Papers by Author: Myeong Woo Cho

Abstract: Subminiature fuel cell is a representative eco-friendly power source which possesses the merits of having the potential of being implemented in portable electronic devices and other subminiature devices, as well as having high energy density as compared to existing secondary cells. The size of the bipolar plate which is implemented in a subminiature fuel cell should be of a small size according to the size of the fuel cell, and the flow channel inside the bipolar plate also has be reduced to match the overall size of the bipolar plate. Therefore, a higher level of difficulty in the manufacturing process is involved in producing bipolar plate for subminiature fuel cells and mass production is extremely difficult. In this study, powder injection molding was applied to the production of subminiature metal bipolar plate to verify the possibility of mass production. For that, a bipolar plate and mold were designed and then a test injection forming was carried out. This was attempted to validate the feasibility of a subminiature metallic bipolar plate prepared through a debinding and sintering process by evaluating its electric conductivity and density.

Abstract: In this study, Magneto-rheological (MR) fluid was modified by mixing and adding the additives. The researches on MR fluid have been active progress all around the worlds, especially for the application to many devices on the various fields. For the success of application, tribological characterisitcals are important to be satisfied. There are many progress mainly aimed to the methods to improve the stability and performance of MR fluid in application. In this study, the experiments on friction and wear with pin-on-disk tester as well as SRV tester are conducted in different operation and magnetic. From the results, tribological performance of modified MR fluid is analyzed.

Abstract: In this paper, the effect of particle concentration and magnetic field on the tribological behavior of magneto-rheological (MR) fluid is investigated using a pin-on-disc tribometer. The wear loss and friction coefficient are measured to study the friction and wear properties of MR fluid. The morphology of the worn pin is also observed by scanning electron microscope (SEM) in order to analyze the wear mechanism. The results obtained in this work show that the wear loss and friction coefficient decrease with increasing particle concentration under the magnetic field. Furthermore, it is demonstrated that the magnetic field has a significant effect on improving tribological properties of MR fluid, especially the one with high particle concentration. The predominant wear mechanism of the MR fluid has been identified as abrasive wear.

Abstract: This experimental investigation presents wear characteristics of magnetorheoloigcal (MR) fluid under boundary lubrication contact condition. Three different specimens; aluminum, copper and steel pins are prepared and wear test is performed under consideration of several operational factors such as normal load, sliding distance, and sliding speed by using pin-on-disk test equipment. Wear rate and friction coefficients of the MR fluid are experimentally evaluated with respect to the sliding ranges. Microscopic surface and roughness changes of the worn surface of pin specimens are also analyzed by using the scanning electron microscope (SEM). In addition, Energy Dispersive X-ray Spectroscopy (EDS) analysis is conducted and chemical changes are investigated.

Abstract: In this study, the machining characteristics of developed AlN-hBN composites were investigated in the end-milling and precision lapping processes. To achieve the objectives, material properties of the developed AlN-hBN composites were evaluated according to the variation of hBN contents. And, required experimental works were performed to investigate the machining characteristics of the composites. First, the machiniability of the composite was evaluated in the end-milling process under various cutting conditions, such as spindle speed, feederate, and depth of cut variations. Also, generated micro cracks caused by the cutting process were investigated via SEM photographs. Next, precision lapping experiments were performed under various conditions, and the results were estimated.

Abstract: In this study, micro patterns were formed on the developed AlN-hBN composites using powder blasting techniques, which have been considered as one of the most appropriate micro machining methods for hard and brittle materials such as ceramics. To achieve the objective, first, material properties of the developed AlN-hBN composites were evaluated according to the variation of h-BN contents. And, a series of required experiments were performed, and the results were analyzed. As the results, it was investigated that the machiniability of the developed AlN-hBN composites increased with the increase of the h-BN contents in the composites. From the experimental results, it was possible to determine optimum blasting conditions for micro pattern making on the developed AlN-hBN composites.

Abstract: Recently the polymeric LOC (lab-on-a-chip) has been developed by the bio technology and the micro electro mechanical system technology. It is possible that biological experiment and analysis is executed rapidly and easily using a simple specimen. Therefore, many researches on LOC are progressing actively in various fields such as medical, biotechnical, environmental aspects etc. Up to the present time, most of polymeric LOC has been manufactured by LIGA (lithograpie galvanofomung abformung) or chemical etching processes. However, these methods have several disadvantages which are the complicated fabrication stages, requirements for various special equipments and limitations of materials. In this study, in order to overcome these difficulties, the possibilities of micro fluidic channel features were executed by micro cutting process for the steel. So the micro mold insert with micro fluidic channel was designed and manufactured by micro cutting process for the injection molding.

Abstract: The objective of this research was to apply the artificial neural network algorithm to predict the surface roughness in high speed milling operation. Tool length, feed rate, spindle speed, cutting path interval and run-out were used as five input neurons; and artificial neural networks model based on back-propagation algorithm was developed to predict the output neuron-surface roughness. A series of experiments was performed, and the results were estimated. The experimental results showed that the applied artificial neural network surface roughness prediction gave good accuracy in predicting the surface roughness under a variety of combinations of cutting conditions.

Abstract: Recently, ultra-precision micro patterns and shapes have been widely used in optical field. Various methods which are based on semi-conductor fabrication methods are nowadays used in fabrication of micro shapes and patterns, but micro endmilling technology has lately attracted considerable attention because of various available materials, flexibility of process and high-productivity. For the precision micro endmilling process, analysis of micro cutting error is mandatory. In general, tool deflection is a major factor which causes cutting error and limits realization of the high-precision cutting process. Specially, in micro endmilling process, micro tool deflection generates very serious problems compared to macro tool deflection. In this paper, it is performed to observe the real tool deflection shapes in micro endmilling process, so the trend of micro tool deflection was analyzed using real captured images in this study. To get the real images of micro tool deflection, micro slot cutting processes were executed under various cutting conditions using micro endmill and the real images of tool deflection were obtained during cutting process by high-speed camera. Finally, the extent of tool deflection was calculated by the deflection angle according to cutting conditions and two trends (the point of first tool contact and the cutting stage) of micro tool deflection were analyzed.

Abstract: Recently, the trends of industrial products move towards more miniaturization, variety and mass production. Micro drilling which take high precision in cutting work is required to perform more micro hole and high speed working. Especially, Micro deep hole drilling is becoming more important in a wide spectrum of precision production industries, ranging from the production of automotive fuel injection nozzle, watch and camera parts, medical needles, and thick multilayered Printed Circuit Boards(PCB) that are demanded for very high density electric circuitry. The industries of precision production require smaller holes, high aspect ratio and high speed working for micro deep hole drilling. However the undesirable characteristics of micro drilling is the small signal to noise ratios, wandering motion of drill, high aspect ratio and the increase of cutting force as cutting depth increases. In order to optimize cutting conditions, an experimental study on the characteristics of micro deep hole machining processes using a tool dynamometer was carried out. And additionally, microscope with built-in an inspection monitor showed the relationship between burr in workpieces and chip form of micro drill machining.