• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.232-235
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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 200MW and 135MW respectively and each turbine is supported by 6 journal bearings. However, we had many difficulties because the vibration amplitude of No 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 No 1 steam turbine. Until a recent date, No 3 and 4 bearings which support high pressure turbine for No 1 steam turbine had shown about 135${\mu}$m in vibration amplitude (sometimes it increased to 221${\mu}$m maximum. alarm: 6mils, trip: 9mils) at base load. After applying the study, they decreased to about 40${\mu}$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 stopper 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.

• 한국정밀공학회지
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• 제6권1호
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• pp.45-51
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• 1989

In metal cutting various types of chips are produced in consequence of cutting conditions. According to the type of chips the cutting mechanism is to be changed. Most of the cutting theory is based on the continuous chip because of its convenient analysis, but the occurrence of the saw-toothed chip depends upon the workpiece and/or the cutting conditions, one of which is titanium alloy used widely. Nowadays titanium alloys are used widely with the rapid development of aerospace structural engineering application, whereas the theory of cutting mechanism has not been established yet, and the formatting process has not been understood satisfactorily, either. Unfortunately several misconceptions, conflicting statements and statements needing further clarifi- cation are also found. In this paper an attempt is made to clarify the formation process of saw-toothed chips which are to be produced during the orthogonal cutting process of titanium alloys. They were machined at low speed to avoid the rapid tool wear. We observed the SEM-photographs of chips taken at the quick-st- opping device. It is hoped that a rational model of the mechanics of cyclic chip formation can be developed. The results obtained are as follows. 1. When a saw- toothed chip is formed, the shear band begins at the primary shear zone and trans- fers to the free surface, so that a segment is produced and it is completed by upsetting between the formatting segment and the formatted segment. 2. As the rake angle or the clearance angle increases in the machining of the titanium alloy, the chip approaches to that of the continous type. 3. When the rake angle and the clearance angle are increased the shear energy and the unit friction energy decrease, which shows the same aspect as that of the continuous chip.

• 한국정밀공학회지
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• 제24권4호
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• pp.115-122
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• 2007

Recently, remarkable progress has been made in both technology and production of optical elements including aspheric lens. Especially, requirements for machining glass materials have been increasing in terms of limitation on using environment, flexibility of material selection and surface accuracy. In the past, precision optical glass lenses were produced through multiple processes such as grinding and polishing, but mass production of aspheric lenses requiring high accuracy and having complex profile was rather difficult. In such a background, the high-precision optical GMP process was developed with an eye to mass production of precision optical glass parts by molding press. This GMP process can produce with precision and good repeatability special form lenses such as camera, video camera, aspheric lens for laser pickup, $f-\theta$ lens for laser printer and prism, and me glass parts including diffraction grating and V-grooved base. GMP process consist a succession of heating, forming, and cooling stage. In this study, as a fundamental study to develop molds for GMP used in fabrication of glass lens, we conducted a glass lens forming simulation. In prior to, to determine flow characteristics and coefficient of friction, a compression test and a compression farming simulation for PBK40, which is a material of glass lens, were conducted. Finally, using flow stress functions and coefficient of friction, a glass lens forming simulation was conducted.

• Tribology and Lubricants
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• 제31권4호
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• pp.148-156
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• 2015

In the study, a design process for ensuring optimal clearance in a fuel injection pump(FIP) is suggested. Structure analysis and hydrodynamic lubrication analysis are performed to determine the optimal clearance. The FIP is simulated using Hypermesh, Abaqus 6.12 to evaluate the reduction of clearance when the maximum supply pressure is applied. The reduction in clearance is caused by the difference in the deformations between the barrel and plunger. When the deformation of the plunger is larger than that of the barrel, a reduction in clearance at the head part occurs. On the other hand, the maximum clearance reduction equals the maximum deformation in the stem part, because the deformation of barrel does not occur in this region. The clearance of FIP should be designed to be larger than maximum reduction of clearance in order to avoid contact between the plunger and barrel. In addition, the two-dimensional Reynolds equation is used to evaluate lubrication characteristics with variations of viscosity, clearance and nozzle for a laminar, incompressible, unsteady state flow. The equation is discretized using the finite difference method. The lubrication characteristics of FIP are investigated by comparing film parameter, which is the ratio of the minimum film thickness and surface roughness. The optimal clearance of FIP is to be designed by considering the maximum reduction in clearance, lubrication characteristics, machining limits and tolerance of clearance.

• 대한치과기공학회지
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• 제40권1호
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• pp.1-8
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• 2018

Purpose: The purpose of this study is to evaluate the trueness and fitness of machined prostheses according to drill radius setting in CAD software. Methods: For this study, standard abutment were replicated in Type IV stone. The stone abutment were scanned using a dental scanner. The CAD design software was designed using scanned abutment data. When designing, the drill radius was set to 0.3 mm and 0 mm, respectively, and saved. The saved design data was milled using a milling machine (n=13). The inner surface of the milled crown was scanned. The trueness and fitness were measured using the inner scan data of prostheses. Independent t-tests were performed to identify significant differences in each data. Results: Trueness values of the data saved with 0.3 mm and 0 mm drill radius were $18.9{\pm}2.3{\mu}m$ and $19.1{\pm}0.9{\mu}m$, respectively. There was no statistically significant difference between the groups. Fitness values of the data saved with 0.3 mm and 0 mm drill radius were $65.5{\pm}0.8{\mu}m$ and $33.8{\pm}1.0{\mu}m$, respectively. There was a statistically significant difference between the groups (p<.05). Conclusion : Setting the drill radius is important to produce clinically good fit prostheses.

• 한국전산구조공학회논문집
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• 제25권5호
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• pp.433-438
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• 2012

본 논문은 두 편으로 구성된 치과용 임플란트 구조설계에 대한 논문 중 두 번째 논문으로 첫 번째 논문에서 구조해석 비교연구를 통하여 그 구조적 성능이 확인된 새로운 임플란트 구조모델에 대하여 시험 설계도면을 작성하여 완성하였으며, 이를 근거로 실제로 CNC 공작기계 등을 이용하여 임플란트를 가공 제작하고, 이를 평가함으로써 치과용 임플란트 구조 설계를 완성하였다. 설계도면 작업은 전용 Tool인 MDT를 이용하여 수행하였으며, 가공작업은 CNC 선반, 범용밀링머신, Wire EDM 등을 이용하여 수행하였다. 전자현미경을 이용하여 임플란트 표면의 가공 상태를 최종 평가 확인하였다. 평가 결과 매우 양호한 상태의 임플란트 시험제품을 설계 제작하였다.

• 대한기계학회논문집A
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• 제29권10호
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• pp.1416-1429
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• 2005

In order to meet the increasing competitive pressures coupled with higher demands for component quality, whirling machines have been at the cutting edge of the automobile industry for more than 25 years. The hard whirling process can save on machining time and operation elimination. Hard whirling is done dry, without coolant. The chips carry away nearly all of the heat during cutting, leaving the workpiece cool and minimizing any thermal geometry variations. The surface finish and profile accuracy are close to grinding quality. Whirling machines usually consist of four major parts; 1) loading system that requires the necessary axial speeds, 2) head stock that needs high precision clamping and positioning system at the chuck and tailstock, 3) whirling unit that demands the high cutting speeds and cutting power fer cutting deep thread profiles and 4) unloading system that requires an easy workpiece unloading. Also, capabilities of the whirling machine can be improved by attaching a vision system to the machine. Most of whirling machines in Korean automobile industry are imported from the Leistritz company, Germany and the Hasegawa company, Japan. Tn this paper, a basic research will be performed to improve and enhance the existing whirling machines. Finally, a new Korean whirling machine will be proposed and developed.

• 대한기계학회논문집A
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• 제41권4호
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• pp.251-255
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• 2017

미세 패턴의 기계적 가공에서는 버의 발생이 불가피하고, 패턴과 버의 크기가 매우 작아 이를 구분하기 어렵다. 본 연구에서는 turning machine을 이용하여, 원통형 공작물의 옆면에 패턴의 피치와 높이가 각각 60, $1{\mu}m$인 마이크로 패턴을 제작하고, 패턴의 형상을 이용하여 버의 형상과 생성 메커니즘을 정의하였다. 또한, 자기연마 디버링을 이용하여 마이크로 패턴의 정밀도를 높이고자 하였다. 그 결과, 미세패턴의 제작 시, 공구의 이송방향에 따라 버의 생성방향이 결정됨을 확인할 수 있었다. 또한, 자기연마디버링 공정에서 자속밀도와 공구의 회전속도가 각각 40mT, 1600rpm일 때, 패턴의 높이는 $1.018{\mu}m$로 측정되었으며, 자기연마 디버링에 가장 적합한 조건임을 알 수 있었다.

• 대한금속재료학회지
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• 제49권12호
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• pp.983-988
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• 2011

Failure analyses of the screws in spinal fixation devices were carried out. The fractured screws were retrieved from a patient who had spinal surgery in the thoracic vertebrae from number 10 to 15. The failure occurred one month after the removal of the braces. Microstructures and fracture surfaces were examined by optical and scanning electron microscopy. The microstructures of the screws corresponded to annealed Ti-6Al-4V bar. However, in the vicinity of the screw surface, there was an insufficient number of fine precipitates. Fracture surfaces showed typical fatigue failure modes. Regarding the fact that no machining defects were detected, fatigue crack initiation might have been caused by the lack of precipitates near the screw surfaces. Only the fourth of five fixed screws was severely stress-concentrated by the action of the spinal bones, while the stress of the 4th screw was decreased to half of its acceptable level when the screw was supplemented by one more, which might have been fixed in the 6th vertebra under the 5th position by the switching of its position. The stress simulation was conducted by ANSYS with 3D CAD of PRO/E in order to understand the stress concentration behavior and to provide an effective spinal surgery guide.

• 소성∙가공
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• 제27권6호
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• pp.370-378
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• 2018

Cold forging, carried out at room temperature, leads to high dimensional accuracy and excellent surface integrity as compared to other forging methods such as warm and hot forgings. In the cold forging process, WC-Co (Tungsten Carbide-Cobalt) alloy is the mainly used material as a core dies because of its superior hardness and strength as compared to other structural materials. For cold forging, die life is the most significant factor because it is directly related to the manufacturing cost due to periodic die replacement in mass production. To investigate die life of WC-Co alloy for cold forging, mechanical properties such as strength and fatigue are essentially necessary. Generally, uniaxial tensile test and fatigue test are the most efficient and simplest testing method. However, uniaxial tension is not efficiently application to WC-Co alloy because of its sensitivity to alignment of the specimen due to its brittleness and difficulty in thread machining. In this study, shape of specimen, tools, and testing methods, which are appropriate for uniaxial tensile test for WC-Co alloy, are proposed. The test results such as Young's modulus, tensile strength and stress-strain curves are compared to those in previous literature to validate the proposed testing methods. Based on the validation of test results it was concluded that the newly developed testing method is applicable to other cemented carbides like Titanium carbides with high strength and brittleness, and also can be utilized to carry out fatigue tests for further investigation on die life of cold forging.