Aspherical Optics Parts Ultra-precision Machining Technology

1.1 The role of aspherical optical components

Aspherical optical components are a very important optical component. Commonly used are parabolic mirrors, hyperbolic mirrors, ellipsoidal mirrors, and the like. Aspherical optics can achieve unparalleled good imaging quality of spherical optics. It can well correct many aberrations in optical systems, improve image quality, improve system discrimination ability, and it can use one or several aspherical surfaces. Parts replace multiple spherical parts to simplify the instrument structure, reduce costs and effectively reduce the weight of the instrument.

The application of aspherical optical components in military and civilian optoelectronic products is also extensive, such as in photographic lenses and viewfinders, television camera tubes, zoom lenses, movie playback lenses, satellite infrared telescopes, video recorder lenses, video and audio recording heads. , bar code readers, fiber optic connectors, medical equipment, etc.

1.2 Status of Ultra-Precision Machining Technology for Foreign Aspherical Parts

Since the 1980s, there have been many new kinds of aspheric ultra-precision machining technologies, mainly including: computer numerical control single-point diamond turning technology, computer numerical control grinding technology, computer numerical control ion beam forming technology, computer numerical control ultra-precision polishing technology and aspherical surface Copying technology, etc., these processing methods basically solve the problems in the processing of various aspherical mirrors. The first four methods use numerical control technology. They all have high machining accuracy and high efficiency, and are suitable for mass production.

When aspherical parts are machined, factors such as the material, shape, accuracy, and bore diameter of the parts to be machined are taken into account. For soft materials such as copper and aluminum, superfinishing can be performed using single point diamond cutting (SPDT). Or plastics, etc., currently use the first ultra-precision processing of its mold, and then use the forming method to produce aspherical parts. For other high-hardness brittle materials, it is mainly through ultra-precision grinding and ultra-precision grinding, polishing and other methods. Processed, additional. There are also special processing techniques for aspherical parts such as ion beam polishing.

Many foreign companies have integrated ultra-precision turning, grinding, grinding and polishing processes, and developed ultra-precision composite processing systems, such as Nanoform300, Nanoform250 manufactured by Rank Pneumo, Nanocentre developed by CUPE, AHN60-3D of Japan, ULP-100A(H) has a composite processing function, which can be more flexible in the processing of aspherical parts.

1.3 Status Quo of Ultra-precision Machining Technology for Non-spherical Parts in China

China began to research on ultra-precision machining technology since the early 1980s, which is 20 years behind that of foreign countries. In recent years, the units that have performed well in this work include the Beijing Institute of Machine Tools, the China Aviation Precision Machinery Research Institute, Harbin Institute of Technology, and the Changchun Optics Laboratory Applied Optics Laboratory of the Chinese Academy of Sciences.

In order to better carry out research on this ultra-precision machining technology, the National Commission for Science, Technology and Industry for Defence first established China's first key laboratory for research on ultra-precision machining technology at the China Aviation Precision Machinery Research Institute in 1995.

2. Aspherical Parts Ultra-precision Machining Technology

In 1972, Union Carbide Corporation of the United States successfully developed the R-θ aspherical ingenuity machine tool. This is a two-axis CNC lathe with position feedback, which can change the rotation angle θ and radius R of the tool holder guide in real time to realize aspherical mirror processing. Processing diameter of φ380mm, the accuracy of the shape of the workpiece is ±O. 63 μm, surface roughness Ra 0.025 μm.

In 1980, Moore first developed an M-18AG aspherical machine tool that uses three coordinate controls. This machine can machine a variety of aspherical metal mirrors with a diameter of 356mm.

Rank Pneumo of the United Kingdom introduced to the market in 1980 the use of laser feedback control of the two-axis linkage processing machine (MSG-325), the machine can process a diameter of 350mm aspherical metal mirror, processing workpiece shape accuracy of 0.25- 0.5μm, surface roughness Ra 0.01-O. Between 025μm. Subsequently introduced ASG2500, ASG2500T, Nanoform300 and other machine tools. The company also developed Nanoform600 in 1990 based on the above-mentioned machine tools. The machine can process aspherical mirrors with a diameter of 600mm, and the shape accuracy of machined workpieces is better than 0. .1μm, the surface roughness is better than 0.01μm.

On behalf of today's high-level ultra-precision diamond lathe is the United States Lawrence. Developed in 1984 by the Livermore (LLNL) laboratory, LODTM can process workpieces up to 2100mm in diameter and weighing up to 4500kg with processing accuracy of 0.25μm and surface roughness RaO. 0076μm, the machine can process plane, spherical and aspherical, mainly used for processing laser fusion engineering parts, infrared device parts and large astronomical reflectors.

The large ultra-precision diamond right mirror-cutting machine tool developed by the Institute of Precision Engineering (CUPE), Cranfield University, UK, can process aspherical mirrors for large-scale X-ray astronomical telescopes (cone mirrors with a maximum diameter of 1400 mm and a maximum length of 600 mm). The Institute has also successfully developed a diamond cutting machine that can be machined for use on X-ray telescopes with inboard rotary paraboloids and outboard rotary hyperboloid mirrors.

Ultra-precision machining tools developed in Japan are mainly used to process lenses and reflectors for consumer products. At present, the Japanese-manufactured machining tools include: ULG-l00A(H) developed by Toshiba Machines, ASP-L15 and Toyota Works. AHN10, AHN30×25, AHN60―3D aspherical processing machine tools, etc.

3. Ultra-precision Grinding Technology for Aspheric Parts

3.1 Non-spherical parts super-precision grinding device

British Rank Pneumo Company developed the improved ASG2500, ASG2500T, and Nanoform300 machine tools in 1988. These machines not only can be machined, but also can be ground with diamond grinding wheels. They can process aspherical metal mirrors with a diameter of 300mm to machine workpieces. The accuracy of the shape is 0.3-O. 16μm, surface roughness Ra 0.01μm. Recently introduced the Nanoform250 ultra-precision machining system, which is a two-axis ultra-precision CNC machine that can perform both ultra-precision turning and super-abrasive grinding. Ultra-precision polishing is also possible. The most prominent feature is the ability to directly grind hard and brittle material optics with optical surface quality and surface accuracy that meet optical system requirements. The machine uses a number of advanced Nanoform600, Optoform50 design ideas, the largest machine tool workpiece diameter up to 250mm, it through a lifting device to make the machine's largest workpiece diameter 450mm, in addition by controlling the vertical hydrostatic guide (Y axis ) Can also grind non-axisymmetric parts. The resolution of the CNC system is up to O. 001μm, the position feedback element uses a grating with a resolution of 8.6nm or a laser interferometer with a resolution of 1.25nm. The precision of the surface profile of the machined workpiece reaches 0.25μm, and the surface roughness is better than that of Ra0.01μm.

Nanocentre250, Nanocentre600 is a three-axis ultra-precision CNC aspherical surface forming device, which can meet the requirements of single point and ductile grinding in two aspects, through the rationalization of machine tool structure design, the use of high rigidity servo drive system and hydrostatic bearing The machine has a high closed-loop stiffness, resolution of 1.25nm for the x and z axes, and this machine is considered to comply with modern process specifications. Nanocentre aspheric optical parts processing machine produced by CUPE, processing diameter up to 600mm. The surface accuracy is better than 0.1μm, and the surface roughness is better than Ra0.01μm. CUPE also researched, designed and produced the world's largest ultra-precision large-scale CNC optical parts grinder “0AGM2500” for Kodak Company. The machine is mainly used for the processing of hard and brittle materials such as optical glass, which can process and measure 2.5m× 2.5m×O. With a 61m workpiece, it can machine a 2m square asymmetric optical mirror with a mirror shape error of only 1μm.

The AHN60-3D developed by Japan Toyoko Machine Tool is a CNC three-dimensional truncated grinding and turning machine that can grind and turn axially symmetrically shaped optical components under X, Y, and Z axis control. Y-axis and Z-axis control and grinding of non-axisymmetric optical parts under the control of two half-axes. The truncation accuracy of the machined workpiece is 0.35unl, and the surface roughness is Ra 0.016μm. In addition, ULG-100A(H) ultra-precision composite processing device developed by Toshiba Machine Co., Ltd. realizes the cutting and grinding of aspherical lens molds by separately controlling two axes. The strokes of the X-axis and Z-axis are respectively 150mm and 100mm, the position feedback element is a grating with a resolution of 0.01 μm.

3.2 ELID mirror grinding technology of aspheric optical components

The Japanese scholar Omori et al. studied the superabrasive grinding wheel in 1987 and developed a grinding method using electrolysis In Process Dressing (ELID) to achieve high grade mirror grinding and ductile grinding of hard and brittle materials. This method has been successfully applied to the ultra-precision machining of spherical, aspherical lenses and molds.

1 ELID mirror grinding principle

ELID grinding systems include: metal bond ultra-fine grain superabrasive grinding wheel, electrolytic dressing power supply, electrolytic trimming electrode, electrolyte (also known as grinding fluid), electric brush and machine tool equipment. During the grinding process, the grinding wheel is connected to the positive pole of the power supply through the connecting brush. The dressing electrode installed on the machine tool is connected to the negative pole of the power supply, and the electrolyte is poured between the grinding wheel and the electrode. In this way, the power source, grinding wheel, electrode and grinding wheel The electrolyte between the electrodes forms a complete electrochemical system.

When grinding with ELID, there are special requirements for the grinding wheel, power supply and electrolyte used. It is required that the bonding agent of the grinding wheel has good conductivity and electrolysis, the hydroxide or oxide of the bonding agent element is non-conducting, and it is insoluble in water. The power source used for ELID grinding can be electrolytic power-processed DC power supply or various Waveform pulse power or DC base pulse power. In the ELID grinding process, in addition to the grinding fluid, the electrolyte plays a role in lowering the temperature in the grinding zone and reducing the friction. ELID grinding generally uses water-soluble grinding fluid, and is entirely based on the mechanical properties of the bonding agent grinding wheel. The strength is high. By setting a suitable amount of electrolysis, the wear of the grinding wheel is small. At the same time, high shape accuracy can be achieved. Using this principle, ultra-precision mirror grinding of various shapes of optical elements from flat to aspherical surfaces can be realized.

2ELID mirror grinding experimental system

On Rank Pneumo's ASG-2500T machine tool, it is equipped with a grinding wheel, power supply, electrode, grinding fluid, etc. Daxon's entire ELID system uses 400# for rough rough forming and 1000# or 2000# for semi-finishing machining. Mirror grinding uses 4000# (average particle size approx. 4μm) or 8000# (average particle size approx. 2μm) cast iron binder diamond grinding wheel for electrolytic trimming power supply (ELID power supply) using DC high frequency pulse voltage Special power supply, working voltage is 60V, current is lOA. The grinding fluid used requires the use of pure water to dilute the water-soluble grinding fluid AFH-M and CEM 50-fold.

3 ELID mirror grinding experimental methods and experimental results

For the aspherical processing, only the forming body of the flat grinding wheel is formed by a bowl-shaped grinding wheel (325#W2mm cast iron bonded diamond wheel) mounted on the workpiece shaft, and after the initial electrolysis in 10 minutes, 400# is passed. Coarse grinding and semi-finishing of 1000#, and finally ELID mirror grinding with 4000#, on the ultra-precision aspherical processing machine, with the aid of ELID grinding technology, successfully processed the optical glass BK - 7 aspherical lens . Surface accuracy is better than o. 2μm, surface roughness up to Ra20nm, but for slightly softer materials such as LASFN30 and Ge aspheric surface processing, can also achieve surface accuracy better than O. 2-O. 3μm, surface roughness Ra30nm.

4. Ultra-precision polishing (grinding) technology for aspherical parts

Ultra-precision polishing is a processing method with extremely slow processing speed. It is not suitable for shape processing. In recent years, due to the rapid development of short-wavelength optical elements, OA instruments, and AV machines, higher requirements have been placed on the surface roughness of parts. So far, there is no better than ultra-precision polishing. Practical methods, especially when the surface roughness requirement of the part is better than 0.01μm, this method is indispensable. For workpieces with high requirements for shape accuracy, if the method of forced feed is used for cutting or grinding The shape accuracy will be directly affected by the accuracy of the feed positioning of the machine tool, to reach the reaction, and the resulting processing effect, there is the same tiny concave part on the surface of the workpiece, in general, can only obtain larger ripple fluctuations s surface.

Prof. Mori Mori, Professor of the Faculty of Engineering, Osaka University, Japan, and others have used EEM to develop a three-axis (x, z, C) numerical control optical surface modeler that controls the retention of polyurethane balls on the surface of the workpiece while processing the device. At the same time, the whole area of ​​the object to be processed is scanned using a polyurethane ball, and a high-precision arbitrary curved surface can be machined using this device.

5. CVM (Chemical Vaporization Machining) Technology for Aspheric Surface Parts Plasma

Currently widely used cutting, grinding, polishing and other mechanical processing methods, due to the presence of fine cracks in the processing materials or character defects in the crystal, no matter how to improve the processing accuracy, and improve the processing device, there are always some limitations, for this reason, Prof. Mori Yongzheng of the Faculty of Engineering at Osaka University in Japan proposed a new processing method for chemical gas processing called Plasma CVM. This is a technique that uses atomic chemical reactions to obtain super-precision surfaces. Its processing principle. Like plasma etching, in the plasma, the activated free radicals and the surface of the workpiece react to become volatile molecules, and the processed plasma is generated under high pressure by vapor evaporation. It can generate radicals with very high density, so this processing method can achieve processing speed comparable to that of mechanical processing methods. Under high pressure, the plasma is confined near the electrodes due to the extremely small mean free path of the gas molecules. Therefore, O. can be processed by electrode scanning. 01μm accuracy of any shape of the parts, in addition can be processed at a speed of 50μm/min single crystal silicon surface, the workpiece surface roughness up to 0.1nm (Rrms).

In the next century, CVM technology will be used in many fields such as silicon chip processing and aspheric lens processing for semiconductor exposure devices. At present, some people are studying the use of a combination of CVM and EEM to process atoms such as X-ray mirrors for synchrotron accelerators. Flat arbitrary surface.

6. Aspherical part replication technology

The polishing (grinding) method that controls the thickness removal can produce high-precision aspherical parts, but compared with the general optical parts processing method, the processing efficiency of this method is very low, and one of the methods to solve this problem is copying technology. , namely plastic injection molding and glass molding technology, this technology can produce a part of the aspheric lens. Plastic lens injection molding is a method in which molten resin is injected into a mold and cooled and solidified while applying pressure. This method enables inexpensive, mass-produced production. However, there are some problems with plastic itself, such as temperature change and moisture absorption. The refractive index of the lens changes.

Molding glass is an optimal mass production method for small parts instead of cutting, grinding, and grinding lenses and prisms. The press molding technique is to control the temperature in the mold below the softening temperature of the stamping glass transition temperature and below the softening temperature in the mold, enter the flowing glass, press molding, and maintain this state for more than 20 s until the formed glass The temperature distribution is uniform, the shape accuracy of the mold is set to 0.1 μm, and the surface roughness is set to 0.01 μm or less. Under the above conditions, the mold is press-formed to produce a part having a precision close to that of the mold.