A Subsurface Damage Detection Method for Small Curved Parts Based on Small-Speckle Magnetorheological Finishing

Abstract

A method for detecting subsurface damage of small curved surface parts based on small-spot magnetorheological polishing, which relates to the field of ultra-precision machining. To solve the problems that the detection accuracy of existing methods is greatly affected by the resolution and motion accuracy of the system and the detection accuracy is relatively low. The method includes the following processes: performing single-point polishing on the small curved surface part, and etching the small curved surface part; obtaining the three-dimensional topographic profile of the polished spot on the etched small curved surface part, and intercepting its two-dimensional profile to obtain the cross-sectional profile of the polished spot, where the cross-sectional profile of the polished spot includes a grinding area and a polishing area; using a microscope to observe the microscopic surface topography of the polished spot after etching, and measuring the horizontal distance D from the position where the subsurface crack disappears at the polished spot to the polishing boundary line; reconstructing the fitting contour curve of the removed material, calculating the shortest distance from the position where the crack disappears on the cross-sectional profile of the polished spot to the fitting contour curve of the removed material, and obtaining the result of the subsurface damage depth.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-precision machining, and in particular, to a method for detecting subsurface damage of small curved surface parts based on small-spot magnetorheological polishing. Background Art

[0002] With the continuous progress of manufacturing technology, the core devices applied in the fields of aerospace, ships, satellites, etc. show the characteristics of miniaturization. The hemispherical resonator is a sensitive element of the hemispherical resonator gyroscope, and is a typical small-diameter thin-walled complex component. High-precision form and position accuracy and surface quality are obtained by ultra-precision grinding and magnetorheological polishing of high-purity fused quartz with a small-diameter ball-end grinding wheel. However, due to the hard and brittle characteristics of the fused quartz material, subsurface damage will inevitably occur during the ultra-precision grinding process. The existence of the damaged layer not only affects the efficiency of subsequent polishing, but also affects the quality factor and frequency splitting of the hemispherical resonator, reducing the service performance of the gyroscope. Therefore, comprehensively, accurately, and quantitatively characterizing the subsurface damage generated by hard and brittle materials such as fused quartz during the grinding process is a key issue in the research of grinding processes.

[0003] Currently, common subsurface damage detection technologies include two categories, namely destructive detection and non-destructive detection. Among them, destructive detection means include angle polishing method, magnetorheological polishing method, chemical etching method, etc. With the application and popularization of optical microscopes, many biological and medical imaging technologies have gradually been used for the detection of subsurface damage. The optical imaging technology combined with digital image processing technology provides technical ideas for the non-destructive, on-line, and efficient detection of subsurface damage, and thus non-destructive detection methods such as total internal reflection microscopy, optical coherence tomography, laser scattering confocal technology, fluorescence detection method, and polarized laser confocal technology have been derived. However, whether it is destructive detection or non-destructive detection means, most of them are for flat parts, and there is less research on the detection of subsurface damage of curved surface parts, especially the subsurface damage of such anisotropic thin-walled components as hemispherical resonators. When using traditional destructive detection means to detect thin-walled complex parts such as hemispherical resonators, it is difficult to machine a wedge-shaped inclined surface or a flat cross-section at any position on the outer curved surface, and the machining process is difficult to control. At the same time, the vast majority of destructive detection means are prone to generate new damage, thus affecting the measurement results of the subsurface damage depth. Therefore, traditional destructive detection means are not suitable for the detection of subsurface damage of hemispherical resonators. Non-destructive detection, that is, using the corresponding differences of subsurface damage to acoustic waves, light fields, and X-rays to achieve its detection, its accuracy is greatly affected by the resolution and motion accuracy of the system, and the detection accuracy is low. At present, the detection of sub-micron damage is still a difficult problem. Summary of the Invention

[0004] The technical problem to be solved by the present invention is:

[0005] Existing detection methods for damage to curved surface parts have problems such as the detection accuracy being greatly affected by the resolution and motion accuracy of the system, and the detection accuracy being relatively low.

[0006] The technical solution adopted by the present invention to solve the above technical problems:

[0007] The present invention provides a sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing, including the following steps:

[0008] S1. Perform single-point polishing on the small curved surface part after grinding to form a polishing spot;

[0009] S2. Clean the small curved surface part after single-point polishing, immerse the cleaned small curved surface part in an etching solution for etching to expose the sub-surface cracks covered under the polished hydrolysis layer. After the etching is completed, clean the surface of the small curved surface part to remove the residual etching solution on the surface;

[0010] S3. Obtain the three-dimensional topography profile of the polishing spot of the small curved surface part after etching to obtain the three-dimensional topography data of the polishing spot, and intercept the two-dimensional profile of the three-dimensional topography profile of the polishing spot to obtain the cross-sectional profile of the polishing spot. The cross-sectional profile of the polishing spot includes a grinding area and a polishing area;

[0011] S4. Measure the horizontal distance D from the position where the sub-surface crack disappears at the polishing spot to the polishing boundary line, and determine the position of the corresponding crack disappearance point on the cross-sectional profile of the polishing spot according to the polishing boundary point;

[0012] S5. Utilize the contour data of the grinding area, and based on the cross-sectional profile of the polishing spot, fit and reconstruct the profile of the removed material to obtain the fitted profile curve of the removed material, and calculate the shortest distance from the position where the crack disappears on the cross-sectional profile of the polishing spot to the fitted profile curve of the removed material to obtain the sub-surface damage depth of the small curved surface part.

[0013] Further, the fitted profile curve of the removed material in S5 is:

[0014]

[0015] where x i and y i are the coordinates of the i-th point on the fitted profile of the removed material, g(x, y) is the curve equation of the fitted profile of the removed material, R is the fitted profile radius, x o and y o are the center of the fitted circle;

[0016] The relationship between the crack disappearance point and the polishing boundary point on the cross-sectional profile line of the polishing spot is:

[0017] xcv = x pi + D

[0018] where x cv and x pi are respectively the horizontal coordinates of the crack disappearance point and the polishing junction point;

[0019] Calculate the shortest distance from the crack disappearance point on the cross-sectional profile of the polishing spot to the fitted contour curve of the removed material as:

[0020]

[0021] where x c and y c are the intersection points of the line connecting the crack disappearance point and the center of the fitted circle with the fitted circle, that is:

[0022]

[0023] Furthermore, the single-point polishing of the small curved surface part after grinding in S1 includes the following steps:

[0024] S11. Perform single-point polishing tool setting on the small curved surface part after grinding;

[0025] S12. Configure the magnetorheological fluid;

[0026] S13. Adjust the position of the bamboo joint pipe nozzle so that the magnetorheological fluid drops towards the polishing head; make the polishing head rotate at a preset speed, and the workpiece spindle does not rotate, that is, the small curved surface part remains stationary, and perform single-point polishing on the small curved surface part;

[0027] Furthermore, S11 performing single-point polishing tool setting on the small curved surface part after grinding includes the following steps:

[0028] S111. According to the grinding wheel movement trajectory during the last grinding process, draw the two-dimensional contour of the outer surface of the small curved surface part, and offset the outer curve of the contour by a distance R1 + Δd in the normal direction to the outside, where R1 is the radius of the polishing head and Δd is the polishing gap between the polishing head and the workpiece, to obtain the movement trajectory of the polishing ball center for actually polishing the outer surface of the small curved surface part;

[0029] S112. Select a certain position on the two-dimensional contour of the outer surface of the small curved surface part as the single-point polishing position, and record the coordinates (x1, y1) of this position in the workpiece coordinate system. The intersection point (x2, y2) of the single-point polishing position coordinates (x1, y1) in the normal direction of the contour and the movement trajectory of the polishing ball center is the polishing ball center position corresponding to polishing the point (x1, y1);

[0030] S113. Convert the position coordinates (x2, y2) of the polishing head in the workpiece coordinate system to the machine tool coordinate system to obtain the position coordinates (x3, y3) of the center of the polishing ball in the machine tool coordinate system;

[0031] S114. With the help of a CCD camera and a magnifying lens, adjust the height of the center of the polishing ball to be the same as the height of the rotation axis of the hemispherical resonator;

[0032] S115. Move the polishing head to the position (x3 + δ, y3) in the machine tool coordinate system, where δ is the reserved value for the tool setting error;

[0033] S116. Move the polishing head in the direction close to the workpiece at a certain feed rate f to reduce the gap between the polishing head and the surface of the small curved surface part;

[0034] S117. Manually rotate the workpiece spindle. If no collision sound is generated, execute S116 again; if a periodic sound appears, directly execute the next step;

[0035] S118. At the current position, move the X-axis workbench to move the polishing head away from the workpiece by a distance of Δd to complete the tool setting.

[0036] Further, the polishing gap Δd between the polishing head and the workpiece in S111 is 80 μm to 100 μm, preferably 80 μm.

[0037] Further, the method for configuring the magnetorheological fluid in S13 is as follows: Add 3.5 parts by mass of cellulose to 500 parts of hot water at 100 °C, stir evenly, then add 412 parts of normal temperature water at 20 °C, stir evenly, add 168 parts of cerium oxide polishing powder, stir evenly, and finally add 2100 parts of hydroxyl iron powder and stir evenly to obtain the magnetorheological fluid.

[0038] Further, the time for single-point polishing of the small curved surface part in S14 is 15 min to 20 min.

[0039] Further, the etching solution in S2 contains 1% HF and 15% NH4F by mass fraction.

[0040] Further, the etching time in S2 is 12 min to 15 min.

[0041] Further, in S3, a white light interferometer is used to measure the three-dimensional topography profile of the single-point polishing area of the small curved surface part.

[0042] Compared with the prior art, the beneficial effects of the present invention are:

[0043] A method for detecting subsurface damage of small curved surface parts based on small-spot magnetorheological polishing. By using magnetorheological spot polishing, the measurement of crack depth is transformed into the measurement of the horizontal extension distance of millimeter-scale cracks. Especially when the subsurface damage depth is in the micron range, it can play a role in amplifying the subsurface damage. At the same time, the magnetorheological polishing method adopted in the present invention effectively avoids the problem of new damage introduced during the processing by traditional methods, exposes the subsurface damage under the grinding surface to the surface, and realizes high-precision detection of subsurface damage of hemispherical resonators during grinding.

[0044] The method of the present invention can measure the subsurface damage depth at various positions of small curved surface parts, provide parameter basis for the subsurface damage depth that needs to be removed by the polishing process, and has high feasibility and accuracy. At the same time, the method of the present invention can provide accurate parameter basis for the optimization of the grinding process.

[0045] The method of the present invention has a certain universality. It can not only be extended to the detection of subsurface damage of small-diameter (φ20 - φ50mm) rotary parts, but also has strong adaptability to the detection of subsurface damage of most curved surface parts. Brief Description of the Drawings

[0046] Figure 1 It is a flowchart of the method for detecting subsurface damage of small curved surface parts based on small-spot magnetorheological polishing in the embodiment of the present invention;

[0047] Figure 2 It is a schematic structural diagram of the processing equipment in the embodiment of the present invention;

[0048] Figure 3 It is a schematic diagram of small-spot magnetorheological polishing of small curved surface parts in the embodiment of the present invention;

[0049] Figure 4 It is a schematic diagram of single-point polishing of hemispherical resonators in the embodiment of the present invention;

[0050] Figure 5 It is a three-dimensional topographic contour map of the polishing spot in the embodiment of the present invention;

[0051] Figure 6 It is a cross-sectional contour map of the polishing spot in the embodiment of the present invention;

[0052] Figure 7 It is a microscopic image and subsurface crack map of the polishing spot area in the embodiment of the present invention;

[0053] Figure 8 It is a schematic diagram of the principle of subsurface damage measurement in the embodiment of the present invention;

[0054] Figure 9 It is a subsurface damage map of the end face of the hemispherical resonator blank in the embodiment of the present invention.

[0055] Description of the reference numerals: 1 - C-axis turntable, 2 - U-axis connecting frame, 3 - V-axis, 4 - tool spindle fixing bracket, 5 - tool spindle, 6 - polishing head, 7 - magnetorheological fluid recovery base, 8 - horizontal workbench, 9 - workpiece spindle protection cover, 10 - workpiece to be machined, 11 - workpiece spindle, 12 - bamboo joint pipe nozzle, 13 - U-axis protection cover, 14 - U-axis. Detailed implementation manners

[0056] In the description of the present invention, it should be noted that in the embodiments of the present invention, the terms "first", "second", and "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first", "second", and "third" may explicitly or implicitly include one or more of such features.

[0057] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention will be given with reference to the accompanying drawings.

[0058] Specific implementation manner 1: The present invention provides a method for detecting subsurface damage of small curved surface parts based on small-spot magnetorheological polishing. As Figure 3 shown, small-spot magnetorheological polishing refers to performing magnetorheological single-point polishing on the grinding surface of small curved surface parts using a small-diameter permanent magnet polishing tool and forming polishing spots on the workpiece surface; as Figure 1 shown, it includes the following steps:

[0059] S1. Perform single-point polishing on the small curved surface parts after grinding to form polishing spots;

[0060] S2. Clean the small curved surface parts after single-point polishing, immerse the cleaned small curved surface parts in an etching solution for etching to expose the subsurface cracks covered under the polished hydrolysis layer. After the etching is completed, clean the surface of the small curved surface parts to remove the residual etching solution on the surface;

[0061] S3. Obtain the three-dimensional morphology profile of the polishing spots of the small curved surface parts after etching to obtain the three-dimensional morphology data of the polishing spots, and intercept the two-dimensional profile of the three-dimensional morphology profile of the polishing spots to obtain the cross-sectional profile of the polishing spots. The cross-sectional profile of the polishing spots includes a grinding area and a polishing area;

[0062] S4. Measure the horizontal distance D from the disappearance point of the subsurface cracks at the polishing spots to the polishing junction line, and determine the position of the corresponding crack disappearance point on the cross-sectional profile of the polishing spots according to the polishing junction point;

[0063] S5. Using the contour data of the grinding area, based on the cross-sectional contour of the polishing spot, fitting and reconstructing the contour of the removed material to obtain the fitted contour curve of the removed material, calculating the shortest distance from the position where the crack disappears on the cross-sectional contour of the polishing spot to the fitted contour curve of the removed material, and obtaining the subsurface damage depth of the small curved surface part.

[0064] As Figure 2 shown, in this implementation, an existing device is used: an ultra-precision grinding device with an inclined and spatially rotatable grinding wheel spindle (application number: CN201710186959.2). This device is a four-axis linkage magnetorheological finishing machine tool, including three linear moving platforms, a C-axis turntable 1, two precision fine-tuning linear feed platforms U-axis 14 and V-axis 3, as well as a tool spindle 5 and a workpiece spindle 11. The three linear moving platforms are the X-axis linear platform, the Y-axis linear platform, and the Z-axis linear platform respectively. The horizontal workbench 8 is installed on the X-axis linear platform and the Y-axis linear platform. By controlling the X-axis linear platform and the Y-axis linear platform through the control system, the horizontal workbench 8 can be linearly moved along the X-axis direction and the Y-axis direction. Both the X-axis direction and the Y-axis direction are horizontal directions. The tool spindle 5 is installed on the Z-axis linear platform, and the tool spindle 5 can be linearly moved along the Z-axis direction through the control system. The Z-axis direction is the vertical direction. The C-axis turntable 1 is used to realize the rotational movement of the tool spindle 5 along the Z-axis. The polishing head 6 is installed at the output end of the tool spindle 5 through a precision chuck to realize high-speed rotation during processing. The included angle between the tool spindle 5 and the horizontal plane is 40°. The tool spindle 5 is suspended and installed below the U-axis 14 through the tool spindle fixing bracket 4, and the U-axis 14 is provided with a U-axis protective cover 13. The U-axis 14 is rigidly connected to the lower part of the C-axis turntable 1 through the U-axis connecting bracket 2. The V-axis 3 is installed below the C-axis turntable 1, and the lower end of the V-axis 3 is connected to the tool spindle fixing bracket 4. The tool spindle 5 is finely adjusted in the X-axis direction and the Y-axis direction through the U-axis 12 and the V-axis 3. The workpiece spindle 11 is provided with a workpiece spindle protective cover 9 and is installed on the upper surface of the horizontal workbench 8 through the workpiece spindle protective cover 9. The axis direction of the workpiece spindle is parallel to the Y-axis movement direction. The workpiece to be processed 10 is clamped at the output end of the workpiece spindle 11. This device is provided with a bamboo joint pipe nozzle 12 and a magnetorheological fluid recovery base 7. The magnetorheological fluid recovery base 7 is arranged below the bamboo joint pipe nozzle 12. The bamboo joint pipe nozzle 12 is adjusted to make the magnetorheological fluid drip towards the polishing head to realize the supply of the magnetorheological fluid. The magnetorheological fluid recovery base 7 is used to recover the magnetorheological fluid flowing out of the bamboo joint pipe nozzle 12 and the magnetorheological fluid splashed by the polishing head during the polishing process. In this implementation, after the grinding process of the small curved surface part is completed, the numerical control system is controlled to move the upper wheel away from the small curved surface part along the X-axis direction of the machine tool, and the grinding wheel installed on the tool spindle is replaced with a small-diameter permanent magnet polishing head 6.

[0065] In this implementation, in S2, deionized water, dilute hydrochloric acid, and alcohol are respectively used to clean the magnetorheological fluid remaining on the surface of the small curved parts after single-point polishing; deionized water and absolute ethanol are respectively used to clean the etching solution remaining on the surface of the small curved parts after the etching is completed.

[0066] The small curved parts in this implementation are hemispherical resonators, which are composed of a central rod and a thin-walled hemispherical shell.

[0067] Specific implementation two: The fitted contour curve of the material to be removed in S5 is:

[0068]

[0069] Among them, x i and y i are the coordinates of the i-th point on the fitted contour of the material to be removed, g(x, y) is the curve equation of the fitted contour of the material to be removed, R is the radius of the fitted contour, x o and y o are the centers of the fitted circle;

[0070] The relationship between the crack disappearance point and the polishing junction point on the cross-sectional contour line of the polishing spot is:

[0071] x cv = x pi + D

[0072] Among them, x cv and x pi are respectively the horizontal coordinates of the crack disappearance point and the polishing junction point;

[0073] Calculate the shortest distance from the crack disappearance point on the cross-sectional contour of the polishing spot to the fitted contour curve of the material to be removed as:

[0074]

[0075] Among them, x c and y c are the intersection points of the line connecting the crack disappearance point and the center of the fitted circle with the fitted circle, that is:

[0076] Other parts of this implementation are the same as those of the first specific implementation.

[0077] Specific implementation three: In S1, single-point polishing is performed on the small curved parts after grinding, including the following steps:

[0078] S11. Perform tool setting for single-point polishing on the small curved parts after grinding;

[0079] S12. Configure the magnetorheological fluid;

[0080] S13. Adjust the position of the knurled tube nozzle so that the magnetorheological fluid droplets move towards the polishing head. Rotate the polishing head at a preset speed while keeping the workpiece spindle stationary, i.e., the small curved part remains stationary, and perform single-point polishing on the small curved part. Other aspects of this implementation are the same as those of the first specific implementation.

[0081] This implementation also includes the following process: After the single-point polishing tool setting is completed, connect the stirrer, supply peristaltic pump, and universal knurled tube nozzle in sequence according to the magnetorheological fluid flow direction using a silicone hose. Then connect the magnetorheological fluid recovery base, recovery peristaltic pump, and stirrer in sequence, and place the magnetorheological fluid recovery base under the universal knurled tube nozzle to form a closed magnetorheological fluid circulation loop. Before performing single-point polishing, place magnetorheological fluid baffles around the small curved part to prevent the splashing of magnetorheological fluid during the processing. Turn on the supply peristaltic pump and the recovery peristaltic pump to perform single-point polishing on the small curved part.

[0082] The preset speed of the polishing head in this implementation is 7000 r / min.

[0083] Specific implementation four: S11 performs single-point polishing tool setting on the small curved part after grinding, including the following steps:

[0084] S111. According to the grinding wheel movement trajectory during the last grinding process, draw the two-dimensional contour of the outer surface of the small curved part. The outer curve of the contour is offset outward by a distance R1 + Δd along the normal direction, where R1 is the radius of the polishing head and Δd is the polishing gap between the polishing head and the workpiece, to obtain the movement trajectory of the polishing ball center for actually polishing the outer surface of the small curved part.

[0085] S 112. Select a certain position on the two-dimensional contour of the outer surface of the small curved part as the single-point polishing position and record its coordinates (x1, y1) in the workpiece coordinate system. The intersection point (x2, y2) of the single-point polishing position coordinates (x1, y1) along the contour normal direction and the polishing ball center movement trajectory is the polishing ball center position corresponding to polishing the (x1, y1) point.

[0086] S113. Convert the polishing head position coordinates (x2, y2) in the workpiece coordinate system to the machine coordinate system to obtain the position coordinates (x3, y3) of the polishing ball center in the machine coordinate system.

[0087] S114. With the help of a CCD camera and a magnifying lens, adjust the height of the polishing ball center to be consistent with the height of the hemispherical resonator rotation axis.

[0088] S115. Move the polishing head to the position (x3 + δ, y3) in the machine coordinate system, where δ is the reserved value for the tool setting error.

[0089] S116. Move the polishing head towards the workpiece at a certain feed rate f to reduce the gap between the polishing head and the surface of the small curved surface part.

[0090] S117. Manually rotate the workpiece spindle. If no collision sound is generated, execute S116 again; if a periodic sound appears, directly execute the next step.

[0091] S118. At the current position, move the X-axis workbench to move the polishing head away from the workpiece by a distance of Δd to complete the tool setting. Other aspects of this implementation scheme are the same as those of the third specific implementation scheme.

[0092] As Figure 4 shown is a schematic diagram of single-point polishing of a hemispherical resonator, and the feed rate f used is 1 μm.

[0093] Specific implementation scheme five: The polishing gap Δd between the polishing head and the workpiece in S111 is 80 μm to 100 μm, preferably 80 μm. Other aspects of this implementation scheme are the same as those of the fourth specific implementation scheme.

[0094] Specific implementation scheme six: The method for configuring the magnetorheological fluid in S13 is as follows: Add 3.5 parts by mass of cellulose to 500 parts of hot water at 100 °C, stir evenly, then add 412 parts of normal temperature water at 20 °C, stir evenly, add 168 parts of cerium oxide polishing powder, stir evenly, and finally add 2100 parts of hydroxyl iron powder, stir evenly to obtain the magnetorheological fluid. Other aspects of this implementation scheme are the same as those of the third specific implementation scheme.

[0095] Specific implementation scheme seven: The time for single-point polishing of the small curved surface part in S14 is 15 min to 20 min. Other aspects of this implementation scheme are the same as those of the third specific implementation scheme.

[0096] Specific implementation scheme eight: The etching solution described in S2 contains 1% HF and 15% NH4F by mass fraction. Other aspects of this implementation scheme are the same as those of the first specific implementation scheme.

[0097] Specific implementation scheme nine: The etching time in S2 is 12 min to 15 min. Other aspects of this implementation scheme are the same as those of the first specific implementation scheme.

[0098] Specific implementation scheme ten: In S3, a white light interferometer is used to measure the three-dimensional topography profile of the single-point polishing area of the small curved surface part. Other aspects of this implementation scheme are the same as those of the first specific implementation scheme.

[0099] As Figure 5 shown is the measurement of the three-dimensional topography profile of the polishing spot of the hemispherical resonator using a white light interferometer, and the intercepted part is as Figure 6The cross-sectional profile A-A’ of the polished spot shown, the cross-sectional profile of the grinding area is determined by the profile shape of the spherical shell of the hemispherical resonator after grinding. Theoretically, it is an arc curve, but actually it includes the peaks and valleys of the roughness features. The cross-sectional profile of the polished area is a polished pit formed by typical magnetorheological single-point polishing. Since no new damage is generated to the material during the magnetorheological polishing process, the cross-sectional profile of the polished area is smooth and continuous.

[0100] As Figure 7 shown, a microscope is used to observe the microscopic surface morphology of the polished spot after chemical etching. It can be seen from the figure that there is a relatively clear polished boundary line between the grinding area and the non-damaged area, which can be used as a reference for measuring the subsurface damage. By measuring the horizontal distance D from the polished boundary line when the subsurface crack disappears at the cross-section A-A’, as Figure 8 shown, the horizontal distance D is used to calculate the subsurface damage depth. In order to further verify the accuracy and superiority of the subsurface damage detection method provided by the present invention for the detection of curved surface parts, a traditional detection method is used to measure the subsurface damage of the end face (plane) of the hemispherical resonator blank. The cross-section polishing method is adopted, that is, the cross-section of the surface to be measured is obtained by cutting, and then the surface to be measured is polished. After polishing, the hydrolysis layer generated on the surface during the polishing process is removed by chemical etching to expose the subsurface cracks. In order to more clearly evaluate the subsurface damage at the cross-section, a scanning electron microscope is used for measurement, and it is judged whether the surface is sputtered with gold before measurement according to the conductivity of the surface to be measured (for surfaces with poor conductivity, the surface needs to be sputtered with gold before measurement). The results are as Figure 9 shown in and Table 1. The deviation between the subsurface damage depth detected by the subsurface damage detection method of small-spot magnetorheological polishing of the present invention and the subsurface damage depth result measured by the traditional subsurface damage measurement method for the outer spherical surface is 5.4%. It can be seen that the method of the present invention has high accuracy in detecting the subsurface damage depth of the curved surface of the part, and the subsurface damage detection method of small-spot magnetorheological polishing of the present invention can measure the subsurface damage depth at different positions of the curved surface of the part, which has more advantages compared with the traditional subsurface damage detection method.

[0101] Table 1

[0102]

[0103]

[0104] Although the present invention is disclosed as above, the protection scope of the present invention is not limited thereto. Those skilled in the art of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention, and these changes and modifications will all fall within the protection scope of the present invention.

Claims

1. A method for detecting subsurface damage of small curved surface parts based on small-spot magnetorheological polishing, characterized in that, It includes the following steps: S1. Perform single-point polishing on the small curved surface parts after grinding to form polishing spots; S2. Clean the small curved surface parts after single-point polishing, immerse the cleaned small curved surface parts in an etching solution for etching to expose the subsurface cracks covered under the polished hydrolysis layer. After etching, clean the surface of the small curved surface parts to remove the residual etching solution on the surface; S3. Obtain the three-dimensional topography profile of the polishing spots on the small curved surface parts after etching to obtain the three-dimensional topography data of the polishing spots. Intercept the two-dimensional profile of the three-dimensional topography profile of the polishing spots to obtain the cross-sectional profile of the polishing spots, and the cross-sectional profile of the polishing spots includes a grinding area and a polishing area; S4. Measure the horizontal distance D from the position where the subsurface cracks disappear at the polishing spots to the polishing interface line, and determine the position of the corresponding crack disappearance point on the cross-sectional profile of the polishing spots according to the polishing interface point; S5. Use the contour data of the grinding area to fit and reconstruct the profile of the removed material based on the cross-sectional profile of the polishing spots to obtain the fitted profile curve of the removed material. Calculate the shortest distance from the position where the cracks disappear on the cross-sectional profile of the polishing spots to the fitted profile curve of the removed material to obtain the subsurface damage depth of the small curved surface parts; 2. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 1, characterized in that, The fitted profile curve of the removed material in S5 is: where x i and y i are the coordinates of point i on the fitted profile of the material to be removed, g(x, y) is the curve equation of the fitted profile of the material to be removed, R is the radius of the fitted profile, x o and y o is the center of the fitted circle; The relationship between the crack disappearance point and the polishing interface point on the cross-sectional profile of the polishing spots is: x cv = x pi + D where x cv and x pi are the horizontal coordinates of the crack disappearance point and the polishing junction point, respectively; The calculation of the shortest distance from the position where the cracks disappear on the cross-sectional profile of the polishing spots to the fitted profile curve of the removed material is: where x c and y c are the intersection points of the line connecting the crack disappearance point and the center of the fitted circle with the fitted circle, i.e.:

3. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 1, characterized in that, In S1, the single-point polishing of the small curved surface parts after grinding includes the following steps: S11. Perform tool setting for single-point polishing on the small curved surface parts after grinding; S12. Configure the magnetorheological fluid; S13. Adjust the position of the bamboo joint tube nozzle so that the magnetorheological fluid drops onto the polishing head; make the polishing head rotate at a preset speed and the workpiece spindle does not rotate, that is, the small curved surface parts remain stationary, and perform single-point polishing on the small curved surface parts; 4. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 3, characterized in that S11 for performing tool setting for single-point polishing on the small curved surface parts after grinding includes the following steps: S111. According to the grinding wheel movement trajectory during the last grinding process, draw the two-dimensional profile of the outer surface of the small curved surface parts. The outer curve of the profile is offset outward by a distance R1 + Δd along the normal direction, where R1 is the radius of the polishing head and Δd is the polishing gap between the polishing head and the workpiece, to obtain the movement trajectory of the polishing ball center for actually polishing the outer surface of the small curved surface parts; S112. Select a certain position on the two-dimensional profile of the outer surface of the small curved surface parts as the position for single-point polishing, and record the coordinates (x1, y1) of this position in the workpiece coordinate system. The intersection point (x2, y2) of the coordinates (x1, y1) of the single-point polishing position along the profile normal direction and the movement trajectory of the polishing ball center is the position of the polishing ball center corresponding to polishing the point (x1, y1); S113. Convert the coordinates (x2, y2) of the polishing head position in the workpiece coordinate system to the coordinates (x3, y3) of the polishing ball center in the machine tool coordinate system; S114. With the help of a CCD camera and a magnifying lens, adjust the height of the polishing ball center to be the same as the height of the hemispherical resonator rotation axis; S115. Move the polishing head to the position (x3 + δ, y3) in the machine tool coordinate system, where δ is the reserved value for the tool setting error; S116. Move the polishing head towards the workpiece direction with a certain feed rate f to reduce the gap between the polishing head and the surface of the small curved surface part; S117. Manually rotate the workpiece spindle. If no collision sound is generated, execute S116 again; if a periodic sound appears, directly execute the next step; S118. At the current position, move the X-axis table to move the polishing head away from the workpiece by a distance of Δd to complete the tool setting.

5. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 4, wherein The polishing gap Δd between the polishing head and the workpiece in S111 is 80μm - 100μm.

6. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 3, characterized in that The method for configuring the magnetorheological fluid in S13 is as follows: Add 3.5 parts by mass of cellulose to 500 parts of hot water at 100°C, stir evenly, then add 412 parts of normal temperature water at 20°C, stir evenly, add 168 parts of cerium oxide polishing powder, stir evenly, and finally add 2100 parts of hydroxyl iron powder and stir evenly to obtain the magnetorheological fluid.

7. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 3, characterized in that The time for single-point polishing of the small curved surface part in S14 is 15min - 20min.

8. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 1, wherein The etching solution in S2 contains 1% HF and 15% NH4F by mass fraction.

9. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 1, characterized in that The etching time in S2 is 12min - 15min.

10. The sub-surface damage detection method for small curved surface parts based on small-spot magnetorheological polishing according to claim 1, characterized in that, In S3, a white light interferometer is used to measure the three-dimensional topography profile of the single-point polishing area of the small curved surface part.

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