Papers by Author: Hye Jin Lee

Abstract: This research explores the micro-forming process of spiral groove pattern on Fluid Dynamic Bearing(FDB), which is utilized in precision driving part of the hard disk drive(HDD), using micro desktop forming system. While EDM and ECM process has been widely used to engrave the precision pattern which generates dynamic pressure on FDBs, micro forming process is newly proposed in this study to increase the productivity and to reduce the product costs. At first, desktop forming system is designed for spiral groove pattern forming. FE simulations are followed in order to evaluate the feasibility of micro-forming. The simulation results show that forming loads of 1,500Kgf is required to fabricate micro patterns with the depth of 15 μm. Finally the formability test is carried out with various forming loads. Deformed shapes and forming loads obtained from the test are compared with those from the analysis. The results fully demonstrate that micro pattern forming techniques are available to fabricate micro spiral groove patterns in FDB.

Abstract: In this paper, finite element analyses of thread rolling process for the micro-sized screw with the diameter of 800 μm and the thread pitch of 200 μm are firstly carried out. For the finite element analysis of the thread rolling process, hardening behavior of SUS has been estimated by the compression and tensile tests. And then, process design has been conducted with the aid of simulations and the prototype of the micro screw finally has been fabricated. In order to verify the CAE based process design for micro-sized screws, the deformed shape and dimensions obtained from the CAE are compared with those from the experiment. The deformed shapes of the pitch part and the top part of threads can be demonstrated the feature of experimental result.

Abstract: High precision micro screws have been widely required for the fastening parts of electronic storage devices. Teeth’s forming of the micro screw is generally utilized by a cold thread rolling process with a flat die. This process for micro sized parts has some difficulties such as the design of die shape, alignment between dies, and process managements. In this paper, it is focused on the effect of the alignment between dies and material, and process parameters in simulation. The investigated parameters are the friction coefficient and the relative position between stationary and moving dies using a numerical simulation. The simulations provide that the shear friction factor of 0.9 is proper for preventing slip between dies and raw material and the relative position of two dies has to be set to the half length of the pitch for maintaining the continuous thread profiles. The deformed shapes of the pitch part and the top part of threads can be demonstrated the feature of experimental result. The folding phenomenon appeared at the top part of threads, which seemed to be induced by a lack of the metal flow under micro sized deformation.

Abstract: This paper is concerned with formability of AZ31B magnesium alloy sheets at various temperatures. In order to acquire the forming limit curve for quantifying the formability of the magnesium alloy sheet, hemispherical punch forming tests are performed under various temperatures. The initial shape of the blank is rectangular and six different types of specimens are used with their length along the rolling direction of 175mm and widths along the transverse direction of 25, 50, 75, 100, 120 and 175mm. With the equipment of on a SIMADZU 100-ton universal testing machine, hemispherical punch forming tests are performed and a forming limit diagram is constructed at the temperature of 100°C, 200°C and 250°C. FE analyses of hemispherical punch forming tests are also conducted by considering ductile failure model in order to compare the forming limit curve obtained from the experiment and numerical analysis. The comparison examines problems in the current material model and it suggests a future research direction.

Abstract: Mechanical properties of micro-electro-mechanical systems (MEMS) materials are increasingly important with the wide use of miniaturized systems. This paper deals with development of the strain measurement module for micro materials tests. Vision strain method (VSM) and actuating stroke method (ASM) are adopted in the strain measurement module in order to reduce the discrepancies between the measured deformation and actual one in the specimen. With the developed strain measurement system, a number of tensile tests were concluded for 99.9% nickel thin films with the thickness of 500µm. Specimens with various widths from 1 mm to 2mm and with various lengths from 2mm to 4mm were used in the tests. From the experiment, material properties such as yield strength, tensile strength, fracture strength, fracture strain and young’s modulus are analyzed with stress-strain curves of 99.9% nickel thin film.

Abstract: In this paper, Experimental results on the measurement of mechanical properties of fine patterns in the MEMS structure are described. The mechanical properties of embossing patterns on metallic thin foil is measured using the nano indentation system, that is developed by Korea Institute of Industrial Technology(KITECH). These micro embossing patterns are fabricated using CIP(Cold Isostatic Press) process on micro metallic thin foils(Al-1100) that are made by rolling process. These embossing patterned metallic thin foils(Al-1100) are used in the reflecting plate of BLU(Back Light Unit) and electrical/mechanical MEMS components. If these mechanical properties of fine patterns are utilized in a design procedure, the optimal design can be achieved in aspects of reliability as well as economy.

Abstract: The existing forming press uses a hydraulic actuator and high powered mechanical actuator, therefore occupying a large space because of its size. This type of system is inefficient for manufacturing micro size and precision products. As forming components are small in size, forming equipment must also be small in size because the forming die and load must be small. The micro forming manufacturing system is an ultra precision forming equipment the size of several micros to millimeters and precision of sub-micro to micrometer. This micro forming manufacturing system has the advantage of minimization in manipulating distance and working space. As equipment and tools become smaller in size, minute inertia force and high natural frequency can be obtained. Therefore, high precision forming performance can be obtained. This allows the factory to quickly provide the customer with goods because the manufacturing system and process are reduced. To construct a micro manufacturing system, many technologies are necessary such as high stiffness frame, high precision actuating part, structural analysis, high precision tools and system control. In this paper Research development about a micro metal forming manufacturing system has been developed. To coincide with the purpose to be more practical, we set the development of the equipment including micro deep drawing, micro punching and micro restriking process to the goal. To achieve this goal, the miniaturized micro metal forming manufacturing system is designed and made with miniaturized size system. A micro deep drawing process and system dynamic characteristic experiments are researched using this miniaturized micro forming system. A micro deep drawing experiment is performed using micro thin foil materials (Al-1100, SUS-304). If this miniaturized micro forming technology is used, efficient material practical use in the micro forming field which uses the micro metal thin foil is possible.

Abstract: This paper is concerned about the precision material property measurement of a micro metal thin foil used in MEMS technology. Evaluations of reliability such as the life cycle endurance test, impact test, and residual stress test are necessary for MEMS components, because MEMS components require great precision and accuracy. However, in practical reliability tests, we should consider various factors that make them hard to perform. In this manner, we use the analytical method to evaluate the reliability of MEMS component rather than actual testing. In general, the analytical method is utilized by software tools. Nevertheless, it is obviously necessary to acquire fundamental properties of materials through real test methods. In this paper, we proposed the micro scale material property measurement system to measure the oriented mechanical properties of aluminum thin foil.

Abstract: Many micro technology researches have been concentrated in the field of materials and a process field. But the properties of micro materials should be understood to give still more advanced results. Among the various material properties, mechanical material properties such as tensile strength, elastic modulus, etc., is the basic property. To measure mechanical properties in micro or nano scale, actuating must be very precise. Piezo is a famous actuator, frequently used to measure very precise mechanical properties in micro research field. But piezo has a nonlinearity called hysteresis. Not precision result is caused because of this hysteresis property in piezo actuator. Therefore feedback control method is used in many researches to prevent this hysteresis of piezo actuator. Feedback control method produces a good result in processing view, but causes a loss in a resolution view. In this paper, hysteresis is compensated by using an open loop control method. To apply the open loop control method to piezo actuated nano scale material testing machine, hysteresis property is modeled in a mathematical function, and a compensated control input is constructed using inverse function of original data. The reliability of this control method can be confirmed by testing nickel, aluminum, and copper micro thin foil that is used in MEMS material broadly. If these MEMS material properties are used in a MEMS research field, more economical and high performance MEMS materials can be obtained.

Abstract: A micro-alloyed non-heat-treated material does not need post heat treatment processes such as quenching and tempering after the forming process in production stages. This material can be called a green material since it can reduce industrial costs and harmful pollutants generated from post heat treatments. In this paper, near-net-shape forming processes were studied in order to make an automotive part using a micro-alloyed material. The cold forging technique using a former was utilized for the main shaping, and the cold incremental forming technique using a cross wedge rolling machine was adopted for the enhancement of strength and the final shaping of the part. In order to get more adequate process, the cross wedge rolling process is compared to the swaging process for the micro-alloyed steel and general carbon steels through experiments.