Tsunemoto Kuriyagawa

Micro grooving on single-crystal germanium for infrared Fresnel lenses

Single-crystal germanium is an excellent optical material in the infrared wavelength range. The development of germanium Fresnel lenses not only improves the optical imaging quality but also enables the miniaturization of optical systems. In the present work, we developed a ductile-mode micro grooving process for fabricating Fresnel lenses on germanium. We used a sharply pointed diamond tool to generate the micro Fresnel structures under three-axis ultraprecision numerical control. By adopting a small angle between the cutting edge and the tangent of the objective surface, this method enables the uniform thinning of the undeformed chip thickness to the nanometric range, and thus provides complete ductile regime machining of brittle materials. Under the present conditions, a Fresnel lens which has a form error of 0.5 µm and surface roughness of 20–50 nm Ry (peak-to-valley) was fabricated successfully during a single tool pass.

Effects of Ultrasonic Vibration in Truing and Dressing of CBN Grinding Wheel Used for Internal Grinding of Small Holes

This paper describes an experimental investigation of the effects of ultrasonic vibration in the truing and dressing of the small CBN grinding wheel used for the internal ultrasonic grinding of small holes. In the precision machining of small holes measuring several millimeters in diameter, improvement in the wheel truing accuracy is significantly levels off when using a single diamond dresser or a rotary GC wheel dresser. In the present work, a new truing and dressing technique was proposed, by which the grinding wheel is ultrasonically vibrated in its axial direction during the truing operation using a rotary GC cup dresser. In order to validate the proposed new technique, experiments were carried out. During experimental operations, the GC cup wheel was traversed along the vitrified CBN grinding wheel axis with an in-feed motion toward the grinding wheel in the wheel radial direction. The influences of the truing parameters on the truing force, the run-out of grinding wheel and the grinding wheel surface properties were investigated. As a result, it was found that applying ultrasonic vibration to the grinding wheel decreased the truing force by more than 22%, and the run-out of grinding wheel decreased from an original value of 150µm to a final one of less than 0.8µm, while that obtained without ultrasonic vibration was more than 1.1µm. As well, better surface properties of the grinding wheel were obtained by the application of ultrasonic vibration.

Determination of an Optimum Geometrical Arrangement of Workpiece in the Ultrasonic Elliptic-Vibration Shoe Centerless Grinding

This paper clarifies the influence of the geometrical arrangement of the workpiece on workpiece roundness in the ultrasonic elliptic-vibration shoe centerless grinding, and determines an optimum geometrical arrangement for minimizing the roundness error of the workpiece. The influence of the geometrical arrangements ( , , ) of the workpiece on workpiece roundness were investigated by computer simulation involving a cylindrical workpiece of 5 mm in diameter with an initial roundness error of 25 μm. The results indicated that the final roundness error of the workpiece after grinding reaches a minimum at + =7° for various values of . It was found that the smaller the blade angle , the more precise the workpiece in terms of final roundness. Practical grinding operations involving pin shaped workpieces, such as SKH51, 5 mm in diameter and 15 mm in length, were carried out on the experimental apparatus previously developed. The experimental results agreed closely with those obtained by the simulation, showing that the optimum geometrical arrangement of the workpiece can be determined at + =7° and =60°, in which the workpiece roundness was improved from an initial roundness error of 25 μm to the final one of approximately 0.6 μm.

A Novel Form Error Compensation Technique for Tungsten Carbide Mould Insert Machining Utilizing Parallel Grinding Technology

Mould inserts with high form accuracy can be produced with ease using modem grinding technologies. However, several grinding cycles are often required to reduce the form error to an acceptable value, significantly dependent on the tool path compensation technique used. This paper reports on a novel form error compensation technique for tungsten carbide mould insert machining utilizing a parallel grinding method. In this technique, a newly developed program is used to process the profile data measured using a Form Talysurf profilometer, and to further generate the NC tool path for form error compensation. The developed technique focuses on the compensation of form error resulted by two major error sources, wheel radius and waviness errors. Using the developed technique, the initial residual form error upon the completion of primary grinding is minimized. Subsequently, the residual form error is compensated by modifying the NC tool path. With this technique, the speed of convergence of the residual form error has improved markedly. The grinding result shows that, after just one compensation cycle, a form error of approximately 0.3 mu m in PV is achieved.

Ductile Regime Machining of Single-Crystal CaF2 for Aspherical Lenses

Single-crystal calcium fluoride (CaF2) is an important optical material used for dark-field imaging systems and short-wavelength photolithography systems. The fabrication of aspherical lenses using CaF2 effectively eliminates the problems of color aberration and spherical aberration. The present paper describes a ductile regime machining system developed for fabricating aspherical surfaces on CaF2 by single point diamond turning. In this system, the aspherical surface is enveloped by a straight-nosed diamond tool under three-axis simultaneous numerical control. The formation mechanism of the machining form error is analyzed and a rapid tool setting system is developed to aid the improvement of machining accuracy. The experimental results show that the system is effective for high-efficiency manufacturing of ultra-precision CaF2 optics. Introduction Single-crystal CaF2 is an optical material with high permeability over a wide wavelength range, from ultraviolet to infrared. It is used as lens substrate for applications such as dark-field imaging devices, home-use cameras and videos, microscopes, and photolithography systems. CaF2 has unique optical properties including extremely low color aberration. This function is due to the abnormal partial dispersion characteristics of CaF2, by which the degree of dispersion differs according to the light wavelength. For high-quality short wavelength optical systems, CaF2 lenses are indispensable. Up to date, most CaF2 lenses are spherical lenses, which are finished by polishing. However, spherical lenses have the inherent problem of spherical aberration. Although spherical aberration can be avoided by a combination of multiple concave and convex lenses, in order to avoid spherical aberration using a single lens, the lens must be aspherical. The use of CaF2 aspherical lenses solves the problems of both color aberration and spherical aberration simultaneously. Therefore, the development of the fabrication technology for CaF2 aspherical lenses will contribute significantly to the improvement of imaging quality, miniaturization, and the production cost of optical products. A potential method for machining CaF2 aspherical optics would be single-point diamond turning (SPDT). SPDT is capable of ultra-precision machining of ductile materials such as nonferrous metals and plastics. However, CaF2 is a highly brittle material and is extremely sensitive to temperature changes, leading to difficulties in machining. In a previous paper [1], one of the authors (J. Yan) carried out SPDT experiments on CaF2 and found that thermal fracturing occurs during low tool feed wet cutting, which makes ductile machining difficult to achieve. To avoid thermal fracturing, two processing conditions were applicable: dry cutting and high tool feed wet cutting. Since dry cutting may cause rapid tool wear, the method of high tool feed wet cutting will be industrially beneficial. The latter requires that the tool feed rate must be sufficiently high while the undeformed chip thickness is kept extremely small. This paper presents an aspherical surface machining system that can satisfy these demands. The effectiveness of the system will be demonstrated through the fabrication of a CaF2 aspherical lens. Key Engineering Materials Online: 2004-02-15 ISSN: 1662-9795, Vols. 257-258, pp 95-100 doi:10.4028/www.scientific.net/KEM.257-258.95

New Compensation Grinding of Axisymmetric Aspherical Lenses with High NA Value

Aspherical parts are installed in various optical instruments. At present, a higher form accuracy is required for aspherical parts to improve the resolution of the optical instruments. To meet this demand, an arc envelope grinding method has been developed. In the arc envelope grinding process, a spherical shaped grinding wheel is used and the form error of the cross-sectional profile of the grinding wheel is transcribed to the workpiece profile. Therefore, the grinding wheel should be trued previously. However, the form error of the grinding wheel cannot be removed perfectly. To reduce the affect of the form error of the grinding wheel, compensation grinding must be carried out. In this work, the wheel path of a new compensation grinding method is proposed for a high NA value of the workpiece. Tests using the new compensation grinding method demonstrate the reduction in the form error of aspherical parts.

New technologies for aspherical grinding/polishing of micro/meso optics

New technologies for ultra-precision aspherical grinding and polishing of aspherical optical lenses and molding dies are introduced, which are