Processing method of large-aperture high-precision infrared aspheric optical imaging lens

Abstract

A machining method of a large-caliber high-precision infrared aspheric optical imaging lens comprises the steps that S1, according to a design drawing, rough machining is conducted on the double-face contour and the outer circle datum of a lens blank through a milling and grinding machine, and the center thickness allowance is reserved; s2, by taking the excircle and the convex / concave platform of the lens as a reference, mounting the lens on a spindle of a single-point diamond lathe, performing primary turning on the surface type of the excircle and the concave / convex surface of the lens to enable the cylindricity of the excircle to be superior to 0.005 mu m, and after the primary turning of the surface type is completed, testing the surface type by adopting a contourgraph and performing compensation turning according to the error of the surface type; s3, turning by taking the outer circle of the lens and the concave surface / convex surface platform as the positioning reference, and turning the convex surface / concave surface type in the same way as that in the S2; s4, performing uniform and non-eccentric smooth polishing on the surface type of the lens until the turning subsurface damage is completely removed; and S5, installing the lens structural member, re-testing the surface shape, and carrying out magneto-rheological fine polishing.

Description

Technical Field

[0001] The invention belongs to the field of precision optical manufacturing, and in particular relates to a method for processing a large-aperture high-precision infrared aspherical optical imaging lens. Background Art

[0002] As the performance requirements of optical systems in fields such as optical remote sensing continue to increase, the demand for large-aperture, high-precision aspheric lenses in infrared optical imaging systems is increasing. This type of lens is significantly different from lenses used in ordinary infrared fields in the following aspects: 1) In terms of aperture, the lens size mentioned in this article is much larger than ordinary infrared lenses, generally exceeding 100mm; 2) In terms of processing indicators, the surface accuracy of the lens mentioned in this article is required to be better than λ / 50rms, and the surface finish is required to reach level IV, the surface roughness is better than 2nm, and the effective aperture unit margin is less than 2mm. Its processing accuracy requirements are much higher than those of ordinary infrared lenses; 3) In terms of processing methods, ordinary infrared lenses often use processes such as molding, turning or classical polishing, and rarely involve combined processing of multiple processes. They cannot meet the processing indicators of the lenses mentioned in this article, and a technical route of multi-process combination must be adopted.

[0003] According to current literature and patents, the existing full-link CNC machining process route for large-aperture lenses is based on the process route of milling-turning-robotic composite polishing-ion beam fine polishing, but the following problems still exist: 1) The lens profile accuracy after turning can usually be better than λ / 10rms. Although the use of robotic rough polishing can remove the sub-surface damage left over from turning, it is impossible to retain the surface accuracy and surface roughness after single-point diamond turning. It still requires a long period of robotic smoothing and fine polishing to repair it. There is inevitable accuracy degradation in the process, and the technical advantages of the single-point diamond turning process cannot be fully utilized; 2) Since the lens undergoes multiple rounds of rough polishing, smoothing and fine polishing processes in the robotic polishing stage, the damage caused by robotic polishing The edge effect gradually emerges, and the mid- and high-frequency errors caused by the superposition of processing trajectories gradually increase, which increases the difficulty of subsequent ion beam fine polishing. It is inevitable to use a removal function with a smaller half-width to correct the mid- and high-frequency errors and edge effects, which will lead to a significant increase in the ion beam fine polishing time, thereby reducing the surface finish of the lens; 3) During the assembly and adjustment of the optical system, the installation of the lens and structural parts may cause the mirror surface to deform and affect the imaging quality. In order to simulate the deformation during lens assembly, the best strategy is to pre-assemble the structural parts before fine polishing, re-test the surface shape after the adhesive is cured, and then perform fine polishing. However, the surface temperature of the workpiece increases during ion beam polishing, and there is a risk of extrusion and rupture of the structural parts or failure of the structural adhesive, which cannot meet the requirements of component-level lens polishing. Summary of the invention

[0004] The technical problem solved by the present invention is to overcome the deficiencies of the prior art and provide a method for processing a large-aperture and high-precision infrared aspherical optical imaging lens.

[0005] The technical solution of the present invention is: a method for processing a large-aperture high-precision infrared aspherical optical imaging lens, comprising:

[0006] S1. According to the design drawings, the double-sided contour and outer circle reference of the lens blank are rough-machined by a milling machine, and the center thickness margin is reserved; after milling and forming, the lens shape and position tolerance is better than 10μm, and the double-sided surface contour is better than 30μm pv;

[0007] S2. Using the outer circle of the lens and the convex / concave platform as the reference, install it on the spindle of a single-point diamond lathe, and first perform preliminary turning on the outer circle and concave / convex surface of the lens to make the outer circle cylindricality better than 0.005μm. After the preliminary turning of the surface is completed, use a profilometer to test the surface and perform compensatory turning according to the surface error;

[0008] S3, turning the surface with the outer circle of the lens and the concave / convex surface platform as the positioning reference, and turning the convex / concave surface in the same way as in S2;

[0009] S4, the lens surface is polished evenly without eccentricity until the turning subsurface damage is completely removed and the surface roughness is better than 2nm;

[0010] S5. Install the lens structure and retest the surface shape, and perform magnetorheological polishing. The process parameters of magnetorheological polishing are selected as polishing wheel speed 150-250rpm, ribbon thickness controlled at 1.25-1.75mm, magnetorheological fluid penetration depth of 0.15-0.3mm, and iterative processing to meet the lens processing indicators.

[0011] Preferably, the center thickness margin retained by the rough milling process is 0.1 to 0.2 mm.

[0012] Preferably, the tool parameters for preliminary turning of the outer circle and any surface of the lens are a rake angle of -15 to -25°, a back angle of 10 to 15°, an arc radius of 1.5 to 2 mm, a waviness of 90 to 120 nm, and the turning process parameters are a rotation speed of 1500 to 3000 rpm, a feed speed of 5 to 10 mm / min, a feed depth of 10 to 20 μm, and a step spacing of 0.005 to 0.01 mm.

[0013] Preferably, the compensating turning tool is the same as the preliminary turning tool, and the blade on the other side is replaced and set in the opposite spindle rotation and feed direction during preliminary turning for compensating turning. The turning process parameters are a rotation speed of 1500 to 3000 rpm, a feed speed of 2 to 5 mm / min, a feed depth of 2 to 5 μm, and a step spacing of 0.002 to 0.005 mm.

[0014] Preferably, a non-eccentric polishing tool head is used for smooth polishing, the non-eccentric polishing tool is a non-eccentric polishing tool disk of φ10-30mm, the polishing medium is asphalt + black damping cloth, the polishing auxiliary material is cerium oxide polishing liquid, the particle size is 0.5-2μm, the rotation speed of the non-eccentric polishing head is 100-200rpm, the spacing between the motion trajectory points is 1 / 10-1 / 5 of the caliber of the polishing tool disk, the single point residence time does not exceed 15s, and the difference between the lens surface error after non-eccentric uniform polishing and the rear surface error after turning is not more than 5%.

[0015] Preferably, the polishing auxiliary materials are gradually transitioned according to the principle of particle size from large to small.

[0016] Preferably, after double-sided turning in S3, the center thickness of the lens should meet the tolerance requirements on the drawing, and the double-sided surface accuracy should be better than λ / 10rms, and the sub-surface damage should be less than 2μm.

[0017] Preferably, the iterative processing of magnetorheological polishing is 2-3 rounds.

[0018] Preferably, after the non-eccentric polishing, the lens structure is installed first and then the surface shape is retested, followed by magnetorheological polishing to avoid precision loss caused by structural deformation during lens installation and adjustment.

[0019] The beneficial effects of the present invention compared with the prior art are:

[0020] The present invention proposes an improvement on the high-precision processing method of large-aperture infrared lenses. Through the optimization of the process chain, not only the digital deterministic processing of the entire chain is realized, but also the problems of repeated precision in the processing process, the mutual constraints between surface quality and processing cycle, and the inability to achieve component-level precision polishing are overcome.

[0021] The present invention adopts non-eccentric polishing, which can not only effectively avoid the edge effect in the polishing stage that affects the effective light aperture of the lens, but also has a good conformal polishing effect, which can enable the lens to maintain good surface accuracy after turning after polishing, and overcome the process defect of reduced surface accuracy after robot polishing, thereby avoiding the process bottlenecks such as reduced surface accuracy due to polishing surface damage and accumulation of medium and high frequency errors due to multiple rounds of polishing in the original process.

[0022] The present invention pre-assembles structural parts before fine polishing of the lens, which can truly reflect the deformation of the lens during installation and adjustment. At this time, magnetorheological polishing is used instead. Compared with the current method of using ion beam fine polishing, the problem of reduced mirror surface finish caused by heat accumulation in multiple rounds of ion beam fine polishing is avoided. The most critical thing is that the fine polishing of the lens at the component level is realized, ensuring the consistency of the lens processing and installation surface accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1A process flow chart of a method for processing a large-aperture, high-precision infrared aspherical optical imaging lens;

[0024] Figure 2 Typical lens design drawings of the present invention (implementation examples);

[0025] Figure 3 The surface quality and shape of the lens after turning in the present invention;

[0026] Figure 4 The surface quality and shape of the lens after non-eccentric polishing of the present invention;

[0027] Figure 5 The lens is deformed after the structural parts are installed;

[0028] Figure 6 The final surface accuracy of the lens processed by the method of the present invention;

[0029] Figure 7 Comparison case: Surface quality and shape of lens after robot polishing;

[0030] Figure 8 Comparison case: Surface quality and shape of lens after ion beam polishing;

[0031] Fig. 9 Comparison case: Final surface shape of the lens during assembly. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with the embodiments.

[0033] Figure 2 This is an implementation example of the present invention, which is a meniscus lens, and the center thickness requirement is 20.5±0.02mm. The other design values ​​are shown in the drawings.

[0034] A method for processing a large-aperture high-precision infrared aspherical optical imaging lens, such as Figure 1 As shown, the following steps are included:

[0035] 1. Milling

[0036] According to the design drawings, the lens blank is formed by a milling machine, which mainly includes rough machining of the double-sided contour and the outer circle reference, and retaining a certain center thickness margin (0.1-0.2mm). In this case, the center thickness margin of the lens aperture is 0.15mm, and the average center thickness margin of the concave and convex surface of the lens is 0.075mm, which is removed during subsequent turning. After milling and forming, the lens shape and position tolerance is better than 10μm, and the double-sided surface profile is better than 30μm pv.

[0037] 2. Turning

[0038] Taking the outer circle and convex platform of the lens as the reference, it is installed on the spindle of a single-point diamond lathe. The outer circle and concave surface (surface 2) of the lens are first turned to make the outer circle cylindricity better than 0.005μm. Natural diamond arc tools are used for turning. The tool parameters are generally a front angle of -15 to -25°, a back angle of 10 to 15°, an arc radius of 1.5 to 2mm, and a corrugation of 90-120nm. The turning process parameters are generally a rotation speed of 1500 to 3000rpm, a feed speed of 5 to 10mm / min, a feed depth of 10 to 20μm, and a step spacing of 0.005 to 0.01mm.

[0039] against Figure 2 In the case, the tool rake angle is -25°, the back angle is 10°, the measured tool arc radius is 1.561mm, the waviness is 112nm, the turning process parameters are rotation speed 2500rpm, feed speed 7mm / min, feed depth 15μm, step spacing 0.01mm, the single-sided center thickness removal is controlled between 60 and 70μm, and the remaining 5 to 15μm of center thickness allowance is left to be removed during compensatory turning.

[0040] 3. Compensation turning

[0041] After the preliminary turning of the concave surface (surface 2) is completed, the profile is tested with a profiler and compensatory turning is performed according to the surface error. The diamond tool remains unchanged during compensatory turning, and the blade on the other side is replaced and set to the opposite spindle direction and feed direction of the preliminary turning for compensatory turning. The compensatory turning uses fine turning parameters. The turning process parameters are generally a rotation speed of 1500 to 3000 rpm, a feed speed of 2 to 5 mm / min, a feed depth of 2 to 5 μm, and a step spacing of 0.002 to 0.005 mm, which simultaneously improves the turning surface accuracy and surface quality.

[0042] against Figure 2 In the case, the compensation turning process parameters are rotation speed 2500rpm, feed speed 3mm / min, feed depth 5μm, step spacing 0.005mm, and the single-side center thickness removal is controlled at about 10μm.

[0043] 4. Turning

[0044] After the concave surface (surface 2) is turned, the convex surface (surface 1) is turned over with the outer circle of the lens and the concave surface platform as the positioning reference, and the corresponding surface error compensation turning and finishing parameters are adopted (refer to steps 2 and 3); after turning, the double-sided surface accuracy of the lens is λ / 10rms, there is no obvious collapse or warping, and the sub-surface damage is less than 2μm. Figure 3As shown in the figure, the surface shape is λ / 10rms at this time, but there is obvious turning damage on the surface, which is manifested as rainbow color lines and chromatic aberration distributed in rings on the material surface. In the cross-section line, there are dense sawtooth surface shape changes, indicating that there are more mid- and high-frequency errors in the surface shape at this time, which will limit the convergence effect of subsequent refinement of the surface shape.

[0045] 5. Non-eccentric polishing of the robot

[0046] An eccentric polishing tool head is integrated on the industrial manipulator robot. A 10-30mm eccentric polishing tool disc is selected to perform uniform conformal polishing on the surface damage after turning. The polishing medium is asphalt + black damping cloth, and the polishing auxiliary material is cerium oxide polishing liquid with a particle size of 0.5-2μm. The polishing auxiliary material can be gradually transitioned according to the principle of particle size from large to small. The rotation speed of the eccentric polishing head is generally 100-200rpm, and the point spacing of the motion trajectory is generally about 1 / 10-1 / 5 of the diameter of the polishing tool disc. The single-point dwell time is generally not more than 15s. Evenly polish until there is no obvious sub-surface damage, dense pitting and other defects on the lens surface when observed with a strong flashlight. Since the eccentric uniform polishing is only used to remove surface damage after turning, the single-side center thickness removal depth is about 5-10μm, and the subsequent magnetorheological polishing has almost no effect on the center thickness of the lens, so it can be guaranteed that the final center thickness value of the lens meets the tolerance requirements. The difference between the surface error of the lens after eccentric uniform polishing and the surface error after turning is no more than 5%. The results after polishing are as follows Figure 4 As shown, the surface shape is still λ / 10rms at this time, and the accuracy has hardly changed, indicating that the uniform polishing without eccentricity has good conformality and basically does not lose the surface shape accuracy of turning; in addition, it can be seen that after the uniform polishing without eccentricity, there is no rainbow pattern or annular chromatic aberration on the mirror surface, and the entire mirror surface presents a smooth and uniform mirror effect, indicating that the sub-surface damage of turning has been completely removed and the surface finish of the lens has been significantly improved; the dense serrated surface shape in the cross-section line has been completely removed, and the surface shape curve after polishing becomes gentle, indicating that the uniform polishing without eccentricity has a good smoothing effect.

[0047] 6. Structural parts installation

[0048] After the double-sided polishing of the lens is completed, the lens and the structural parts are bonded with structural adhesive, and the lens surface error is retested after curing. Figure 5 As shown in FIG. 1 , the lens is slightly deformed due to the force of the structural parts, and the surface accuracy of the lens is 0.129λrms.

[0049] 7. Magnetorheological polishing

[0050] After the robot has polished the lens without eccentricity, the surface finish and surface roughness of the lens have met the requirements. The installation of the structural parts does not change the surface finish and surface roughness. After the structural parts are installed, the lens changes due to the force surface accuracy, which is generally between λ / 5 and λ / 10rms, and still meets the magnetorheological entrance accuracy requirements. The process parameters for magnetorheological fine polishing generally select a polishing wheel speed of 150 to 250rpm, a ribbon thickness of 1.25 to 1.75mm, and a magnetorheological fluid penetration depth of 0.15 to 0.3mm. Iterative processing of 2 to 3 rounds can meet the final requirements.

[0051] Figure 6 The lens surface accuracy obtained by the method of the present invention can be seen from the figure that the surface shape is better than λ / 50rms at this time, and there is no edge effect. The mirror quality is almost the same as that after non-eccentric polishing, without obvious pitting and other defects, and meets the index requirements. Since the structural parts are installed in advance and the surface shape errors introduced by the deformation of the structural parts are processed away, the consistency of the surface shape after the optical system is also guaranteed.

[0052] In contrast, for Figure 2 The case in this article adopts the milling-turning-manipulator composite polishing-ion beam polishing process. The effect of the lens after turning is similar to Figure 3 The surface shape accuracy and surface quality after the composite polishing by the robot are as follows: Figure 7 As shown in the figure, although the rainbow pattern and annular chromatic aberration on the surface after turning have been completely removed, many irregularly distributed scratches are produced on the mirror surface during the composite polishing process of the robot, and the surface accuracy is degraded to 0.164λrms. Due to the characteristics of the robot eccentric wheel polishing, the edge has obvious warping.

[0053] After the robot composite polishing, ion beam fine polishing is performed. After 3 to 5 rounds of fine polishing, the shape no longer converges. At this time, the surface accuracy and surface quality are as follows Figure 8 As shown. It can be seen that the warping after robot polishing limits the convergence effect of ion beam fine polishing. The shape accuracy after fine polishing is 0.041λrms, and the edge warping cannot be completely removed. In addition, due to the accumulation of heat caused by the bombardment of the ion beam during the ion beam processing, the mirror surface has snowflake-shaped pitting defects after multiple rounds of ion beam processing, and the mirror surface finish is reduced. During system assembly, due to the deformation of the mounting structure, the lens surface shape degraded to 0.059λrms, which was 0.02λrms worse than the surface shape accuracy at the time of delivery after fine polishing. Fig. 9 shown.

[0054] It can be seen that the method of the present invention effectively ensures the manufacturing accuracy, surface quality and efficiency of the lens by improving the process route.

[0055] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0056] Parts not described in detail in the present invention belong to common knowledge of those skilled in the art.

Claims

1. A method for processing a large-aperture, high-precision infrared aspherical optical imaging lens, characterized in that include: S1. According to the design drawings, the double-sided contour and outer circle reference of the lens blank are rough-machined by a milling machine, and the center thickness margin is reserved; after milling and forming, the lens shape and position tolerance is better than 10μm, and the double-sided surface contour is better than 30μm pv; S2. Using the outer circle of the lens and the convex / concave platform as the reference, install it on the spindle of a single-point diamond lathe, and first perform preliminary turning on the outer circle and concave / convex surface of the lens to make the outer circle cylindricality better than 0.005μm. After the preliminary turning of the surface is completed, use a profilometer to test the surface and perform compensatory turning according to the surface error; S3, turning the surface with the outer circle of the lens and the concave / convex surface platform as the positioning reference, and turning the convex / concave surface in the same way as in S2; S4, the lens surface is polished evenly without eccentricity until the turning subsurface damage is completely removed and the surface roughness is better than 2nm; S5. Install the lens structure and retest the surface shape, and perform magnetorheological polishing. The process parameters of magnetorheological polishing are selected as polishing wheel speed 150-250rpm, ribbon thickness controlled at 1.25-1.75mm, magnetorheological fluid penetration depth of 0.15-0.3mm, and iterative processing to meet the lens processing indicators.

2. The processing method according to claim 1, characterized in that The center thickness allowance retained by rough milling is 0.1 to 0.2 mm.

3. The processing method according to claim 1, characterized in that: The tool parameters for preliminary turning of the outer circle and any surface of the lens are as follows: rake angle -15~-25°, back angle 10~15°, arc radius 1.5~2mm, waviness 90~120nm, turning process parameters are rotation speed 1500~3000rpm, feed speed 5~10mm / min, feed depth 10~20μm, and step spacing 0.005~0.01mm.

4. The processing method according to claim 3, characterized in that: The compensating turning tool is the same as the preliminary turning tool. The blade on the other side is changed and set in the opposite spindle rotation and feed direction during preliminary turning for compensating turning. The turning process parameters are a rotation speed of 1500 to 3000 rpm, a feed speed of 2 to 5 mm / min, a feed depth of 2 to 5 μm, and a step spacing of 0.002 to 0.005 mm.

5. The processing method according to claim 1, characterized in that: A non-eccentric polishing tool head is used for smooth polishing. The non-eccentric polishing tool is a non-eccentric polishing tool disk of φ10-30mm, the polishing medium is asphalt + black damping cloth, the polishing auxiliary material is cerium oxide polishing liquid, the particle size is 0.5-2μm, the rotation speed of the non-eccentric polishing head is 100-200rpm, the spacing between the motion trajectory points is 1 / 10-1 / 5 of the diameter of the polishing tool disk, the single point residence time does not exceed 15s, and the difference between the lens surface error after non-eccentric uniform polishing and the rear surface error after turning is not more than 5%.

6. The processing method according to claim 5, characterized in that: Polishing auxiliary materials are gradually transitioned according to the principle of particle size from large to small.

7. The processing method according to claim 1, characterized in that: After double-sided turning in S3, the center thickness of the lens should meet the tolerance requirements on the drawing, and the double-sided surface accuracy should be better than λ / 10rms, and the sub-surface damage should be less than 2μm.

8. The processing method according to claim 1, characterized in that: The iterative processing of magnetorheological polishing is 2-3 rounds.

9. The processing method according to claim 1, characterized in that: After the non-eccentric polishing, the lens structure is installed first and then the surface shape is retested, followed by magnetorheological polishing to avoid precision loss caused by structural deformation during lens installation and adjustment.

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