Method for polishing large-aperture potassium dihydrogen phosphate crystal elements

Abstract

The application provides a polishing method for large-diameter potassium dihydrogen phosphate crystal elements, which can realize fast convergence of crystal surface shape precision, and comprises the following steps: 1) configuring an oil-based polishing liquid; 2) pasting a vacuum adsorption pad on the surface of a substrate with the same size as the crystal element; 3) pasting a light-induced debonding protective tape on the back surface of the crystal element, adsorbing the crystal element on the vacuum adsorption pad, and mounting the crystal element on a tool of a machine tool; 4) performing fast polishing on the processed surface of the crystal element; 5) after the fast polishing is completed, lifting a workpiece disc, and taking the crystal element and the substrate off the polishing disc; and 6) performing fast polishing on the crystal element after turning over. The crystal element is processed by using the method of full-diameter polishing of a hard polishing disc, the full surface of the crystal element is in contact with the polishing tool at the same time in the polishing process, fast processing based on the rigid pair of research is realized, the crystal surface shape precision is fast converged, and the method can be widely applied to the processing of various specifications of optical elements.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical manufacturing, in particular to a polishing method of a large-aperture potassium dihydrogen phosphate crystal element. BACKGROUND

[0002] Functional crystal materials can realize the interaction and mutual conversion of electric, magnetic, acoustic, optical, force, thermal and other energy forms, and are a kind of core materials in the field of optoelectronics, which have been widely used in various fields. With the rapid development of optical technology, higher and higher requirements are put forward for the processing quality and efficiency of functional crystal optical elements. Nonlinear laser materials represented by potassium dihydrogen phosphate crystals (including KDP crystals, DKDP crystals, CLBO crystals, etc.) have material characteristics such as softness, brittleness, easy hydrolysis, anisotropy, etc., and the processing difficulty is extremely great.

[0003] The existing fine processing of large-aperture ultra-thin potassium dihydrogen phosphate crystal elements of high-power laser devices is mainly based on diamond fly cutting, but diamond fly cutting is limited by clamping deformation, and it is difficult to control the flatness precision of the crystal, and other polishing methods have insufficient processing efficiency for large-aperture elements and are suitable for small sizes. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a polishing method of a large-aperture potassium dihydrogen phosphate crystal element, which can realize rapid convergence of the flatness precision of the crystal.

[0005] The technical solution adopted by the present application to solve the technical problem is: a polishing method of a large-aperture potassium dihydrogen phosphate crystal element, the method comprising the following steps:

[0006] 1) configuring an oil-based polishing liquid;

[0007] 2) attaching a vacuum adsorption pad to the surface of a substrate with the same size as the crystal element;

[0008] 3) attaching a light-induced debonding protective tape to the back surface of the crystal element, adsorbing the crystal element on the vacuum adsorption pad, and mounting the crystal element and the substrate as a whole on the tooling of the machine tool;

[0009] 4) rapidly polishing the processed surface of the crystal element to improve the flatness to within 10 μm, and the rapid polishing parameters are: rotation speed 15-20 rpm, processing pressure 800-1200 Pa, abrasive particle size ≥1 μm, and processing time 20-40 min;

[0010] 5) after the rapid polishing is completed, lifting the workpiece disc, taking the crystal element and the substrate off the polishing disc, and irradiating the tape with UV light to make the crystal element and the light-sensitive tape non-destructively peel off;

[0011] 6) flipping the crystal element for rapid polishing to improve the flatness of both surfaces to within 10 μm.

[0012] 7) Precision polishing the processed surface of the crystal element to improve the flatness to 2μm, the precision polishing parameters are: rotation speed 3-5rpm, processing pressure 200-600Pa, abrasive particle size 0.3-1μm, processing time 10-20min.

[0013] Further, after step 7) above, there is step 8) below: repeating steps 3)-7) above to perform secondary iteration processing.

[0014] Further, after step 8) above, there is step 9) below: using a brush to thoroughly clean the polishing disc, and after the polishing disc and the polishing liquid supply circuit are dried, using pure anhydrous oil to perform non-abrasive polishing, further removing abrasive particles and impurities embedded on the surface of the crystal element.

[0015] Further, before step 1) above, there is a step: before polishing, using displacement compensation turning to turn the polishing disc into a micro-convex parabolic surface, and the actual shape of the micro-convex parabolic surface and the shape error of the theoretical curve are controlled within 2 microns.

[0016] Further, after turning is completed, square grooves are opened on the surface of the polishing disc, and the groove spacing is less than 6mm.

[0017] Further, the oil-based polishing liquid in step 1) is configured by polishing oil and abrasive in a ratio of 100:5-100:1, and 0.1% of abrasive particle dispersant by mass fraction is added, and the water content is controlled within the range of 50ppm-1000ppm.

[0018] Further, the flatness of the substrate in step 2) is better than 0.2μm or better than one-tenth of the target accuracy of the crystal element.

[0019] Further, the crystal element and the stop block have a sub-millimeter gap in step 3), and the front end of the stop block is processed into a circular arc shape, so that the crystal element is adaptively floating and pressurized.

[0020] Further, a composite oil liquid filtration system is configured during the polishing process in step 7), the filtration accuracy is better than 1μm, and large particle impurities and water are filtered synchronously.

[0021] Further, the surface of the vacuum adsorption pad is grooved, and the polishing disc uses a hard metal composite polishing disc.

[0022] The beneficial effect of the present application is that the crystal element is processed by the method of full-caliber polishing with a hard polishing disc, the full surface of the crystal element is in contact with the polishing tool at the same time in the polishing process, the high point of the crystal element is pressed large, the material removal is relatively fast, the processing precision of the crystal element depends on the shape precision of the polishing disc, the fast processing based on the rigid pair of research mode is realized, the crystal surface shape precision is quickly converged, and the flattening of the full surface is finally achieved; the flatness of the 300-600mm caliber crystal element is better than 2um, and the processed surface shape is smooth, the processing efficiency is high, the process stability is good, and the method can be widely applied to the processing of various specifications of optical elements, and is especially suitable for the polishing of large-caliber potassium dihydrogen phosphate type hydrolysis crystal element. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a polishing schematic diagram of the present application.

[0024] Figure 2 is a flatness schematic diagram of the crystal element after polishing in example 1.

[0025] Figure 3 is a roughness schematic diagram of the crystal element after polishing in example 1.

[0026] Figure 4 is an initial flatness schematic diagram of the crystal element in example 2.

[0027] Figure 5 is a flatness schematic diagram of the crystal element after polishing in example 2. DETAILED DESCRIPTION

[0028] As shown in Figure 1 , the polishing method of the large-caliber potassium dihydrogen phosphate type crystal of the present application adopts an adaptive polishing device, which comprises a base plate 1, a workpiece disc 3, a connector 5, a stop hook 7, a stop block 8, a vacuum adsorption pad 10 and a polishing disc 12, wherein the upper end of the stop hook 7 is installed on the workpiece disc 3 by a screw, the stop block 8 is installed at the lower end of the stop hook 7 by a screw, the connector 5 is installed at the center position of the workpiece disc 3 by a screw, the crystal element 11 is arranged on the base plate 1 by the vacuum adsorption pad 10, the stop block 8 and the lower end of the stop hook 7 limit the position of the crystal element 11, the connector 5 is connected with the main shaft of the machine tool, and the main shaft of the machine tool drives the connector 5 to rotate, so as to drive the workpiece disc and the crystal element 11 to complete the rotary motion.

[0029] The stopper 8 is preferably made of flexible polytetrafluoroethylene, and the front end of the stopper is arc-shaped to protect the side of the crystal element 11; the base plate 1 is made of high-quality aluminum with a thickness greater than 25 mm to maintain high structural rigidity and shape accuracy; the vacuum adsorption pad 10 is slotted on the surface to make the crystal element 11 and the vacuum adsorption pad 10 point contact, and the overall contact area is not greater than 40% to reduce the deformation of the crystal element 11; the polishing disc 12 is made of hard metal composite polishing disc; in order to increase the polishing pressure, the base plate connecting rod 4 and the weight plate can also be provided, and the plurality of base plate connecting rods 4 pass through the workpiece disc 3 and are connected with the weight block.

[0030] The polishing method of the large-diameter potassium dihydrogen phosphate crystal of the present application comprises the following steps:

[0031] 1) Configure an oil-based polishing liquid: polish oil and abrasive are configured in a ratio of 100:5-100:1, and about 0.1% of a mass fraction of a grinding particle dispersant is added. The grinding particle dispersant can improve the suspension properties of the grinding particles, and the water content is controlled in the range of 50-1000 ppm. In precision polishing, a polishing liquid with low water content and fine particle size abrasive can be used; in rapid polishing, a polishing liquid with high water content and large particle size abrasive can be used to improve the reaction efficiency through micro-dissolution chemical action. The abrasive generally can be diamond, alumina, etc., and the abrasive form should be spherical as much as possible to improve the surface quality of the processing;

[0032] 2) Process a base plate 1 with the same size as the crystal element 11. The flatness of the base plate 1 is better than 0.2 μm or better than one-tenth of the target accuracy of the crystal element 11, and a layer of flexible vacuum adsorption pad 10 with the same thickness is attached to the surface of the base plate 1;

[0033] 3) The processed crystal element 11 is wiped clean with anhydrous ethanol or other analytical pure organic solvents, and a layer of light-induced adhesion loss protective tape is attached to the back. The crystal element 11 is adsorbed on the vacuum adsorption pad 10, and the crystal element 11 and the base plate 1 are integrally installed on the tooling of the machine tool to realize low-strain rapid clamping. The crystal element 11 and the stopper 8 have a sub-millimeter gap, so that the crystal element 11 self-adapts to floating pressure, the polishing pressure is derived from the gravity of the crystal element 11 and the base plate 1, the pressure at each point of the crystal element 11 is uniform, and the processing process will not produce edge collapse;

[0034] 4) Rapidly polish the processed surface of the crystal element 11 to remove residual lines or deep defects in the previous stage, and increase the flatness to within 10 μm. The rapid polishing parameters are: rotation speed 15-20 rpm, processing pressure 800-1200 Pa, abrasive particle size ≥1 μm, and processing time 20-40 min;

[0035] 5) After the rapid polishing is completed, the workpiece disc 3 is lifted, the crystal element 11 and the substrate 1 are taken off from the polishing disc 12 as a whole, and the UV light is used to irradiate the adhesive tape, so that the crystal element 11 is peeled off from the photosensitive adhesive tape without damage, and then the surface of the crystal element 11 is cleaned by using a soft and clean cloth;

[0036] 6) The crystal element 11 is turned over and rapidly polished, and the double-side flatness is improved to within 10 μm;

[0037] 7) The processed surface of the crystal element 11 is precisely polished, and the flatness is improved to about 2 μm. The precise polishing parameters are as follows: the rotating speed is 3-5 rpm, the processing pressure is 200-600 Pa, the abrasive particle size is 0.3-1 μm, and the processing time is 10-20 min. During the polishing process, a composite oil liquid filtering system is configured, and the filtering precision is better than 1 μm, so that large-particle impurities and moisture can be filtered synchronously;

[0038] 8) In order to improve the final processing precision, the above steps 3)-7) can be repeated for two times of iteration processing. Different from conventional processing, the processing process does not need to be detected, the operation is simple, and the processing efficiency is high;

[0039] 9) The polishing disc 12 is completely cleaned by using a brush disc, and after the polishing disc 12 and the polishing liquid supply circuit are dried, non-abrasive polishing is performed by using pure anhydrous oil liquid, so as to further remove the abrasive particles and impurities embedded in the surface of the crystal element 11, and improve the surface quality.

[0040] In the present application, the metal composite polishing disc is used as the polishing tool. Different from the flexible polishing pad in the conventional optical processing, the polishing disc 12 is a rigid polishing tool, the processing precision of the crystal element 11 is mainly affected by the shape precision of the polishing disc 12, the removal efficiency is higher, and the edge collapse effect is less. The polishing disc 12 will not be deformed during the processing process, and the shape precision of the polishing tool can be kept for a long time. The metal composite polishing disc has a low elastic modulus and a porosity of not less than 30%, and the abrasive can be embedded in the polishing tool. The abrasive can be embedded in the polishing disc 12, so as to improve the removal efficiency, and avoid that the abrasive causes brittle cracks of the crystal element 11, and the processing surface quality is good. The metal composite polishing disc has oil resistance, and avoids the failure and delamination of the conventional polyurethane, polishing cloth and pitch disc under the erosion of the organic oil liquid.

[0041] In the present application, the polishing disc 12 is first turned to a specified shape by using the displacement compensation turning method before polishing, the shape precision of the polishing disc 12 is controlled to within 2 μm by combining the non-contact precise detection of the polishing disc 12, then the crystal element 11 is rapidly polished by using the anhydrous polishing liquid, and the iteration convergence of the two large surfaces of the crystal element 11 can be realized by the double-side alternate turning method. The processing method has a large tolerance for the crystal inlet shape, and the crystal element 11 does not need to be precisely detected during the processing process.

[0042] To improve the precision convergence efficiency of the crystal element 11, the polishing disc 12 is preferably a micro-convex parabolic surface, and the actual shape of the micro-convex parabolic surface is controlled to be within 2 microns of the shape error of the theoretical curve. After turning, the surface of the polishing disc 12 is opened into a square groove with a groove spacing of less than 6 mm, which can improve the transport capacity of the abrasive and reduce the material removal non-uniformity. After the polishing disc 12 is used for a certain period of time, it needs to be turned again to remove the surface passivation layer, improve the processing efficiency, and restore the profile shape and surface processing characteristics.

[0043] Example 1:

[0044] 1) Install the synthetic tin polishing disc on the main shaft of the machine tool, the porosity of the polishing disc material is ≥30%, the radius of the polishing disc 12 is equivalent to the aperture of the crystal element 11; combined with the non-contact precision detection of the polishing disc 12, the polishing disc 12 is turned into a convex parabolic surface by using displacement compensation turning method, the height value of the high and low points of the parabolic surface is not more than 5 microns, and the shape error of the actual shape of the parabolic surface is controlled to be within 2 microns of the theoretical curve; a 3mm diameter high-speed milling cutter is used for slotting, the groove shape is square, the groove spacing is 6mm, and the groove depth is 5mm;

[0045] 2) Configure oil-based polishing liquid: ① Fast polishing polishing liquid: polishing oil, abrasive is configured in a proportion of 50:1, and a 0.1% mass fraction of abrasive dispersant is processed, the water content is 800ppm, and the abrasive is 1 micron alumina; ② Precise polishing polishing liquid: polishing oil, abrasive is configured in a proportion of 50:1, and a 0.1% mass fraction of dispersant is processed, the water content is 300ppm, and the abrasive is 0.3 micron alumina;

[0046] 3) Prepare a 200mm×200mm×30mm aluminum substrate 1, the flatness of the substrate 1 is better than 0.1 microns, the surface of the substrate 1 is pasted with a vacuum adsorption pad 10, and the surface of the vacuum adsorption pad 10 is grooved to make the vacuum adsorption pad 10 evenly distributed in small square blocks;

[0047] 4) Rough cutting of 200mm×200mm×8mm polishing crystal element 11, the flatness of the crystal element 11 is less than 50 microns, the crystal element 11 is wiped clean with anhydrous ethanol, and a layer of light-induced adhesion protection tape is pasted on the back. The crystal element 11 is adsorbed on the vacuum adsorption pad 10, and the crystal element 11 and the substrate 1 are integrally installed on the tooling of the machine tool, and there is a 0.5mm gap between the crystal element 11 and the flexible polytetrafluoroethylene stopper 8, so that the crystal element 11 realizes floating pressurization;

[0048] 5) Install a weight block on the lower surface of the workpiece disk 3. When the workpiece disk 3 descends, the weight block first slowly presses on the substrate 1, and then descends further, so that the stop block 8 is basically flush with the crystal element 11. At this time, the conical surface of the substrate connecting rod 4 is separated from the inner hole of the workpiece disk 3, that is, the weight block is pressed on the substrate 1 by gravity.

[0049] 6) Perform rapid polishing on the machined surface to remove residual lines or deep defects from the previous stage and improve the flatness to within 10μm. The rapid polishing parameters are: rotation speed 15rpm, processing pressure 800Pa, polishing fluid supply flow rate 200ml / min, and processing time 30min.

[0050] 7) After rapid polishing is completed, lift the workpiece disk 3. The workpiece disk 3 drives the substrate connecting rod 4 to rise a few centimeters. The conical surface of the substrate connecting rod 4 is stuck in the inner hole of the workpiece disk 3, and then drives the weight block to rise until it leaves the substrate 1.

[0051] 8) Remove the crystal element 11 and the substrate 1 from the polishing disc 12 as a whole, irradiate the tape with UV light to peel the crystal element 11 off from the photosensitive tape without damage, and then wipe the surface of the crystal element 11 clean with a soft and clean silk cloth.

[0052] 9) Repeat steps 4)-8) above to complete the rapid polishing of the reverse side of crystal element 11, so that the flatness of both sides is improved to within 10μm;

[0053] 10) Remove the counterweight and place the crystal element 11 into the fixture as in step 4);

[0054] 11) Perform precision polishing on the machined surface to improve the flatness to about 2μm. The precision polishing parameters are: rotation speed 3rpm, processing pressure 200Pa, polishing fluid supply flow rate 150ml / min, and processing time 20min.

[0055] 12) Repeat steps 10)-11) to perform precision polishing on the other side of the crystal element 11;

[0056] 13) After precision polishing, abrasive-free polishing is performed using pure anhydrous oil to remove abrasive particles and impurities embedded on the surface of crystal element 11. An oil filtration system with a filtration accuracy better than 1μm is used during polishing to simultaneously filter large particles and moisture. The polishing parameters are: rotation speed 10 rpm, processing pressure 200 Pa, polishing fluid supply flow rate 250 ml / min, and processing time 5 min.

[0057] 14) After double-sided polishing, wipe the surface of crystal element 11 clean with a soft, clean silk cloth, and test crystal element 11: the flatness PV of crystal element 11 reaches 728nm, as shown. Figure 2 As shown, the surface has no obvious defects, and the roughness Ra is within 2 nm.Figure 3 as shown.

[0058] Example 2

[0059] 1) Install the synthetic tin polishing disc to the main shaft of the machine tool, the porosity of the polishing disc material is greater than or equal to 30%, the radius of the polishing disc 12 is equivalent to the aperture of the crystal element 11; in combination with the non-contact precision detection of the polishing disc 12, the polishing disc 12 is turned into a convex parabolic surface by using the displacement compensation turning method, the height difference of the high and low points of the parabolic surface is not more than 6μm, and the shape error between the actual shape of the parabolic surface and the theoretical surface is controlled within 2μm; a 5mm diameter high-speed milling cutter is used for slotting, the slot shape is square, the slot spacing is 5mm, and the slot depth is 5mm;

[0060] 2) Configure the oil-based polishing liquid: ① Fast polishing polishing liquid: polishing oil, abrasive are configured in a ratio of 100:5, and a mass fraction of 0.1% abrasive dispersant is processed, the water content is 800ppm, and the abrasive is 2μm alumina. ② Precise polishing polishing liquid: polishing oil, abrasive are configured in a ratio of 50:1, and a mass fraction of 0.1% dispersant is processed, the water content is 300ppm, and the abrasive is 0.3μm alumina;

[0061] 3) Prepare a 400mm×400mm×30mm aluminum substrate 1, the flatness of the substrate 1 is better than 0.1μm, a vacuum adsorption pad 10 is attached to the surface of the substrate 1, and the vacuum adsorption pad 10 is evenly distributed in the form of small squares by slotting the surface of the vacuum adsorption pad 10;

[0062] 4) The crystal element 11 is wiped clean with anhydrous ethanol on both sides, and a layer of light-induced adhesion-reducing protective tape is attached to the back. The crystal element 11 is adsorbed on the vacuum adsorption pad 10, and the crystal element 11 and the substrate 1 are integrally installed on the tooling of the machine tool, with a 0.2mm gap between the crystal element 11 and the flexible polytetrafluoroethylene stopper 8, so that the crystal element 11 realizes floating pressurization;

[0063] 5) Install the weight block on the lower surface of the workpiece disc 3, when the workpiece disc 3 descends, the weight block first slowly presses on the substrate 1, and then further descends, so that the stopper 8 is basically flush with the crystal element 11, at this time the tapered surface of the substrate connecting rod 4 is separated from the inner hole of the workpiece disc 3, that is, the weight block is completely pressed on the substrate 1 by gravity;

[0064] 6) Fast polishing the machined surface to remove the residual lines or deep defects of the previous stage, and increase the flatness to within 10μm, the fast polishing parameters are: speed 20rpm, processing pressure 1100Pa, polishing liquid supply flow 400ml / min, processing time 40min;

[0065] 7) After rapid polishing is completed, lift the workpiece disk 3. The workpiece disk 3 drives the substrate connecting rod 4 to rise a few centimeters. The conical surface of the substrate connecting rod 4 is stuck in the inner hole of the workpiece disk 3, and then drives the weight block to rise until it leaves the substrate 1.

[0066] 8) Remove the crystal element 11 and the substrate 1 from the polishing disc 12 as a whole, irradiate the tape with UV light to peel the crystal element 11 off from the photosensitive tape without damage, and then wipe the surface of the crystal element 11 clean with a soft and clean silk cloth.

[0067] 9) Repeat steps 4)-8) above to complete the rapid polishing of the reverse side of crystal element 11, so that the flatness of both sides is improved to within 10μm;

[0068] 10) Remove the counterweight and place the crystal element 11 into the fixture as in step 4);

[0069] 11) Perform precision polishing on the machined surface to improve the flatness to about 2μm. The precision polishing parameters are: rotation speed 5rpm, processing pressure 300Pa, polishing fluid supply flow rate 150ml / min, and processing time 20min.

[0070] 12) Repeat steps 10)-11) to perform precision polishing on the other side of the crystal element 11;

[0071] 13) After precision polishing, abrasive-free polishing is performed using pure anhydrous oil to remove abrasive particles and impurities embedded on the surface of crystal element 11. An oil filtration system with a filtration accuracy better than 1μm is used during polishing to simultaneously filter large particles and moisture. The polishing parameters are: rotation speed 10 rpm, processing pressure 200 Pa, polishing fluid supply flow rate 250 ml / min, and processing time 5 min.

[0072] 14) After polishing, wipe the surface of crystal element 11 clean with a soft, clean silk cloth, and inspect crystal element 11: the initial flatness PV of the large-diameter crystal element 11 is approximately 20μm, such as... Figure 4 As shown, the flatness PV value converges to within 2μm after processing, as... Figure 5 As shown.

Claims

1. A polishing method for a large-diameter potassium dihydrogen phosphate class crystal element, characterized by, The method comprises the following steps: 1) configure the oil-based polishing liquid; 2) paste the vacuum adsorption pad (10) on the surface of the substrate (1) with the same size as the crystal element (11); 3) paste the light-induced debonding protective tape on the back of the crystal element (11), adsorb the crystal element (11) on the vacuum adsorption pad (10), and install the crystal element (11) and the substrate (1) as a whole on the tool of the machine tool; 4) quickly polish the processed surface of the crystal element (11) to improve the flatness to within 10μm, and the quick polishing parameters are: rotation speed 15-20rpm, processing pressure 800-1200Pa, abrasive particle size ≥1μm, and processing time 20-40min; 5) after the quick polishing is completed, lift the workpiece disc (3), take down the crystal element (11) and the substrate (1) from the polishing disc (12), and irradiate the tape with UV light to make the crystal element (11) and the tape be stripped without damage, and the polishing disc (12) adopts a hard metal synthetic polishing disc; 6) turn over the crystal element (11) and quickly polish to improve the flatness of both surfaces to within 10μm; 7) precisely polish the processed surface of the crystal element (11) to improve the flatness to 2μm, and the precise polishing parameters are: rotation speed 3-5rpm, processing pressure 200-600Pa, abrasive particle size 0.3-1μm, and processing time 10-20min.

2. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to claim 1, characterized by, After the above step 7), there is a step 8) of repeating the above steps 3)-7) for secondary iteration processing.

3. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to claim 2, characterized by, After the above step 8), there is a step 9) of completely cleaning the polishing disc (12) by using a brush disc, and after the polishing disc (12) and the polishing liquid supply circuit are dried, polishing without abrasive is carried out by using pure anhydrous oil to further remove the abrasive particles and impurities embedded on the surface of the crystal element (11).

4. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to Claim 1, wherein Before the above step 1), there is a step of turning the polishing disc (12) into a micro-convex parabolic surface by using displacement compensation turning before polishing, and the shape error between the actual shape of the micro-convex parabolic surface and the shape of the theoretical curve is controlled within 2μm.

5. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to claim 4, wherein After turning, square grooves are opened on the surface of the polishing disc (12), and the groove spacing is less than 6mm.

6. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to Claim 1, wherein The oil-based polishing liquid in step 1) is configured by using polishing oil and abrasive in a proportion of 100:5-100:1, and 0.1% of abrasive particle dispersant is added, and the water content is controlled within the range of 50ppm-1000ppm.

7. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to Claim 1, wherein The flatness of the substrate (1) in step 2) is better than 0.2μm or better than one-tenth of the target accuracy of the crystal element (11).

8. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to Claim 1, wherein The crystal element (11) has a sub-millimeter gap with the stop block (8), and the front end of the stop block (8) is processed into a circular arc shape, so that the crystal element (11) is adaptively floated and pressurized.

9. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to Claim 1, wherein A composite oil liquid filtering system is configured during the polishing process of step 7), the filtering accuracy is better than 1μm, and large particle impurities and water are filtered synchronously.

10. The polishing method of a large-aperture potassium dihydrogen phosphate class crystal element according to Claim 1, wherein The surface of the vacuum adsorption pad (10) is grooved.

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