Papers by Author: Yunn Shiuan Liao

Abstract: Micro milling has been known to behave quite differently from conventional macro milling. The effect of cutting speed on the machined surface roughness in micro milling of ductile materials was investigated in this paper. The micro flat end milling cutters of the diameters of 0.5 mm and 0.3 mm were taken to machine 6061 aluminum alloy and S20C low carbon steel. For comparison purpose, the cutter of the diameter of 10 mm was tested as well. It is found that the machined surface finish is closely related to the cutting speed. For a large size cutter, the roughness decreases with the increase of cutting speed as expected. But the decreasing trend is valid only up to a certain speed for the small diameter cutters. It is also noted that beyond this speed limit broken and powder type chips form easily. These chips will adhere to the machined surface and tool face and deteriorate machined surface as a result. Based on this finding, an approach by directing the cutting fluid to the cutting zone via high pressure air assistance is proposed. It is verified that the cutting force and its variation are greatly reduced, tool wear becomes less, and finish of the machined surface is significantly improved.

Abstract: The cutting of precision threads is an important manufacturing process. Several passes are needed to complete the cutting of a thread and the choice of appropriate cutting speed and depth of cut for each cutting pass is essential. The cutting efficiency and tool life are significantly affected by these two parameters, especially when cutting threads in difficult-to-cut materials, such as titanium alloy. This paper proposes the concept of an equal undeformed chip area for all cutting passes, in order to determine the depth of cut for each pass. The principal goal is to maintain the same cutting force throughout the cutting process. Using tool geometry, the relationship between the cumulative depth of cut and the undeformed chip area for each cutting pass are derived. The depth of cut of each corresponding cutting pass can be determined, once the dimensions of the thread and the number of cutting passes are specified. Experiments were conducted to cut an ISO metric screw thread, with a pitch of 0.5 mm, on a 40 mm in diameter bar. It was found that, for the same total number of cutting passes, the tool wear was less than that suggested by the tool makers, when a depth of cut for each pass was determined using the proposed method. The thread could be cut using a higher cutting speed, resulting in a much shorter machining time. In addition, the proposed strategy also allowed completion of cutting using less cutting passes. A 25% increase in efficiency was noted for the specific thread used in the experiment.

Abstract: The mechanical chemical polishing (MCP) process which uses soft abrasives to polish hard workpiece have been employed recently to polish silicon carbide (SiC) ceramic for various applications. An ultrasonic vibration assisted apparatus is designed to investigate the effects of ultrasonic vibration on the efficiency of MCP of SiC by ferric oxide (Fe2O3) abrasives. Experimental results show that the ultrasonic vibration can effectively improve polishing efficiency; the material removal rate is increased by about 60~70%. But it does not lead to a better final surface finish. The effect is more obvious under a higher working pressure condition. It is also found that the output power of ultrasonic transducer is positively correlated with material removal rate. The rise of polishing efficiency is attributed to the increase of local asperities temperatures which promote chemical reaction of silicon carbide with oxygen to form passivation layers. The layers are removed by ferric oxide abrasives afterward.

Abstract: Microtechnologies have been vigorously developed recently. Micro products have been widely used in many fields, for instance, the sensors in electronic, medical, optical and automotive applications. Thermocouples, which are used for temperature measurement, are usually fabricated by utilizing the joining technology of argon arc welding. However, the weld bead is so big that the conductivity and repeatability of temperature signals would be degraded. Laser welding may fix the weld bead problem, but the cost of equipment is relatively high. This study is about butt-welding thin brass wire of 250 m by mean of micro discharge. At first a low temperature plasma torch is formed to ignited arc, then discharge happens to complete the welding. Argon is added at ambient atmosphere during the welding process. The joined interfaces and heat affect zone of the welding is examined by optical and scanning electronic microscopies. Tensile and hardness tests as well as the microscopy examination indicate that this technology is feasible for thin brass welding. The features of this technology include low cost, easy to maneuver, and applicable in industry. This CIMDW technology offer choice for the applications of micro discharge in the field of microtechnology.

Abstract: The use of cutting fluid containing nano particles in wet grinding and MQL grinding of Ti-6A1-4V is studied. For comparison purpose, experiments by applying the general purpose water-based cutting fluid are also conducted. The loading of the grinding wheel and morphology of the ground surface are observed, and the grinding forces and roughness of the ground surface are measured for analysis. It is found that the use of cutting fluid containing nano particles results in less loading of the wheel and better ground surface as compared with those with the use of the general purpose water-based cutting fluid because of smaller grinding forces and coefficient of friction originated from “lotus effect” of nano particles. MQL leads to better results than wet grinding. This is attributed to the possibility of more nano particles to effectively reach the grinding zone by the assistance of the high pressure air of MQL, which in turns enhances the functions of nano particles in alleviating wheel loading and decreasing coefficient of friction. Based on the experimental results, it is concluded that the use of cutting fluid containing nano particles has positive effects on grinding. Especially the MQL, not only can it achieve the best performance, it is also cost effective and environmentally attractive.

Abstract: Sintered polycrystalline diamond cubicles were oriented to make CMP pad conditioners. The dressing experiments demonstrated the capability of fast pad cutting and efficient removal of glazed layer that is formed by polishing wafers with slurry on dressed pad.

Abstract: This paper presents a layer manufacturing technology called selective laser gelling (SLG) to fabricate metal-ceramic composites green parts which are difficult to construct using traditional methods for fabricating composites. When a layer of metal-ceramic slurry is scanned via Nd: YAG laser radiation, the metal particles are gelled together by the silica sol to form a composites part. In comparison with other composites processes, the features of this process include lower laser forming energy, faster fabrication speed, less dimensional variations. The material composition is mixing by the stainless steel powder and a silica sol in a proportion of 75 to 25 wt. %. A series of experiments was conducted to obtain the smallest pave-able layer thickness of 50 μm on an experimental rapid prototyping (RP) machine. The feasibility of this process was demonstrated by manufacturing a gear shaped prototype with a surface finish of 18μm under a laser energy density of 3.5 J/mm2.

Abstract: The pad surface will become grazed because of the accumulated debris in the chemical mechanical polishing (CMP) process. It results in the reduction of the wafer removal rate, and a pad conditioner or diamond disk must be frequently employed to refresh the pad surface. The wear behavior of a diamond with respect to its location, original relative protruded height and protruded shape on a diamond disk was investigated in this thesis. It was found from experimental data that the diamonds which were located outside, originally protruded more highly and with crest lines oriented upward wore faster. Accordingly, four suggestions were proposed to obtain the uniform diamond wear. They are: lowering the protruded heights of the outside diamonds, replacing the current flat substrate by a curved surface substrate, changing the diamond distribution on the disk from the current uniform one to a higher concentration around outside diameter, and orienting the diamonds such that the flats are protruded upward for the outside diamonds and the crest lines are projected upward for the inside diamonds. Experiment had verified that the pad conditioner with center-protruded substrate led to a stable and higher dressing rate.

Abstract: The material in powder state has long been used by selective laser sintering (SLS) for making rapid prototyping (RP) parts. A new approach to fabricate smoother surface roughness RP parts of ceramic material from slurry-sate has been developed in this study. The silica slurry was successfully laser-gelling in a self-developed laser sintering equipment. In order to overcome the insufficient bonding strength between layers, a strategy is proposed to generate ceramic parts from a single line, a single layer, to multi-layers of gelled cramic in this paper. It is found that when the overlap of each single line is 25% and the over-gel between layers is 30%, stronger and more accurate dimensional parts can be obtained under a laser power of 15W, a laser scanning speed of 250 mm/s, and a layer thickness of 0.1 mm. The 55:45 wt. % of the proportion between the silica powder and silica solution results in suitable viscosity of the ceramic slurries without precipitation. Furthermore, the effects of process parameters for the dimensional accuracy and surface roughness of the gelled parts are investigated and appropriate parameters are obtained.

Abstract: This article proposes a rapid prototyping apparatus of selective laser sintering for forming silica ceramic green parts. The main differences between the proposed and other RP processes for forming ceramic part are the slurry material used to obtain fine layer thickness and the capability of constructing support structure to increase the dimensional accuracy of the workpiece having an overhang. The RP apparatus developed by us comprises a laser scanning system, a material paving system, and a computer control system. A CO2 laser is adopted to scan over a mixture made of a silica sol and silica powder. The silica sol acts as a binder to gel the silica powder together, which forms a 3D object using laser gelation method. A series of experiments were carried out to obtain the optimal process parameters. An SEM is employed to analyze the microstructure of the ceramic part. It has been found that the smallest layer is of 100 μm thick. The results show that both the accuracy of the material paving mechanism and the optimal process parameters can fulfill the requirements of the RP processes.