• Journal of the Korean Society of Manufacturing Process Engineers
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• v.16 no.1
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• pp.36-41
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• 2017

In this paper, $Si_3N_4/hBN$ ceramics with various hexagonal boron nitride (hBN) contents (0, 10, 20, or 30 wt%) were fabricated via spark plasma sintering (SPS) at $1500^{\circ}C$, 50MPa, and 10m holding time. The material properties such as the relative density, hardness, and fracture toughness were systematically evaluated according to the hBN content in the $Si_3N_4/hBN$ ceramics. The results show that relative density, hardness, and fracture toughness continuously decreased as the hBN content increased. In addition, peak-step drilling (with tool diameter $500{\mu}m$) was performed to observe the effects of hBN content in micro-hole shape and cutting force. A machined hole diameter of $510{\mu}m$ (entrance) and stable cutting force were obtained at 30 wt% hBN content. Consequently, $Si_3N_4/30wt%$ hBN ceramic is a feasible material upon which to apply semi-conductor components, and this study is very meaningful for determining correlations between material properties and machining performance.

• The Transactions of the Korean Institute of Power Electronics
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• v.6 no.2
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• pp.132-140
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• 2001

A universal motor is a small dc series machine motor that is designed to operate from an ac machine. The characteristics of universal motors are high no-load and staring torque. Because of the high operating speed, the size of these motors for a given hp rating is typically smaller that other fractional hp ac machine, making it ideal for hand-held tools and appliances where weight, compactness, and speed are importance factors. A phase-angle control with AC drive system gains a high popularity due to their simple implementation, but contains the disadvantage of their poor input power factor, subharmonic current. Pulse width modulation control with DC drive systems increase the power factor as without delay phase angle. This paper analyzed the subharmonic characteristics of the phase angle control system that is controlled by zero voltage crossing similar to traditional method, and the dc chopper system that is used PWM.

• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.4
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• pp.373-379
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• 2010

EMAT has been applied in various fields for flaw detection and material characterization because it has noncontact property in wave generation and a good mode selectivity. Unfortunately, however, EMAT shows low signal to noise ratio relative to commercial contact transducer because of low energy conversion efficiency. If the phase matching through the control of time delay between each coil consisting of the array EMAT is accomplished, it is expected that it will be a solution for the improvement of low signal to noise ratio. In this experiment, the phased array EMATs which consists of 3 or 4 meander coils and one big magnet were fabricated for surface and vertical shear wave generation. Effect of phased delay control on signal directivity and amplitude enhancement was verified. A slit with the depth of 0.5 mm and a side-drill hole of 0.5 mm diameter were clearly detected by fabricated phased array EMATs, respectively.

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2003.11a
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• pp.671-675
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• 2003

In accordance with the KRR-1 & 2 decommissioning project, the decontamination and dismantling activities of the KRR-2 auxiliary facilities, radioisotope production facilities, were completed from Aug 2001 to Dec 2002. The auxiliary facilities were composed of the concrete hot-cell, lead hot-cells and several laboratories for the radioisotope production. The dismantling objects are home hoods, experimental desks, sinks, and contaminated inner facilities. For the purpose of the safe decommissioning activity, the method statements and working procedures were set up. The manpower of the total 20,933 man-hour was required and several dismantling equipments were also. The maximum surface contamination is: 9.24 Bq/$\textrm{cm}^2$ in removable contamination and 350,000 cpm in fixed contamination. The total amount of 62.146 Ton was raised as dismantled waste with kinds of the concretes, wood, steels, etc. The collective dose was evaluated as 0.33 mam-mSv during this period.

• Explosives and Blasting
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• v.21 no.2
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• pp.1-13
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• 2003

Overbreak or underbreak is one of the most important factors in evaluating the results of a tunnel blasting. Overbreak, which depends on the quality of rock, the type and quantity of explosives, and drilling conditions, has been a target of challenge to many blasting engineers because it directly affects construction cost. Drilling is generally known as one of the primary factors to generate overbreak. This study presents a real working model to reduce overbreak based on the analysis of drilling accuracy and overbreak generated from various working methods related to drilling. As the first step of the study, 45 experiments have been performed. The factors investigated are: marking contour line, the position of perimeter holes, the change of look-out with drilling rig position, and the proper space between perimeter holes. It is concluded that workers and engineers＇ will and efforts are the most important factors to reduce overbreak and that improving drilling method and pattern could reduce overbreak to a considerable amount.

• Journal of Technologic Dentistry
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• v.40 no.1
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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.

• Design & Manufacturing
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• v.14 no.3
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• pp.44-49
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• 2020

One of display trends today is development of high pixel density. To get high PPI, a small size of pixel must be developed. RGB pixel is arranged by evaporation process which determines pixel size. Normally, a fine metal mask (FMM; Invar alloy) has been used for evaporation process and it has advantages such as good strength, and low thermal expansion coefficient at low temperature. A FMM has been manufactured by chemical etching which has limitation to controlling the pattern shape and size. One of alternative method for patterning FMM is laser micromachining. Femtosecond laser is normally considered to improve those disadvantages for laser micromachining process due to such short pulse duration. In this paper, a femtosecond laser drilling for thickness of 16 ㎛ FMM is examined. Additionally, we introduce experimental results for controlling taper angle of hole by vibration module adapted in laser system. We used Ti:Sapphire based femtosecond laser with attenuating optics, co-axial illumination, vision system, 3-axis linear stage and vibration module. By controlling vibration amplitude, entrance and exit diameters are controllable. Using vibrating objective lens, we can control taper angle when femtosecond laser hole drilling by moving focusing point. The larger amplitude of vibration we control, the smaller taper angle will be carried out.

• Special Issue of the Society of Naval Architects of Korea
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• 2011.09a
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• pp.12-16
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• 2011

During the shipbuilding construction, the repair works will be occurred by various reasons such as the natural disaster, mistakes of engineers or workers, defect of material, and so on. The one of the engineer's responsibilities is to design considering every possibility and to prevent the repair works due to the mistakes during construction stages, but actually it is very difficult and impossible to avoid every mistake. However, it is also the responsibility of engineers to find the best solution for the unavoidable mistakes which is to maintain the capacity of vessel and guarantee the safety for the repair works considering the construction schedules and economic cost. In this paper, it will be introduced the brief of repair works to the structural damage of the moon-pool structure in drillship which is built in our shipyard. For the verification of the structural strength, the fatigue analysis has been carried out based on the guide of Classification society. Furthermore, the modifications of structure design and welding procedure have been applied to avoid the stress concentration in the moon-pool structure.

• Journal of the Ergonomics Society of Korea
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• v.26 no.3
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• pp.27-33
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• 2007

The purpose of this study was to investigate influence of wrist position on the electromyographic(EMG) activities of the flexor digitorum superficialis (FDS) and extensor digitorum (ED) during drilling and pinch grip. Eighteen healthy subjects had been recruited and each subject performed two tasks (drilling and pinch grip) at three different wrist positions ($30^{\circ}$ flexion, neutral, $30^{\circ}$ extension). The EMG amplitude of each muscle was normalized to the amplitude in the maximal voluntary contraction (MVC). Repeated one-way ANOVA was used to compare the differences of EMG across wrist positions. The FDS EMG activity in both drilling and pinch grip was lowest at $30^{\circ}$ wrist extension and the highest at $30^{\circ}$ wrist flexion. The ED EMG activity was lowest when the wrist was neutral in both tasks. From the results of this study, we can conclude that the desirable wrist positions for drilling and pinch grip tasks are slightly extended or neutral position. Therefore, flexed wrist position should be avoided to reduce the excessive work load on the finger muscles during the tasks.

• Journal of the Korean Geotechnical Society
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• v.21 no.5
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• pp.133-142
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• 2005

The research concentrates on improving the in-hole seismic probe, which has been developed in past five years, to be used in stiffness measurements of tunnel-face materials. The size of probe was reduced to be fit in 45-mm diameter holes (or BX) drilled by a jumbo-drill, which is used to drill holes to install explosives for tunneling. Also trigger system was improved by using a down-speeding motor for operating convenience and air packing system was replaced with a set of plate-springs to eliminate supply of compressed air. These modifications are to adjust the probe for the unfavourable environment inside of tunnels and to test without any further drilling cost. The probe and testing procedure were successfully adopted with horizontal holes drilled by a jumbo-drill at a tunnel-face to evaluate the stiffness of rock mass. The measured shear wave velocities can be used to estimate deformation properties of rock mass for tunnel analyses.