Micro-Machines For 3D Micro-Parts

Transcription

1 Micro-Machines For 3D Micro-Parts Kaushal Chandak and Suhas S. Joshi Department of Mechanical Engineering Indian Institute of Technology, Bombay Powai, MUMBAI (India) Abstract: Limitations on the resources of energy, space and material coupled with the recent trend of miniaturization of mechanical devices, have provided impetus to the development of meso- or micro-sized machines to manufacture micro-parts. It is observed that in these machines, the vibrations, inertia and thermal effects decrease significantly with the scale of machines. This paper focuses on the function and design features of various components of a micro-machine system. The discussion is based on the relevant examples from literature and the authors work. 1. INTRODUCTION Recent developments in engineering technology have lead to downsizing of many systems in the electronic and optical fields. The large size computers of one time have now been scaled down to lap-top or palm-top sizes. To effectively pursue this trend of miniaturization, mechanical components or systems also need to be miniaturized too. Therefore, use of meso- or micro-sized machines, popularly known as micro-machines, is likely to increase considerably in the near future. In fact, with the limitations on resources of energy, material and space, miniaturization will become a necessity in virtually every field. Consequently, smaller machines will be required for manufacturing of smaller components because the use of big machines to manufacture small components results in an undue loss of space, energy and material [1]. It has been observed that the vibration and inertia effects are reduced with the scale of machine. The inertia and elastic forces decrease as the fourth and second power of the scaling factor respectively [2]. Further, the thermal deformations were found to decrease linearly (or faster) with a decrease in the size of a machine tool [3]. The miniaturization and integration is found to increase the degree of freedom of product design thereby facilitate modifications in the system design [4]. The Mechanical Engineering Laboratory, (MEL, Japan) had come up with a concept of micro-factory in 1990 [1, 4-6]. In 1996, MEL developed a miniature lathe called micro-lathe with overall dimensions of only one cubic inch in size. Subsequently, prototypes of small production machines viz. micro-milling and micro-press were also made. Similarly, Lu and Yoneyama [7] of Kanazawa University, Japan have developed a micro-lathe turning system of length 200 mm. There have been other efforts to develop meso- or micro-scale machines such as pocket size EDM [8] and micro-equipment [3]. It is clear that the emphasis of these developments is on the miniaturization of production

2 machines. These machines are capable of producing a variety of 3D micro-shapes virtually on any work material; unlike, the very high resolution new technologies such as lithography, etching [9], etc. that are capable of producing 2D shapes on a few select work materials such as Si and Ge [3]. Therefore, over the last few years, development of meso- or micro-scale machines to manufacture micro-parts has gained momentum all over the world. This paper presents recent developments in the area of miniaturization of production machines with relevant inputs from the literature and authors work. The emphasis here is on identifying functions and design features of various components of typical micromachines. Based on this understanding, development of a conceptual micro-lathe is discussed. 2. PRINCIPLES OF MICRO-MACHINE DEVELOPMENT There are a number of reasons for which the miniaturization or development of micromachines is always attempted at. Firstly, to date, the manufacturing of micro-components is mainly facilitated by lithography based techniques. However, these processes can produce two-dimensional components on a select few materials. Whereas, many microscopic actuation devices and mechanisms require three-dimensional components made of materials of varying characteristics. In the past, these have been manufactured using precision or ultra-precision machine tools. However, the ability of a machine tool to produce a precise micro-component is proportional to its size. The smaller machines can produce smaller components more accurately than larger machines. This is because the inertial forces decrease proportional to the fourth power of the scaling factor. Consider a schematic of disc fixed on a rotating shaft on as shown in Figure 1 [a]. Fig. 1 [a] Schematic of a disc fixed on a rotating shaft [b] Free body diagram [3]. The center point of the disc is displaced relative the center point of the shaft by amount e. The inertia force F i of the disc is given by F i 2 = mω e (1)

3 where, m is the mass of the disc and ω is the angular speed of disc. Assuming the shaft to be a cantilever beam loaded at end by the inertial force F i, the deflection from bending (see Fig. 1[b]) is given by 3 4 z = CL Fi ED (2) where, C is constant, E is the elastic modulus of shaft material, L is shaft length, D is shaft diameter and F i is the inertial force. Therefore, the deflection of machine tool A made of the same material and design as that of B but S times smaller than machine tool A is given by z = S (3) A z B Therefore, the inertial forces produce deflection in the machine tool B is at least S times smaller than that of in the machine tool A. Also, with the reduction in size, the machines are subjected to reduced thermal deformations [3]. If there are two machine tools of size A and B that have the same design and are made of same material. If B is S times smaller that of A. The thermal deformation of a component length L can be given by L = α L T (4) Therefore, the length component of B is S times smaller than that of A, therefore L A = SL B (5) Since it is easier to maintain constant temperature in the smaller volume than in the larger volume, therefore, the internal temperature difference machines can be represented as T >= (6) ea T eb Therefore, the thermal deformation of A will be larger than that of B, as given by L >= S (7) A L B Thus, the thermal deformations decrease linearly (or faster) with the decrease in the size of the machine tool. Similarly, the elastic forces decrease proportional to the second power of the scaling factor [10]. The reduced inertia of the smaller machines, facilitate higher speeds and increased flexibility in manufacturing. They also provide large extend of savings in the energy (see Table 1 for examples), space and environment.

4 Table 1: Comparison of average energy consumption due to miniaturization to 1/X [11] Sr. Energy Average consumption Energy saving effect (1/X no. in actual factories (%) miniaturization) 1. Operating energy 13 1/X 3 2. Environmental energy Illuminating 23 1/(1.5*X 3 ) Air conditioning 56 1/(3*X 3 ) 3. Processing energy and others 8 1 However, the miniaturization of machines is never free of complications as limitation of space imposes numerous constraints on the machine development process. The power consumed by a mechanical system is expected to be proportional to square or cube of its dimensions. However, experimental results with previously developed micro-machines reveal that amount of energy saved by downsizing is not as per the expectations. This could be due to power loss in the developing highly accurate mechanisms. 3. BASIC COMPONENTS OF A MICRO-MACHINE Development of a micro-machine system could involve integration and development of a number of components as summarized in Fig. 2. Setting and resetting of work Machine Surveillance Actuation (Power supply) and control unit Micro-machine System Miniaturization of tools Human handling of machine Fig. 2 Subjects for the development of a cutting system of micro-parts [5]. Specifically, for a micro-machine for metal cutting purpose, requirements of the system can be listed as below Actuation and Control Unit: It involves spindle supported by appropriate bearings and coupled to a drive of variable speed. It also involves the actuation mechanism and drives for machine tool slides. In the world of micro-machines, better and efficient actuators are always necessary. Conventional actuators like electromagnetic motors have serious problems when they are miniaturized. For example, with a high-speed electric motor, the associated gears require appropriate reduction to obtain desired torque. Here, motion

5 characteristics of the device depend on the accuracy of manufacturing of micro-gears and their reliable coupling. A solution to this problem is to use direct drive actuation. Among the various types of actuators, piezoelectric based actuators with high response speed (100kHz), large yield (40N/mm 2 ) and very compact size are the best suited for micro-machines [12]. Other actuation drives which fall in non-direct driven category include electro-mechanical, electro-pneumatic drive, electro-hydraulic drive, micro-stepper motors and linear motors. However, these drives facilitate limited miniaturization. Miniaturization of Tools: The micro-machine system requires specially manufactured cutting tools of smaller size. They must be stronger than work material. Since in such processes the metal removal rate is very small, the breakage strength of work material approaches its theoretical value G/2π (where G is shear modulus) [13]. Investigations have also shown that unpredictable tool-life and premature tool failure are the main commons in micro-machining operations using solid cutting tools. The FEM analysis of various types of micro-end mill geometry, shows that during the machining operation, the end mills are subjected to cutting forces of varying magnitude rather than localized loads [14]. The analysis shows that failure of cutting tools can be easily detected by monitoring the surface finish. Setting and Resetting of Work: At micro level, loading and unloading of work cannot be done by hand. Hence dedicated manipulators to handle workpieces are required along with the micro-machine. Machine Surveillance: Microscope or computer controlled cameras are required along with the micro-machine system to monitor or to grasp the working situation directly. This is usually achieved by an optical microscope connected to some display unit. Human Handling of Machines: It is required for the movement of machines for relocation or integration. A number of aspects of selection or design of components of a typical micro-machine can be understood from a careful study and analysis of various micro-machines or micromechanisms developed earlier. A summary of various micro-machines and their salient features is presented in Table DESIGN OF A PROPOSED MICRO-LATHE The selection or design of the components for the desired micro-machine can be arrived at after a careful study and analysis of various micro-machines or micro-mechanisms developed earlier. It is therefore proposed to develop a micro-lathe with following preliminary specifications (see Table 3). The design and development activity for the proposed micro-lathe is already underway and initial concept of spindle design for this machine is presented in this paper.

6 Table 2: A summary of micro-machine features. Machine Micro-lathe [1, 5] Micro-lathe [7] Micromilling [1] Micro-press [1] Dimensions (mm) L:32 W:25 H:30.5 L:200 L:170 W:170 H:102 L:111 W:66 H:170 Spindle speed (rpm) Min. diameter achieved Spindle power Achievable Surface roughness (µra) ~ µm 1.5 W 1.5µm ~ µm -- 1µm ~ W DC W AC Other informati on Weight 100 gm Load 3kN Speed - 60 strokes / min. Table 3. Specifications of the proposed micro-lathe. Sr. No. Machine Feature Specifications 1. Machining Process Turning 2. Capability to generate Internal cylindrical 0.1 to 3 mm features External cylindrical 1 to 5 mm 3. Approximate area 1200 mm 2 4. Maximum Spindle Speed 10, 000 rpm Consider a static model of the micro-machine spindle with outer diameter D and inner diameter d as shown in Fig. 3. The distance between the bearing supports is a and overhang of the spindle is b. If F is the cutting force, E is Young s modulus of spindle the material and I is the moment of inertia of the cross-section of the spindle, then the cutting point static flexibility is given by [15] α = F (8) where, is the net spindle deflection at the cutting point. If s is the deflection at the cutting point due to bending and B is the deflection at the cutting point due to the bearings, then the net spindle deflection is given by = S + B (9) For the system under consideration, the net spindle deflection is given by [14]

7 ( b + ab ) 3EI + F( a + b) ak1 Fb a k2 = F + (10) where, k 1 and k 2 are stiffness of bearing. Therefore, from Eq. (8) we get, α = ( b + ab ) 3EI + F( a + b) ak1 + Fb a k2 (11) D d a b Fig. 3. Schematic of micro-machine spindle. Now, consider dimensions of the proposed spindle such as - a = 60 mm, b = 40 mm, D = 8 mm, d = 6 mm. Let the non-dimensional bearing stiffnesses K 1 and K 2, and are given by K 1 = k 1 / ED and K 2 = k 2 /ED (12) and typically the range of K 1, K 2 is given by 0.01 K 1, K Therefore, the cutting point static flexibility (α) for the proposed system varies from µm/n to 2.09 µm/n. It implies that during micro-cutting, if the magnitude of cutting force is of the order of 10 mn, the cutting point deflection would be about 2 X 10-2 µm, which is fairly acceptable. The dynamic flexibility (α D ) of a machine tool determines its susceptibility to chatter. To prevent from chattering, following condition needs to be satisfied. 1/α D > K F where, K F is the cutting force factor, and has almost linear dependence upon the width of cut for a given workpiece and geometry [15]. The dynamic flexibility of a machine can be calculated on the similar lines. 5. CONCLUDING REMARKS 1. Development of micro-machines is vital to manufacture three-dimensional microcomponents on virtually any type of work material.

8 2. Use of small machines to manufacture of small components has a number of useful effect and results in a considerably savings on resources. 3. A micro-machine is a system rather than an individual machine. It needs a number of additional components to facilitate human interface with the system. In that respect, development of a micro-machine is always a challenge. 4. Based on the understanding from the literature, design of a micro-lathe for performing a micro-turning operation is proposed. 6. REFERENCES 1. Tanaka, M., Development of desktop machining microfactory, RIKEN Review,34, 46-49, Ishihara, H., Arai, F. and Fukuda, T., Micro-mechatronics and micro-actuators, IEEE/ASME Trans. Mechatron., 1, 68 79, Kussul, E., Baidyk, T., Ruiz-Huerta, L., Caballero-Ruiz, A., Velasco, G. and Kasatkina, L., Development of micromachine tool prototypes for microfactories, J. of Micromechanics and Microengineering, 12, , Ishikawa, Y., and Kitahara, T., Present and Future of Micromechatronics, IEEE International Symposium on Micromechatronics and Human Science, , Kitahara, T., Ishikawa, Y. Terada, T., Nakajima, N. and Furuta, K., Development of Micro-Lathe, Journal, Mechanical Engineering Laboratory, 50(5), , Kawahara, N., Suto, T., Hirano, T., Ishikawa, Y., Kitahara, T., Ooyama, N. and Ataka, T., Microfactories: new applications of micromachine technology to the manufacture of small products, Microsystem Technologies,37 41, Lu, Z. and Yoneyama, T., Micro cutting in the micro lathe turning system, International J. of Machine Tools and Manufacture, 39(7), , Higuchi, T., Furutani, K., Yamagata, Y. and Takeda, K., Development of pocket size Electro-discharge Machine, Annals of CIRP, 40(1), , Mohamed, Gad-el-Hak, MEMS Handbook, Boca Raton: CRC Press, Ishihara H, Arai F and Fukuda T 1996Micro mechatronics and micro actuators IEEE/ASME Trans. Mechatron. 1, Kawahara, N., Suto, T., Ishikawa, Y., Kitahara, T., Ooyama, N., Ataka, T., Microfactories; new applications of micromachine technology to the manufcture of small products, Microsystem Technologies, 37-41, Dario, P., Vallegii, R., Carrozza, M., Montesi, M., Cocco, M., Microactuators for microrobots: a critical survey, J. of Micromechanics and Microengineering, 2, , Masuzawa, T., State of the art of Micromachining, Annals of CRIP(49), Fang, F., Wu, H., Liu, X., Liu, Y., and Ng, S., Tool geometry study in micromachining, J. of Micromechanics and Microengineering,13,, , Wardle, F., Lacey, S., Poon, S., Dynamic and Static characteristics of a wide speed range machine tool spindle, Precision Engineering 5, ,1983