Ultra-precision Machining and its Applications in Precision Optics
超精密加工技術及其在光學產品中的應用
Originated from the US Government weapons laboratories in the early 1970’s, ultra-precision machining with single-point diamond tool was traditionally used in the manufacture of high-quality components and parts for systems in a number of advanced science, defence, aerospace and technology applications. Since 1980’s, the technique was applied for the manufacture of a variety of optical components for its high precision, versatility and lower overall manufacturing cost.
Ultra-precision machining based on single-point diamond turning (SPDT) is a machining process making use of a monocrystal diamond cutting tool which possesses nanometric edge sharpness, form reproducibility and wear resistance. The depth of cut being used is in the order of a micrometer or less. The process is capable of producing components with micrometer to sub-micrometer form accuracy and surface roughness in nanometre range. Non-ferrous work materials like aluminium, copper alloy, silver, gold, electroless nickel and PMMA plastics can be directly machined to optical quality without the need for subsequent post-polishing. Some “infrared materials” such as silicon and germanium can also be finished to a surface roughness of a few tens of nanometers. The superior surface finish and form accuracy of ultra-precision diamond turning allow the technology to be widely adopted for the manufacture of a variety of precision mechanical and optical parts. Examples are now seen in the manufacturing of optical components like photoconductor drums in copier, scanner mirrors, compact camera lenses, contact lenses, magnifiers, telescopes, mould inserts for injection-moulding plastic camera lenses and substrates for memory disks etc. More recent applications are for the manufacture of CCD camera lenses for uses in tele-conferencing, surveillance, security, machine vision, video camera and digital cameras. A significant increase in demand has been found on PC camera used for multimedia applications. On the other hand, with the application of aspheric and diffractive optics elements, the number of spherical lenses in complex optical systems can be significantly reduced. However, aspherics and diffractive optics elements are extremely difficult to be made to high precision in any materials using conventional machining technology. Ultra-precision machining based on SPDT has become an economically viable solution of making aspherics and diffractive optics. Numerical control with the aid of tool path generation software allows direct lathing of aspheric and diffractive optical surfaces to the minute details.
In SPDT, the superior surface finish is produced as a result of extremely high fidelity in transferring the machine motions and the tool profile into a workpiece. The excellent form accuracy is the result of the ability to maintain the machine spindle to operate at sub-micron rotational accuracy and to control the table motions at nanometric resolutions. Although ultra-precision machines develop from the conventional machine tool design concepts, special attention is given to the precision and integration of the system components to provide a very stiff machine structure and stable (thermally and dynamically) machine bases with vibration isolation and constant temperature control facilities. An air-bearing spindle for minimising the rotational run-out and vibration, a vacuum chuck for minimising the workpiece distortion, hydrostatic slides for reducing the static friction between bearing surfaces and nanometric precision position sensing and control for the machine motions are essential features that allow the machining of optical quality surface with sub-micrometer form accuracy and nanometric surface roughness. The success of ultra-precision machining technology is not only relying on the sophisticated machine design and control but also depends on its associated metrology. For nanometre level surface roughness and sub-micrometre level form accuracy, extremely high precision metrology instruments are needed to inspect the quality of the machined surfaces.
In the past, there were only a few companies and institutions in the Far East which possess the ultra-precision machining and nano-metrology facilities for the manufacture of high quality precision lenses and components. Most of the local manufacturers were forced to source their components from overseas markets like USA, UK, Taiwan and Japan. The long processing time and the additional delivery cost incurred on the imported lenses and precision components increase the manufacturing costs and products. In 1995, the Industry and Technology Development Council (ITDC) the Hong Kong Special Administrative Region HKSAR (formerly the Hong Kong Government) approved a project with a grant of a HK$9.5 million to establish an Ultra-precision Machining Centre (UMC) in The Hong Kong Polytechnic University. The Centre is the first of its kind in Hong Kong engaging in the promotion and application of ultra-precision machining and nano-metrology technologies for precision mould and optics industries. It is jointly developed by the Department Manufacturing Engineering of The Hong Kong Polytechnic University and Hong Kong Productivity Council (HKPC) with support from the Industry Department of HKSAR.
The Centre is well equipped with state-of-the-art facilities in ultra-precision machining and nano-metrology. These include a Nanoform 300 two-axis CNC/interferometer controlled ultra-precision machine and an Optoform 30 two-axis CNC ultra-precision lathe from Taylor Hobson Pneumo Co. The machines are capable of performing single-point diamond turning on a wide variety of non-ferrous metals, polymers and crystals. The Centre is also equipped with advanced metrology equipment for form and surface roughness measurement. These include a Zygo Interferometric Non-contact Profiler System, a WYKO TOPO-3D Non-contact Microsurface Measurement System and a Form Talysurf Laser Interformetric Form/Surface Texture Measuring System. With these state-of-the-art facilities, a whole range of ultra-precision machining and measurement activities and technical support can be offered to local precision mould makers, optics and optical goods designers and manufacturers, manufacturers of computer peripheral products, etc.
Apart from providing vital industrial support to local industries, the UMC is also actively engaged in the advancement of the ultra-precision machining and nano-metrology technologies. A number of research projects has been launched in various aspects of ultra-precision machining and surface metrology. These include the investigation of cutting mechanics and chip formation in SPDT; brittle material machining; modelling and simulation of nano-surface generation in ultra-precision machining; the development of new methodologies for surface characterisation; the investigation of the effect of crystallographic orientation of work materials in ultra-precision machining; virtual machining and inspection of precision optical products, etc.
With Hong Kong industry undergoing a transformation from low-end to up-market high technology products, higher precision and accuracy in manufacturing are becoming more important to enable a wider scope of advanced technology products to be launched. As a result, ultra-precision machining and nano-metrology technologies become an indispensable tool for the design and the manufacturing of these high-value-added products. With the availability of the ultra-precision machining facilities in Hong Kong, local manufacturers no longer exclusively depend on the supply of expensive lenses and related precision components from overseas vendors. This would strengthen the competitive edge of Hong Kong industry in the area of high-tech and high-value-added products. The establishment of the Ultra-precision machining centre plays an increasingly important role to assist local industry with the provision of a full range of ultra-precision facilities and technical services for the rapid development and testing of high-quality ultra-precision products.
Honorary Chairman
Ultra-precision Machining
