Suet To

Ultraprecision diamond turning of aluminium single crystals

Abstract Ultraprecision diamond turning is an important technology to generate a high surface finish in precision components and optics. In this paper, the diamond turning of aluminium single crystal rods with crystallographic axes normal to , and is analysed. The effect of the crystallographic anisotropy on the machining of these single crystals is investigated in the light of the mechanics of chip formation, of the cutting force and of the surface microtopography. Continuous chips were formed under all cutting conditions for all of the crystals examined. However, differently orientated crystals exhibit differences in the cutting force and in the quality of the machined surface. The implications of these findings on the improvement of the surface finish that can be obtained in the diamond turned surface are discussed.

Influence of material swelling on surface roughness in diamond turning of single crystals

Abstract The effect of material swelling on the surface roughness in ultraprecision diamond turning has been investigated. Experimental results from the power spectrum analysis indicate that the profile of the tool marks is distorted by the effect of swelling of the materials being cut. A good correlation exists between the surface roughness and the amount of swelling that has occurred in the machined layer. Radically different surface roughness profiles were obtained when machining aluminium and copper single crystals with the same cutting plane and tool shape. The difference in the machining behaviour could not be accounted for by elastic recovery alone but could be explained by considering the plastic deformation induced in the machined layer.

Modelling and simulation of freeform surface generation in ultra-precision raster milling

Abstract Optical freeform surfaces are large-scale surface topologies with shapes generally possessing non-rotational symmetry with submicrometric form accuracy and nanometric surface finish. Due to the geometrical complexities, the prediction of form accuracy in ultraprecision raster milling of ultra-precision freeform surfaces is more difficult than conventional machining. Nowadays, the achievement of submicrometre form accuracy still depends largely on the experience and skills of the machine operators through an expensive trial-and-error approach. This paper presents a model-based simulation system for the prediction of form accuracy in ultra-precision raster milling of optical freeform surfaces. The system takes into account the cutting mechanics, cutting strategy, and the kinematics of the cutting process. Experimental work has been undertaken to verify the system and the predicted results agree well with the experimental results. The successful development of the model-based simulation system allows the optimum cutting parameters and cutting strategies to be determined without the need to conduct massive and costly trial-and-error cutting tests.

Anisotropy of surface roughness in diamond turning of brittle single crystals

This paper deals with an investigation of the effect of crystallographic orientation and process parameters on the surface roughness of brittle silicon single crystals in ultraprecision diamond turning. The process parameters involve the depth of cut, feed rate, and spindle speed. Experimental results indicate that anisotropy in surface finish occurs when the cutting direction relative to the crystal orientation varies. There exists a periodic variation of surface roughness per workpiece revolution, which is closely related to the crystallographic orientation of the crystals being cut. Such an anisotropy of surface roughness can be minimized with an appropriate selection of the feed rate, spindle speed, and depth of cut. The findings provide a means for the optimization of the surface quality in diamond turning of brittle silicon single crystals.

Large-scale fabrication of micro-lens array by novel end-fly-cutting-servo diamond machining

Fast/slow tool servo (FTS/STS) diamond turning is a very promising technique for the generation of micro-lens array (MLA). However, it is still a challenge to process MLA in large scale due to certain inherent limitations of this technique. In the present study, a novel ultra-precision diamond cutting method, as the end-fly-cutting-servo (EFCS) system, is adopted and investigated for large-scale generation of MLA. After a detailed discussion of the characteristic advantages for processing MLA, the optimal toolpath generation strategy for the EFCS is developed with consideration of the geometry and installation pose of the diamond tool. A typical aspheric MLA over a large area is experimentally fabricated, and the resulting form accuracy, surface micro-topography and machining efficiency are critically investigated. The result indicates that the MLA with homogeneous quality over the whole area is obtained. Besides, high machining efficiency, extremely small volume of control points for the toolpath, and optimal usage of system dynamics of the machine tool during the whole cutting can be simultaneously achieved.

Analysis of surface generation in the ultraprecision polishing of freeform surfaces

Abstract The use of freeform designs in engineering surfaces has become increasingly popular over the last decade. Applications of freeform shapes range from aesthetics of components to the bending of light rays through advanced optic designs. The fabrication of the components for these applications requires submicrometre form accuracy, in some cases with surface roughness at nanometric levels. Ultraprecision polishing is an emerging technology for the fabrication of high-precision and high-quality freeform surfaces. However, the factors affecting nanosurface generation in ultraprecision polishing have received relatively little attention. Moreover, the quality of the polished surface relies heavily on appropriate selection of process conditions and polishing strategies. This paper presents an analytical study of the factors affecting surface generation in ultraprecision polishing. A series of polishing experiments have been designed and undertaken, allowing the relationships between various factors and the surface quality of the workpiece to be determined. The results of the study provide a better understanding of nanosurface generation, as well as the strategy for optimizing surface quality, in the ultraprecision polishing of freeform surfaces.

Static Electropulsing-Induced Microstructural Changes and Their Effect on the Ultra-Precision Machining of Cold-Rolled AZ91 Alloy

The effects of electropulsing on the phase transformations of a cold-rolled Mg-9Al-1Zn alloy were studied using X-ray diffraction, back-scattered electron microscopy, transmission electron microscopy, and optical microscopy techniques. The results indicated that with increasing frequency of electropulsing, the decomposition and precipitation of β phase were tremendously accelerated sequentially. Electropulsing accelerated the decomposition of β phase by a factor of approximately 3600 times. The effects of the electropulsing-induced microstructural changes on machinability of the alloy, by single-point diamond turning, was discussed.

Measuring ultra-precision freeform surfaces using a robust form characterization method

Ultra-precision freeform surfaces are complex surfaces that possess non-rotational symmetry and are widely used in advanced optics applications. However, there is a lack of a surface characterization method that measures the form accuracy of the ultra-precision freeform surfaces with micrometre to sub-micrometre form accuracy. Due to the high precision requirement of the ultra-precision freeform surfaces, this inevitably involves the outliers in the measured data that would significantly affect the accuracy and the performance of the form characterization method. Although some research work has been found in the development of the form characterization method, most workers have not considered the influence of outliers. It is vital to incorporate robust estimation in the surface characterization method for catering for the influence of outliers. In this paper, a robust form characterization method (RFCM) is presented to characterize the form accuracy of the ultra-precision freeform surfaces. A series of computer simulation and experimental analyses were undertaken to verify the RFCM. The theoretical results agree well with the simulation and experimental results.

Measuring optical freeform surfaces using a coupled reference data method

Flat optical freeform surfaces usually possess non-rotational symmetry with a small curvature and lack of strong features for surface alignment. Due to the lack of strong features and small curvature, it is difficult to align the design and measured surfaces for characterizing the surface quality of flat optical freeform surfaces with sub-micrometre form accuracy. The traditional least squares method (LSM) generally produces large errors as there is a lack of strong features as reference for the alignment of the design and measured surfaces. This paper proposes a novel and practical method named the coupled reference data method (CRDM) to evaluate flat optical freeform surfaces with high efficiency and precision in the nanometre scale. The method couples reference data to the workpiece of the freeform surface designed model and the concerning reference features are machined together with the workpiece. By aligning the reference data, the proposed CRDM carries out fast surface matching. This makes good preparation for the next matching optimization which is conducted by the least-squares and minimax zone method. After the precise surface matching, the flat optical freeform surface can be evaluated by 3D form error topography and parameters. As compared with a traditional freeform measurement method such as LSM, it is interesting to note that the accuracy and the stability of the measurement can be significantly enhanced by the CRDM.

An experimental investigation of surface generation using an integrated ultra-precision polishing process and different polishing trajectories

Ultra-precision freeform polishing (UPFP) is an emerging technology for the machining of ultra-precision freeform surfaces with submicrometre form accuracy and surface roughness in the nanometre range. However, our understanding of the surface generation and process planning for UPFP is still far from complete. This paper presents an experimental study of the surface generation for different polishing processes, which include mechanical polishing (MP) and fluid jet polishing (FJP). In addition, the generation of desirable structured surfaces of UPFP has also been experimentally investigated to examine if different trajectory planning affects surface hydrophobicity. The experimental results show that both MP and FJP can produce similar surface roughness but differences in surface texture after polishing. Desirable surface roughness and structured surfaces can be produced by appropriate planning and control of the combination of MP and FJP, and tool path trajectories in UPFP. The results also prove that tool path trajectories affect the hydrophobicity of polished surfaces. The results provide an important means for the development of an integrated polishing process to achieve the desirable surface roughness and surface texture based on process planning and control in UPFP.