A method for determining the micro-milling repair process parameters of the full plastic domain of surface defects of KDP crystals

Abstract

A method for determining the micro-milling repair process parameters of the full plastic domain of KDP crystal surface defects, which relates to the field of optical element processing. It aims to solve the problems that the existing methods do not establish the maximum undeformed chip thickness model under multiple milling methods and do not consider the influence of surface defects on the undeformed chip thickness. The method includes the following steps: Step 1, measure the depth of the KDP crystal surface defects; Step 2, select the process parameters of ball-end micro-milling repair; Step 3, respectively construct and calculate the maximum undeformed chip thickness models under four milling modes of up-milling, down-milling, climb milling and conventional milling; Step 4, adjust the repair process parameters until the maximum undeformed milling thickness of each milling method is less than the critical cutting depth of brittle-ductile transition; Step 5, determine the repair process parameters of the full plastic domain of the surface defects. Based on the analysis results of the maximum undeformed chip thickness model, the present invention determines the micro-milling repair process parameters of the full plastic domain of KDP crystal, which can improve the repair surface quality.

Description

Technical Field

[0001] The present invention relates to the technical field of optical element processing, and in particular, to a method for determining the micro-milling repair process parameters of the full plastic domain of the surface defects of KDP crystals. Background Art

[0002] Large-sized KH2PO4 (KDP) crystals, with their excellent optical properties, can be used as end-frequency converters and optoelectronic switches and are widely used in inertial confinement fusion (ICF). At present, single-point diamond fly-cutting technology is mainly used at home and abroad to obtain the ultra-smooth surface of KDP crystals. However, due to the mechanical properties of KDP crystals that are not conducive to machining, such as low hardness, high brittleness, and obvious anisotropy, a large number of micro-defects, such as pits and scratches, will be generated during the fly-cutting process. Under subsequent high-power laser irradiation, the size of the processing surface defects will increase exponentially, ultimately leading to the failure of the entire optical element. Considering the high cost of crystal growth, the most economical solution is to use advanced processing technology to remove the original surface defects to achieve the purpose of repairing KDP crystals. After comparing femtosecond laser ablation, CO2 laser processing, water etching, and micro-mechanical repair methods, Lawrence Livermore National Laboratory (LLNL) in the United States believes that ball-end micro-milling is the most promising repair method for removing surface defects of KDP crystals. However, undoubtedly, the soft and brittle KDP crystal is a typical difficult-to-machine material because it not only has significant anisotropy but also is prone to deliquescence. Therefore, achieving the full plastic domain repair of the soft and brittle KDP crystal poses a huge challenge to the ball-end micro-milling processing technology.

[0003] During the micro-milling repair process, the fly-cutting processing defects will change the undeformed cutting thickness of the KDP crystal surface, thereby affecting the material removal mechanism. In addition, since the surface machining process of the repair profile consists of two parts: layer milling and helical milling (for the definition of the repair process, please refer to the patent "Micro-milling repair process method for laser damage on the surface of large-aperture KDP crystal components"), when the tool moves along the preset path, the relative position between the KDP crystal and the tool will change, resulting in four milling methods in the micro-milling repair process: up milling, down milling, conventional milling, and climb milling, which will change the undeformed cutting thickness in different milling repair areas. Usually, the undeformed cutting thickness under different cutting parameters (spindle speed, feed rate, and cutting depth) is compared with the critical cutting thickness (critical brittle-ductile transition cutting depth) at which the plastic domain transforms into the brittle domain to achieve plastic cutting. Xiao Yong, Cheng Jian and others obtained the critical brittle-ductile transition cutting depth value through the indentation critical load theory and the elastic-plastic contact theory [1]. However, the current undeformed cutting thickness calculation models mainly focus on the maximum undeformed cutting thickness models for single abrasive grains, turning processes, and conventional milling and climb milling in the cutting process of flat-end mills. For the complex ball-end micro-milling repair process, no theoretical model of the maximum undeformed cutting thickness under multiple milling methods has been established. On the other hand, the existing theoretical models all assume that the unprocessed surface of the workpiece is an ideal surface (defect-free surface). For KDP crystal optical components, the influence of surface defects on the undeformed cutting thickness is not considered in the actual repair process. Therefore, calculating the maximum milling thickness of the KDP crystal defect surface under different milling methods is of great significance and engineering value for determining the plastic domain repair process parameters and achieving the full plastic domain repair process. Summary of the Invention

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

[0005] The existing undeformed cutting thickness calculation models mainly focus on the maximum undeformed cutting thickness models for single abrasive grains, turning processes, and conventional milling and climb milling in the cutting process of flat-end mills. For the complex ball-end micro-milling repair process, no theoretical model of the maximum undeformed cutting thickness under multiple milling methods has been established. On the other hand, the existing theoretical models often assume that the unprocessed surface of the workpiece is an ideal surface (defect-free surface), and do not consider the influence of surface defects on the undeformed cutting thickness in the actual repair process.

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

[0007] The present invention provides a method for determining the full plastic domain micro-milling repair process parameters of the KDP crystal surface defects, including the following steps:

[0008] Step 1. Measure the depth of the surface defect of the KDP crystal;

[0009] Step 2. Select the process parameters for ball-end micro-milling repair;

[0010] Step 3. Respectively construct theoretical models of the maximum undeformed chip thickness under four milling modes of up-milling, down-milling, conventional milling and climb milling, and calculate the maximum undeformed chip thickness under each milling mode in combination with the micro-milling repair process parameters and the surface defect depth;

[0011] Step 4. Respectively compare the obtained maximum undeformed milling thicknesses of up-milling, down-milling, conventional milling and climb milling with the critical cutting depth of brittle-ductile transition. If the maximum undeformed milling thickness of a milling mode is greater than or equal to the critical cutting depth of brittle-ductile transition, re-select the process parameters for ball-end micro-milling repair for calculation until the maximum undeformed milling thicknesses of all milling modes are less than the critical cutting depth of brittle-ductile transition;

[0012] Step 5. Determine the process parameters for full plastic domain repair of the surface defect.

[0013] Furthermore, the surface defect of the KDP crystal in Step 1 is a scratch defect.

[0014] Furthermore, in Step 1, the depth of the surface defect of the KDP crystal is measured by a white light interferometer.

[0015] Furthermore, the process parameters for ball-end micro-milling repair in Step 2 include spindle speed, feed rate, milling depth and tool inclination angle.

[0016] Furthermore, the up-milling in Step 3 is a milling mode in which the feed direction and the tool inclination direction are the same, and the constructed theoretical model of the maximum undeformed chip thickness for up-milling is:

[0017]

[0018] where d s is the defect depth, a p is the cutting depth, α is the rotation angle, R is the tool radius, f z is the feed per tooth, and its calculation method is:

[0019] f z = f·(zn) -1

[0020] In the formula, f is the feed rate, z is the number of cutting edges, and n is the spindle speed.

[0021] Furthermore, the down-milling in Step 3 is a milling mode in which the feed direction and the tool inclination direction are opposite, and the constructed theoretical model of the maximum undeformed chip thickness for down-milling is:

[0022]

[0023] Furthermore, the up milling in Step 3 is a milling method when the tool feed direction is perpendicular to the spindle inclination direction and the velocity direction passing through the lowest point is the same as the feed direction; while the machining mode when the velocity direction is opposite to the feed direction is defined as down milling. Since the difference between up milling and down milling lies only in the relative direction between the tool and the workpiece and the maximum undeformed chip thickness of the two is the same, the theoretical model of the maximum undeformed chip thickness for up milling or down milling is as follows:

[0024]

[0025] In the formula, N is the actual cutting depth, and f t is the feed per tooth.

[0026] Furthermore, the critical cutting depth value of the brittle-ductile transition in Step 4 is selected as 230 nm obtained through the indentation critical load theory and the elastic-plastic contact theory.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] A method for determining the micro-milling repair process parameters of the full plastic region of KDP crystal surface defects in the present invention. First, calculate the maximum undeformed chip thickness on the scratched defect surface of KDP crystal under different milling methods through a mathematical model, providing a basis for determining the subsequent full plastic region repair process parameters. Second, analyze the influence of process parameters such as spindle speed, feed per tooth, and milling depth on the maximum undeformed chip thickness under different milling methods, and determine the full plastic region repair process parameters on the premise that the maximum undeformed chip thickness under all milling methods is less than the critical cutting depth of the brittle-ductile transition. Third, this method has a certain universality and can be extended to determine the full plastic region repair process parameters of various KDP crystal surface defects such as pits and protrusions. Fourth, determining the micro-milling repair process parameters of the KDP crystal full plastic region based on the analysis results of the maximum undeformed chip thickness model can improve the repair surface quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a flow chart of a method for determining the micro-milling repair process parameters of the full plastic region of KDP crystal surface defects in an embodiment of the present invention;

[0030] Figure 2 is a schematic diagram of four milling methods during the relative position change between the tool and the workpiece in the micro-milling process in an embodiment of the present invention;

[0031] Figure 3 is a schematic diagram of the maximum undeformed chip thickness on the scratched defect surface of KDP crystal in the up milling mode in an embodiment of the present invention;

[0032] Figure 4 Schematic diagram of the maximum undeformed cutting thickness on the scratched defect surface of KDP crystal in the negative milling mode in the embodiment of the present invention;

[0033] Figure 5 Schematic diagram of the maximum undeformed cutting thickness on the scratched defect surface of KDP crystal in the down milling mode in the embodiment of the present invention;

[0034] Figure 6 Surface topography map of microgroove machining under different milling methods in the embodiment of the present invention. Detailed implementation manners

[0035] 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 specifying 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.

[0036] 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.

[0037] Detailed implementation manner 1. As Figure 1 shown, a method for determining the micro-milling repair process parameters of the fully plastic region of the KDP crystal surface defects includes the following steps:

[0038] Step 1. Measure the depth of the KDP crystal surface defects;

[0039] Step 2. Select the ball-end micro-milling repair process parameters;

[0040] Step 3. Respectively construct theoretical models of the maximum undeformed cutting thickness under four milling modes of up milling, down milling, conventional milling, and climb milling, and calculate the maximum undeformed cutting thickness under each milling method in combination with the micro-milling repair process parameters and the surface defect depth;

[0041] Step 4. Respectively compare the obtained maximum undeformed milling thicknesses of up milling, down milling, conventional milling, and climb milling with the critical cutting depth of brittle-ductile transition. If the maximum undeformed milling thickness of a milling method is greater than or equal to the critical cutting depth of brittle-ductile transition, re-select the ball-end micro-milling repair process parameters for calculation until the maximum undeformed milling thicknesses of each milling method are all less than the critical cutting depth of brittle-ductile transition;

[0042] Step 5. Determine the repair process parameters of the fully plastic region of the surface defects.

[0043] The surface defect of the KDP crystal described in Step 1 is a scratch defect. The depth of the surface defect of the KDP crystal is measured by a white light interferometer, and the obtained result is 5 μm.

[0044] The ball-end micro-milling repair process parameters described in Step 2 include spindle speed, feed rate, milling depth, and tool inclination angle.

[0045] Combining the obtained defect size and the actual situation of the machine tool performance, the following two groups of micro-milling repair process parameters are initially selected:

[0046] ① Tool inclination angle 30°, spindle speed 50000 r / min, milling depth 8 μm, feed rate 0.8 mm / s

[0047] ② Tool inclination angle 30°, spindle speed 50000 r / min, milling depth 8 μm, feed rate 5.0 mm / s

[0048] When the relative position of the tool and the workpiece changes, various milling methods will affect the material removal mechanism. There are four milling modes in the micro-milling process, as Figure 2 shown. It can be seen that when the tool inclination direction is parallel to the feed direction, the cutting process is up-milling (+α) or down-milling (-α). If the tool inclination direction is perpendicular to the feed direction, the cutting process is climb milling (+β) or conventional milling (-β).

[0049] As Figure 3 shown, when the ball-end mill moves relative to the workpiece to the right and its inclination angle is +α, the up-milling mode appears in the repair process. The coordinate origin o is the center of the tool ball (the (j - 1)th tooth). The coordinate system oxy rotates clockwise by an α angle around point o to obtain the ox1y1 coordinate system. When the tool moves the feed per tooth f t , the center of the ball o moves to o1 (the jth tooth). A1 is the intersection point of the cutting edge contour line of the (j - 1)th tooth and the bottom surface of the crystal defect area. A straight line parallel to the x1 axis is drawn through point A1. The intersection points of the straight line and the straight line where the translated y1 axis is located and the cutting edge contour line of the jth tooth are A and A2 respectively. The distance h A1A2 between A1 and A2 is the maximum undeformed chip thickness under up-milling.

[0050] During the micro-milling repair process on the surface of the KDP crystal scratch defect, the milling depth is a p , and since the scratch defect depth is d s , the actual cutting depth N is:

[0051] N = a p - d s (1)

[0052] In the coordinate system oxy, point A1 is on curve I (the ball-end contour line of the (j - 1)th gear), and its coordinates can be obtained from the geometric relationship:

[0053]

[0054] Since the coordinate system ox1y1 is formed by rotating the coordinate system oxy by an angle α, the position of point A1 in the ox1y1 coordinate system can be calculated by homogeneous transformation, and its coordinate formula is:

[0055]

[0056] Since point A2 is on curve II, the coordinate relationship of point A2 in the coordinate system ox1y1 is expressed as:

[0057]

[0058] where f z is the feed per tooth, and its calculation method is:

[0059] f z = f·(zn) -1 (5)

[0060] In the formula, f is the feed rate, z is the number of cutting edges, and n is the spindle speed.

[0061] Since point A1 and point A2 have the same ordinate in the coordinate system ox1y1, therefore:

[0062]

[0063] In the ox1y1 coordinate system, the actual maximum undeformed chip thickness can be expressed as:

[0064]

[0065] Finally, h A1A2 is calculated from equations (1) to (7) as:

[0066]

[0067] As Figure 4 shown, when the tool moves left relative to the workpiece and its inclination angle is -α, a negative milling mode appears in the repair process, and the coordinate system oxy rotates clockwise by an angle α around point o to obtain the ox1y1 coordinate system. Point o is translated left by f t to point o1. Point B1 is the intersection of the j-1 cutting edge profile line and the bottom surface of the crystal defect area. A straight line parallel to the x1 axis is drawn through point B1, and the intersections of the straight line with the straight line where the translated y1 axis is located and the jth tooth cutting edge profile line are B and B2 respectively. In the coordinate system oxy, point B1 is on curve I, and its coordinates can be calculated from geometric relationships as:

[0068]

[0069] In addition, since point B2 lies on curve II, the coordinate relationship of point B2 in the coordinate system ox1y is expressed as:

[0070]

[0071] Points B1 and B2 have the same ordinate in the coordinate system ox1y. Therefore, in the ox1y coordinate system, the actual maximum undeformed chip thickness can be obtained as:

[0072]

[0073] As Figure 5 shown, when the workpiece feed direction is perpendicular to the tool rake angle direction and is the same as the velocity direction when the cutting edge rotates to the lowest point, a down milling mode appears in the repair process, the rake angle is +β, and the deepest point of the microgroove is located on the plane formed by the feed direction and the direction perpendicular to the KDP crystal surface. The ball end mill moves from left to right with a feed per tooth of f t , and the maximum undeformed chip thickness h CF is located within the cross-section. Point A is the intersection of the bottom surface of the crystal defect area and the y-axis, and point B is the intersection of the bottom surface of the crystal defect area and the cutting edge profile of the (j - 1)-th tooth. In the oxy coordinate system, l AB can be expressed as:

[0074]

[0075] is:

[0076]

[0077] Point C is the intersection of the straight line o1B and the cutting edge profile of the j-th tooth. Therefore, l BC can be obtained as:

[0078]

[0079] In the B-C view, point C is the lowest point of the cutting edge. After rotating o1C clockwise by β angle around point o1 to obtain a straight line, draw a perpendicular line CE from point C to this straight line, and the intersection of CE and the bottom surface of the crystal defect area is point F. The actual maximum undeformed chip thickness h CF (the distance between points C and F) on the scratched defect surface of the KDP crystal is:

[0080]

[0081] Finally, the actual maximum undeformed chip thickness in the down milling mode can be calculated as:

[0082]

[0083] Since the difference between up milling and down milling lies only in the relative direction of the cutting tool and the workpiece, on the defective surface of the KDP crystal, the actual maximum undeformed chip thickness of both is the same.

[0084] Substitute the scratch defect depth, spindle speed, feed per tooth, and milling depth into the theoretical model of the maximum undeformed chip thickness under different milling methods to obtain the maximum milling thickness under different milling methods. The results are shown in Table 1.

[0085] Table 1

[0086]

[0087] The critical cutting depth value of the brittle-ductile transition is selected from existing research, and the value of 230 nm obtained through the indentation critical load theory and the elastic-plastic contact theory.

[0088] 1) When the feed rate is 5.0 mm / s, the maximum undeformed chip thickness of up milling is 719.7 nm, which is much greater than 230 nm, so new process parameters are reselected; when the feed rate is 0.80 mm / s, the maximum undeformed chip thickness of up milling is 122.5 nm, which is less than 230 nm.

[0089] 2) When the feed rate is 0.8 mm / s, the maximum undeformed chip thickness of down milling is 211.7 nm, which is less than 230 nm.

[0090] 3) When the feed rate is 0.8 mm / s, the maximum undeformed chip thickness of climb milling or conventional milling is 153.5 nm, which is less than 230 nm.

[0091] Since when the feed rate is 0.8 mm / s, the maximum undeformed chip thickness of all milling methods is less than the critical cutting depth of the brittle-ductile transition, a micro-milling repaired surface in the full plastic region can be obtained using this set of process parameters.

[0092] The process parameters for micro-milling repair in the full plastic region obtained are a tool inclination angle of 30°, a spindle speed of 50000 r / min, a milling depth of 8 μm, and a feed rate of 0.8 mm / s. Based on these parameters, micro-milling experiments were carried out on the scratched defect surface of the KDP crystal, and the surface morphology of the machined surface was observed with a scanning electron microscope, as Figure 6As shown. It can be seen from the figure that under up milling, there are no cracks and brittle fractures at the bottom of the micro-groove surface, and the boundary contours of the micro-groove on the cutting-in side and cutting-out side of the scratch defect are complete, with only a small amount of chips adhering to the surface. Under down milling, the micro-groove surface is smooth and the tool marks are clear, belonging to the removal in the plastic domain. In addition, under climb milling and conventional milling, there are also no brittle fracture points and cracks on the micro-groove surface. However, it can be clearly seen that under the conventional milling mode, a large amount of chips adhere to the micro-groove surface. The reason is that when the tool rotation direction is opposite to the workpiece feed direction, it is not conducive to chip evacuation. To sum up, the obtained full-plastic domain micro-milling process parameters can obtain a high-quality repaired surface, and there are no cracks and brittle fracture pits in the cutting process under all milling methods.

[0093] The above steps use the determination process of the present invention and successfully determine the full-plastic domain process parameters for the ball-end micro-milling repair of KDP crystal surface defects, providing a reference for the selection of the best process parameters in the actual surface defect repair process, and helping to obtain a super-smooth surface of KDP crystal components and improve their laser damage resistance.

[0094] 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 disclosure, and these changes and modifications will all fall within the protection scope of the present invention.

[0095] Literatures cited in the present invention:

[0096] [1]Xiao Y, Chen M J, Yang Y T, et al. Research on the critical condition of brittle-ductile transition about micro-milling of KDP crystal and experimental verification[J]. International Journal of Precision Engineering and Manufacturing, 2015, 16(2): 351-359.

Claims

1. A method for determining the micro-milling repair process parameters of the full plastic domain of surface defects of KDP crystals, characterized in that, It includes the following steps: Step 1: Measure the depth of surface defects of the KDP crystal; Step 2: Select the process parameters for ball-end micro-milling repair; Step 3: Respectively construct theoretical models of the maximum undeformed chip thickness under four milling modes of up-milling, down-milling, climb milling and conventional milling, and calculate the maximum undeformed chip thickness under each milling mode in combination with the micro-milling repair process parameters and the surface defect depth; Step 4: Respectively compare the maximum undeformed milling thicknesses of up-milling, down-milling, climb milling and conventional milling obtained with the critical cutting depth of brittle-ductile transition. If the maximum undeformed milling thickness of a milling mode is greater than or equal to the critical cutting depth of brittle-ductile transition, re-select the process parameters for ball-end micro-milling repair for calculation until the maximum undeformed chip thicknesses of all milling modes are less than the critical cutting depth of brittle-ductile transition; Step 5: Determine the process parameters for full plastic domain repair of surface defects; In Step 3, the up-milling is a milling mode in which the feed direction and the tool tilt direction are the same, and the constructed theoretical model of the maximum undeformed chip thickness in up-milling is: Among them, d s is the defect depth, a p is the cutting depth, α is the rotation angle, R is the tool radius, f z is the feed per tooth, and its calculation method is as follows: f z = f·(zn) -1 In the formula, f is the feed rate, z is the number of cutting edges, and n is the spindle speed; In Step 3, the down-milling is a milling mode in which the feed direction and the tool tilt direction are opposite, and the constructed theoretical model of the maximum undeformed chip thickness in down-milling is: In Step 3, the climb milling is a milling mode when the tool feed direction is perpendicular to the spindle tilt direction and the velocity direction passing through the lowest point is the same as the feed direction; while the machining mode when the velocity direction is opposite to the feed direction is defined as conventional milling. Since the difference between climb milling and conventional milling is only in the relative direction between the tool and the workpiece and their maximum undeformed chip thicknesses are the same, the theoretical model of the maximum undeformed chip thickness for climb milling or conventional milling is: Where N is the actual cutting depth and f t is the feed per tooth.

2. The method for determining the micro-milling repair process parameters of the full plastic domain of the KDP crystal surface defects according to claim 1, characterized in that The surface defect of the KDP crystal in Step 1 is a scratch defect.

3. The method for determining the micro-milling repair process parameters of the full plastic domain of the KDP crystal surface defects according to claim 2, characterized in that, In Step 1, the depth of the surface defect of the KDP crystal is measured by a white light interferometer.

4. The method for determining the micro-milling repair process parameters of the full plastic domain of the KDP crystal surface defects according to claim 1, characterized in that The process parameters for ball-end micro-milling repair in Step 2 include spindle speed, feed rate, milling depth and tool inclination angle.

5. The method for determining the micro-milling repair process parameters of the full plastic domain of the KDP crystal surface defects according to claim 1, characterized in that The value of the critical cutting depth of brittle-ductile transition in Step 4 is selected as 230 nm obtained through the indentation critical load theory and the elastic-plastic contact theory.

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