Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 UDK - UDC 621.914+621.941 Pregledni znanstveni članek - Review scientific paper (1.02) Mehanska mikroobdelava s frezanjem, žično erozijo, potopno erozijo in diamantnim struženjem Mechanical Micro-Machining Using Milling, Wire EDM, Die-Sinking EDM and Diamond Turning Alberto Herrero1 - Igor Goenaga1 - Sabino Azcarate1 - Luis Uriarte1 - Atanas Ivanov2 - Andrew Rees2 - Christian Wenzel3 - Claas Miiller4 (1Fundacion Tekniker, Spain; 2Cardiff University, United Kingdom; 3Fraunhofer Institute for Production Technology IPT, Germany; 4Albert-Ludwigs-University Freiburg, Germany) Med vsemi različnimi mikroobdelavami se ta prispevek osredotoča na tehnike, ki se lahko štejejo kot običajni obdelovalni postopki. Obravnavani so naslednji obdelovalni postopki: zelo natančno rezkanje, obdelava s tanko žično erozijo, potopna erozija ter diamantno struženje. Kljub splosnosti postopkov ima vsaka izmed njih posebno lastnost, ki izboljša njihove zmožnosti obdelave. Postopki so v tem prispevku analizirani, prikazane pa so tudi njihove razlike s splošnimi obdelovalnimi postopki nato pa so prikazane se njihove omejitve. Predstavljene so tudi najnovejše uporabe in potencialni trgi. Čisto na koncu je prikazana se primerjava teh postopkov z drugimi izdelovalnimi postopki, kakor je litografija z namenom poudariti mogoče pristojnosti in dopolnila. © 2006 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: obdelave zelo natančne, mikroobdelave, rezkanje, struženje diamantno, obdelave elektroerozijske) From among all the different techniques currently applied for micro-manufacturing, this paper focuses on those techniques that can be considered as conventional techniques because of their direct relationship with standard machine tools. The following processes are discussed: ultra-precision milling, thin-wire EDM, die-sinking micro-EDM and diamond turning. Despite the mentioned relationship with conventional machine tools, they all have special characteristics that enhance the capability of the machining principle. The processes are analysed, showing the differences with respect to the corresponding conventional processes and stating their current limitations. A review of the state of the art, applications and potential markets is presented. Finally, the capabilities of these technologies and the other micro-manufacturing techniques (lithographic processes) are compared in order to highlight the possible competences and complementariness that they present. © 2006 Journal of Mechanical Engineering. All rights reserved. (Keywords: ultra-precision machining, micromachining, milling, diamond turning, EDM) 0 INTRODUCTION During recent years, the so-called “micro-technologies” have invaded the production of different components in some strategic sectors like automotive, electronics are medical ([1] and [2]). The name has been used during the past 5 years, and brings together all those technologies that are capable of producing small parts. In any case, even today, it has an ambiguous meaning due to the lack of clearly defined dimensions. The discussion about the meaning of “micro-technologies” is not new ([3] and [4]), and the dimensional limits are still fuzzy. The reasonably accepted limits are shown below: • Micro-technologies: from 0.5 to 499 mm in 2D/3D. ¦ Nano-technologies: from 0.5 to 499 nm in 2D/3D. In order to keep these dimensions in perspective: 1 micrometer is 0.001 millimetres, and 1 nanometre is 106 millimetres (1 nm = 109 m) or 10 angstroms. An illustrative example is the dimension of a human hair (Fig. 1), with a usual diameter of 80 to 100 mm. Micro-technologies are capable of achieving tolerances smaller than 0.5 mm and an average sur- 484 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 Fig. 1. A hair machined by a femto-second laser (image captured by x20 confocal microscope) face roughness of less than 50 nm. The corresponding conventional processes achieve 1 to 10-mm tolerances (if CNCs are used, 10 to 100 for manual machines) and a 50 to 100-nm average roughness. Micro-technologies are usually divided into two groups: 1) Lithographic processes: they have evolved from the 2D technologies applied in the production of circuit boards. They can produce stair-by-stair (referred as 2.5D) structures with very high resolution in XYZ. Most of these technologies must be applied in a clean-room environment. They can machine a limited range of materials (silicon being the most documented material) with a maximum aspect ratio close to 1:100 (which can be improved for some techniques, like LIGA). 2) Ultra-precision processes: they are the evolution of the usual industrial production technologies for precision components. They were initially applied by watchmakers, but now other sectors like surgical equipment, aerospace or nuclear vessels make use of them. The applied machinery is adapted for micro-machining: smaller tools, higher resolution, environmental isolation, etc. Some technologies belonging to the second group are analysed below: micro-milling, diamond turning and EDM. 1 LITHOGRAPHIC VS ULTRA-PRECISION PROCESSES From searching the available information about micro-technologies, it seems like lithographic and ultra-precision processes are competitors in the production of microsystems. Nevertheless, if a deeper analysis is performed, these technologies are found to be complementary, and only compete in some particular cases; just as happens between different machining technologies. Analysing the geometry of the obtainable features, ultra-precision technologies can produce complex free-form 3D profiles that cannot be obtained with lithographic processes. Considering the maximum dimensions of the part, the lithographic processes can only machine parts on very flat wafers and cannot take an accurate reference with respect to other features of the part. The ultra-precision technologies have an important market in the machining of small accurate features or textures of bigger parts (moulds, dies, punches, etc.) obtained by conventional machining methods. Considering the machinable range of materials, the clean-room processes can machine silicon, Pyrex, glass, chrome, nickel, gold, etc., with some of them being brittle and difficult to machine using ultra-precision techniques. On the other hand, ultra-precision techniques can machine most plastics, metals and ceramics. The material of a micro-part is an important specification when choosing the right machining process. Concerning the part’s accuracy, generally speaking, the clean-room processes obtain a better accuracy (one order of magnitude better: ±0.1 mm vs ±1 mm) than the mechanical processes, which are limited by the tolerances of the part-to-tool stiffness loop. On the other hand, this consideration must be carefully analysed, because, despite the high precision of lithographic processes when projecting a mask, the mask accuracy itself should be considered as an error in the process. This error can be very small (<50 nm when using an ion beam, but it is a very expensive process) but sometimes the mask is produced by ultra-precision techniques like laser or milling. Considering the production yield, lithographic processes are easier to apply for high-yield production, but they are very expensive for low-yield production; the ultra-precision processes being more suitable for the production of small series. Mehanska mikroobdelava s frezanjem - Mechanical Micro-Machining Using Milling 485 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 The integration of all these technologies in industry is very slow. They are mainly applied by universities and research centres due to the high cost of the required equipment and their low productivity, added to the required skills for the process application, which leads to the important cost of manpower. Those countries that have an active industry based on microelectronics are more active in the research of lithographic processes; however, those countries with a tradition in the metal-processing industry are more active in the research of ultra-precision technologies. 2 DIAMOND TURNING Diamond turning is a cutting process capable of producing an absolute accuracy of better than 1 mm, and a 0.002 to 0.005 mm average roughness in some metals, plastics and ceramics [6]. Its application is the production of mirror surfaces in optical-quality components, moulds or reference parts. The machine must be able to provide high stiffness, thermal and kinematic stability (lack of straightness errors, angular errors or vibrations, hydrostatic bearings are usual for this purpose) and high resolution [7] (~0.01 mm). The tool geometry must be accurate, the control of the edge radius and the tool tip radius being the key parameters to obtaining a mirror finish. The control must be performed with an accuracy of 3 to 75 nm. 2.1 Diamond for Turning Diamond presents some special features that make it ideal for cutting: high stiffness (E = 700 to 1200 GPa; G = 300 GPa), high thermal conductivity at room temperature (2000 W/mK), easy to work and easy to obtain flat surfaces and sharp edges in its crystallographic directions. Diamond can machine for long periods, keeping the tool geometry and providing both high precision and low roughness (depending on the tool radius and the process parameters). On the other hand, diamond is a brittle material and the tool can break easily if it receives an impact (tool higher than the part’s axis, excessive feeds, important variations in the depth of cut, etc.), with this being catastrophic. As a cutting tool diamond can be found as: - Natural Mono-crystalline Diamond: presents different properties in different crystallographic directions and can have some impurities that reduce the tool’s service life. - Poly-crystalline Diamond (PCD): is a cermet composed of cobalt as a binder and small diamond grains. - Synthetic Diamonds: are obtained using pure graphite that is pressurised (55000 bar) and heated (1500"C). The obtained diamond has a perfect crystallographic structure with almost no impurities. In the diamond-turning process, natural and synthetic diamond tools are usually applied, with the latter ones being more expensive (no imperfections). In other processes, like grinding or cutting tools, PCD and synthetic diamond are applied as hard coatings. 2.2 Diamond Machinable Materials Diamond has a low reactivity with many other materials. At high temperature it reacts with those metals that have an affinity for the carbon in its structure, forming carbides that contaminate the tool, which loses its properties and wears ([8] to [10]). Favourable materials are: - Metals: aluminium alloys, brass, bronze, copper, gold, silver, zinc, beryllium, lead, tin, indium, pluto-nium, magnesium - Plastics: metacrylate (PMMA), polycarbonate, Teflon, PVC, polypropylene, polyester - Glass: silicon, germanium Glass machining produces a higher tool wear [11]. The machining of plastics can introduce some internal tensions that cause the subsequent deformation of the plastics. In all these materials an average surface roughness of 3 to 6 nm and an absolute accuracy of 1to 2 mm can be obtained. With regard to the materials that have an affinity for carbon, these include: steel, nickel, titanium, molybdenum, cobalt, chrome, vanadium, rhodium and tungsten. 2.3 Diamond Turning Process The machine configuration and the process concept are similar to a conventional lathe, but the process parameters are different [7]. The part must be pre-machined in another lathe, with an excess of material of around 0.3 to 0.5 mm. The tool-tip radius must be controlled with a very high precision [12], the other parameters of the tool are as follows (Fig. 2): the rake angle is close to 0° (slightly positive for plastics ~5°) and the clearance angle is close to 6 to 10°. Diamond makes it possible to obtain an edge radius close to 20 n be- 486 Herrero A. - Goenaga I. - Azcarate S. - Uriarte L. - Ivanov A. - Rees A. - Wenzel C. - Muller C Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 Fig. 2. Typical tool geometry (solid rendering) tween the clearance and the rake planes. Such a small value makes it possible to cut with a very small depth of cut. Rough machining for diamond turning implies a depth of cut of 40 to 50 mm, in finishing it can be 1 to 2 mm. The usual feeds are 5 to 40 mm/rev, and the spindle speed range is 1500 to 2000 rev/min. The coolant is an air-oil mixture targeted at the tool tip. It will remove the chips, lubricate the cutting area, and cool the tool. The chips must be removed because otherwise they would cause marks if they stayed stuck to the tool edge. Special care must be taken when orienting the coolant nozzles. 2.4 Diamond Turning Applications The process can be applied for two different purposes: 1) The machines are stiff, stable and can move with very high precision. These specifications make it suitable for micro-machining (Fig. 3, right). 2) Using the correct tools, mirror finishing can be obtained, avoiding several operations (turning-grinding-polishing) (Fig. 3, left). For micro-machining the process is suitable for producing small diameter shafts (0.2 to 0.02 mm) and small slots (using small tailor-made tools). Part cutting becomes an important issue. Mirror finish- ing is currently its main market, and some applications are as follows: laser-driving optics, wavelength-filtering surfaces, moulds for components of optical quality, etc. 3 MILLING The ultra-precision milling process is very similar to conventional milling, being an intuitive process that can easily be assimilated by any operator. Despite cutting chip widths of just a few nanometres, the effect of inter-atomic forces, sometimes considered by other authors, is negligible. The milling machine has some special characteristics. At present, there are a few commercial solutions ([14] and [15]). Knowledge about the machine is linked to the process knowledge, and it is important to understand what happens at the tip of the tool. Just as with diamond turning, the process parameters, the tools and the cutting process itself have some differences with respect to conventional milling. 3.1 Tools and Auxiliary Systems Micro-milling depends a lot on the auxiliary components needed for the process, which is why the entire group (machine, components, tools, etc.) must Fig. 3. Mirror surface machined in aluminium (shape error <0.08 mm in spherical zone, <0.05 mm in flat zone); Miniature 00.75 mm brass part. Mehanska mikroobdelava s frezanjem - Mechanical Micro-Machining Using Milling 487 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 be analysed as a whole. Apart from the machine, this includes the tools, the spindle, the collet and the referencing system. Tools The tool market is very active in the development of new tools (drills, mills) for micro-machining. In the past several years the minimum diameter of mills has decreased from 0.25 mm to 0.1 mm, the range includes spherical and straight 2-flute mills made of carbide with different coatings (TiAl, TiAlN, CBN, CBD, etc.). In any case, the range is limited and there are no different geometries for different materials. This is an important problem because most of the tools are designed for steel machining. It is important to point out that grain size has a large influence on the tool’s performance. Commercial tools have a well-defined geometry with small tolerances. The tolerances indicated in the catalogues for the sum of the geometrical error plus the run-out error are de ±10 mm. (The tolerance-to-size ratio is poor when compared to conventional high-speed machining mills.) The real errors are usually smaller (±5 mm). A second option is to use tailor-made tools. These tools are provided by specialised companies at a higher cost. These mills can present one (engraving tool) or two flutes with a custom geometry (face angle, helix angle, rake angle) and the diameters can be as small as 0.01 mm. The geometry of the tools has a high dispersion, which is an important issue when changing the tool to continue a machining process. Spindle and collets Because of the use of such small tools, the spindle must rotate at high revolutions (120000 to 160000 rev/min) to achieve an appropriate cutting speed for most materials. Apart from the speed, the spindle must be stiff (>25 N/mm) and it must have a small run-out (<1 mm) in order to ensure the high precision of the cutting process. To reach such high speeds, the spindles have ceramic ball bearings that are continuously cooled and lubricated. They are usually low-power electro-spindles (200 to 500 W) or aerostatic spindles. Usually, the tool is clamped manually using special collets to reduce the run-out. The most common form of collets are the precision ER type collets (clamp a small range of diameters close to a nominal value) and the super-precision ER type collets (only clamp the nominal diameter). The precision collets can present big run-out errors that depend on the clamped diameter, the super-precision collets have run-out errors smaller than 2 mm. Tool wear during micro-milling is relatively high, and that is the reason why it is usual to use two or more mills per operation (one for rough machining, the other for finishing). Tool change is a critical operation because the tool run-out, tool height and collet run-out are modified, thus it must be performed carefully, cleaning the cone, collet, tool and nut and applying controlled torques. Referencing system In many cases the micro-milling operations must be referenced to other operations, surfaces or part features that have previously been machined. Part referencing is performed in the same way as in conventional milling: the tool is moved until it is “touching” the part in different axes. Commercial touch triggers have errors close to 5 to 10 mm, but this error is very big for micro-machining. Alternatively, the trigger is done optically. The machine resolution being much better than in conventional systems (approaching micron-by-micron), it is more difficult to identify the first chip that is cut in the part. In order to assist this action, the micro-milling machines are equipped with high magnification (x100 to 200) vision systems that are used for referencing and also to inspect the machining process. Depending on the applied magnification, both the field of view and the depth of view get very reduced, and the working distance must be fixed with higher precision. It is typical to use zoom systems capable of augmenting the image from x60 to x200: the highest magnification is only used for referencing, while the lower magnification provides a greater depth of view that is adequate for process inspection. 3.2 Materials for Milling The application of different coatings for the tools and different cutting speeds makes it possible to machine metals, plastics and ceramics ([14] and [15]). The greater limitations of the process are the lack of tool geometries adapted to machine different materials and the high wear produced at the tool tip (specially for hardened steels and ceramics). 3.3 Micro-milling Process As was mentioned for diamond turning, the machine configuration and the geometrical concept are similar to the corresponding macro-process, al- 488 Herrero A. - Goenaga I. - Azcarate S. - Uriarte L. - Ivanov A. - Rees A. - Wenzel C. - Muller C Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 though there are some important differences in the The forces acting on the tool will be relatively machine components and the process parameters larger than during normal milling: the specific cutting ([16] and [17]). pressure (ps) increases as the depth of cut decreases (it Considering the process parameters, the is experimentally tested for normal machining). Unfor-depth of cut is very reduced for roughing (<10 mm, tunately, the values for micro-milling are still unknown. depending on the part material and the tool diam- If the values are extrapolated assuming an exponential eter) and finishing (2 to 3 mm). These values would relation, the cutting forces are close to 5 to 10 mN cause part sticking and wrong cutting during nor- when milling hardened steels. Considering the tool di-mal machining. The feeds are reduced (<40 mm/min) ameter, these forces cause important deflections that and the spindle rotates at high speed (>40000 rev/ produce higher tool wear and tool breakage. min). The cutting speeds are close to 25 m/min for Micro-milling can obtain an average surface steels, and the chip thickness is small (<0.5 mm). roughness, Ra, close to 0.1 to 0.05 mm, burr formation Tool geometry has a large influence on the cut- being a key issue for this technology. Burrs tend to ting performance. An important effect that is not usu- appear on the edges when machining boxes, they are ally considered in milling is the edge radius between small chips that could not be evacuated and were the rake face and the clearance face. This radius can be stuck to the piece walls by the next flute. Deburring is the source of important errors during the milling proc- a complex task that can be minimised with optimised ess, because the tool instead of the cutting ploughs tool paths and using a new tool for finishing. the surface material when the depth of cut is very small. The edge radius must be smaller than the chip thick- 3.4 Micro-milling applications ness in order to cut (~0.1 mm, Fig. 4). Analysing the distribution of the cutting Comparing micro-milling to other micro-ma-forces acting on the flute of the tool (Fig. 4) it can be chining technologies, this process is capable of appreciated that when cutting a 2-mm depth of cut, machining freeform 3D shapes (Fig. 5 left) that can-the mean rake angle will be different to the nominal not be obtained with most of the other processes value. The mean rake angle will depend on the rake (2D or 2.5D). face, the tool edge radius [18], tending to be nega- In the other processes the machinable range tive and so plough the material. of the materials is limited (electrically conductive Fig. 4. Geometry of a 2-flute straight mill, detail of the edge radius. 0 0.2 mill captured by confocal microscope (x100) Mehanska mikroobdelava s frezanjem - Mechanical Micro-Machining Using Milling 489 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 Fig. 5. Bracket made of polysulfone (3x3x2 mm); mould for a drug-delivery system (channel height 0.1 mm; channel width 0.3 mm; wall thickness 0.2 mm; accuracy ±2 mm; roughness 0.15 mm Ra). materials for EDM, diamond-compatible materials for circuits) can produce pulses of some microseconds. diamond turning, etc.), but micro-milling can be ap- Most of micro-EDM machines use RC generators plied to a wider range of materials by using mills with small condensers (<10 pF), reducing pulse time with different coatings. This is also important be- and energy. cause it can process not only miniaturised parts, but One of the most important applications for also precision features in big parts. micro-EDM is micro-drilling with small electrodes Another advantage of micro-milling is its simi- (00.1 mm, with an aspect ratio 50:1) made of tung- larity with the conventional process. This makes the sten. These can be used for fuel injectors, air injec- process easier to introduce in industry. Many appli- tors, precision dispensers, ink-jet printing, filters, etc. cations of micro-milling involve the machining of Smaller holes can be made by electrode-dressing medical parts (Fig. 5, right) and surgical tools (the techniques (Fig. 6 left). Among these techniques are sector in which this technology has opened new the following: slab milling and wire electro-discharge opportunities), moulds and dies, scientific research, grinding (WEDG - Wire Electro Discharge Grind- etc. ing, developed in 1985, by T. Masuzawa of Tokyo University). WEDG is a technique that incorporates 5 DIE-SINK EDM AND MICRO-EDM a wire electro-discharge unit horizontally in the sinking EDM working area. Changing the electrode po- The lack of cutting forces and the capability of larity, it can be dressed against the wire electrode, removing small portions of material per spark makes decreasing its diameter to 10 to 20 mm. Controlling EDM a perfect process for micro-machining. The die- the Z axis rotation, it is possible to produce form sinking EDM process was studied by many compa- electrodes [22]. nies and institutes, with new machines and auxiliary A second research field in the sinking EDM systems capable of machining smaller features appear- process has started to use the electrode to sculpt ing. Comparing the process to conventional EDM, the complex 3D shapes, controlling its position in space main differences are the electrode dimensions, the (similar to the milling process) (Fig. 6, right). This higher resolution of the machine and the capability to process is named EDM-milling. In EDM-milling the produce less energetic pulses (~100 nJ). As T. tip of the electrode wears and loses its initial shape. Masuzawa explained [19], the pulse energy is propor- Adjusting the process parameters, the modified tional to the voltage, the intensity and the spark dura- shape is maintained within the process and only the tion. The process is also different in terms of sludge tool height must be compensated. The compensa- removal, process parameters and electrode wear. tion depends on process parameters and part mate- The current and voltage must have some rial; electrode wear characterisation and trajectory minimum values to overcome the resistance of the planning being the key issues ([23] and [24]). It is a cables and connections and produce the spark. Thus, slow process (depth of cut ~20 mm in roughing, 3 mm to reduce the pulse energy, the time must be control- in finishing) that obtains an accuracy of ±2 mm. led. Transistorised generators can reduce the pulse The difference with respect to conventional interval to 0.5 ms, while the relaxation circuits (RC EDM machines, apart from the machine precision, is 490 Herrero A. - Goenaga I. - Azcarate S. - Uriarte L. - Ivanov A. - Rees A. - Wenzel C. - Muller C Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 Fig. 6. 00.060 mm hole by s-EDM (00.2 mm tungsten electrode dressed to 00.036 mm). 1x1 mm EDM-milled pyramid with 0.2 mm stairs (captured by confocal microscopy x20). that micromachining systems can interpolate in 3D and erode in any spatial direction, controlling the gap width. Commercially the range of such systems is limited [25], and only two companies offer products focused on micro-EDM. The process limitations are that vertical walls and sharp edges cannot be produced (unless special shaped electrodes are used for finishing) and the electrode wear/part wear ratio is higher than in conventional EDM because finishing parameters are applied during the whole process (positive polarity high frequency and low energy). The combination of electrode dressing and EDM-milling makes it possible to machine very small freeform surfaces in conductive materials. The electrode can be dressed to diameters not obtainable with cutting tools, making it possible to machine smaller features. 4.1 Electrodes and Auxiliary Tools The applied electrodes are made of different materials and the machine is equipped with some special systems that are different from normal EDM. The most used material for micro-EDM electrodes is tungsten, due to its high stiffness and fusion temperature. On the market there are cylindrical electrodes down to 00.06 mm and tube electrodes (holed) down to 00.1 mm. Handling electrodes below 00.1 mm is almost impossible, but there are feeding systems for this purpose. The manual collets are specially designed to clamp small diameters and have fine-regulation systems to reduce the rotation run-out. Apart from the collets, it is common to use ceramic guides that make it possible to work with longer electrodes that minimise the need for manual feeding. Optical micro- scopes are another usual accessory for micro-EDM machines, apart from being used to reduce the electrode run-out, they are also used to align the collet and the ceramic guide. The machines have rotary spindles to ensure the highest circularity when drilling (<2 mm for 00.2 mm electrodes). In some cases the spindle rotation can be controlled (C axis) to produce shaped electrodes. The dielectric is usually oil, but some machines apply de-ionised water to drill higher aspect ratios in steel. 4.2 Micro Die-sinking EDM Applications The main application for this process is the machining of high-aspect-ratio small holes (>00.15 mm) for injection. A second important application is performing drills for subsequent wire threading in the WEDM process. Concerning the EDM-milling process, the application is mould machining in hard-to-machine materials. 5 THIN-WIRE EDM The thin-wire EDM process (sometimes call mi-cro-WEDM) is very similar to the conventional WEDM process. The wire electrodes that are used have smaller diameters (<0.050 mm) making it possible to machine miniaturised complex ruled surfaces with an aspect ratio greater than 10:1, and micrometric accuracy [26]. The machining systems are an evolution of conventional WEDM machines that achieve higher accuracy (1-3 mm, depending on the part height) in small travels and fine adjustable wire traction [27]. All those wires with a diameter smaller than 0.05 mm are considered thin wires. The spark generator is usually an RC type generator that can provide high-frequency and low-energy pulses. Mehanska mikroobdelava s frezanjem - Mechanical Micro-Machining Using Milling 491 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 The applied dielectric fluid is oil because it roundness errors being 1 to 2 mm. This makes the proc- has a higher resistivity than water, reducing both the ess less accurate than expected. Finally, wire threading energy of the spark reaching the part and the gap is difficult to perform. In most commercial machines the width. Thus, it is possible to reach a higher precision automatic threading is only reliable up to 00.05 mm and a smaller surface roughness. A disadvantage is (some companies like Agie or Makino have a threading that by using oil the productivity is 10 times lower. system for 00.020 mm), making manual threading a The thin wires, due to their small section and complex task that can last for ±10 minutes; this limits mass, cannot be tensioned with high forces to reduce the industrialization of the process. the effect of the process forces. All these forces cause wire vibration and deformation levels that are larger 5.2 Thin-Wire EDM Applications than in conventional WEDM. Process optimisation can be done choosing the right cutting strategies, In thin-WEDM, the wire is continuously re-parameters, dielectric flow and wire tension for each newed and the process achieves high accuracy. It kind of material and each part height. can machine any ruled surface but, when the machining is performed inside a part, a threading hole 5.1 Wire Electrodes and Auxiliary Tools must be machined previously. The process can machine hardened materials and is used in the machin- The minimum machinable feature depends on ing of precision features of moulds, dies and wire diameter, wire tension, wire guiding and the skills punches. It can also machine mechanical compo- of the operator to thread it correctly. Usually, the wire nents (Figs.7 and 8), small connectors, etc. electrode is made of tungsten (Fig. 7, left), although there are wires made of molybdenum or brass-coated 6 CONCLUSIONS steel. The minimum market-available wire diameter was 0.030 mm (±1 mm), until last year, when two smaller All these technologies present important ca- dimensions entered the market: 00.025 and00.020 mm. pabilities that can be applied mainly to the develop- The limit is not in the wire manufacturing (Bedra pre- ment of precision miniaturised moulds, punches and sented some demonstrators of 00.015 mm wire in ISEM dies. Most of them keep a strong relationship with XIV) but in the machine’s capability to work with such the corresponding conventional technologies, and small wires (guides and tension). It is important to point this makes it easier for them to be assimilated by the out that the dimensional tolerances are similar to con- metal-processing industry. ventional wires (±2 mm for00.25 mm wires), the toler- The limits of these technologies are not in ance/diameter ratio being worse for thin wires. their positioning accuracy, but in the development Wire guides are another important issue; most of improved tools and referencing systems. At machines are designed to work with00.030 mm, there present, all these processes are being actively re-are no existing consumables to use thinner wires. The searched and their introduction in some industrial wire guides are machined by laser and cannot be con- sectors (surgical tools, car sector, etc.) has started trolled better than conventional ones, the diameter and the initiation of a strong market around them. Fig. 7. Used and new 00.030 mm tungsten wire; 00.5 mm nominal diameter gear cut by 00.030 mm WEDM. 492 Herrero A. - Goenaga I. - Azcarate S. - Uriarte L. - Ivanov A. - Rees A. - Wenzel C. - Muller C. Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 Fig. 8. Thin-Wire EDM-ed components for a micro-car transmission 7 REFERENCES [I] Wechsung R. et al. (1998) Market analysis for microsystems, 1996-2002, A NEXUS Task Force Report. Nexus Office (eds). Grenoble. [2] Wechsung R. et al. (2002) Market analysis for microsystems II, 2000-2005, A NEXUS Task Force Report. Nexus Office (eds). Grenoble. [3] Bueno R, Corta, R, Herrero, A. (1999) Microtechnologies in Spain. Semaine Nationale des Nano et Micro Technologies. Menrt, Paris. Noviembre 1999. [4] Bueno R, Corta R, Herrero A. (1999) MICROMACHINE: Perspectivas en torno a las maquinas, microfabricacion y fabricacion de precision, Fundacion Tekniker. Eibar. [5] Masuzawa T (2000) State of the art micromachining. Colibri A.G. (eds) Annals of the CIRP 49-2. Uitendorf, Switzerland, pp 473–488. [6] Benjamin, R. J. (1983) Diamond machining applications and capabilities. SPIE Vol. 433, pp. 24-31, 1983. [7] Weck M. et al. (1988) Performance assessment in ultraprecision micro-machining. Annals of the CIRP, 37/ 1/1988, p. 499. [8] Moriwaki T et al. (1990) Effect of cutting heat on machining accuracy in ultra precision diamond turning. Annals of the CIRP, 39/1/1990, p. 81. [9] Evans C, Bryan J.B. (1991) Cryogenic diamond turning of stainless steel. Annals of the CIRP 40/1/1991, p. 571. [10] Shimada S. et al. (2000) Suppression of tool wear in diamond turning of copper under reduced oxygen atmosphere. Annals of the CIRP, 49/1/2000, p. 21. [II] Nakasuji T et al. (1990) Diamond turning of brittle materials for optical components. Annals of the CIRP, 39/1/1990, p. 89. [12] S. Asai S. et al. (1990) Measuring the very small cutting edge radius for a diamond tool using a new type of SEM having two detectors. Annals of the CIRP, 39/1/1990, p. 85. [13] Herrero A., Bueno R. (2001) Development of a three axes travelling column ultra-precision milling machine, Proceedings of the 10th ICPE. Yokohama. Japan. 18-20 July, 2001. [14] Spath D. et al. (2000) Micro-milling of steel for mould manufacturing - influences of material, tools and process parameters. EUSPEN 2000. [15] Takeuchi Y. et al. (1996) Ultraprecision 3D machining of glass. Annals of the CIRP, Vol. 45/2/1996. [16] L. Uriarte, A. Herrero, R. Onate (2004) Fresadora de ultraprecision de 3 ejes de columna movil. XV Congreso de Maquinas-Herramienta y Tecnologias de Fabricacion. ISBN 931828-7-7. San Sebastian, October 2004. [17] Weck M., y Fischer, S. (1997) Development of an ultraprecision milling machine. Proceedings of the 9th IPES/4th UME. Braunschweig. Alemania. 26-30 Mayo, 1997. [18] Lucca DA., Seo Y.W., Komanduri R. (1993) Effect of tool edge geometry on energy dissipation in Mehanska mikroobdelava s frezanjem - Mechanical Micro-Machining Using Milling 493 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)7-8, 484-494 ultraprecision machining. Annals of the CIRP, 42/1/1993, p. 83. [19] Masuzawa, T. (2001) Micro EDM, Proceedings of the ISEM XIII Vol I., p.3-19 [20] Masuzawa T., Tsukamoto J., Fujino M. (1989) Drilling of deep microholes by EDM. Annals of the CIRP, 38/1/1989, p. 195. [21] Masuzawa T., Fujino M. Et al. (1985) Wire electro-discharge grinding for micro-machining. Annals of the CIRP, 34/1/1985, p. 431. [22] S Piltz S., Uhlmann E. (2004) Micro machining of cylindrical parts by electrical discharge grinding. ISEM XIV. [23] Zhao W., Yang Y. et al. (2004) A CAD/CAM system for micro ED-Milling of Small 3D Freeform Cavity. ISEM XIV. [24] Bleys P., Kruth JP., Lauwers B. (2004) Sensing & compensation of tool wear in milling EDM. ISEM XIV. [25] Beltrami I. (2004) Micro and nano electride-discharge machining. ISEM XIV. [26] Klocke F, Lung D., Antonoglou G.. (2004) Using ultra thin electrodes to produce micro-parts with wire- EDM. ISEM XIV. [27] Mu-Tian Y, Huang CW. Et al. (2004) Development of a prototype micro wire-EDM machine. ISEM XIV. Authors’ Address: Mag. Alberto Herrero Mag. Igor Goenaga Mag. Sabino Azcarate Fundacion Tekniker Micro & Nano technologies Dep. Avda. Otaola, 20 20600 Eibar, Spain Mag. Luis Uriarte Fundacion Tekniker Mechatronics Dep. Avda. Otaola, 20 20600 Eibar, Spain Dr. Atanas Ivanov Mag. Andrew Rees Cardiff University Queen’s Building PO Box 925 Cardiff CF24 3AA, Wales Mag. Christian Wenzel Fraunhofer-Institut fur Produktionstechnologie IPT Steinbachstrasse 17 52074 Aachen, Germany Dr. Claas Miiller MTEK Alber-Ludwigs University Freiburg Georges Kohler Allee 103 79110 Freiburg, Germany Prejeto: Sprejeto: Odprto za diskusijo: 1 leto Received: 2.11.2005 Accepted: 22.6.2006 Open for discussion: 1 year 494 Herrero A. - Goenaga I. - Azcarate S. - Uriarte L. - Ivanov A. - Rees A. - Wenzel C. - Muller C.