CN118559556A - An ultra-precision grinding tool for complex surfaces with large aspect ratio and high steepness and a method of using the same - Google Patents

Abstract

The invention discloses an ultra-precise grinding tool for a complex surface with large length-diameter ratio and high gradient and a use method thereof, wherein the grinding tool comprises a grinding wheel which is of a hemispherical structure, the side of the grinding wheel is provided with a plurality of microstructure grooves distributed along the circumferential direction of the center of the grinding wheel, the microstructure grooves extend from the center to the periphery in a divergent mode, and the intervals between adjacent grooves are gradually changed according to rules. According to the invention, on one hand, the arrangement spacing of the micro grooves is changed to adapt to grinding areas with different curvatures, and on the other hand, the chip thickness is thinned to realize wide plastic domain removal, so that the roughness and waviness of the grinding surface are obviously reduced; in addition, in the case of keeping the material removal rate unchanged in the use method, the machining efficiency of the grinding process can also not be reduced. The invention can be popularized to ultra-precise machining of complex surfaces of various optical elements such as glass, crystals, ceramics and the like.

Description

An ultra-precision grinding tool for complex surfaces with large aspect ratio and high steepness and its use method

Technical Field

The invention relates to the field of optical processing technology, and in particular to an ultra-precision grinding tool for complex surfaces with large aspect ratio and high steepness, and a use method thereof.

Background Art

Compared with conventional metals and polymer materials, hard and brittle materials have stable physical and chemical properties, high hardness, high strength mechanical properties, excellent wear resistance, corrosion resistance and other advantages. Hard and brittle materials represented by optical glass, advanced ceramics and ceramic-based composites are widely used in components in the fields of aerospace, optical imaging, biomedicine, etc. However, due to their high hardness, brittleness and anisotropy, as well as the application requirements for high precision and low damage to their surfaces, ultra-precision machining technology for hard and brittle materials is quite difficult.

At present, domestic and foreign scholars have conducted extensive research on ultra-precision grinding technology for hard and brittle materials. The research mainly focuses on the analysis of grinding mechanism and optimization of process methods for plane or small curvature surfaces. However, in the ultra-precision grinding process of high-steepness components with large aspect ratios, the change in the curvature of the workpiece surface in the grinding path causes the contact position between the workpiece and the grinding wheel to change, resulting in inconsistency in the maximum undeformed chip thickness corresponding to single-grain cutting, resulting in unstable grinding force, which seriously affects the quality of the workpiece's machining surface and greatly reduces the surface shape accuracy of the machining surface. Therefore, for the ultra-precision grinding of complex surfaces with large aspect ratios and high steepness, it is necessary to develop an ultra-precision grinding tool for complex surfaces with large aspect ratios and high steepness and its use method to solve the above problems.

Summary of the invention

The present invention aims to solve the problems of unstable grinding force and low quality of processed surface in the ultra-precision processing technology for complex surfaces of optical elements in the prior art, and provides an ultra-precision grinding tool for complex surfaces with large aspect ratio and high steepness and a method of using the same.

On the one hand, to achieve the above-mentioned purpose, the present invention provides an ultra-precision grinding tool for complex surfaces with large aspect ratio and high steepness, including a grinding wheel. The grinding wheel is a hemispherical structure, and the side of the grinding wheel is provided with a plurality of grooves distributed circumferentially along its central axis, and the two ends of the grooves extend to the large end and the small end of the grinding wheel respectively, and the spacing between adjacent grooves gradually increases in the direction away from the small end.

Preferably, the distance between adjacent grooves gradually increases in a direction away from the small end and ranges from 10 μm to 42.3 μm.

On the other hand, to achieve the above-mentioned purpose, the present invention also provides a method for using an ultra-precision grinding tool for a complex surface with a large aspect ratio and high steepness, comprising the following steps:

Acquire first wear data and second wear data of the grinding wheel, wherein the first wear data includes a surface profile error of the grinding wheel, and the second wear data includes an average depth of groove wear;

When it is determined that the first wear data is greater than a first preset value, repairing the grinding wheel;

When it is determined that the second wear data is greater than a second preset value, repairing the groove;

When it is determined that the first wear data is not greater than a first preset value and the second wear data is not greater than a second preset value, the surface of the workpiece is ground using the grinding tool according to claim 1 or 2 based on the set trajectory.

Preferably, the method for obtaining the first wear data and the second wear data of the grinding wheel comprises:

The grinding wheel surface profile error and the average depth of the groove wear are obtained based on a laser micrometer.

Preferably, the first preset value is 80 μm, and the second preset value is 10 μm.

Preferably, the grinding wheel comprises a first body and a second body connected to each other, the first body is a top arc portion of the grinding wheel, and the second body is a peripheral arc portion of the grinding wheel.

Preferably, before acquiring the first wear data, the method further includes performing contour splicing on the first body and the second body to obtain the grinding wheel.

Preferably, when it is determined that the first wear data is greater than a first preset value, the method for repairing the grinding wheel includes:

The first wear data is fitted and then compared with the first preset value to obtain wear distribution data. Based on the wear distribution data, the surface profile of the grinding wheel is compensated and corrected by a repair tool.

Preferably, the particle size of the repair tool is larger than that of the wear tool, the rotation axes of the repair tool and the wear tool are perpendicular to each other, and the rotation speed of the grinding tool is larger than that of the repair tool.

Preferably, the set trajectory includes a process route of rough grinding, semi-finishing grinding or fine grinding.

Compared with the prior art, the present invention has the following advantages and technical effects:

(1) The microstructured ball-end grinding wheel of the present invention has the function of reducing the average chip thickness, thereby realizing low-damage processing, and can also ensure constant grinding force throughout the grinding path; the process parameters of the ultra-precision machining process are optimized to reduce the roughness and waviness of the surface of the optical element after grinding, thereby realizing ultra-precision machining of complex optical surfaces;

(2) Since the surface of the present invention is microstructured, the number of diamond micro-edges is increased, and the average chip thickness can be reduced at the same grinding feed depth, while the material removal rate is not reduced. On the one hand, the surface processing accuracy of the optical element is improved, which is conducive to obtaining a better processing surface quality, and on the other hand, it does not affect the processing efficiency of the optical element;

(3) Due to the variable pitch arrangement of the micro-blade array on the surface of the present invention, the maximum undeformed chip thickness and grinding force of the material in the processed area can be accurately controlled during the processing, which is conducive to obtaining better processing surface quality. In addition, the same grinding depth at each point on the grinding path is achieved by increasing the tool compensation amount Δδ on the machine tool control console to achieve the purpose of stabilizing the grinding force. This method is more efficient and has higher point position accuracy than moving the machine tool guide rail;

(4) The present invention has strong versatility. The arrangement of the microstructure groove array of the tool is adjusted according to the processing path function for different optical elements. It can be used for ultra-precision grinding of various complex surfaces such as aspheric surfaces, free-form surfaces and structural surfaces. The material can be suitable for a variety of hard and brittle difficult-to-process materials such as glass, ceramics, and crystals.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

FIG1 is a structural diagram of a microstructured grinding tool according to an embodiment of the present invention;

FIG2 is a schematic diagram of the relative position relationship and motion trajectory of a grinding tool and a workpiece according to an embodiment of the present invention;

FIG3 is a schematic diagram of a conventional grinding tool abrasive cutting process according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of an abrasive cutting process of a microstructured grinding tool according to an embodiment of the present invention.

DETAILED DESCRIPTION

It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

As shown in Figures 1 to 4, the present invention proposes an ultra-precision grinding tool for complex surfaces with a large aspect ratio and high steepness, including a grinding wheel. The grinding wheel is a hemispherical structure, and the side of the grinding wheel is provided with a plurality of grooves distributed circumferentially along its central axis, and the two ends of the grooves extend to the large end and the small end of the grinding wheel respectively, and the spacing between adjacent grooves gradually increases in the direction away from the small end.

The distance between adjacent grooves gradually increases in the direction away from the small end and ranges from 10 μm to 42.3 μm.

Furthermore, the grinding wheel is a diamond spherical grinding wheel, and its structure includes a grinding wheel hemispherical main section, a cylindrical transition section and a handle base section; the surface of the grinding wheel hemispherical main section is microstructured by picosecond laser.

The grooves are microstructure grooves, and the spacing of the microstructure grooves is distributed along the axial direction of the grinding wheel as shown in Figure 2. They are arranged with a pitch of 10μm-42.3μm from the top of the grinding wheel to the large end. The microstructure groove spacing _dt is proportional to the maximum undeformed chip thickness _hmax , so that the workpiece material removal is uniform on the grinding path, thereby making the overall grinding force constant.

The microstructure groove spacing _dt can be obtained by the following formula:

d _t = k·h _max

d _t is the spacing between the microstructure grooves, k is the proportionality coefficient, and h _max is the maximum undeformed chip thickness.

The width of the microstructure groove is controlled in the range of 7-10 μm, and the depth of the groove is controlled in the range of 15-20 μm; the width and depth of the microgroove are kept consistent in the grinding area of the grinding wheel.

The microstructure groove width d and depth h can be obtained by the following formula:

_R0 is the radius of the laser spot, _Id is the peak energy density of the picosecond laser, _It is the critical laser energy density for material removal, α is the absorption coefficient of the workpiece material, d is the width of the microstructure groove, and h is the depth of the microstructure groove.

In order to ensure that the grinding area of the ball-end grinding wheel has sufficient chip space, it is necessary to ensure that the microstructure groove depth h is greater than the maximum undeformed thickness h _max ; at the same time, in order to ensure that the microstructure abrasive grains of the grinding wheel have sufficient cutting strength, it is necessary to ensure that the back angle of the grinding edge is greater than 90°, and it is necessary to ensure that the micro-edge height h is less than the spacing d _t , as shown in the following formula:

d _t ＞h＞h _max

As shown in Figure 3, in this embodiment, the workpiece coordinate system _O1 (x, y, z) is set to define the position of the workpiece with a large aspect ratio and high steepness and the area to be processed, and the tool coordinate system is set to define the ball-end grinding wheel. The processing target surface, i.e., the aspherical generatrix equation, is defined in the workpiece coordinate system and used as the motion trajectory equation of the spherical grinding wheel.

The aspheric generatrix equation is:

Wherein, c is the paraxial curvature, k is the eccentricity function, k=-e ² (e is the eccentricity), A ₄ and A ₆ are aspheric coefficients.

The tool coordinate system O ₂ (p, q, r) is established at the center of the grinding wheel, and the grinding wheel surface profile curve is:

In this embodiment, since the material to be processed by the present invention is a brittle material, in order to obtain nanoscale surface roughness during processing, the material removal process must be plastic domain removal, that is, the maximum undeformed chip thickness h _max during processing should be less than the critical depth η _th of the plastic-brittle transition of the material.

According to the plastic processing theory of brittle materials, the critical depth of plastic-brittle transition of the material can be obtained as _follows :

As shown in Figure 4, the maximum undeformed chip thickness h _max during the single abrasive cutting process can be expressed by the following formula:

The equivalent radius of the contact area, _rev, can be calculated by the following formula:

At the grinding depth δ, the intersection area of the spherical grinding wheel and the arc surface is the material removal amount of the workpiece at this processing point. Since the grinding depth is at the sub-nanometer level and the tool contact area is small enough, it can be assumed that the curvature radius of each point in the processed area of the workpiece is the same.

The radius of curvature _r1 can be solved by the following formula:

The average pressure _Pave in the grinding area can be expressed by the following formula:

The normal grinding force _Fn can be obtained as follows:

F _n = S· _Pave

Since other process parameters remain unchanged and the grinding performance of the grinding wheel on the processing path remains basically unchanged, the friction coefficient μ on the grinding wheel surface remains unchanged; according to the formula F _t = μF _n, it can be seen that the tangential friction force remains stable during the grinding process. It can be seen that when the grinding depth at each point on the grinding path is consistent, the maximum undeformed chip thickness remains unchanged, so that the total grinding force is stable throughout the process.

On the one hand, after the surface of the grinding tool is microstructured, the number of micro-edges cut by the abrasive increases, which reduces the average cutting material thickness of the micro-edges at the same grinding depth, making it easier to achieve plastic domain removal of the workpiece surface material without reducing the grinding efficiency; in the process of microstructured abrasive cutting workpiece materials, the maximum undeformed chip thickness corresponding to the chips formed by micro-edge cutting is inversely proportional to the number of micro-edges in the contact area. After microstructuring, the abrasive particles form micro-edges _g1 , _g2 , and _g3 , which cut the workpiece surface in turn to form chip thicknesses _hg1 , _hg2 , and _hg3 , achieving the effect of refining the maximum undeformed chip thickness _hmax corresponding to a single abrasive particle; in addition, the microstructure grooves are arranged according to a set spacing rule to adapt to the changes in the rotation speed of the grinding tool contact point on the grinding path and the changes in the curvature of the workpiece processing point, thereby keeping the maximum undeformed chip thickness constant during the grinding process; and because the grinding force is related to the maximum undeformed chip thickness, the total grinding force of the entire path can be kept stable.

On the other hand, after the abrasive grains on the surface of the grinding tool are microstructured, the micro-cutting edges are evenly arranged. For the curvature of different processing areas on the surface of optical elements with large aspect ratio and high steepness, the chip thickness formed by the micro-cutting edges can be uniformed; the appropriate grinding parameters are selected in combination with the maximum deformation chip thickness formula, so that the micro-blade chip thickness is less than the critical depth of the plastic-brittle transition of the material removal mode, and the plastic domain removal of hard and brittle materials can be achieved, thereby reducing or avoiding the generation of sub-surface cracks, and meeting the requirements of high-precision and low-damage processing of hard and brittle materials. The specific parameters can refer to the fine grinding parameters in Table 1.

A picosecond laser with Gaussian intensity distribution was used; the laser power was 1.0W, the repetition frequency was 5kHz, the pulse width was 140ps, the laser wavelength was 532nm, the processing was positive defocus, and the defocus amount was 0.6mm; the relationship between the laser ablation groove width d and the laser energy density can be expressed by the following formula:

Where _R0 is the laser spot radius, _Id is the laser power, and _{It is} the critical laser power for material removal.

This embodiment also provides a method for using an ultra-precision grinding tool for a complex surface with a large aspect ratio and high steepness, which specifically includes:

Acquire first wear data and second wear data of the grinding wheel, wherein the first wear data includes a surface profile error of the grinding wheel, and the second wear data includes an average depth of groove wear;

Further, when it is determined that the first wear data is greater than a first preset value, the grinding wheel is repaired;

When it is determined that the second wear data is greater than a second preset value, repairing the groove;

When it is determined that the first wear data is not greater than the first preset value and the second wear data is not greater than the second preset value, grinding is performed on the surface of the workpiece based on the set trajectory.

The first preset value includes a standard value compared with the surface profile error of the grinding wheel, which is 80μm in this embodiment. If the current value is greater than the standard value of 80μm, the grinding wheel needs to be repaired; the second preset value is a standard value compared with the average depth of groove wear, which is 10μm in this embodiment. When the average depth of groove wear is greater than 10μm, the groove is repaired.

Install the ball-end grinding wheel and the fairing blank with a large aspect ratio and high steepness, as well as the detection, dressing, laser microstructuring and other components on the processing equipment, and determine the relative positions of each component; install the ball-end grinding wheel on the horizontal rotating axis of the processing equipment, and then fix the workpiece with a large aspect ratio and high steepness and complex surface on the spindle of the processing equipment; install the grinding tool surface detection component, contour dressing component and in-situ laser structuring component on the processing equipment respectively.

The grinding tool surface is divided into a top arc part and a peripheral arc part, and the two parts are inspected separately and then the contours are spliced.

The grinding wheel surface profile error is detected by a laser micrometer, and when it is determined that the first wear data is greater than a first preset value, the grinding wheel is repaired.

Specifically, the surface profile error of the grinding wheel is detected by a laser micrometer, and the measured data is fitted. The fitting radius and residual value correspond to the measured radius and profile error of the grinding tool respectively; the profile error can be obtained by comparing the detection data after the grinding tool is worn with the profile before wear; when the average depth of the microstructure groove wear on the surface of the grinding tool is greater than 10μm, it needs to be trimmed to maintain good grinding performance.

The grinding wheel is compensated and corrected by a repair tool, specifically:

The repair tool is a cylindrical diamond grinding wheel with a larger grain size, and the grain size of the dressing grinding wheel is selected to be 105μm; the grinding wheel of the repair tool and the rotation axis of the grinding tool are perpendicular to each other, the grinding wheel of the repair tool rotates at a speed of 7000rpm/min, and the grinding tool rotates at a speed of 10000rpm/min. At the same time, the grinding wheel of the repair tool is fed along the normal section profile of the grinding tool to complete the dressing, so as to achieve the flattening of the abrasive grains on the tool surface and the consistency of the spherical profile.

Combined with the laser displacement sensor in-situ detection and centering error compensation, the ball head grinding wheel profile error is controlled to be less than 3μm; the tool surface is re-microstructured using an in-situ picosecond laser, and the microgrooves are arranged in a regular variable pitch. Among them, the centering error is the distance difference between the rotating spindle of the dressing wheel and the rotating spindle of the spherical grinding wheel during the tool setting process.

The steps of detecting the spherical surface shape of the grinding wheel before the groove repair work include: using a laser micrometer to measure the microstructured tool surface in situ; detecting the top arc part and the peripheral arc part of the grinding tool respectively and then performing contour splicing; fitting the measured data, and the output fitting radius corresponds to the real-time radius of the grinding tool; taking the difference between the real-time radius of the grinding tool and the radius before wear as Δr ₁ as the tool compensation amount, and inputting it into the processing equipment controller to realize compensation correction of the processing path.

The rough grinding, semi-finishing grinding and finishing grinding process routes are adopted to iteratively process the surface of the workpiece with large aspect ratio and high steepness. In this embodiment, the corresponding grinding tool particle sizes are 35μm, 20μm and 7μm respectively, and the selected grinding process parameters are shown in Table 1 below:

Table 1

Based on the plastic processing mechanism, the grinding depth is controlled to be less than the critical depth of the ductile-brittle transition of the material, δ _th . Therefore, the grinding depth is kept within 15nm in the fine grinding process, thereby achieving low-damage processing in the grinding process. The pre-set processing trajectory program is transferred to the motion control system of the processing equipment, so that the grinding tool grinds the workpiece surface according to the set trajectory. The inspection, dressing and microstructuring work are repeated many times during the grinding process. The repeated cycle processing program completes the ultra-precision processing of the workpiece surface with a large aspect ratio and high steepness.

The present invention establishes a functional relationship between the microstructure rate of the grinding tool surface and the maximum undeformed chip thickness, and uses an in-situ picosecond laser to perform microstructuring on the surface of the grinding tool. The grinding method practice shows that the effect of stabilizing the grinding force can be achieved; compared with other grinding tools, the tool can achieve extensive plastic domain removal by refining the chip thickness on the one hand, and significantly reduce the grinding surface roughness and waviness by changing the microgroove arrangement spacing to adapt to grinding areas with different curvatures on the other hand; in addition, the method keeps the material removal rate unchanged to ensure that the processing efficiency of the grinding process is not reduced. The method of the present invention can be extended to ultra-precision machining of complex surfaces of various optical components such as glass, crystals, and ceramics.

The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims (10)

1. The ultra-precise grinding tool for the complex surface with the large length-diameter ratio and the high gradient is characterized by comprising a grinding wheel, wherein the grinding wheel is of a hemispherical structure, a plurality of grooves are formed in the side surface of the grinding wheel and distributed along the circumferential direction of the grinding wheel, two ends of each groove extend to the large end and the small end of the grinding wheel respectively, and the distance between every two adjacent grooves is gradually increased along the direction away from the small end.

2. A grinding tool according to claim 1, wherein the distance between adjacent grooves becomes progressively larger in a direction away from the small end in the range of 10 μm to 42.3 μm.

3. A method of using an ultra-precise grinding tool for high aspect ratio high steepness complex surfaces, characterized in that the method comprises the following steps based on the grinding tool of claim 1 or 2:

Acquiring first wear data and second wear data of a grinding wheel, wherein the first wear data comprise grinding wheel surface contour errors, and the second wear data comprise average depth of groove wear;

repairing the grinding wheel when the first abrasion data is determined to be larger than a first preset value;

Repairing the groove when the second abrasion data is determined to be larger than a second preset value;

When it is determined that the first wear data is not greater than a first preset value and the second wear data is not greater than a second preset value, grinding the surface of the workpiece with the grinding tool according to claim 1 or 2 based on the set trajectory.

4. A method of using as claimed in claim 3, wherein the method of obtaining the first and second wear data of the grinding wheel comprises:

and acquiring the surface profile error of the grinding wheel and the average depth of the abrasion of the groove based on a laser micrometer.

5. A method of use according to claim 3, wherein the first preset value is 80 μm and the second preset value is 10 μm.

6. A method of use according to claim 3, wherein the grinding wheel comprises a first body and a second body connected, the first body being a top arcuate portion of the grinding wheel and the second body being a peripheral arcuate portion of the grinding wheel.

7. The method of claim 6, further comprising contour stitching the first body and the second body to obtain the grinding wheel prior to obtaining the first wear data.

8. The method of claim 7, wherein the method of repairing the grinding wheel when the first wear data is determined to be greater than a first predetermined value comprises:

And fitting the first abrasion data, comparing the first abrasion data with the first preset value to obtain abrasion distribution data, and compensating and correcting the surface profile of the grinding wheel through a repairing tool based on the abrasion distribution data.

9. The method of claim 8, wherein the repair tool grain size is greater than the wear tool grain size, the repair tool and the wear tool axis of rotation are perpendicular to each other, and the grinding tool has a rotational speed greater than the repair tool rotational speed.

10. A method of use according to claim 3, wherein the set trajectory comprises a course of a rough, semi-refined or refined process.