Key Engineering Materials Vol. 679

Abstract: In this paper, adaptive diamond turning (ADT) is proposed for micro-structured surfaces to obtain homogeneous surface quality without loss of efficiency. With consideration of the local shape variation of desired surfaces, the adaptation in feeding direction is to actively control motions along the x-axis to obtain constant scallop heights, whereas the adaptation in forwarding direction is to vary angular sampling intervals to obtain constant linearization errors. The basic principle of ADT is demonstrated, and unique features of the corresponding motions of the machine tool are detailed by numerical simulation. Two degree-of-freedoms fast tool servo or slow slide servo-based turning is strongly recommended to meet the requirements of the ADT method.

Abstract: The fabrication of ultra-precision freeform surfaces possessing non-rotationally symmetric geometry and sub-micrometre form accuracy requires an efficient positioning method for precisely locating the position of the workpiece on the machine with high repeatability during the manufacturing cycle. This paper presents an initial attempt to develop a fiducial aided positioning (FAP) method for precisely locating the freeform workpiece on multi-axis machine tools in order to improve the accuracy and efficiency in the manufacturing of ultra-precision freeform surfaces. The FAP method makes use of fiducials to establish intrinsic surface features of the freeform workpiece which are used to determine the coordinate transformation between the coordinate frame of the workpiece and the machine in the machining process and the measurement process. A preliminary simulation experiment was conducted to verify the technical feasibility of the proposed FAP method. The results show that the FAP method is technically feasible to establish a link between the machining and measurement process so as to improve machining accuracy.

Abstract: In this paper, we present a new approach for modeling the hysteresis nonlinearity of a piezo-positioning systems. Because of its nonlinear hysteresis effect, the tracking control accuracy of the precision positioning system is difficulty achieved. Hence it is desirable to take hysteresis effect into consideration for improving the trajectory performance. A model reference adaptive optimal control scheme based on online actor-critic algorithm is developed. The proposed controller can be constructed without a nonlinear hysteresis dynamic equation to compensate the hysteresis effect. Simulation results show the effectiveness of the new approach.

Abstract: Ultra-precision grinding is an effective method to machine the optical micro v-groove, which is one of microstructures applied to the fiber-optics connectors, displays and other photonics devices. The ultra-precision grinding technology directly obtains high surface quality for brittle materials when the grinding process is under the ductile mode. This paper introduces general aspects of ultra-precision grinding technology in the fabrication of the micro v-grooves structures and introduces the essential features of ultra-precision grinding. The process of the manufacturing of the optical micro v-grooves components is presented in this paper. It contains the prediction models of surface roughness and form accuracy in the ultra-precision grinding and the optimization model under the consideration of the influences of grinding parameters,grinder factors and the material properties on the surface quality and machining efficiency. This study therefore contributes to providing a further understanding on the mechanisms of material removal and surface generation in ultra-precision girnding.

Abstract: A Collins formula method with a scaling factor between the target-and source-plane has been proposed for the laser propagation in the optical system design, which can be used to evaluate the laser optical system performance and the tolerance analysis. The laser propagation in the optical systems can be calculated by the Collins integral formula, and an angular spectrum method has been derived by coordinate substitution. A scaling factor m is introduced to make the choice of the observation-plane more flexibility and the calculation more accurate. The laser optical system has been designed, and its tolerance analysis is conducted by the Angular-Spectrum method. The evaluating criterion is the laser spot radius in the far field, which is defined by 86.5% power in bucket (PIB). The radius of the laser spot in 90m between 0.8mm and 1.4mm with the expectation of 0.92mm is calculated by the tolerance analysis, and the experimental result is 1.01mm. In the distance of 47m, the radius is between 0.42mm and 0.73mm with the expectation of 0.48mm calculated by the tolerance analysis, the experimental result is 0.46mm. The experiment agrees the result of the tolerance analysis well. The focal shift for laser propagation in the optical systems is validated. The experiment results confirm the calculation and it proved the use of the method in laser focus optical system design.

Abstract: Embossing by patterned rollers is one of the most efficient machining approaches for manufacturing plastic films with patterned microstructures, and the precision roller with patterned microstructures is the key tooling component in the roll-to-roll process. Single-point diamond turning with a slow or fast tool servo is an enabling and efficient ultra-precision machining process to fabricate microstructures through a simple process to achieve optical surface finish directly. Most of the current studies on slow or fast tool servo machining processes either focus on face machining of patterned microstructures, or on tool servo machining of microstructures in the radial direction of precision rollers. Relatively little research work is found in relation to machining patterned microstructures on a precision roller by using the tool servo in the axial direction. This paper presents a pilot study on the development of a tool path generator for machining precision rollers with wavy patterned microstructures by using slow tool servo machining in the axial direction on a precision roller. The machining mechanism is firstly explained, based on which the tool path generator is developed for machining wavy patterned microstructures on a precision roller surface. Preliminary experimental work was conducted to study the generation of wavy patterned microstructures on a precision roller using a four-axis ultra-precision machine. The machined wavy microstructures on the roller surface are measured and analyzed to successfully validate the performance of the proposed tool path generator.

Abstract: The free-form surface closed to a sphere of brittle material has been used widely, but it is difficult for machining and the efficiency of processing is insufficient. In order to get a product, several processes are needed, such as rough machining, semi-finishing and finishing. Axisymmetric curved surface can take place of the free-form surface in roughing or semi-finishing for wiping off the mass allowance efficiently. Therefore, a spherical approximation algorithm of free-form surface closed to sphere is presented in which free-form surface optical lens will be replaced by a spherical surface in semi-finishing and get the approximate sphere of the free-form surface. It can be certified in the test that this method is simple and reliable. The efficiency and precision in machining is excellent and the distribution of allowance for finishing is uniform in the whole surface, which has great practical significance in machining of optical free-form surface of brittle materials.