Ultra precision machining technology of the hottes

Ultra precision machining technology of aspheric optical parts

1 overview

1.1 the role of aspheric optical parts

aspheric optical parts are very important optical parts, commonly used paraboloid mirror, hyperboloid mirror, ellipsoid mirror and so on. Aspheric optical parts can obtain unparalleled good imaging quality of spherical optical parts. In the optical system, it can well correct a variety of aberrations, improve imaging quality, and improve system identification ability. It can replace multiple spherical parts with one or several aspheric parts, so as to simplify the structure of the instrument, reduce costs, and effectively reduce the weight of the instrument

aspheric optical parts are also widely used in military and civil optoelectronic products, such as photographic lenses and viewfinders, television camera tubes, zoom lenses, movie playback lenses, satellite infrared telescopes, video camera lenses, video and recording disc readers, bar code readers, optical fiber connectors for optical fiber communication, medical instruments, etc

1.2 current situation of ultra precision machining technology of aspheric parts abroad

since the 1980s, there have been many new ultra precision machining technologies of aspheric surfaces, mainly including:

computer numerical control single point diamond turning technology, computer numerical control grinding technology, computer numerical control plasma beam forming technology, computer numerical control ultra precision polishing technology, aspheric copy technology, etc, Basically solve the problems existing in the processing of various aspheric mirrors. The first four methods use numerical control technology, which have the characteristics of high machining accuracy and high efficiency, and are suitable for mass production

when machining aspheric parts, factors such as the material, shape, accuracy and diameter of the parts to be machined should be considered. For soft materials such as copper and aluminum, the method of single point diamond cutting (SPDT) can be used for ultra precision machining. For glass or plastic, at present, ultra precision machining of its mold is mainly used first, and then forming method is used to produce aspheric parts. For other brittle materials with high hardness, At present, it is mainly processed by ultra precision grinding, ultra precision grinding, polishing and other methods. In addition, there are special processing technologies for aspheric parts, such as ion beam polishing

many foreign companies have integrated ultra precision turning, grinding, grinding and polishing, and developed ultra precision composite machining systems, such as nanoform300 produced by rank pneumo, nanoform250 developed by cupe, nanocentre developed by Japan's ahn60-3d and ulp-100a (H) have composite machining functions, which can facilitate the processing of aspheric parts more flexibly

1.3 current situation of ultra precision machining technology of aspheric parts in China

China began to study ultra precision machining technology in the early 1980s, which is 20 years behind foreign countries. In recent years, the units that have carried out this work well include Beijing Machine Tool Research Institute, China Aerospace Precision Machinery Research Institute, Harbin Institute of technology, Changchun Institute of Optics and mechanics, Chinese Academy of Sciences, and the Key Laboratory of Applied Optics

in order to better carry out the research on this ultra precision machining technology, the Commission of science, technology and industry for national defense first established the first domestic key laboratory engaged in ultra precision machining technology research at the China Institute of Aeronautical precision machinery in 1995

2 ultra precision machining technology for aspheric parts

Union Carbide Company of the United States successfully developed R ― in 1972 θ Aspheric surface generation processing machine tool. This is a two coordinate CNC lathe with position feedback, which can change the angle of the tool holder guide rail in real time θ And radius r to realize mirror machining of aspheric surface. Machining diameter up to φ 380mm, and the shape accuracy of the processed workpiece is ± 0.63 μ m. The surface roughness is Ra0.025 μ m。

UK rank pneumo company introduced a two axis linkage machining machine tool (msg-325) controlled by laser feedback to the market in 1980. This machine tool can process aspheric metal mirrors with a diameter of 350mm, and the shape accuracy of the workpiece can reach 0..5 μ m. The surface roughness Ra is 0.01-o.025 μ M. Subsequently, asg2500, asg2500t, nanoform300 and other machine tools were introduced. Based on the above machine tools, the company developed nanoform600 in 1990. This machine tool can process aspheric mirrors with a diameter of 600mm, and the shape accuracy of processed workpieces is better than 0.1 μ m. The surface roughness is better than 0.01 μ m。

the ultra precision diamond lathe representing the high level of today's workers is the lodtm developed by Lawrence Livermore (LLNL) laboratory in the United States in 1984. It can process workpieces with a diameter of 2100mm and a weight of 4500kg, and its machining accuracy can reach 0.25 μ m. Surface roughness rao.0076 μ m. The machine tool can process plane, spherical and aspheric springs with small loads. It is mainly used to process parts required for laser nuclear fusion engineering, parts for infrared devices and large celestial reflectors

the large ultra precision diamond right mirror cutting machine developed by the Institute of Precision Engineering (cupe) of the University of Cranfield in the UK can process aspheric mirrors (conical mirrors with a maximum diameter of 1400mm and a maximum length of 600mm) for large X-ray celestial telescopes. The Institute has also successfully developed a diamond cutting machine tool that can be used to machine the inner rotating paraboloid and outer rotating hyperboloid mirrors of x-ray telescopes

ultra precision machining machines developed in Japan are mainly used to process lenses and reflectors required for civil products. At present, machining machines made in Japan include: urg-l00a (H) developed by Toshiba machinery, asp-l15 of bueryue company, ahn10 and ahn30 of Toyota machinery × 25. Ahn60 - 3D aspheric machining machine tool, etc

3 ultra precision grinding technology for aspheric parts

3.1 ultra precision grinding device for aspheric parts

UK rank pneumo company developed improved asg2500, asg2500t, nanoform300 machine tools in 1988. These machine tools can not only be used for cutting, but also can be ground with diamond wheels. They can process aspheric metal mirrors with a diameter of 300mm, and the shape accuracy of the machined workpiece is 0.3-o.16 μ m. The surface roughness reaches ra0.01 μ m。 Recently, the nanoform250 ultra precision machining system has been launched, which is a two axis ultra precision CNC machine tool. On this machine tool, ultra precision turning, ultra dense grinding and ultra precision polishing can be carried out. The most outstanding feature is that it can directly grind hard and brittle optical parts with optical surface quality and surface accuracy that can meet the requirements of the optical system. The machine tool adopts many advanced design ideas of nanoform600 and optoform50. The maximum machining workpiece diameter of the machine tool is 250mm. It makes the maximum machining workpiece diameter of the machine tool reach 450mm through a lifting device. In addition, it can grind non axisymmetric parts by controlling the hydrostatic guide rail (Y axis) in the vertical direction. The resolution of the numerical control system of the machine tool reaches o.001 μ m. The position feedback element adopts a grating with a resolution of 8.6nm or a laser interferometer with a resolution of 1.25nm, and the surface accuracy of the machined workpiece is up to 0.25 μ m. The surface roughness is better than ra0.01 μ m。

nanocentre250 and nanocentre600 are three-axis ultra precision CNC aspheric generating devices, which can meet the requirements of single point and ductile grinding. Through rationalizing the structural design of the machine tool, using high stiffness servo drive system and hydrostatic bearing, the machine tool has a high closed-loop stiffness, and the resolution of X and Z axes is 1.25nm. This machine tool is considered to meet the modern process specifications. The machining machine for aspheric optical parts produced by cupe has a machining diameter of 600mm, and the surface accuracy is better than 0.1 μ m. The surface roughness is better than ra0.01 μ m。 Cupe also researched, designed and produced the world's largest ultra precision large CNC optical parts Grinder "0agm2500" for Kodak, which is mainly used for the processing of hard and brittle materials such as optical glass, and can process and measure 2.5m × 2. There are examples of manufacturing metal matrix composites abroad.5m × O. 61m workpiece, it can process 2m square asymmetric optical mirror, and the shape error of the mirror is only 1 μ m。

ahn60-3d developed by Toyota machine tool is a CNC three-dimensional profile grinding and turning machine. It can grind and turn optical parts with axial symmetrical shape under the control of X, y, and Z axes, and grind and turn non axisymmetric optical parts under the control of X, y, and Z axes. The profile accuracy of the processed workpiece is 0.35unl, and the surface roughness reaches ra0.016 μ m。 In addition, the ulg-100a (H) ultra precision composite machining device developed by Toshiba machinery realizes the cutting and grinding of aspheric lens mold by controlling two axes respectively. The stroke of X axis and Z axis are 150mm and 100mm respectively, and the resolution of position feedback element is 0.01 μ M grating

3.2 ELID mirror grinding technology for aspheric optical parts

Japanese scholar Masahiro Ohmori and others have studied the superhard abrasive wheel since 1987, developed the grinding method using electrolytic in process dressing (ELID), and realized the high-grade mirror grinding and ductile grinding of hard and brittle materials. Now this method has been successfully applied to the ultra precision machining of spherical, aspheric lenses and molds

① ELID mirror grinding principle

elid grinding system includes: metal bond ultra-fine grained super hard abrasive wheel, electrolytic dressing power supply, electrolytic dressing electrode, electrolyte (also used as grinding fluid), electric brush and machine tool equipment. In the grinding process, the grinding wheel is connected with the positive pole of the power supply through the electric brush, the dressing electrode installed on the machine tool is connected with the negative pole of the power supply, and the electrolyte is poured between the grinding wheel and the electrode. In this way, the power supply, the grinding wheel, the electrode, the electrolyte between the grinding wheel and the electrode form a complete electrochemical system

when ELID grinding is used, there are some special requirements for the grinding wheel, power supply and electrolyte used. It is required that the bond of the grinding wheel has good conductivity and electrolysis, and the hydroxide or oxide of the bond element is non-conductive and insoluble in water. The power supply used for ELID grinding can adopt the DC power supply of electrochemical machining or the pulse power supply of various waveforms or DC basic pulse power supply. In ELID grinding process, in addition to being used as grinding fluid, electrolyte also plays a role in reducing the temperature of grinding area and friction. ELID grinding generally adopts water-soluble grinding fluid, which belongs to base bond grinding wheel with high mechanical strength. By setting the appropriate amount of electrolysis, the wear of grinding wheel is small. At the same time, high shape accuracy can be obtained. Using this principle, ultra precision mirror grinding of optical elements of various shapes from plane to aspheric surface can be realized

② ELID mirror grinding experimental system

on the asg-2500t machine tool of rank pneumo company, the taimori ELID system, which is composed of grinding wheel, power supply, electrode, grinding fluid, etc., is equipped with 400 #, 1000 # or 2000 #, and 4000 # (average particle size is about 4) for rough forming, semi finishing and mirror grinding μ m) Or 8000 # (average particle size is about 2 μ m) Cast iron bonded diamond grinding wheel, electrolytic sharpening power supply (ELID power supply), using a DC high-frequency pulse voltage special power supply, the working voltage is 60V, the current is loa. When using the grinding fluid used, it is required to dilute the water-soluble grinding fluids afh-m and CEM by 50 times with pure water

③ ELID mirror grinding experimental method and results

When Machining Aspheric surfaces, through the bowl shaped grinding wheel (325) installed on the workpiece shaft#