Aspherical parts superfine machining skills

November 04, 2021

1 Overview

1.1 The effect of aspherical optical parts Aspherical optical parts are a very important optical part; commonly used parabolic mirrors, hyperbolic mirrors, ellipsoidal mirrors, etc. Aspherical optical parts can achieve unparalleled outstanding imaging of spherical optical parts Quality; can correct a variety of aberrations well in optical systems; improve image quality; improve system discrimination; it can replace multiple spherical parts with one or several aspherical parts; then simplify instrument layout; reduce cost and useful Reduce the instrument component.
Aspheric optical components are also widely used in military and civilian optoelectronic products; such as in shooting lenses and viewfinders, television cameras, zoom lenses, movie shooting lenses, satellite infrared telescopes, video recorder lenses, video and audio recording discs. Surface, bar code reading surface, fiber optic communication fiber optic connector, medical equipment, etc.
1.2 Status of ultra-fine processing skills of foreign aspherical parts
Since the 1980s, many new aspheric superfine machining skills have been presented; firstly: computer numerical control single point diamond turning skills, computer numerical control grinding skills, computer numerical control ion beam forming skills, computer numerical control superfine polishing skills and aspheric surfaces Copying skills, etc.; these processing methods; basically deal with the problems in the processing of various aspherical mirrors. The first four methods use numerical control skills; both have high processing precision; high-power features; suitable for mass production.
When processing aspherical parts; think about the materials, shape, accuracy, and caliber of the parts being processed; about soft materials such as copper and aluminum; superfinishing can be performed by single point diamond cutting (SPDT); Or plastics; at that time, the first choice was to use ultra-fine processing of the mold; then use the forming method to produce aspherical parts; for other high-hardness brittle materials; at that time, it was super-fine grinding and ultra-fine grinding and polishing. Processed; otherwise. There are also special processing skills for aspherical parts such as ion beam polishing.
Many foreign companies have integrated ultra-fine turning, grinding, grinding and polishing; and announced the development of ultra-fine composite processing systems; such as Nanoform300, Nanoform250 produced by Rank Pneumo, Nanocentre developed by CUPE, AHN60D3D and ULP of Japan. 100A (H) has a composite processing function; this makes the processing of aspherical parts more sensitive.
1.3 The status of ultra-fine processing skills of aspherical parts in China China has only started research on super-fine processing skills since the early 1980s; it has been 20 years behind in foreign countries. These years, the better units are discussed in Beijing Machine Tool. Institute of China Aviation Fine Machinery Research Institute, Harbin Institute of Technology, Chinese Academy of Sciences, Changchun Institute of Optics, the use of optical points laboratory.
In order to better carry out the research on this superfine processing skill; in 1995, the National Defense Science and Technology Commission established the first domestic laboratory for the research of ultra-fine processing skills at the China Aviation Fine Machinery Research Institute.
2. Ultra-fine machining technology for aspherical parts In the United States, Union Carbide successfully developed the RDθ method for aspherical surface-created machining machines in 1972. This is a two-axis CNC lathe with azimuth response; it can change the rotation angle of the tool holder rail in real time. And radius R; complete aspherical mirror processing. The processing diameter is φ380mm; the shape accuracy of the machined workpiece is ±O. 63μm; appearance roughness is Ra0. 025μm.
In 1980, Moore Corporation announced the first MD18AG aspherical machining machine with three coordinates; this machine can process a variety of aspherical metal mirrors with a diameter of 356mm.
In 1980, the British company Rank Pneumo introduced a two-axis linkage machining machine (MSGD325) with laser reverberation control. The machine can process aspherical metal mirrors with a diameter of 350mm. The shape accuracy of the workpiece is 0.25-0. 5μm; appearance roughness Ra is 0.01-O. Between 025μm. Then ASG2500, ASG2500T, Nanoform300 and other machine tools were introduced; the company is based on the above machine tools; Nanoform600 was announced in 1990; the machine can process aspherical mirrors with a diameter of 600mm; the shape of the workpiece The accuracy is better than 0.1μm; the surface roughness is better than 0.01μm.
The ultra-fine diamond lathe representing the high level of today's staff is Lawrence of the United States. The LLNL laboratory developed the LODTM in 1984; it can process diameters up to 2100mm; workpieces weighing up to 4500kg can be processed to 0.25μm; surface roughness RaO. 0076μm; the machine can process plane, spherical and aspherical surfaces; it is mainly used for machining parts required for laser nuclear fusion engineering, parts for infrared equipment and large-scale astronomical mirrors.
Large ultra-fine diamond right-mirror cutting machine developed by Cranfield University's Fine Engineering Research Institute (CUPE); aspherical mirror for large X-ray astronomical telescopes (maximum diameter up to 1400mm; maximum length 600mm cone). The research institute also developed a diamond cutting machine that can process the inner reverse paraboloid and the outer reverse hyperboloid mirror for the X-ray telescope.
The ultra-fine processing machine developed in Japan is mainly used for processing lenses and mirrors for civilian products. At the time, the machine tools made in Japan were: ULGDl00A (H) developed by Toshiba Machinery, ASPDL15 of the company, and AHN10 of Toyota Machine. , AHN30×25, AHN60D3D aspherical processing machine tools, etc.
3. Ultra-fine grinding machining skills for aspherical parts
3.1 Aspherical parts super-precision grinding equipment Rank Pneumo Company of the United Kingdom developed the improved ASG2500, ASG2500T, Nanoform300 machine tools in 1988; these machines can not only be cut; but also can be ground with diamond grinding wheels; It is a 300mm aspherical metal mirror; the shape accuracy of the machined workpiece is 0.3-O. 16μm; the surface roughness reaches Ra0.01μm. The Nanoform250 ultra-fine processing system is introduced. The system is a two-axis ultra-fine CNC machine tool. It can perform ultra-fine turning and ultra-lifting grinding on this machine. Ultra-fine polishing is also possible. The most outstanding feature is the ability to directly grind hard and brittle optical parts with optical appearance quality and surface accuracy that meet the requirements of optical systems. The machine uses many leading Nanoform600 and Optoform50 depictions. The maximum machining workpiece diameter of the machine tool is up to 250mm; it passes through a lifting device to make the machine's largest workpiece diameter reach 450mm; other hydrostatic guides (Y-axis) that are manipulated in the straight direction can grind non-axisymmetric parts; The resolution of the numerical control system reaches O. 001μm; the azimuth response component uses a grating with a resolution of 8.6nm or a laser interferometer with a resolution of 1.25nm; the surface accuracy of the processed workpiece is 0.225μm; the surface roughness is better than Ra0.01μm.
Nanocentre250, Nanocentre600 is a three-axis ultra-fine CNC aspherical surface forming device; it can satisfy the requirements of both single-point and ductile grinding; it is characterized by rationalized machine tool layout, using high-stiffness servo drive system and hydrostatic bearing The machine has a high closed-loop stiffness; the x and Z axes have a resolution of 1.25 nm; this machine is considered to be compatible with modern process standards. CUPE produces Nanocentre aspheric optical parts machining machines; processing diameters up to 600 mm. The surface accuracy is better than 0.1μm; the surface roughness is better than Ra0. 01μm. CUPE also researched, depicted and produced the world's largest ultra-fine large-scale CNC optical parts grinding machine “0AGM2500” for Kodak Company; Processing of hard and brittle materials such as optical glass; processing and measuring 2.5m × 2.5m × O. 61m workpiece; it can process 2m square asymmetric optical mirror; the shape error of the mirror is only 1μm.
The AHN60D3D developed by Toyota Motor Corporation of Japan is a CNC three-dimensional cutting and turning machine; it can grind and turn axially symmetrical shaped optical parts under X, Y and Z three-axis control; it can be used in X, Y and Grinding and turning non-axisymmetric optical parts under the control of two half-axis of Z-axis; the cutting precision of the machined workpiece is 0.35unl; the roughness of the surface is Ra0.016μm. Other ULGD100A(H) ultra-fine composite developed by Toshiba Machine Processing equipment; it uses the method of controlling the two axes; completes the cutting and grinding of the aspherical lens mold; the X-axis and Z-axis strokes are 150mm and 100mm; the azimuth response component has a resolution of 0.01μm Raster.
3.2 ELID Mirror Grinding Skills for Aspherical Optical Parts Japanese expert Dasen Confucius studied the superabrasive grinding wheel from 1987; developed the grinding method using electrolytic In Process Dressing (ELID); completed the hard Crisp data high-grade mirror grinding and ductility grinding; today the method has been successfully applied to spherical, aspherical lenses, mold ultra-fine processing.
1 ELID mirror grinding principle
ELID grinding system includes: metal bond ultra-fine particle size super-hard abrasive wheel, electrolytic dressing power supply, electrolytic dressing electrode, electrolyte (also used as grinding fluid), electric brush and machine tool equipment. During grinding; The electric brush is connected to the positive pole of the power source; the trimming electrode mounted on the machine tool is connected to the negative pole of the power source; the electrolyte is poured between the grinding wheel and the electrode; thus; the electrolyte composition between the power source, the grinding wheel, the electrode, the grinding wheel and the electrode An intact electrochemical system.
When ELID grinding is used; there are some special requirements for the grinding wheel, power supply and electrolyte used. The bonding agent of the grinding wheel has excellent conductivity and electrolysis, the hydroxide or oxide of the bonding element is not conductive; and it is insoluble. In water; ELID grinding power supply; can choose electrolytic DC power supply or pulse power supply or DC base pulse power supply with various waveforms. In ELID grinding process; electrolyte is used as grinding fluid; The effect of lowering the temperature of the grinding zone and reducing the friction; ELID grinding usually uses water-soluble grinding fluid; the mechanical strength of the all-base bond grinding wheel is high; the appropriate amount of electrolysis is set; the grinding wheel wears little. High shape accuracy. Apply this principle; it can complete ultra-fine mirror grinding from plane to aspheric surface; optical components of various shapes.
2ELID mirror grinding test system is used on the ASGD2500T machine tool of Rank Pneumo; it is equipped with grinding wheel, power supply, electrode, grinding fluid, etc. It is used in the rough processing of the Ossen ELID system. 400# is used in the rough forming process, and 1000# is used in the semi-finishing process. 2000#, for mirror grinding, use 4000# (uniform particle size is about 4μm) or 8000# (uniform particle size is about 2μm) cast iron bond diamond grinding wheel; electrolysis sharpening power supply (ELID power supply); use DC High-frequency pulse voltage type power supply; operating voltage is 60V; current is lOA. The grinding fluid used; the water-soluble grinding fluid AFHDM and CEM are diluted 50 times with pure water.
3 ELID mirror grinding test method and test results for aspherical machining; through the bowl-shaped grinding wheel (325# cast iron bonder diamond grinding wheel φ30×W2mm) mounted on the workpiece shaft, the flat grinding wheel is only shaped; 10min After the initial stage of electrolysis, after 400# rough grinding and 1000# semi-finishing; finally use 4000# for ELID mirror grinding; on ultra-fine aspherical processing machine; with ELID grinding skills; successfully processed Out of the optical glass BKD7 aspherical lens. The surface accuracy is better than o. 2μm; surface roughness up to Ra20nm; and aspherical processing on slightly softer materials such as LASFN30 and Ge; the same reachable surface accuracy is better than O. 2-O. 3μm; surface roughness up to Ra30nm.
4. Ultra-fine polishing (abrasive) skills for aspherical parts Ultra-fine polishing is a very slow processing method. It is not suitable for shape-forming processing; these years; because of the rapid development of short-wavelength optical components, OA instruments and AV machines; There is a higher demand for the surface roughness of the part; by that time there is no better way than ultra-fine polishing; especially when the surface roughness requirement of the part is better than 0.01 μm; this method is not short For workpieces with high requirements on shape accuracy; if forced feeding is used for cutting or grinding; the shape accuracy will be directly affected by the positioning accuracy of the machine feed; the arrival location will be reverberated; The processing effect; there are some similar small concaves on the surface of the workpiece; under normal circumstances; only the rugged and rugged appearance can be obtained.
Prof. Sen Yongjun from the Faculty of Engineering, Osaka University, Japan, used EEM to develop a three-axis (x, z, C) numerical control optical appearance device; when using this equipment; while manipulating the polyurethane ball on the surface of the workpiece At the same time; while scanning the whole object of the target with a polyurethane ball; using this equipment can process high-precision and comfortable surfaces.
5. CVM (Chemical Vaporization Machining) skills of aspherical parts plasma are widely used in the cutting, grinding, polishing and other mechanical processing methods; because of the existence of fine cracks in the processing resources or the quality defects in the crystallization; no matter how to improve the processing accuracy; Improvement of processing equipment; there are always certain limitations; for this reason; Prof. Mori Masahiro, Faculty of Engineering, Osaka University, Japan, proposed a new processing method using chemical gas processing; called plasma CVM method; this is an application of atomic chemistry Reverberation; a skill in obtaining a super-fine appearance; the processing principle is the same as that of plasma etching; in the plasma; the activated radicals and the appearance of the workpiece are reverberating; turning them into volatile molecules; The process of transpiration is completed; the plasma that occurs under high pressure; can produce free radicals with very high density; so this processing method can reach the processing speed comparable to the mechanical processing method. Under high pressure; because of gas molecules Uniform free travel stroke. The soil is still pouring. So you can scan through the electrode; The O. 01μm precision of the shape of the part; other can process the single crystal silicon plane at 50μm / min; the surface roughness of the workpiece can reach 0. 1nm (Rrms).
The next century; in the field of silicon chip processing and aspheric lens processing for semiconductor exposure equipment; CVM skills will be used; at that time some people are studying the combination of CVM and EEM; X-ray mirrors for processing synchrotrons, etc. Level flat surface.
6. Aspherical parts imitation skills can be used to produce high-precision aspherical parts by polishing the thickness (polishing) method; however, compared with the usual optical parts processing methods; the processing power of this method is very low; the solution to this problem One has imitation skills; that is, plastic injection molding and glass molding skills; this skill can make a somewhat aspherical lens. Plastic lens injection molding is to write molten resin into the mold; while applying pressure; while cooling and solidifying Processing methods; this method can be sold, mass production; but there are some difficulties in the plastic itself; such as temperature changes, moisture absorption caused by changes in the refractive index of the lens.
Molding of glass is the best way to mass-produce small-sized parts instead of cutting, grinding, grinding, and prisms. The molding technique is to control the temperature inside the mold below the p-softening temperature of the stamped glass handling temperature; In the mold; enter the fluid glass; press forming; and adhere to this condition for more than 20s; until the formed glass temperature spreads evenly; the shape accuracy of the mold is made to 0.1 μm; the surface roughness is made to 0 .01μm or less; press forming under the above conditions; can process parts near the mold precision.

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Ms. Ivy Wang

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