. Part Management 4. Cutting Fluids and Fluid Dynamics. Machine resolution, control, construction and ancillary tools all become much more critical to the success in producing micro parts. Besides being able to machine micro features and parts, simply handling micro parts and tools will pose unique challenges. This unfortunately would have an impact on the repeatability of a process that has a desired tolerance of less than a micron. USING NANOMETER RESOLUTION Although high resolution feedback systems have improved accuracy and eliminate servo drift there is more that need to be done. Figure 4 demonstrates the difference in servo drift between 1µ and 0.1µ feedback resolution. It is also no longer good enough to have feedback systems and resolutions only in the micron range. When machining in the micro machining range, sub-micron control and feedback systems are necessary. Due to this cumulative effect of errors, a good rule of thumb is that the systems employed for manufacturing should be 10 times more accurate than the repeatable tolerance desired. This would mean that to achieve a ±0.00020" (±5 micron) tolerance would require a system that would provide at least 0.000020" (0.5 micron) precision. Feedback resolutions in the 10 ~ 50 nanometer range are now available to improve "resolution" accuracy on new machine tools, however this does not necessarily make a machine accurate. Resolution is the digital accuracy of the machine tool and does not correct for alignment problems. Alignment is critical in producing accuracy, and depending upon the number of axes that are combined in a system, the perpendicularity, parallelism and straightness of the positioning axes to each other are just as critical to precision as the resolution. Figure 5 shows the volumetric accuracy of a Vertical Machining Center. ENVIRONMENTAL CONTROL We all know that the shop environment can change during the day. Even a single degree of change will affect accuracy when machining in the submicron level. Simply monitoring the room is not enough. Structural changes caused by temperature variations are affected by the mass of the machine. The rate of change (convection) will vary from system to system based on mass. For this reason, it makes sense to incorporate a machine thermal enclosure to maintain the machine in its own controlled environment. In Figure 6, demonstrates the thermal stability that can be achieved with a multi-layer thermal enclosure that combines insulation with a layer of air. Air temperature monitoring and flow is controlled to maintain uniform temperature distribution. Direct monitoring of minute temperature variations that occur in the machining system can be accomplished through the use of monitors (thermisters) embedded in the machine frame or casting. SUBMICRON TOOLING Precision work and tool (electrode) holding systems capable of positional repeatability in the 1 micron range have been commercially available since the late 1980’s. Originally designed for the EDM industry, these systems are highly reliable, even in the harsh EDM environment, and are now found on virtually every type of machining system. From machining centers to grinders, to coordinate measuring machines, these holding systems can be found in every type of manufacturing environment. Due to the demands for better accuracy required for Micro Machining, new innovations in tooling with submicron repeatability are necessary. Figure 8 (Courtesy of Erowa Technologies) demonstrates the advances in workholding technology being developed for the micro machining market. According to the manufacturer, this new tooling system is twice as accurate as previously available systems. MICRO MILLING Micro machining using conventional technologies, such as milling present unique challenges in manufacturing. Cutting forces and tool pressures when using micro tools create a whole new realm of problems. Any variation in axis position during the cut can be disastrous. The spindle must be stable and minimize thermal expansion, tool change variation and vibration. Any vibration or run-out at the tool tip will adversely influence the surface finish and accuracy. One of the problems associated with micro milling is the amount of foce associated with removing material at the particulate level. One solution developed several years ago is a direct tool-change type spindle. By eliminating the use of a tool holder, it is possible to reduce total run-out caused by tool holder variation and is ideal for micro machining due to the elimination stack up issues. Whether using a direct type, or one that uses a tooling system such as HSK, the spindle must be stable and minimize thermal expansion, tool change variation and vibration. The method of cooling and lubricating the spindle will have a direct impact on spindle growth and movement during the machining process. The Inside – Out cooling provided by Spindle Core Cooling takes advantage of centrifugal force to move cooling fluid though the spindle. Traditional laser tool measurement systems have shown some limitations when measuring cutting tools below .020" (0.5 mm) common when machining micro parts. New hybrid ATLM systems combines the benefits of touch and non-contact measuring in order to verify tool tip position in the micron range regardless of spindle thermal expansion. Figure 11 demonstrates the submicron precision obtained over a four hour period even when completing multiple tool changes. MICRO WIRE EDM ED machining has been a mainstay of manufacturing for more than 50 years, providing unique capabilities to the job shop and manufacturer alike. Miniature parts and components have been produced for many years using the non-contact capabilities of EDM systems. In fact, Life magazine published a photograph in the early sixties of a series of micro holes that were EDM’d through a needle and spelled out the magazines name "LIFE". The ability to automatically wire thread and machine parts with a 20 µ (0.00078") wire and achieve corner radii of less than 15 µ would have been unthinkable just a few years ago. The problems of small hole threading and hole proximity can now be conquered. A key to this technology is the horizontal inclination of the Wire and the utilization of air and vacuum rather than a fluid to thread the Wire. This is a radical departure from conventional designs. Machining in the horizontal plane provides several advantages including an integrated "C" axis for work holding and automated part loading with slug removal systems that improve automation. The open design of a V type wire guide improves automatic Wire threading reliability. The guide controls the cutting tool and must therefore be highly accurate. The V type guide provides 3-Point contact with the wire for superior wire alignment. As discussed earlier, an environmental system for precise temperature control (within ± 0.5º C) must be used when machining ultra fine details. Lastly, an Oil dielectric instead of de-ionized water in order to provide - A smaller spark gap due to the added insulation strength of Oil. Oil produces superior surfaces due to the quenching characteristics Oil Eliminates the problems of rust during long unattended operation MICRO EDM SINKING Today’s CNC die sinking EDM system is much more capable than their manual predecessors for conventional mold manufacturing and micro machining of complex parts for a wide range of applications. When talking about machining in the micron range it is necessary to look at machine design and construction, as well as the ability to produce parts efficiently in a production environment. Successful EDM’ing requires minute orbital motion in order to achieve the speed and surface finish to make micro machining economical. Machining of micro holes has become an immensely important micro machining application. From micro start holes for newer wire EDM’s to production ED drilling of small holes, EDM’ing has produced tremendous results. Figure 11 pictures a Silver Tungsten electrode that was ED dressed in the EDM machine to 6-µm diameter for producing an 11 µm (0.00043") diameter hole. Holes having an L to D of 100:1 are possible by incorporating a "High Pressure" dielectric pumping (>800 psi) system and using micro tubing down to .1mm. holes. High-pressure seals in the rotating head and high precision rotation are necessary as well. CONCLUSIONS Micron and sub-micron manufacturing requirements will continue to grow offering unique challenges and immense opportunities to a wide group of manufacturers. The designs and construction of many machine tools, work and tool holders, cutting tools and electrodes will naturally evolve as greater demands are placed on them when machining these miniature parts. Many of the challenges will evolve around economically controlling the micro manufacturing process. Many of these systems would not have been developed had it not been for the demands of industry for more and more capability. For these systems to perform successfully in the "Real World" requires cooperation and imagination on everyone’s part. In the end, it is the user that challenges the expertise of the OEM to develop effective micro machining systems, processes and applications techniques to support the business.