Papers by Author: Ji Wang Yan

Abstract: Precision slicing tests were performed for single-crystal silicon by using a newly developed dicing wire saw system and diamond wires. The developed dicing wire saw enables slicing thick workpiece of hard and brittle materials which could not be sliced by conventional dicing machines. To achieve high precision and efficiency, the dicing wire saw system adopted tension control and high speed control technologies which provides a maximum wire feeding speed of 2000m/min. In this study, the diamond wire was driven in a single direction at a speed of 750-1750m/min and the slicing force, wire wear and workpiece surface roughness after slicing were investigated experimentally. The results showed that as a new slicing system, the developed dicing wire saw was useable for high-precision slicing of thick workpiece.

Abstract: Powder jet deposition (PJD) method is one of the blasting methods to generate surface coatings. The optimization of PJD conditions has been reported in our previous research. However, the deposition mechanism in PJD is still under investigation. Impact phenomena between an alumina particle with the mean particle size of 2 μm and a glass substrate has been successfully simulated by smoothed particle hydrodynamics (SPH) method. From the simulation result, we have deduced that a cubic particle is fractured by an impact, and it is adhered on to the substrate. It has been also deduced that substrate is removed by a spherical particle impact. Furthermore, PJD experiments of alumina particles blasted onto a glass substrate were also conducted. The particle size distribution of rectangular particles before and after impact was measured. It was found that the particle sizes after impact averagely became smaller than those before impact. The substrate was partly removed when spherical particles impact. From the results of the simulation and the experiment, we believe that the rectangular particles are fractured due to the impacts at the moment blasting onto the substrate, and then, firmly deposited on the substrate.

Abstract: Glass molding press (GMP) was applied to the fabrication of glass aspherical Fresnel lens for high-efficiency mass production. A pair of molds for the Fresnel lens was precisely fabricated on the Nickel Phosphorous (Ni-P) plating layer by the single diamond cutting with multi-axis machining center. A kind of low transition temperature (T_g) glass was heated to several tens degree centigrade above its T_g and compressed between the two molds. In this way, the shape of the mold was replicated to the glass lens surface. The molding process was simulated by finite element method (FEM), and the molding condition was optimized to minimize the residual stress.

Abstract: On-machine measurement and statistical analysis of wheel topography are important for real-time monitoring the wheel condition during grinding and predicting the performance of grinding process. In this paper, the surface topographies of grinding wheel in the different stages of the grinding of fused silica and BK7weremeasured with a white light interferometer, which was set on the surface grinder. Height parameters of 3D surface roughness were proposed to evaluate the wheel wear. Wheel wear mechanisms were essentially determined by analyzing the different material removal modes of optical glasses. Grinding of BK7 mainly causes severe attritious wear of grains, while grinding fused silica, self-sharpening effect makes grinding sustain for a long time.

Abstract: Micro-grinding is a machining method to create submillimeter structures, and it can be applicable to various kinds of materials with a flexible forming ability. However, the micro tool may be easily bended by the grinding force, which results in relative low form accuracy and worse ground surface roughness. To solve these problems, ultrasonic assistance was applied to micro-grinding. This paper proposes a method to calculate ground surface roughness theoretically using statistical approach, and the ground surface improving mechanism of ultrasonic assistance was clarified. Finally, the theoretical calculation was confirmed experimentally using a developed ultrasonic-assisted micro-grinding device.

Abstract: Tool wear reduction is very important in cutting of ultra hard and brittle materials. In this paper, a tool-swinging cutting method was applied to reduce the wear of round-nosed diamond tools in cutting of tungsten carbide (WC). In this method, the geometrical center of the cutting edge was adjusted to be in coincidence with the rotation center of the B-axis table, and thus the cutting point could be changed along the cutting edge by swinging the tool about the B-axis center. Experimental results showed that the width of the flank wear land was greatly reduced compared to that in the conventional cutting. This work can shed light on ultraprecision machining WC parts without or with less need for subsequent polishing process.

Abstract: This paper presents a glass molding press (GMP) method to fabricate microgroove array and micropyramid array on the glass plate by replicating the shape of the mold to the glass surface. The differences between microgroove forming and micropyramid forming were investigated by experiments and finite element method (FEM) simulations. Microgroove arrays and micropyramid arrays were generated on the flat glass plate in the GMP process by using an electroless-plated Nickel Phosphorus (Ni-P) mold, on which the microstructures are fabricated by micro cutting. Furthermore, FEM simulations were used to trace the stress distribution and the strain distribution during the glass deformation, which illustrates the glass material flow in the microgrooves and the micropyramids on the mold during pressing. By comparing the processes between microgroove forming and micropyramid forming, the differences between them observed in the experiments were explained by the simulation results. Finally, some techniques to improve the forming accuracy were proposed.

Abstract: There is a demand for high-efficiency and high surface integrity grinding of fused silica. Ductile grinding is an ideal method for producing a mirror finished surface on hard and brittle materials to significantly decrease polishing time. However, the fused silica is still difficult to ductile grind because of its high brittleness. A creep feed taper grinding method was applied to investigate the relationship between maximum grit depth of cut and surface integrity of fused silica. Ductile mode grinding was achieved on fused silica. When the depth of cut exceeds the critical wheel depth of cut, the surface suddenly changes from the ductile mode to the brittle mode. At the same ratio of wheel speed and table speed, the critical wheel depth of cut is noticeably increased by increasing the wheel speed which caused an increase in the temperature at the interface of grains and workpiece. The depth of subsurface damage (SSD) was investigated by polishing the ground surface. The experiment results show that the depth of SSD is deepest in transition mode and stables in brittle mode.

Abstract: Fused silica is an important material for optics of ultraviolet laser transmission. It is difficult to be ground in ductile mode because of a high brittleness. In this paper, ductile mode grinding was achieved by a taper grinding method when the initial surface was a polished surface. However, when the initial surface was an unpolished surface, ductile mode grinding could not be realized because of the existence of surface and subsurface damages. The damages on the ground fused silica surface were repaired by CO2 laser irradiation with optimal laser power and scanning velocity. The optimal parameters were determined by studying the viscosity of fused silica and etching depth of laser irradiated groove. Besides laser processing parameters, surface roughness of laser irradiated surface is also greatly influenced by the initial surface roughness.

Abstract: Amorphous NiP plate is used as a mold of precision optical parts owing to the superior machinability. In the amorphous NiP plate, some pores, whose diameter is about 100m, are generated occasionally. At present, the amorphous NiP plates with pore are rejected. However, because size of the mold becomes large in recent years, the possibility of pores becomes high. In addition, the cost of the amorphous NiP plate also becomes high. Therefore, reparation method of the pore in the amorphous NiP plate should be developed. In this paper, laser assist powder jet deposition is proposed as a reparation method of the amorphous NiP plate. And fundamental experiments are carried out.