Diamond-turning Process Research Articles

Monitoring of machining process anomalies in diamond turning of Ti6Al4V alloy using transfer learning-based algorithms

In-process monitoring of anomalies in Ultra Precision Machining (UPM) can improve and ensure product quality and lower manufacturing costs, which is essentially important for industrial-scale ultraprecision manufacturing. UPM is found to be extremely sensitive towards minute instabilities; if such nascent anomalies are not detected can cause irrecoverable defects. However, the classification of the diamond-turning process through conventional monitoring techniques is challenging. This study explores the use of spectrogram-based deep learning to enable real-time, intelligent process monitoring in UPM. The vibrational signals obtained from machining are transformed into log-spectrogram images. These images obtained during machining allow the rendering of more accurate and richer features of signals, as most of the time domain signal obtained in UPM is susceptible to noise and exhibits several non-linearities. The current approach also uses Transfer Learning (TL) to address the feature selection problem. TL is adopted by using the deep learning (DL) models, which have already been developed for classifying different Images. DL pre-trained networks, including VGG19, ResNet50 and Densenet201, are studied for classifying the anomalies. These TL models are applied to the spectrogram images for the classification of normal and abnormal machining in UPM. Among the TL models, the VGG19 model yields the highest classification accuracy at 90%, which demonstrates the potential feasibility of the TL for monitoring process anomalies in UPM.

Turning Parameters Optimization for Diffraction Effect Suppression of Diamond-Turned Surface Combining Surface Micro-Topography Model and Scattering Theory

Abstract The diamond-turning process is a mean optical surface generation technique with high figure accuracy and surface finish. The diamond-turned surface has a significant diffraction effect introduced by the tool marks remaining on the surface, which heavily degrade the optical performance in the visible wavelength spectrum. The traditional approach that was used to eliminate this effect was polishing. In this paper, we present a method to find turning parameters that can generate an optical surface without diffraction effect directly by coupling a surface micro-topography model of a turned surface via the scattering theory. The surface micro-topography model of the turned surface reveals the relationship between tool marks and the diamond-turning parameters (DTPs). The scattering theory reveals the relationship between diffraction intensity distributions (DIDs) and surface micro-topography of the turned surface. Therefore, we obtained the relationship between DIDs and DTPs. The diffraction effect is considered to be eliminated when the first-order diffraction intensity is less than 0.01% of incidence intensity. The criterion of turning parameters for diffraction elimination is then obtained. Finally, turning experiments are performed to confirm the effectiveness of this method, and the diffraction-free surface finish is achieved.

Diamond turning of micro-lens array on the roller featuring high aspect ratio

Slow slide servo (S3)-assisted diamond turning combining roll-to-roll replication has been extensively regarded as a very promising technique for the generation of micro-lens arrays (MLAs) for optical applications. However, to avoid undesired interference between the flank surface of the tool and the machined structure, the aspect ratio (AR) (ratio of height to aperture) of machined MLAs is usually restricted to be very small as for the diamond-turning process. To generate MLAs with high AR on the roller, a new method is proposed to increase the practical clearance angle during turning by lowering the position of the cutting tool tip in vertical direction. The optimal tool path determination algorithm is developed with full consideration of tool radius as well as tool position. Effectiveness of the proposed method is demonstrated well experimentally through turning a MLA with AR of 0.158 (79-μm height to 500-μm aperture).

Description and reimplementation of real freeform surfaces

Freeform surfaces are becoming an increasingly exciting opportunity in optical design, in particular when correcting systems with off-axis geometries. Nevertheless, especially when coming to commercial use, the challenges for manufacturing are difficult to handle. The optical quality of a system is perturbed by typical deformations, such as localized figure errors and regular mid-spatial-frequency ripples, that come from the diamond-turning process. In this proposal, we investigated a workflow for analyzing the impact of real optical surfaces on the optical performance for even complex systems. Based on a simple and robust description, the surface is implemented back into the design. While the more localized deviations are analytically described by radial basis functions, the residual ripple structures are covered by a new approach based on the power spectral density. The reimport of optical surfaces back into the design software allows simple estimations for the requirements on manufacturing and the analysis of the realistic impact on system performance.

Holographic projection based on diamond-turned diffractive optical elements

We introduce an approach to generate holographic data for diffractive optical elements fabricated by means of a diamond-turning process. The aim is to project a predefined intensity distribution in the far-field domain of the corresponding diffractive surface. The method takes into consideration typical constraints that result from the fabrication process, such as the spiral path of the turning tool and the fact that only the phase distribution of the incident light can be manipulated.

Null test of an off-axis parabolic mirror I Configuration with spherical reference wave and flat return surface

We demonstrate the precise figure measurement of a one-inch (25.4 mm) diamond-turned 90 masculine off-axis commercial-quality parabolic mirror. The test is carried out with a phase-shifting Fizeau interferometer fitted with a spherical reference surface, auxiliary components and a flat return mirror. We present a detailed and systematic appraisal of the necessary steps for alignment and calibration of the instrument and the alignment of the parabolic mirror. Alignment errors and interferometric sensitivity variations are characterised and corrected, and the results give some insight into the diamond-turning process.

Broadband antireflective surface-relief structure for THz optics

The requirements for a broadband antireflective structure in the THz spectral region are derived. Optimized structural parameters for a surface-relief grating adapted to the spectrum of an intended THz pulse are deduced. The effect of a structure fabricated into Topas((R)) by a single-point diamond-turning process is demonstrated.

Stability of Precision Diamond Turning Processes That Use Round Nosed Tools*

A system of retarded differential equations model is developed for a diamond-turning process with chip area cut by a round nosed tool. The results are presented in the form of stability charts that are found to be perturbed forms of those obtained for orthogonal cutting tools.

Scanning tunneling microscopy applied to optical surfaces

The technique of scanning tunneling microscopy has been applied to topographic mapping of two optical surfaces: a ruled grating replica and a diamond-turned gold mirror. We have demonstrated the ability of the scanning tunneling microscope to measure surface topography of a ruled-grating replica over an area of 2 microm x 2 microm. Furthermore, surface structure on a diamond-turned gold mirror was observed that could not be detected by any other type of surface-sensitive microscope. These measurements yield information necessary for gaining a complete understanding of the diamond-turning process.

Machinability Studies of Infrared Window Materials and Metals

Diamond machining of materials for optical applications is becoming an important fabrication process. This report describes current development work in material-removal technology to better understand the mechanics of the diamond-turning process, its limitations, and applications. The technique is presently limited to a select group of metals, most of which are of a face-center-cubic crystal structure. Machinability studies were done which were designed to better understand diamond compatibility and thus expand the range of applicable materials. Nonconventional methods such as ultrasonic tool stimulation were investigated. Work done to determine the machinability of infrared window materials indicates that this is a viable fabrication technique for many materials, although additional effort is needed to optimize the process for particular materials.

Applications of Diamond Turning to Infrared Optical Systems

Diamond turning is immediately applicable to the fabrication of infrared optical components because presently available machines can meet the reduced absolute accuracies required at 10 micrometers. An initial survey of infrared sensor programs at the Honeywell Radiation Center has been conducted to predict the near term and future demand for diamond-turned optical components. Not only does the fabrication process promise significant cost savings as compared to conventional lapping and polishing methods, but, as in the case of aspheric lenses, wider applications are also sought to reduce weight and space requirements. In addition, broader usage of diamond-turned aspherics reduces total parts count and assembly and alignment time, provided proper tools and test equipment are employed. The potential cost savings of diamond-turned vs conventionally fabricated optics are summarized for contracts at the Radiation Center. The savings were calculated by subtracting the difference in fabrication costs and multiplying by the number of items expected to be produced into the mid-1980s. Technical fallout potential of the diamond-turning process is also noted in the apparent ability of a coated sample to pass a 24-hour salt fog test.

Tool feed influence on the machinability of CO_2 laser optics

Influence of tool feed on reflectivity of diamond-machined surfaces was evaluated using materials (gold, silver, and copper) from which CO(2) laser optics are primarily produced. Fifteen specimens were machined by holding all machining parameters constant, except tool feed. Tool feed was allowed to vary by controlled amounts from one evaluation zone (or part) to another. Past experience has verified that the quality of a diamond-machined surface is not a function of the cutting velocity; therefore, this experiment was conducted on the basis that a variation in cutting velocity was not an influencing factor on the diamondturning process. Inspection results of the specimens indicated that tool feeds significantly higher than 5.1 micro/rev (200 microin./rev) produced detrimental effects on the machined surfaces. In some cases, at feeds as high as 13 microm/rev (500 microin./rev), visible scoring was evident. Those surfaces produced with tool feeds less than 5.1 microm/rev had little difference in reflectivity. Measurements indicat d that their reflectivity existed in a range from 96.7% to 99.3% at 10.6 microm.