• Tribology and Lubricants
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• 제32권2호
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• pp.50-55
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• 2016

Recently, a double-side machining process has been adopted in fabricating a sapphire glass to enhance the manufacturability. Double-side lap grinding (DLG) is one of the emerging processes that can reduce process steps in the fabrication of sapphire glasses. The DLG process uses two-body abrasion with fixed abrasives including pallet. This process is designed to have a low pressure and high rotational speed in order to obtain the required material removal rate. Thus, the temperature is distributed on the DLG platen during the process. This distribution affects the shape of the substrate after the DLG process. The coolant that is supplied into the cooling channel carved in the base platen can help to control the temperature distribution of the DLG platen. This paper presents the results of computational fluid dynamics with regard to the heat transfer in a DLG platen, which can be used for fabricating a sapphire glass. The simulation conditions were 200 rpm of rotational speed, 50℃ of frictional temperature on the pallet, and 20℃ of coolant temperature. The five cases of the coolant flow rate (20~36 l/min) were simulated with a tetrahedral mesh and prism mesh. The simulation results show that the capacity of the generated cooling system can be used for newly developed DLG machines. Moreover, the simulation results may provide a process parameter influencing the uniformity of the sapphire glass in the DLG process.

• Journal of Electrical Engineering and Technology
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• 제6권2호
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• pp.263-269
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• 2011

In this study, the compositions for the microwave dielectric materials were investigated to obtain the improved dielectric properties, the high temperature stability, and the sintering temperature of less than $900^{\circ}C$, which was necessary for cofiring with the internal conductor of silver. In addition, the dielectric sheets were prepared by the tape casting technique, after which the sheets were laminated and sintered. In this process, the optimum ratio of powder and binder, laminating pressure, temperature, and possibility for cofiring with the internal conductor were studied. Finally, multilayer chip treplexer filter for the 800-2,000 MHz range were fabricated, and the frequency characteristics of the triplexer filter were investigated. When the $0.6TiTe_3O_8-0.4MgTiO_3+3wt%SnO+7wt%H_3BO_3$ ceramics were sintered at $820^{\circ}C$ for 0.3 hours, the microwave dielectric properties of the dielectric constant of 29.91, quality factor of 33,000 GHz, and temperature coefficient of resonant frequency of -2.76 ppm/$^{\circ}C$ were obtained. Using the Advanced Design System (ADS) and High Frequency Structure Simulator (HFSS), the multilayer chip triplexer filter acting at the range of 800-2,000 MHz were simulated and manufactured. The manufactured triplexer filter had the excellent frequency properties in the CDAM800, GPS and PCS frequency regions, respectively.

• 한국추진공학회지
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• 제11권4호
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• pp.38-45
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• 2007

국내 최초로 액체로켓엔진용 연료 과농 가스발생기가 개발되었으며 이것은 고온, 고압가스(약 900 K, 약 58 bar)를 초당 4 kg이상 발생시킬 수 있다. 고압가스는 터보펌프 터빈을 구동하며, 추진제 탱크에 필요한 열 공급원으로 사용될 수 있다. 본 가스발생기는 개념설계 및 초기 개발시험을 거쳐 최종 형상이 결정되었으며, 구조 및 열 해석이 동시에 진행되었다. 제작은 정밀 기계가공과 표면처리, 특수용접공정을 통해 이루어졌으며, 최종 개발 성능 및 기능 특성 확인을 위해 두 종류의 가스발생기가 제작되어 연소시험을 통한 평가가 진행되었다. 안정적인 점화 및 연소특성과 함께 발생 연소가스의 온도분포 및 평균온도 특성이 양호하며, 개발 요구 조건을 본 개발품이 만족하는 것으로 성공적인 개발이 이루어졌다.

• 한국재료학회:학술대회논문집
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• 한국재료학회 2011년도 춘계학술발표대회
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• pp.58.1-58.1
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• 2011

WC-Co and other similar cemented carbides have been widely used as hard materials in industrial cutting tools and as mould metals; and a number of techniques have been applied to improve its microstructural characteristics, hardness and ear resistance. Cobalt is used primarily to facilitate liquid phase sintering and acts as a matrix, i.e. a cementing phase between WC grains. A uniform distribution of metal phase in a ceramic is beneficial for improved mechanical properties of the composite. WC-Co, starting from initial powders, is vastly used for a variety of machining, cutting, drilling, and other applications because of its unique combination of high strength, high hardness, high toughness, and moderate modulus of elasticity, especially with fine grained WC and finely distributed cobalt. In this study, that started with two different compositions of initial powders, WC-7.5wt%Co and WC-12wt%Co with initial powder size being 1~3 ${\mu}m$, magnetic pulsed compaction followed by subsequent vacuum sintering were carried out to produce consolidated preforms. Magnetic Pulsed Compaction (MPC), a very short duration (~600 ${\mu}s$), high pressure (~4 Gpa), high-density preform molding method was used with varied pressure between 0.5 and 3.0 Gpa, in order to reach an initial high density that would help improve the sintering behavior. For both compositions and varied MPC pressure, before and after sintering, changes in microstructural behavior and mechanical properties were analyzed. With proper combination of MPC pressure and sintering, samples were obtained with better mechanical properties, densification and microstructural behavior, and considerably improved than other conventional processes.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2006년도 제27회 추계학술대회논문집
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• pp.181-185
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• 2006

국내 최초로 개발된 액체로켓엔진용 연료 과농 가스발생기는 900 K 정도의 온도를 갖는 58 bar 수준의 고압가스를 초당 4 kg이상 발생시킬 수 있다. 고압가스는 터보펌프 터빈을 안정적으로 구동할 수 있으며, 추진제 공급탱크 가압에 필요한 열원으로 사용될 수 있다. 본 가스발생기는 개념설계 및 일련의 초기 개발시험을 거쳐 최종 형상이 결정되었으며, 구조 및 열 해석이 동시에 진행되었다. 제작은 정밀 기계가공과 표면처리, 특수용접 공정을 통해 이루어졌으며, 최종 개발 성능 및 기능 특성 확인을 위해 총 다섯 차례의 연소시험이 진행되었다. 시험결과를 통해 안정적인 점화 및 연소특성과 발생 연소가스의 온도분포 및 평균온도 특성이 개발 요구규격을 본 개발품이 만족하는 것으로 판단하였다.

• 한국기계가공학회지
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• 제14권1호
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• pp.70-77
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• 2015

This paper develops and evaluates a mechanical machining process which involves drilling on PVS material. According to the material, two treatment experiments were conducted, one involving drilling in a wet condition or using a lubricant and one involving drilling in a dry condition with no lubricant. Drilling in a dry condition showed better performance in terms of the cutting time than in the wet condition. Otherwise, the wet condition has several advantages. The lubricant influenced the burr diameter size and minimized the temperature on the surface of the work piece. During the wet condition drilling process, a smaller burr diameter size was noted as compared to the dry condition. The temperature showed a linear correlation with the drill bit size, where a least-square analysis provided an $R^2$valuewhichexceeded 0.95. The wet condition required more cutting time than the dry condition. In this condition, the water provides a lubrication effect. A thin layer between the cutting edges and the surface of the work piece is formed. The chip formation is affected by the drilling depth. The color on the tips of the chips was darker than in the initial condition. No correlation between the drilling depth and the bore roughness was noted, but the variation of the cutting speed or the RPM influenced the roughness of the bore. The optimum cutting speed ranged from 40 RPM to 45 RPM in the condition which provided the finest roughness surface.

• 한국기계가공학회지
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• 제14권3호
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• pp.9-14
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• 2015

The Direct Metal Tooling (DMT) process is a kind of additive manufacturing processes, which is developed using various commercial steel powders, such as P20, P21, SUS420, and other non-ferrous metal powders. The DMT process is a versatile process that can be applied to various fields, such as the molding industry, the medical industry, and the defense industry. Among them, the application of the DMT process to the molding industry is one of its most attractive and practical applications, since the conformal cooling channel cores of injection molds can be fabricated at a slightly expensive cost by using the hybrid fabrication method of DMT technology compared with parts fabricated with machining technology. The main objectives of this study are to provide various characteristics of the parts made using the DMT process compared with the same parts machined from bulk materials and evaluate the performance of the injection mold equipped with a conformal cooling channel core fabricated using the hybrid method of the DMT process.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2003년도 춘계학술대회 논문집
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• pp.158-161
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• 2003

Recently, the evolution in production techniques (e.g. high-speed milling), the complex shapes involved in modem production design, and the ever increasing pressure for higher productivity demand a drastic improvement of the dynamic behavior of the machine tool axes used in production machinery. And also machine tools of multi functional and minimized parts are increasingly required as demand of higher accurate in some fields such as electronic and optical components etc. The accuracy and the productivity of machined parts are natural to depend on the linear system of machine tools. The complex workpiece surfaces encountered in present-day products and generated by CAD systems are to be transformed into tool paths for machine tools. The more complex these tool paths and the higher the speed requirements, the higher the acceleration requirements are needed to the machine tool axes and the motion control system, and the more difficult it is to meet the requirements. The traditional indirect drive design for high speed machine tools, which consists of a rotary motor with a ball-screw transmission to the slide, is limited in speed, acceleration, and accuracy. The direct drive design of machine tool axes. which is based on linear motors and which recently appeared on the market. is a viable candidate to meet the ever increasing demands, because of these advantages such as no backlash, less friction, no mechanical limitations on acceleration and velocity and mechanical simplicity. Therefore performance tests were carried out to machine tool axes based on linear motor. Especially, dynamic characteristics were investigated through circular test.

• 한국정밀공학회지
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• 제26권11호
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• pp.99-107
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• 2009

Since a long work piece influences the natural frequency of the entire system with a miniature high speed spindle, a bar-feeder is used for a long work piece to improve the vibration characteristics of a spindle system. Therefore, it is very important to design optimally support positions between a bar-feeder and a long work piece for a miniature high speed spindle system. The goal of the current paper is to present an optimization method for the design of support positions between a bar-feeder and a long work piece. This optimization method is effectively composed of the method of design of experiment (DOE), the artificial neural network (ANN) and the genetic algorithm (GA). First, finite element models which include a high speed spindle, a long work piece and the support conditions of a bar-feeder were generated from the orthogonal array of the DOE method, and then the results of natural vibration analysis using FEM were provided for the learning inputs of the neural network. Finally, the design of bar-feeder support positions was optimized by the genetic algorithm method using the neural network approximations.

• 플랜트 저널
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• 제4권3호
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• pp.66-72
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• 2008

The vibration of steam turbine is caused by Mass unbalance, Shaft misalignment, Oil whip and rubbing etc. But in turbine which is normally operated and maintained, the Mass unbalance component possesses the greatest portion. Our power plant has two steam turbines in capacity of 200 MW and 135 MW respectively and each turbine is supported by 6 journal bearings. However, we had many difficulties because the vibration amplitude of #3 and #4 Bearings was high during the start-up and operation mode change of steam turbine. But, with this study, we completely solved the vibration problem caused by the mass unbalance of #1 steam turbine. Until a recent date, #3 and #4 bearings which support high pressure turbine for #1 steam turbine had shown about $135{\mu}m$ in vibration amplitude (sometimes it increased to $221{\mu}m$ maximum. alarm: 6 mils, trip: 9 mils) at base load. After applying the study, they decreased to about $45{\mu}m$ maximum. It is a result from that we did not change the setting value of bearing alignment and only changed the assembly position of internal parts in Synchro clutch coupling rachet wheel which links between high pressure turbine and low pressure turbine, and increased the internal gap and machining of the Pawl cage surface. In the operation of steam turbine, if the vibration value increases by 1X, we should reduce the vibration of bearing by weight balancing. However, unless the vibration of bearing is declined by the balancing, we will have to disassemble and check the component and find the cause. In this study, we researched the way to lower mass unbalance that is 1X vibration component which has the greatest portion of vibration generated by steam turbine and we got good result by applying the findings of this study.