Ultraprecision machining of microlens arrays with integrated on-machine surface metrology

Rotary-coordinate and shuttling-element cutting strategy for ultra-precision diamond turning of optical microstructures

Toolpath generation techniques have become increasingly critical in ultra-precision diamond turning for optical microstructures due to the dramatically enhanced geometrical complexity of the machined region. However, the conventionally used spiral toolpath is required for interpolation from the structural models, leading to random instability of the feeding axis and additional profile error between the toolpath and designed structures, which means an enlarged effect on the machining quality in ultra-precision machining. In this paper, a rotary-coordinate and shuttling-element cutting strategy based on integrated geometrical modelling and spiral toolpath generation is presented for ultra-precision turning of optical microstructures. Using the innovative rotary-coordinate and shuttling-element cutting method, the point clouds for the micro-structured modelling can be scattered along the spiral shape which can be directly fitted as the final toolpath. A series of simulation and cutting experiments have been carried out to realize the effectiveness of this method, and it is found that the preparation time in diamond turning can be significantly reduced along with ameliorating the machining quality.

Optical design of a monolithic compressed folding imaging lens for infrared/laser dual-band

In order to realize the miniaturization of the dual-band system, the monolithic compressed folding imaging lens (CFIL) is designed for infrared/laser dual-band in this paper. The relationship among the back focal length, field of view, pupil diameter, and central obscuration of the CFIL are derived. The design method of the dual-band CFIL is given, and the stray light of the CFIL can be suppressed by the double-layer hood structure. According to the design method of the CFIL, the infrared/laser dual-band can be applied by a monolithic optical element. The design results show that the minimum MTF for all fields of view in the infrared band is greater than 0.125 at 42lp/mm, the spot uniformity in the laser band is greater than 90%, and the total system length is only 0.305 times the focal length. After tolerance analysis, the MTF of CFIL is greater than 0.1, and the spot diagram is less than 880µm. The working temperature of the system is -20∼50°C, and the compensation distance is given. After stray light optimization, The point source transmittance (PST) value in the infrared band is reduced by 2 to 4 orders of magnitude, and the PST value in the laser band is reduced by 1 to 5 orders of magnitude. Compared with the traditional coaxial reflective system, the infrared/laser dual-band CFIL has only one lens, and the optical structure is compact. It provides a new idea for the integration and miniaturization of the multi-band system.

Ultra-precision manufacturing of microlens arrays using an optimum machining process chain

There are still significant challenges in the accurate and uniform manufacturing of microlens arrays (MLAs) with advanced ultra-precision diamond cutting technologies due to increasingly stringent requirements and shape complexity. In this paper, an optimum machining process chain is proposed based on the integration of a micro-abrasive fluid jet polishing (MAFJP) process to improve the machining quality by single point diamond turning (SPDT). The MLAs were first machined and compensated by SPDT until the maximum possible surface quality was obtained. The MAFJP was used to correct the surface form error and reduce the nonuniformity for each lens. The polishing characterization was analyzed based on the computational fluid dynamics (CFD) method to enhance the polishing efficiency. To better polish the freeform surface, two-step tool path generation using a regional adaptive path and a raster and cross path was employed. Moreover, the compensation error map was also investigated by revealing the relationship between the material removal mechanism and the surface curvature and polishing parameters. A series of experiments were conducted to prove the reliability and capability of the proposed method. The results indicate that the two integrated machining processes are capable of improving the surface form accuracy with a decrease in PV value from 1.67 µm to 0.56 µm and also elimination of the nonuniform surface error for the lenses.

High diopter spectacle using a flexible Fresnel lens with a combination of grooves

In this study, the Fresnel lens was investigated as a potential candidate for vision correction in patients with myopia. A few previous studies have suggested this idea; however, Fresnel lenses are limited by their aesthetics and quality. Therefore, we designed a combination of Fresnel lens grooves with a constant height and pitch of 13 µm and 0.1 mm, respectively, to overcome the limitations caused by ultra-precision machining with a tool nose radius of 30 µm. A thin replicated Fresnel lens with a power of -5 diopter was procured and applied directly as spectacles that are unattached to the normal lens. The optical performance and image quality of the Fresnel lens were compared with those of a conventional lens possessing the same power in both near and far vision. These results extend the applicability for the use of Fresnel lenses as vision-correcting ophthalmological lenses and imaging systems.

Development of computer-controlled atmospheric pressure plasma structuring for 2D/3D pattern on fused silica

Fused silica with structured and continuous patterns is increasingly demanded in advanced imaging and illumination fields because of its excellent properties and functional performance. Atmospheric pressure plasma, based on pure chemical etching under atmospheric pressure, is developed as a promising fabrication technique for fused silica due to its deterministic high material removal rate, controllable removal imprint and no mechanical load. The stable and controllable Gaussian-shape removal function makes computer-controlled plasma tool potential to generate complex structures with high accuracy, efficiency and flexibility. In the paper, computer-controlled atmospheric pressure plasma structuring (APPS) is proposed to fabricate 2D/3D patterns on fused silica optics. The capacitively coupled APPS system with a double-layer plasma torch and its discharge characteristics are firstly developed. By means of multi-physics simulation and process investigation, the stable and controllable Gaussian-shape removal function can be achieved. Two different structuring modes, including discrete and continuous APPS, are explored for 2D/3D patterns. A series of structuring experiments show that different kinds of 2D patterns (including square lens array, hexagon lens array and groove array) as well as complex 3D phase plate patterns have been successfully fabricated, which validates the effectiveness of the proposed APPS of 2D/3D patterns on fused silica optics.

Imaging performance of an ultra-precision machining-based Fresnel lens in ophthalmic devices

This study aims to resolve the trade-off between the constraints and capabilities of ultra-precision machining to achieve ophthalmic Fresnel lenses. A general Fresnel lens pattern has a narrow variable pitch and curved grooves. However, we obviate the limitations of the tool nose radius constraint and the long tool path via ultra-precision machining of the modified Fresnel lens, ensuring a constant pitch of 0.1 mm and varying the height of straight grooves from 0 to 11 µm. Photorealistic raytracing visualization and MTF simulation verified the compatibility of the lens pattern with human perception sensitivity. Copper-coated mold was fabricated using a diamond tool with a tool nose radius of 5 µm. The replicated flexible Fresnel lens demonstrated a relative MTF imaging performance of 89.1% and was attached to the goggles for the qualitative assessment. The proposed Fresnel lens design and fabrication approach can be extended to applications in the visual and infrared ranges as well.

Determination of Selected Texture Features on a Single-Layer Grinding Wheel Active Surface for Tracking Their Changes as a Result of Wear

Measurements of the active surface microgeometry of the grinding wheel by contact and optical methods are commonly used to obtain a cloud of points representing the surface of the examined tool. Parameters that can be determined on the basis of the above-mentioned measurements can be universal parameters, which are commonly used to assess the geometric structure of a surface or parameters taking into account specific properties of the grinding wheel active surface (GWAS) structure. This article proposes a methodology for determining the average level of binder, which allows the definition the cut-off level required to separate from the measurement data: (i) the areas representing grains, (ii) the areas of gumming up of the grinding wheel, and (iii) deep cavities in approximately the same places on the investigated grinding wheel, regardless of the degree of its wear. This, in turn, allows one to track changes in characteristic parameters computed from measurements of texture in the above-mentioned areas due to different GWAS wear processes. The research was based on the analysis of data obtained from measurements of single-layer grinding wheels using the replica technique. The adopted measurement methodology enables measurement of approximately the same (94% coverage) areas of the GWAS at four stages of grinding wheel operation. Errors that were computed related to the determination of the volume of abrasive on the GWAS at various stages of wear using the developed methodology were lower, on average, by 48% compared to the automatic recognition of islands made with a commercial software.

Modeling and Evaluation of Energy Efficiency of New Hybrid Turning-Burnishing Process in Terms of Surface Properties

The combination of the turning and burnishing process is an efficient approach to improve machined quality and productivity. This paper aims to optimize energy efficiency (EF), improved hardness ratio (IHR), and decreased roughness ratio (DRR) of a new hybrid turning-burnishing process. The machining parameters are the feed rate (f), turning speed (v), depth of cut (a), burnishing pressure (p), and the diameter of the compressing ball (d). A new turning-burnishing tool using compressed air has been designed and fabricated. A set of experiments for Aluminum Alloy 5083 were performed using the Taguchi method. The weightage principal component analysis (WPCA) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) were applied to obtain the weight values and optimal outcomes. The results indicated that optimum values of the depth of cut, pressure, diameter, feed rate, and speed are 1.00 mm, 0.4 MPa, 16.00 mm, 0.084 mm/rev, and 120 m/min, respectively. The improvements in the EF and IHR are by 20.75% and 8.23% respectively, while the DDR is decreased by 19.05%, as compared to common values.

Optimization and Analysis of Surface Roughness, Flank Wear and 5 Different Sensorial Data via Tool Condition Monitoring System in Turning of AISI 5140

Optimization of tool life is required to tune the machining parameters and achieve the desired surface roughness of the machined components in a wide range of engineering applications. There are many machining input variables which can influence surface roughness and tool life during any machining process, such as cutting speed, feed rate and depth of cut. These parameters can be optimized to reduce surface roughness and increase tool life. The present study investigates the optimization of five different sensorial criteria, additional to tool wear (V_B) and surface roughness (Ra), via the Tool Condition Monitoring System (TCMS) for the first time in the open literature. Based on the Taguchi L₉ orthogonal design principle, the basic machining parameters cutting speed (v_c), feed rate (f) and depth of cut (a_p) were adopted for the turning of AISI 5140 steel. For this purpose, an optimization approach was used implementing five different sensors, namely dynamometer, vibration, AE (Acoustic Emission), temperature and motor current sensors, to a lathe. In this context, V_B, Ra and sensorial data were evaluated to observe the effects of machining parameters. After that, an RSM (Response Surface Methodology)-based optimization approach was applied to the measured variables. Cutting force (97.8%) represented the most reliable sensor data, followed by the AE (95.7%), temperature (92.9%), vibration (81.3%) and current (74.6%) sensors, respectively. RSM provided the optimum cutting conditions (at v_c = 150 m/min, f = 0.09 mm/rev, a_p = 1 mm) to obtain the best results for V_B, Ra and the sensorial data, with a high success rate (82.5%).

Chromatically multi-focal optics based on micro-lens array design

CCD arrays encode color information via uniformly distributed red, green and blue pixels. Therefore, even a perfectly achromatic system projecting an image onto a CCD plane cannot possibly associate a single object point with the 3 or more discrete pixels encoding color content. Here, we propose and demonstrate a micro-lens array (MLA) design that simultaneously corrects chromatic aberrations and separates color channels to spatially distinct pixels. Starting from a commercially available aspheric condenser lens, methods to design and assess the performance of a few microns deep MLA etched on the convex optical surface are detailed. Actual fabrication is carried out by fluid jet polishing, with an optical form deviation of 0.24 µm rms. Finally, the MLA is assessed with a narrowly collimated beam containing two wavelengths, which produces distinct spots of diameter 10-15 µm as expected.

Development of self-tuned diamond milling system for fabricating infrared micro-optics arrays with enhanced surface uniformity and machining efficiency

Infrared micro-optics arrays (MOAs) featuring large numbers of micro-freeform lenslet are required increasingly in advanced infrared optical systems. Ultra-precision diamond cutting technologies have been widely used to fabricate MOAs with high form accuracy. However, the existing technologies can easily cause the non-uniformly fractured surface of infrared MOAs, due to the inherent low fracture toughness and high anisotropy of infrared materials as well as the time-varying chip thickness induced by ever-changing height and slope of the desired MOAs. In this study, a novel self-tuned diamond milling (STDM) system is proposed to achieve the ductile cutting of infrared MOAs with enhanced the surface uniformity and machining efficiency, and the corresponding toolpath planning algorithm is developed. In STDM system, a dual-axial fast servo motion platform is integrated into a raster milling system to self-adaptively match the maximum chip thickness for each tool rotational cycle with the critical depth of cut of the infrared material according to the local surface topography, thereby obtaining crack-free lenslet with high surface uniformity. Practically, micro-aspheric MOAs free from fractures are successfully machined on single-crystal silicon, a typical infrared material, to validate the proposed cutting concept. Compared with the conventional diamond milling, the proposed STDM is demonstrated to be able to avoid the non-uniform fractures without needing to reduce feed rate, and a smaller surface roughness of 4 nm and nearly double machining efficiency are achieved.

Cutting-based single atomic layer removal mechanism of monocrystalline copper: edge radius effect

The ultimate objective of mechanical cutting is to down minimum chip thickness to single atomic layer. In this study, the cutting-based single atomic layer removal mechanism on monocrystalline copper is investigated by a series of molecular dynamics analysis. The research findings report that when cutting depth decreases to atomic scale, minimum chip thickness could be down to single atomic layer by mechanical cutting using rounded edge tool. The material removal behaviour during cutting-based single atomic layer removal exhibits four characteristics, including chip formation by shearing-stress driven dislocation motion, elastic deformation on the processed surface, atomic sizing effect, and cutting-edge radius effect. Based on this understanding, a new cutting model is proposed to study the material removal behaviour in cutting-based single atomic layer removal process, significantly different from those for nanocutting and conventional cutting. The outcomes provide theoretical support for the research and development of the atomic and close-to-atomic scale manufacturing technology.

Simulation and Experimental Study on the Surface Generation Mechanism of Cu Alloys in Ultra-Precision Diamond Turning

The surface generation mechanism of the Cu alloys in ultra-precision diamond turning is investigated by both simulation and experimental methods, where the effects of the cutting parameters on the surface characteristics are explored, including the workpiece spindle speed, the cutting depth, the feed rate and the nose radius of the diamond tool. To verify the built model, the cutting experiments are conducted at selected parameters, where the causes of the error between the simulation and the machining results are analyzed, including the effects of the materials microstructure and the diamond tool wear. In addition, the nanometric surface characteristics of the Cu alloys after the diamond turning are identified, including the finer scratching grooves caused by the tool wear, the formation of the surface burs and the adhesion of graphite. The results show that the built model can be basically used to predict the surface topography for the selection of the appropriate machining parameters in the ultra-precision diamond turning process.