An equal-thickness precision machining method for an ultra-large diameter-thickness ratio integrated box bottom

Abstract

The application discloses a kind of integral box bottom equal-thickness precision machining methods of super large diameter-thickness ratio, it is related to the field of storage tank processing, including: clamping box bottom, with the horizontal reference line of box bottom blank as reference, according to the inner profile theoretical machining amount of box bottom, the inner profile of box bottom is turned, and the inner profile after processing is obtained;Box bottom is clamped, with the inner profile after processing as reference, the outer profile of box bottom is roughly turned, and the rough turning of outer profile is stopped after reaching the turning allowance;The box bottom after rough turning is heat aged;The outer profile of the box bottom after heat aging is milled by five-axis mirror image milling to mill the thin area and thick area of box bottom.The specific sequence of turning, heat aging, five-axis mirror image milling is cooperated, the high-precision machining of box bottom is realized, the minimum wall thickness 1mm is realized, and the machining precision is ±0.1mm.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of tank bottom processing, in particular to an equal-wall-thickness precision processing method for an overall tank bottom with an ultra-large diameter-thickness ratio. BACKGROUND

[0002] The tank bottom of an active rocket is generally manufactured by adopting a process of first forming and chemically milling thinning single melon petal and then combining and welding to form a tank bottom ring. The chemical milling forming precision is not high, and quality problems such as surface orange peel and water ripples are prone to occur, the profile consistency is poor, and the profile of the melon petal of the tank bottom of the active carrier rocket is over-dimensioned by 30%. As shown in FIG. 1, the melon petal welded tank bottom has a large number of welds, and is affected by welding inherent defects and welding joint strength, so that the overall manufacturing quality and reliability of the tank bottom are not high. Figure 1

[0003] The overall forming tank bottom has the characteristics of short manufacturing cycle and high reliability, and is the development direction of future tank bottom processing. The profile of the tank bottom is mostly an ellipsoid or spherical surface structure, the maximum diameter reaches Φ3350 mm, the profile has a thin-thick area feature, the thin area thickness can reach 1 mm, and the minimum wall thickness precision can reach ±0.1 mm. According to the design weight reduction requirement and precision requirement, a large-area thinning of the inner and outer profile features is required after the overall forming process, so as to ensure the equal-wall-thickness requirement. Numerical control cutting is a green and feasible scheme for ensuring the theoretical thickness of the tank bottom after the overall forming of the tank bottom, but is restricted by the ultra-large diameter-thickness ratio and ultra-high precision requirement of the tank bottom, and a mature and reliable process scheme has been lacking.

[0004] At present, the tank bottom is often thinned in the following two ways after the overall forming process:

[0005] (1) Traditional process-chemical milling The chemical milling is a process method for removing surface metal by using the corrosion action of a solution, which can solve the problem of equal-wall-thickness thinning of the complex thin-thick area feature of the tank bottom, but has two problems: ① the poor surface quality and over-dimensioned wall thickness precision caused by the unstable corrosion process, which cannot meet the design use requirement; and ② the problems of large chemical pollution, large power consumption, consumption of aluminum material that cannot be recycled, and the like, which cannot meet the green and sustainable manufacturing requirement.

[0006] ​(2) Numerical control milling In recent years, numerical control milling is gradually applied in the machining of the whole box bottom. Most of them use multi-point vacuum adsorption clamping or whole mold structure to realize the machining of thin and thick areas. According to the characteristics of the box bottom product profile, numerical control turning machining and numerical control five-axis milling machining are mainly used. However, due to the factors such as the super large diameter-thickness ratio and the super high wall thickness requirement of the whole box bottom, numerical control milling often has three problems: ① The vacuum adsorption clamping is discrete local clamping, and the whole mold clamping has a low mold clamping rate. Both of them cannot realize the constant stiffness clamping of the product; ② The product deformation problem caused by the super large diameter-thickness ratio of the box bottom. The turning machining cannot measure and compensate the deformation, while the milling machining can measure the profile and compensate according to the results. However, this process needs to be repeated many times, and the efficiency is low and the compensation accuracy is not high; ③ Numerical control machining inevitably produces chatter during the machining process due to its mechanism limitation. The cutting depth and surface quality cannot be controlled, especially in the machining of thin-walled box bottom. Therefore, the existing numerical control milling thinning method cannot completely replace the chemical milling. SUMMARY

[0007] The technical problem solved by the present application is to overcome the shortcomings of the prior art and provide an equal-wall-thickness precision machining method for a super large diameter-thickness ratio whole box bottom, which solves the problems of large pollution and poor precision in traditional chemical milling and numerical control milling of the whole box bottom.

[0008] Further, the equal-wall-thickness precision machining method can be used for the equal-wall-thickness precision machining of a super large diameter-thickness ratio whole box bottom with a diameter of Φ3350mm, a thinnest thickness of 1mm and a wall thickness precision of ±0.1mm.

[0009] The technical solution of the present application is:

[0010] An equal-wall-thickness precision machining method for a super large diameter-thickness ratio whole box bottom, comprising

[0011] Clamping the box bottom, taking the horizontal reference line of the box bottom blank as the reference, turning the inner profile of the box bottom according to the theoretical machining amount of the inner profile of the box bottom, and obtaining the machined inner profile;

[0012] Clamping the box bottom, taking the machined inner profile as the reference, rough turning the outer profile of the box bottom, and stopping the rough turning of the outer profile after reaching the turning allowance;

[0013] Performing heat aging on the rough turned box bottom;

[0014] Using five-axis mirror image milling to mill the thin area and thick area of the outer profile of the heat aged box bottom.

[0015] The clamping of the box bottom adopts a plurality of flexible clamping tools, and the plurality of flexible clamping tools are uniformly distributed in the circumferential direction of the box bottom. The flexible clamping tool comprises a square box, a mold block and a pressing plate combination. The pressing plate combination is used to detachably connect the mold block to the square box;

[0016] When the inner profile of the box bottom is machined, the large end bottom of the box bottom is connected with the corner piece, the corner piece supports the box bottom, and the whole turning of the block is performed according to the outer profile of the box bottom, so that the block is attached to the outer profile of the box bottom.

[0017] When the outer profile of the box bottom is machined, the whole turning of the block is performed according to the inner profile of the box bottom, so that the block is attached to the inner profile of the box bottom.

[0018] The outer profile of the box bottom is rough turned by multiple layer cutting, and the clamping of the box bottom is loosened multiple times during the layer cutting to release stress.

[0019] The stress release method comprises:

[0020] A1: turning the outer profile, the turning amount is 30-40% of the current product allowance;

[0021] A2: symmetrically loosening the clamping of the box bottom, and limiting the large end diameter by the pressing plate, and naturally releasing the processing deformation;

[0022] A3: measuring the roundness of the large end, when the roundness is less than or equal to 60% of the product allowance, continue to perform steps A1, A2 and A3, when the roundness is greater than or equal to 60% of the product allowance or the roundness is less than 1mm, it is considered that the stress release is sufficient, and the stress release is ended.

[0023] The current product allowance is the difference between the current wall thickness of the box bottom and the theoretical wall thickness of the box bottom.

[0024] During the rough turning of the outer profile of the box bottom, each layer is segmented and turned according to the set thickness deviation range; after different segmented turning of a layer is completed, the whole turning is performed up and down, and the turning of a layer is completed.

[0025] The determination method of the turning allowance is: after the first rough turning of the outer profile, the wall thickness deviation is measured to obtain an initial tolerance value Q, the initial tolerance value is multiplied by a magnification factor K to obtain an initial tolerance threshold Qk, and the smaller value between the initial tolerance threshold Qk and the recommended value of the mirror milling real-time compensation is taken as the final tolerance threshold Q1;

[0026] After each rough turning, the wall thickness deviation is measured to obtain a real-time wall thickness tolerance value, until the real-time wall thickness tolerance value is not less than the final tolerance threshold Q1, or is lower than the minimum allowance value set by human, the rough turning is ended;

[0027] At this time, the difference between the wall thickness of the box bottom obtained by the rough turning and the theoretical wall thickness of the box bottom is the turning allowance.

[0028] The turning allowance is 3-5mm.

[0029] The thermal aging condition is heating to 140°, holding for 4 hours, and naturally cooling.

[0030] The five-axis mirror milling includes

[0031] S1: adjusting the position of the heat-treated tank bottom on the machine tool to match the machine tool coordinate system;

[0032] S2: laser scanning the inner reference surface of the tank bottom;

[0033] S3: offsetting the inner reference surface according to the theoretical thickness of the overall tank bottom to generate a machining outer surface;

[0034] S4: using five-axis mirror milling to mill the thin and thick areas of the tank bottom according to the machining outer surface.

[0035] The step S4 includes

[0036] S41: rough machining is layered machining, and the thickness of the tank bottom is measured in real time after each layer is machined;

[0037] S42: judging the surface deformation according to the thickness of the tank bottom measured in step S41, if the surface deformation is greater than 50% of the recommended value of the real-time compensation of mirror milling, it is considered that the surface deformation is large, then the inner surface of the tank bottom is scanned until the surface deformation is less than 50% of the recommended value of the real-time compensation of mirror milling, and step S43 is continued;

[0038] S43: fine machining is performed on the thin area of the tank bottom, and the fine machining is layered machining, and the thickness of the tank bottom is measured in real time after each layer is machined, and the next layer is cut according to the real-time measured thickness value until the thickness of the thin area meets the tolerance requirement. Figure 15

[0039] The main technical content of the patent includes:

[0040] (1) A full-process super-large diameter-thickness ratio overall tank bottom machining scheme combining numerical control turning and mirror milling;

[0041] (2) A super-large diameter-thickness ratio overall tank bottom turning process scheme, including an inside-out process flow, a multi-point flexible clamping method, a cyclic stress relief method, and an outer surface turning allowance determination method based on a tolerance threshold.

[0042] (3) A super-large diameter-thickness ratio overall tank bottom mirror milling process scheme, including a mirror milling full-process process method and a milling path planning method based on rapid response compensation.

[0043] In summary, the present application at least includes the following beneficial technical effects:

[0044] ​Rocket tank bottom manufacturing, for the traditional "split milling + welding forming" manufacturing process of product consistency, low strength and other bottleneck problems, innovative box bottom whole forming technology, and based on this, for the weak stiffness clamping, forming error perception, tool irregular vibration, wall thickness precision loss control and other problems in the whole machining of the box bottom, with the whole box bottom machining scheme of numerical control turning combined with mirror milling of large diameter-thickness ratio, the machining index reaches: box bottom diameter 3350mm, minimum wall thickness 1mm, machining precision ±0.1mm. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 For the welding box bottom process in the background art;

[0046] Figure 2 For the whole box bottom structure (Φ3350) of a certain type of launch vehicle in the embodiments of the present application;

[0047] Figure 3 For the flowchart of a large-diameter-thickness ratio whole box bottom equal wall thickness precision machining method in the embodiments of the present application;

[0048] Figure 4 For the box bottom blank with horizontal reference line;

[0049] Figure 5 For the structure diagram of the box bottom blank after welding the corner piece

[0050] Figure 6 a, b are respectively flexible clamping tooling and outer tire clamping structure schematic diagram;

[0051] Figure 7 a, b are respectively flexible clamping tooling and inner tire clamping structure schematic diagram;

[0052] Figure 8 For the stress release method flowchart;

[0053] Figure 9 For the outer surface allowance determination method flowchart based on tolerance threshold;

[0054] Figure 10 For the whole milling of the box bottom;

[0055] Figure 11 For the structure diagram of the box bottom clamping;

[0056] Figure 12 For the milling path planning method flowchart of Z-axis and A-axis constant;

[0057] Figure 13 The ① area is the traditional milling path, and the ② area is the specific milling path of the Z-axis and A-axis constant of the present application;

[0058] Figure 14 This is a mirror milling mechanism;

[0059] Figure 15 This is a flowchart of the precision machining process for the bottom of the box.

[0060] Explanation of reference numerals in the attached diagram: 1. Box bottom; 11. Horizontal baseline; 12. Fork-shaped ring; 13. Corner piece; 14. Large end allowance area;

[0061] 2. Flexible clamping fixture; 21. Square box; 22. Tire block; 23. Pressure plate assembly;

[0062] 31. Support subsystem; 32. Tooling subsystem. Detailed Implementation

[0063] The present application will now be described in further detail with reference to the accompanying drawings and specific embodiments:

[0064] As shown Figure 2 The image shows the integral bottom structure of a certain type of launch vehicle, with a maximum diameter of approximately Φ3350mm. The lower end features a forked ring structure, while the upper end exhibits a thin-section and thick-section characteristic structure. The total thickness is 3.1±0.1mm, with the thin section measuring 1±0.1mm. The inner surface is ellipsoidal. This bottom structure is manufactured using an integral forming process, resulting in a uniformly thick ellipsoidal blank. Both the inner and outer surfaces have significant allowances, necessitating internal and external thinning and machining of features such as the forked ring. Theoretically, the inner surface is ellipsoidal.

[0065] This application discloses a method for precision machining of a single box bottom with uniform wall thickness and an ultra-large diameter-to-thickness ratio, such as... Figure 3 As shown, it includes turning, thermal aging, and five-axis mirror milling.

[0066] like Figure 3 As shown, the overall approach is to perform turning machining on the bottom of the box structure when it has a large allowance and good rigidity to remove the large allowance; then, thermal aging is used to release the residual stress from machining. Finally, when the allowance is small and the rigidity is weak, five-axis mirror milling is performed to machine the thin wall with equal thickness.

[0067] Considering factors such as manufacturing precision, product deformation, and the maximum envelope of the fork-shaped ring, the integral forming process results in a large allowance in the thickness direction of the blank's inner and outer surfaces. Direct traditional milling would be inefficient and cause significant stress release. Therefore, taking into account the product's rigidity and structural characteristics, the inner surface is first machined as the reference for the overall thickness of the box bottom. Then, the blank is flipped and clamped again for machining the outer surface and the dimensions of the fork-shaped ring. As the outer surface thickness decreases, the machining rigidity decreases, and the machining thickness deviation becomes increasingly larger. Therefore, machining allowance must be left during the machining process of the outer surface, while the dimensions of the fork-shaped ring are ensured by direct machining. Finally, the remaining allowance at the bottom of the product is cut off.

[0068] After turning is completed, the large amount of metal removal of the inner and outer surface destroys the material internal stress steady state, and needs to be naturally aged with heat aging to release the product residual stress, so as to minimize the influence of the deformation amount of the next process on the product precision.

[0069] After aging is completed, the outer surface of the overall box bottom is only 4-5mm, and the processed surface has been fully released deformation, and the surface has a certain deviation. The traditional five-axis milling cannot use the milling error caused by the surface deviation, and also basically cannot meet the tool chatter and other machining deformations caused by the thickness of 1mm. Therefore, the five-axis mirror image milling method suitable for thin-walled machining is adopted in the five-axis milling of the patent. First, the product turning and milling reference is coordinated on the mirror image milling equipment, the inner surface is scanned by laser to obtain point cloud data, and the actual inner surface characteristics of the product are obtained by reverse reconstruction based on the point cloud data. Based on this, the equal thickness compensation processing is carried out. At the same time, in order to improve the rigidity of thin-walled part milling, the mirror image milling is carried out by one milling head and one supporting head for mirror image real-time support and milling, which greatly improves the rigidity of milling.

[0070] In summary, based on the overall machining scheme of the super large diameter-thickness ratio overall box bottom, the design requirements of the overall box bottom with a diameter of Φ3350 and a thin wall thickness of 1mm can be realized.

[0071] The specific steps include:

[0072] Step one: turning processing

[0073] The turning processing of the box bottom is the main process in the overall scheme, the allowance removal of which is the largest, and the inner surface reference and fork ring structure are formed. The reference coordination, rigid clamping, turning stress release method and milling allowance determination in the turning process of the super large diameter-thickness ratio overall box bottom are all problems to be solved.

[0074] The turning processing includes:

[0075] S1: transmission of forming reference, a plurality of horizontal reference lines are transmitted on the box bottom blank in the overall forming process, which is used for allowance coordination of turning; as shown in Figure 4 ;

[0076] S2: welding and clamping of angle piece in large end allowance area, considering the clamping of the overall box bottom turning, the position of the lower end of the large end is the large end allowance area; the welding of the angle piece is carried out in the large end allowance area, not less than 8, which is used for clamping the overall box bottom, as shown in Figure 5 ;

[0077] S3: flexible multi-point profiled outer tire fixing, considering the influence of the traditional overall mold tire clamping on the surface error, the flexible multi-point profiled clamping method is set by using the flexible clamping tool. As shown in Figure 6As shown in a and b, the flexible clamping tooling includes a square box, a block, and a pressing plate combination, the pressing plate combination is used to detachably connect the block to the square box; the square box and the block are placed according to the clamping diameter corresponding to the predetermined support height, are fixed, and the overall turning of the block is performed according to the actual outer shape surface size of the box bottom, so that the block is attached to the outer shape surface of the box bottom, and the attachment degree with the part outer shape surface is improved.

[0078] S4: end face and inner shape surface turning, the linear speed changes caused by the diameter changes during the inner shape surface turning, during the turning, the rough machining is performed by segmented turning, and the up-down overall turning is performed at the last inner shape surface forming, the inner shape surface of the box bottom is turned according to the theoretical machining amount of the inner shape surface of the box bottom, and the machined inner shape surface is obtained.

[0079] Specifically, the thickness deviation range is set, when the thickness difference to be turned of the inner shape surface is within the thickness deviation range, it is divided into a segment, and when the thickness difference exceeds the thickness deviation range, it enters the next segment.

[0080] S5: flexible multi-point profiled inner block fixing, the inner side of the box bottom is clamped by using the flexible clamping tooling, the overall turning of the block is performed according to the inner shape surface of the box bottom, so that the block is attached to the inner shape surface of the box bottom. Figure 7 As shown in a and b.

[0081] S6-1: outer shape turning and cyclic stress release method, the outer shape surface has a thin-thick area feature, but the thickness of the thick area is still small, and the turning cannot directly guarantee the accuracy size. Therefore, the outer shape turning of the spherical surface area is still performed according to the equal-thickness turning, and the layer cutting method is adopted, the outer shape surface of the box bottom is coarsely turned according to the machined inner shape surface, and each layer of the layer cutting is segmented turned according to the set thickness deviation range; after the different segmented turning of the layer cutting of one layer is completed, the up-down overall turning is performed, the turning of one layer is completed, and a certain amount of allowance is left, that is, the coarse turning of the outer shape surface is stopped after the turning allowance is reached. The thick fork-shaped ring at the lower end needs to be coarsely and finely turned to the position.

[0082] Due to the thickness size of the fork-shaped ring tip point and the overall forming manufacturing accuracy, the allowance of the outer spherical surface is large, which is often several times of the final thickness, therefore, the product deformation caused by the large turning of the outer spherical surface is a necessary consideration factor, and the machining deformation needs to be maximally released when the thickness of the outer shape surface is not in place in this process. The cyclic stress release method is as shown in Figure 8 During the layer cutting of the outer spherical surface, the machining deformation is released by 2-3 times of loosening of the pressing plate operation according to the allowance condition, but a certain degree of limiting needs to be combined with the deformation condition of the previous times to avoid the irreversible influence of the deformation on the allowance.

[0083] The stress release method is specifically as follows:

[0084] A1: turning the outer shape surface, the turning amount is 30% of the product allowance;

[0085] A2: loosen the clamping of the box bottom symmetrically, and limit the large end diameter with a pressing plate, and naturally release the processing deformation;

[0086] A3: measure the large end roundness, when the roundness is ≤ 60% of the product allowance, continue steps A1, A2 and A3, when the roundness is ≥ 60% of the product allowance or the roundness is less than 1 mm, it is considered that the stress release is sufficient, and the stress release is ended.

[0087] S6-2: an outer surface turning allowance determination method based on a tolerance threshold, the setting of the outer surface turning allowance directly affects the mirror milling process. If the allowance is set too large, the removal amount in the mirror milling process is large, and the processing efficiency is reduced; if the allowance is set too small, the turning rigidity is reduced, the same plate difference of the turning wall thickness is increased, and the effect of the real-time compensation of the mirror milling wall thickness is directly affected. Therefore, the reasonable setting of the outer surface turning allowance is crucial, and the patent provides an outer surface allowance determination method based on a tolerance threshold.

[0088] As shown in the flow Figure 9 After the first turning of the outer surface, the wall thickness deviation is measured to obtain the initial wall thickness tolerance value Q (the theoretical wall thickness of the same circle after turning is a, and the initial wall thickness tolerance value Q is the difference between the maximum and minimum values of the wall thickness after turning), which can be regarded as the normal deviation affected by deformation and other factors in the turning process. Multiply the initial tolerance value by the amplification factor K, which is generally 1.5, to determine the initial tolerance threshold Qk, and compare the value with the recommended value of the real-time compensation of the mirror milling, and the smaller value is determined as the final tolerance threshold Q1. Turn the outer surface multiple times, measure the wall thickness deviation after each turning, until the wall thickness deviation is ≥ the final tolerance threshold Q1, or lower than the minimum allowance value set by the human, stop turning the outer surface, and the difference between the wall thickness of the box bottom obtained by rough turning and the theoretical wall thickness of the box bottom is the turning allowance. Through this value, it can be judged whether the turning is in place, that is, the turning allowance is determined. The final tolerance threshold actually represents the maximum wall thickness deviation that can be reached while meeting the needs of mirror milling and allowing turning.

[0089] Through this method, the overall box bottom allowance setting under different stress conditions can be met. Generally, the final turning allowance determined according to this method is about 3-5 mm.

[0090] S7: turning off and turning off with a bench lathe, that is, removing the allowance part of the lower end of the box bottom.

[0091] Step two: heat aging of the box bottom after rough turning, the heat aging condition is heating to 140°, holding for 4 hours, and naturally cooling.

[0092] The thermal aging is arranged after the turning is finished, so that the internal stress of the turned bottom can be released, and the thermal aging is performed at this time, so that even if the internal surface or external surface of the bottom is deformed, the thickness processing is not affected, because in the process of five-axis mirror milling, the actual internal surface is determined by laser scanning, and the actual internal surface is taken as a reference for processing.

[0093] If the thermal aging is arranged in the turning process, the deformation of the bottom caused by the thermal aging will cause large errors in the subsequent turning process.

[0094] Step three: five-axis mirror milling

[0095] As shown in Figure 10 , after the overall bottom of the super large diameter-thickness ratio is turned according to certain allowance rules, the overall thinning and thin area thinning need to be performed in the milling process, the thickness of the thick area is 3.1mm, the thickness of the thin area is 1mm, and the wall thickness accuracy is ±0.1mm.

[0096] The traditional mechanical milling method has problems such as weak clamping stiffness, irregular tool chatter, and precision control instability in the overall bottom of large diameter-thickness ratio, therefore, the patent provides a process scheme based on the mirror milling mechanism for the overall bottom of super large diameter-thickness ratio, and the process arrangement and processing method are as follows:

[0097] S1: Turning and milling reference coordination

[0098] As shown in Figure 11 , unlike the calibration method of the machine tool adapting to the workpiece in the traditional milling process, the mirror milling has a double-motion structure of a follow-up head and a support head, so the horizontal and vertical references of the machine tool need to be pre-calibrated, and then the workpiece is clamped for position adjustment to match the machine tool coordinate system.

[0099] S2: Internal surface laser scanning

[0100] The milling reference of the remaining thickness is the internal surface, and the accurate perception of the internal surface is an important process for processing precision. The laser head on the follow-up support of the equipment is reciprocally scanned according to the theoretical surface, and the scanning point file is converted into a point cloud file.

[0101] S3: Reverse reconstruction of scanning data

[0102] After a series of measures such as data preprocessing, noise reduction and smoothing of the point cloud by using reverse software, the internal reference surface is generated, the internal reference surface is offset according to the theoretical thickness of the overall bottom at each place to generate the processing external surface, then the thin area boundary of the overall bottom is projected onto the processing surface by using normal projection, and the thin area features are generated by preprocessing. In this way, the offline precision compensation based on laser scanning can be completed.

[0103] S4: Five-axis mirror milling and milling path planning

[0104] Mirror milling has great advantages in the machining of integral box bottom thin wall, especially suitable for solving the problems of weak rigidity milling and tool irregular vibration, and the mechanism is shown in Figure 14 During the process of tool subsystem milling thinning, the support subsystem always maintains mirror symmetry relative to the milling point, playing a supporting role; at the same time, the real-time thickness measuring device is arranged on the auxiliary support subsystem, which can use real-time thickness data to compensate the tool milling depth to ensure the equal wall thickness processing.

[0105] S41: rough machining of the box, the rough machining is layered processing, the ring machining is carried out in the way of constant Z axis and A axis, and the thickness of the box bottom is measured in real time after each layer processing;

[0106] S42: judging the profile deformation according to the thickness of the box bottom measured in step S42, if the profile deformation is greater than 50% of the recommended value of real-time compensation of mirror milling, it is considered that the profile deformation is large, then the inner profile of the box bottom is scanned, the scanning data is used to update the production and processing reference surface, and step S41 is repeated until the profile deformation is less than 50% of the recommended value of real-time compensation of mirror milling, and then step S43 is continued

[0107] S43: fine machining of the thin area of the box bottom, the fine machining is layered processing, and the thin area fine machining is layered processed according to the specific planning path, and the cutting depth is adjusted according to the real-time measured thickness value of the box bottom after each layer processing, and the next layer cutting is carried out until the thickness meets the tolerance requirement.

[0108] The recommended value of real-time compensation is a fixed parameter of the machine tool.

[0109] The method for obtaining the specific planning path is as follows:

[0110] Since the mirror milling equipment servo support head has the functions of real-time thickness measurement and tool cutting depth compensation, but the actual tool cutting is a dynamic process, the response speed of cutting depth compensation determines the precision and quality of the cutting surface. At the same time, due to the existence of many complex thin area features in the integral box bottom, the traditional tool path is affected by the thin area features, and there is a situation that the five-axis interpolation response is too slow during machining, which leads to poor milling surface quality and unstable milling precision.

[0111] Based on this, the patent sets the mirror milling path based on the fast response compensation strategy. In order to improve the response speed of compensation, the number of linkage axes should be reduced as much as possible, and the rotary characteristics of the box bottom are combined, based on this idea, the Z axis and A axis constant milling path is designed, according to its strategy, only three axes are involved in interpolation linkage in most of the milling process, which greatly improves the real-time compensation response speed.

[0112] The method flow is as follows Figure 12As shown, firstly, the single milling range boundary is divided according to the characteristics of the thin-thick zone of the box bottom; secondly, the thin zone section curve is extracted according to the fixed frequency of the Z axis (the milling width of the cutter) (at this time, the thin zone section curve is a complete continuous curve); then, the thin zone characteristics are set with a certain allowance and used as the boundary cutting curve (at this time, the thin zone section curve is cut by the thin zone characteristics to form the boundary cutting curve); then, the milling path conversion is carried out by using the boundary cutting curve, so that constant Z axis cutting in most of the time can be realized; finally, the tool shaft direction is set by using five-axis reprocessing, so that it is always perpendicular to the machining surface, so that in the process of constant Z axis milling, the tool shaft direction is only the C axis linkage.

[0113] As shown in Figure 13 , the cutter path produced by the method is compared, the left side ① area cutting path is affected by the thin zone characteristics, and the conventional cutting path is five-axis linkage; the right side ② area cutting path (the cutting path of the application) is constant in most of the cutting time, only three axes are involved in linkage, which greatly improves the response speed of real-time compensation, the surface roughness reaches at least Ra6.3, and the precision is stable at ±0.1mm.

[0114] Although the present application is disclosed with the above preferred embodiments, it is not intended to limit the present application, and any person skilled in the art can make possible changes and modifications without departing from the spirit and scope of the present application, therefore, the protection scope of the present application should be limited by the scope defined by the claims of the present application.

Claims

1. A method for precision machining of an integral box bottom with uniform wall thickness and an ultra-large diameter-to-thickness ratio, characterized in that: include Clamp the box bottom (1), and take the horizontal datum line (11) of the box bottom (1) blank as the datum. Turn the inner surface of the box bottom (1) according to the theoretical machining amount of the inner surface of the box bottom (1) to obtain the machined inner surface. Clamp the box bottom (1), and rough turn the outer surface of the box bottom (1) with the machined inner surface as the reference. Stop rough turning the outer surface after reaching the turning allowance. The bottom of the box (1) after rough turning was subjected to heat aging; Five-axis mirror milling was used to mill the thick and thin areas of the outer surface of the bottom (1) after heat aging; The rough turning of the outer surface of the box bottom (1) is carried out by multiple layer cutting, and the clamping of the box bottom (1) is loosened multiple times during the layer cutting process to release stress; Stress relief methods include: A1: Turn the outer surface, with a turning allowance of 30-40% of the current product allowance; A2: Symmetrically loosen the clamping of the bottom of the box (1), and limit the large end diameter with a pressure plate to naturally release the processing deformation; A3: Measure the roundness of the large end. When the roundness is ≤ 60% of the product allowance, continue with steps A1, A2 and A3. When the roundness is ≥ 60% of the product allowance or the roundness is less than 1mm, the stress is considered to be fully released and the stress release is over. During the rough turning of the outer surface of the box bottom (1), each layer is cut in sections according to the set thickness deviation range; After the different segments of the layer cutting are completed, the upper and lower parts are machined together to complete the layer cutting. When performing five-axis mirror milling, the following are included: S1: Adjust the position of the bottom of the box (1) after heat aging on the machine tool to match the machine tool coordinate system; S2: Laser scanning of the bottom of the box (1) to obtain the inner reference surface; S3: Based on the theoretical thickness of each part of the overall box bottom (1), offset the inner reference surface to generate the machining outer surface; S4: Use five-axis mirror milling to mill the thick and thin areas of the bottom (1) of the box according to the machining surface; S4 includes: S41: rough machining of the bottom of the box (1), which is a layered machining process, and the thickness of the bottom of the box (1) is measured in real time after each layer is machined; S42: Determine the surface deformation based on the thickness of the box bottom (1) measured in S41. If the surface deformation is greater than 50% of the recommended value of real-time compensation for mirror milling, it is considered that the surface deformation is large. Then perform surface scanning inside the box bottom (1) until the surface deformation is less than 50% of the recommended value of real-time compensation for mirror milling, and continue to step S43. S43: Perform finishing on the thin area of ​​the box bottom (1). The finishing is done in layers. After each layer is finished, the next layer is cut according to the real-time measured thickness value of the box bottom (1) until the thickness of the thin area meets the tolerance requirements.

2. The method for precision machining of an integral box bottom with equal wall thickness and ultra-large diameter-to-thickness ratio according to claim 1, characterized in that: The clamping box bottom (1) adopts multiple flexible clamping fixtures (2), and the multiple flexible clamping fixtures (2) are evenly distributed around the box bottom (1). The flexible clamping fixtures (2) include a square box (21), a tire block (22), and a pressure plate assembly (23). The pressure plate assembly (23) is used to detachably connect the tire block (22) to the square box (21). When machining the inner surface of the box bottom (1), the bottom of the large end of the box bottom (1) is connected to the corner piece (13), which supports the box bottom (1) through the corner piece (13). At the same time, the outer surface of the box bottom (1) is machined as a whole to make the tire block (22) fit with the outer surface of the box bottom (1). When machining the outer surface of the box bottom (1), the tire block (22) is machined as a whole according to the inner surface of the box bottom (1) so that the tire block (22) fits into the inner surface of the box bottom (1).

3. The method for precision machining of an integral box bottom with equal wall thickness and ultra-large diameter-to-thickness ratio according to claim 1, characterized in that: The method for determining the turning allowance is as follows: after the first rough turning of the outer surface, the wall thickness deviation value is measured to obtain the initial wall thickness tolerance value Q. The initial tolerance value is multiplied by the magnification factor K to obtain the initial tolerance threshold Qk. The smaller value between the initial tolerance threshold Qk and the recommended value of real-time compensation for mirror milling is used as the final tolerance threshold Q1. After each rough turning, the wall thickness deviation value is measured to obtain the real-time wall thickness tolerance value. Rough turning ends when the real-time wall thickness tolerance value is not less than the final tolerance threshold Q1 or is lower than the manually set minimum allowance value. At this point, the difference between the wall thickness of the box bottom (1) obtained by rough turning and the theoretical wall thickness of the box bottom (1) is the turning allowance.

4. The method for precision machining of an integral box bottom with equal wall thickness and ultra-large diameter-to-thickness ratio according to claim 3, characterized in that: The turning allowance is 3-5mm.

5. The method for precision machining of an integral box bottom with equal wall thickness and ultra-large diameter-to-thickness ratio according to claim 1, characterized in that: The heat aging conditions are: heating to 140°C and holding for 4 hours.

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