A weak-rigidity laminated structure drilling stepped tool optimal design method

Abstract

This invention belongs to the field of machining technology, specifically relating to an optimized design method for stepped drilling tools for weakly rigid laminated structures. This method addresses the problems of workpiece deformation and interlayer scratches that easily occur during drilling of weakly rigid laminated structures. It uses interlayer clearance control as the core indicator and, based on the coupling relationship between drilling axial force and workpiece deformation, constructs a quantitative mapping relationship between the tool's stepped geometric parameters and the interlayer clearance during machining. Furthermore, by establishing a critical clearance criterion for interlayer damage and combining the deformation characteristics under different drilling modes, the key parameters of the stepped tool, namely the step diameter and step length, are optimized. This ensures that the maximum interlayer clearance during drilling remains below the critical value, effectively suppressing interlayer chip intrusion and interface damage, thereby improving drilling quality. This invention realizes a shift in tool design from experience-driven to mechanism-driven, and is applicable to high-quality hole machining of various weakly rigid laminated structures.

Description

Technical Field

[0001] This invention belongs to the field of machining, and in particular relates to an optimized design method for a stepped drilling tool with a weakly rigid stacked structure. Background Technology

[0002] Reducing structural weight to achieve longer flight range and higher fuel / energy efficiency is a perpetual goal in the aerospace industry. To achieve weight reduction and efficiency improvement, large components such as integrally molded panels and skins are widely used. These components typically have large structural dimensions and thin wall thicknesses, resulting in low overall structural stiffness. Furthermore, when assembling two parts, they must overlap at the connection area, forming a weakly rigid laminated structure, such as composite / composite laminates, composite / metal laminates, and metal / metal laminates. To ensure a stable and reliable assembly connection, connection holes must be integrally machined in the connection area for bolts, rivets, and other connections. Therefore, the quality and precision of the machined holes directly affect the assembly quality and performance of the component. However, due to differences in material properties and low overall stiffness among different parts, the workpiece is prone to significant deformation under drilling axial forces, leading to increased interlayer gaps. Especially when the lower layer material is metal, when the interlayer gap exceeds a certain threshold, hard chips can penetrate the interlayer, causing scratches and chip accumulation at the laminate interface. Therefore, stepped drills are typically used to achieve graded cutting of materials, thereby reducing drilling axial forces and workpiece deformation. However, the geometric parameters such as the diameter and length of the steps significantly affect the drilling axial forces, which in turn induce complex workpiece deformation behavior, making it difficult to effectively control the interlayer gap. Currently, there is no clear correlation mechanism between the design of the tool's stepped geometry parameters and the evolution of the interlayer gap, making the effective suppression of layer interface damage still a significant challenge.

[0003] To address these issues, researchers have developed various tool structures. For example, Jiangxi Jiehao Carbide Tools Co., Ltd.'s patent application (CN201220592135.8), entitled "A Drill Bit for Processing Carbon Fiber Reinforced Composite Materials and Titanium Alloy Laminated Plates," employs a radial large rake angle slotting method to obtain a very sharp cutting edge, thereby effectively suppressing damage such as composite material delamination and tearing. Furthermore, by designing the drill tip with a tri-apex structure, it reduces the axial drilling force generated during processing, thus minimizing machining deformation. However, when drilling the lower layer of material, this tool inevitably produces significant deformation of the lower workpiece and interlayer gaps, leading to problems such as interlayer chip accumulation and scratches. The patent application filed by Danyang Kaiyiyuan Tools Co., Ltd., with application number CN202320213166.6 and patent title "A Stepped Twist Drill with Chip Breaking Groove", proposes a tool with multiple cylindrical and conical surfaces of successively increasing diameters arranged alternately on the outer side of the conical part. The cutting edge is divided along the axial direction, and the cutting resistance is reduced by axial stepped layer cutting. This patent mainly optimizes the chip breaking and chip removal problems in metal processing, but it still lacks targeted design for the workpiece deformation and interface damage problems in hole making of stacked structure parts. The patent number CN201610010005.1 by Wang Fuji et al. of Dalian University of Technology, entitled "Multi-step multi-blade tool for integral hole making of laminated structure parts", further divides the material in both axial and radial directions, and invented a multi-step multi-blade drill bit with drilling, reaming and boring functions. By removing material in stages and multiple times in the axial and radial directions of the drill hole, the cutting force and damage such as delamination and tearing of composite materials are reduced. However, this tool structure is suitable for high-quality hole making of rigid laminated structure parts, and does not consider the problems of drilling deformation and gap control of weak rigid laminated structure parts.

[0004] In summary, stepped cutting tools are currently commonly used for staged cutting of weakly rigid laminated structures to reduce drilling loads and workpiece deformation. However, existing stepped cutting tool design methods are mostly based on experience or rigidity assumptions, lacking quantitative design basis for the interlayer gaps during drilling of weakly rigid laminated structures, making it difficult to effectively control interlayer damage. Therefore, further research is needed on geometric parameter design methods for stepped cutting tools for weakly rigid laminated structures that consider workpiece deformation and interlayer gaps during drilling. Summary of the Invention

[0005] To overcome the problem of interlayer scratches easily caused by the lack of adaptive design for stepped tools to address interlayer gaps when drilling weakly rigid laminated structures, this invention proposes an optimized design method for stepped tools used in drilling weakly rigid laminated structures. This method focuses on controlling the maximum interlayer gap and is based on the coupling relationship between drilling axial force and workpiece deformation. By constructing a quantitative mapping relationship between the geometric parameters of the stepped tool and the maximum interlayer gap, and using the critical interlayer gap threshold that causes damage as a constraint, and combining this with the workpiece drilling deformation behavior under different drilling modes, the diameter and length of the stepped tool are optimized, thereby effectively suppressing interlayer damage and improving drilling quality.

[0006] To achieve the above-mentioned objectives, the technical solution of this invention is as follows: An optimized design method for a stepped drilling tool with a weakly rigid stacked structure includes the following steps: Step 1: Determine the basic parameters of the laminated structure and the machining system, including the material properties of the laminated structure, the combination form of the laminated structure, the material thickness of each layer of the laminated structure, the overall geometric dimensions of the laminated structure, the clamping boundary conditions of the laminated structure, the drilling position, the target hole diameter, the machining speed, and the feed parameters. Step 2: The structural parameters of the stepped tool include the diameter of the first step. D 1. Length of the first step L 1 and the second step diameter D 2, where the diameter of the second step is... D 2 equals the target hole diameter; three drilling modes are preset for the stacked structure to be processed, and the requirements for the relationship between the first step length of the stepped tool and the thickness of the stacked structure under different drilling modes are determined; the three drilling modes include sequential drilling mode, layered synchronous drilling mode, and same-layer synchronous drilling mode; for different drilling modes, steps three to six need to be performed sequentially to obtain the preferred combination of structural parameters of the stepped tool under different drilling modes; Step 3: Analyze the drilling deformation behavior of the laminated structure under the current drilling mode through experiments or experience, analyze the stress on each layer of material and the change in interlayer gap during the drilling process, identify the key stage in the drilling process that generates the maximum interlayer gap and the relative positional relationship between the stepped tool and the laminated structure when the maximum interlayer gap is generated. Step 4: Using the pre-constructed coupled model of drilling axial force and workpiece deformation, input the structural parameters of the stepped tool, the basic parameters of the machining system, and the basic parameters of the stacked structure to calculate the drilling axial force and workpiece deformation when the maximum interlayer gap is generated under the current drilling mode and different first step diameters. Using the workpiece deformation, the deformation difference between the upper and lower layers of material is obtained. The deformation difference is the value of the maximum interlayer gap, thereby establishing a quantitative mapping relationship between the first step diameter of the stepped tool, the maximum drilling axial force, and the maximum interlayer gap. Step 5: Establish the critical gap criterion for interlayer damage; through experimental data fitting or empirical models, determine the critical interlayer gap threshold that induces interlayer chip intrusion and interface scratches under different first step diameters, and establish the mapping relationship between the critical interlayer gap threshold and the first step diameter of the stepped tool. Step 6: Using the critical interlayer clearance threshold from Step 5 as a constraint condition for the optimized design of the first step diameter of the stepped tool, the quantitative mapping relationship between the first step diameter and the maximum interlayer clearance obtained in Step 4 is compared with the mapping relationship between the critical interlayer clearance threshold and the first step diameter of the stepped tool obtained in Step 5. The goal is to ensure that the maximum interlayer clearance generated throughout the drilling process is always less than the critical interlayer clearance threshold, thereby avoiding interlayer damage and optimizing the first step diameter parameter of the stepped tool. The optimized first step diameter of the stepped tool is then combined with the first step length of the stepped tool that satisfies the current drilling mode to obtain the structural parameter combination of the stepped tool. If there is no case where the first step diameter is less than the critical interlayer clearance threshold, the current drilling mode is discarded. Step 7: For the structural parameter combinations of stepped tools obtained under different drilling modes, iterative screening is carried out through multi-faceted engineering rationality judgment to finally optimize the structural parameters of the stepped tools. The effectiveness of the optimized structural parameters of the stepped tools in suppressing interlayer damage is verified through experiments, thus completing the optimized design of the stepped tools.

[0007] Furthermore, in the second step: sequential drilling mode refers to each step of the stepped tool drilling the stacked structure sequentially. The sequential drilling mode requires that the length of the first step of the stepped tool is greater than the sum of the thickness of the stacked structure and the maximum interlayer gap. Layered synchronous drilling mode refers to the two steps of the stepped tool drilling the upper and lower layers of material simultaneously. The layered synchronous drilling mode requires that the sum of the thickness of the stacked structure and the maximum interlayer gap is greater than the length of the first step of the stepped tool, which is greater than the thickness of the lower layer. Same-layer synchronous drilling mode refers to the two steps of the stepped tool drilling the same layer of material simultaneously. The same-layer synchronous drilling mode requires that the length of the first step of the stepped tool is less than the thickness of the lower layer.

[0008] Furthermore, the results of the third step of identification differ under different drilling modes. Specifically: when the current drilling mode is sequential drilling mode, the maximum interlayer gap occurs when the first step is completely drilled into the lower layer material; when the current drilling mode is synchronous drilling mode in the same layer, the influence of the superposition of the drilling axial forces of the two steps on the deformation needs to be considered, and the maximum interlayer gap occurs when the two steps are drilling the lower layer material at the same time; when the current drilling mode is layered synchronous drilling mode, the value of the maximum interlayer gap can be directly determined, and the value of the maximum interlayer gap is the maximum value of the difference in deformation between the lower layer material and the upper layer material.

[0009] Furthermore, in the fourth step, the drilling axial force-workpiece deformation coupling model is used to describe the dynamic coupling relationship between the drilling axial force and the workpiece deformation during the drilling process, laying a theoretical foundation for subsequent optimization of the stepped tool structure. The drilling axial force-workpiece deformation coupling model includes, but is not limited to, analytical models, numerical simulation models, deep learning models, or finite element models. The drilling axial force-workpiece deformation coupling model takes the structural parameters of the stepped tool, the basic parameters of the machining system, and the basic parameters of the stacked structure as inputs, and takes the drilling axial force and workpiece deformation during the drilling process as outputs.

[0010] Furthermore, in the seventh step, the multi-faceted engineering rationality judgment needs to consider the structural strength of the stepped tool, the forming and manufacturing process, the rationality of the chip removal space, and whether the functional structure interferes with each other; the structural parameter optimization of the stepped tool is achieved through parameter traversal, successive approximation method or intelligent optimization algorithm, and the engineering rationality judgment is made in combination with the structural strength, stiffness, manufacturability and chip removal space of the stepped tool.

[0011] The beneficial effects of this invention are as follows: This invention proposes an optimized design method for stepped drilling tools for weakly rigid laminated structures. This method focuses on controlling the interlayer gap that induces damage, establishing a quantitative relationship between the structural parameters of the stepped tool, drilling axial force, workpiece deformation, and interlayer gap. It uses the critical interlayer gap threshold for suppressing interlayer damage as an optimization constraint to achieve optimized design of the stepped tool's structural parameters, transforming the design from experience-driven to mechanism-driven, thus improving the accuracy and reliability of the design. Simultaneously, by analyzing the workpiece drilling deformation behavior and interlayer gap evolution under different drilling modes, it can accurately identify interlayer damage risks and optimize the structural parameters of the stepped tool. This achieves reasonable distribution of drilling axial force and precise control of the interlayer gap during drilling, ensuring that the maximum interlayer gap during machining remains below the critical interlayer gap threshold that induces damage. This effectively suppresses damage such as interlayer chip intrusion and interface scratches, significantly improving the drilling quality of weakly rigid laminated structures. This method is applicable to the machining of weakly rigid laminated structures in various material systems and has significant engineering application value in high-end manufacturing fields such as aerospace. Attached Figure Description

[0012] Figure 1 This is a flowchart of the optimized design process for stepped drilling tools with a weakly rigid stacked structure.

[0013] Figure 2 This is a schematic diagram of a stepped cutting tool structure.

[0014] Figure 3 These are schematic diagrams of different drilling modes; (a) is the sequential drilling mode, (b) is the layered synchronous drilling mode, and (c) is the same-layer synchronous drilling mode.

[0015] Figure 4 It is the relationship between the first step diameter of the stepped tool and the maximum interlayer gap in the layered synchronous drilling mode.

[0016] Figure 5 These are the hole-making effect diagrams before the optimization of the stepped tool structure, where (a) is the stepped tool before optimization and (b) is the hole-making effect before optimization.

[0017] Figure 6 These are the hole-making effect diagrams after the optimization of the stepped tool structure, where (a) is the optimized stepped tool and (b) is the optimized hole-making effect.

[0018] In the diagram: A. First step; B. Second step; C. Tool holder; D 1. Diameter of the first step; L 1. Length of the first step; D 2. Second step diameter. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer and to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. It should be noted that the technical solutions claimed by the present invention include, but are not limited to, the following embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0020] This invention proposes an optimized design method for stepped drilling tools with weakly rigid stacked structures, such as... Figure 1 As shown in the figure. This embodiment selects a carbon fiber reinforced composite / titanium alloy (CFRP / Ti-6Al-4V) weak rigidity laminate structure, which is widely used in typical aerospace assembly, as the research object, and conducts a case study analysis of stepped tool optimization design.

[0021] Step 1: Determine the basic parameters of the laminated structure and machining system. The laminated structure consists of an upper CFRP laminate and a lower Ti-6Al-4V titanium alloy plate. The thickness of the upper CFRP material is 2mm, the thickness of the lower titanium alloy material is 2mm, the width of the laminated structure is 30mm, and the overhang length is 70mm. The workpiece is clamped with both ends fixed and the middle suspended to simulate typical weak-rigid machining conditions. The drilling position is located at the geometric center of the workpiece, and the target hole diameter is 6mm. During drilling, the axial force of drilling is equivalent to a concentrated load acting on the drilling position. The machining parameters are set as follows: spindle speed 300r / min, feed rate 30mm / min.

[0022] Step 2: As Figure 2 As shown, a stepped cutting tool consists of three parts: A) the first step; B) the second step; and C) the tool holder. The structural parameters of the stepped cutting tool include the diameter of the first step. D 1. Length of the first step L 1 and second step diameter D 2. The diameter of the second step is equal to the target hole diameter of 6mm. Three drilling modes are preset for the laminated structure to be machined: sequential drilling mode, layer-by-layer synchronous drilling mode, and same-layer synchronous drilling mode, such as... Figure 3 As shown. The sequential drilling mode refers to each step of the stepped tool drilling CFRP and titanium alloy sequentially. The sequential drilling mode requires that the length of the first step of the stepped tool is greater than the sum of the thickness of the laminated structure and the maximum interlayer gap. The layered synchronous drilling mode refers to the two steps of the stepped tool simultaneously drilling the upper and lower layers of material. The layered synchronous drilling mode requires that the sum of the thickness of the laminated structure and the maximum interlayer gap is greater than the length of the first step of the stepped tool, which is greater than the thickness of the lower layer. The same-layer synchronous drilling mode refers to the two steps of the stepped tool simultaneously drilling the same layer of material. The same-layer synchronous drilling mode requires that the length of the first step of the stepped tool is less than the thickness of the lower layer. In this embodiment, the length of the first step of the stepped tool is selected as 3mm. By comparing it with the thickness of the laminated structure, it can be determined that this stepped tool belongs to the layered synchronous drilling mode. If other first step lengths of the stepped tool are selected, different drilling modes will correspond. For different drilling modes, steps three through six must be performed sequentially to obtain the optimal combination of structural parameters of the stepped tool under different drilling modes.

[0023] Step 3: Analyze the drilling deformation behavior of the laminated structure through experience and experiments. As the stepped tool gradually drills into the workpiece, the first step of the stepped tool first penetrates the upper CFRP layer and generates an initial drilling axial force. When the stepped tool further feeds into the titanium alloy layer, the cutting resistance of the titanium alloy increases significantly, causing the drilling axial force to rise rapidly. This leads to significant flexural deformation in the lower titanium alloy layer, while the upper CFRP layer experiences less flexural deformation under a smaller drilling axial force, thus forming an interlayer gap between the two layers. Analysis of the drilling deformation behavior of the workpiece at different drilling stages shows that in the layered synchronous drilling mode, the interlayer gap reaches its maximum value when the first step of the stepped tool completely penetrates the lower titanium alloy layer. At this point, the second step of the stepped tool also completely penetrates the upper CFRP layer. The maximum interlayer gap value is the difference in deformation between the lower titanium alloy layer and the upper CFRP layer. Record the relative positional relationship between the stepped tool and the workpiece at this time.

[0024] Step 4: Using a pre-constructed coupled model of drilling axial force and workpiece deformation, input the structural parameters of the stepped tool, the basic parameters of the machining system, and the basic parameters of the stacked structure. Calculate the drilling axial force and workpiece deformation at the point of maximum interlayer clearance under the currently set drilling mode and different first-step diameters. Using the workpiece deformation, obtain the deformation difference between the upper and lower layers of material. This deformation difference is the value of the maximum interlayer clearance, thus establishing a quantitative mapping relationship between the first-step diameter of the stepped tool, the maximum drilling axial force, and the maximum interlayer clearance. Figure 4 As shown by the blue dotted line.

[0025] Step 5: Establish the critical interlayer gap criterion for interlayer damage. Through experimental fitting and empirical analysis, a critical interlayer gap criterion for inducing interlayer damage is established. Experiments and experience show that the larger the interlayer gap, the easier it is for chip intrusion to occur. When the interlayer gap reaches a certain critical threshold, interlayer chip intrusion and interface scratches just do not occur. This critical interlayer gap condition is defined as the critical interlayer gap threshold Δ. δ The critical interlayer gap thresholds corresponding to the first step diameter of different stepped cutting tools are different. The relationship between the critical interlayer gap threshold and the first step diameter of the stepped cutting tool was established through experimental fitting, such as... Figure 4 As shown by the middle red line.

[0026] Step 6: Use the critical interlayer gap threshold from Step 5 as a constraint condition for the optimization design of the first step diameter of the stepped tool. This involves comparing the quantitative mapping relationship between the first step diameter of the stepped tool and the maximum interlayer gap obtained in Step 4 with the mapping relationship between the critical interlayer gap threshold and the first step diameter of the stepped tool obtained in Step 5. The goal is to ensure that the maximum interlayer gap is always less than the critical interlayer gap threshold during drilling. Δδ Select the first step diameter of the stepped cutting tool that meets the constraints, such as... Figure 4 As shown, the diameter of the first step of the stepped tool should be greater than 4.3 mm. Figure 4 (in the green area), thus combining it with the first step length of 3mm of the stepped tool that meets the drilling mode requirements to determine the corresponding combination of structural parameters of the stepped tool.

[0027] Step 7: After obtaining the parameter combinations of stepped cutting tools that satisfy the critical interlayer clearance threshold constraint under different drilling modes, the engineering rationality of the stepped cutting tool structure is judged. Taking into account factors such as the strength, stiffness, manufacturing process feasibility, and chip removal space of the stepped cutting tool, parameter combinations that do not meet the engineering application requirements are eliminated. The structural parameter combinations of the stepped cutting tools obtained in this embodiment all meet the above requirements and are stepped cutting tools that satisfy the critical interlayer clearance threshold constraint and have engineering feasibility. To verify the effectiveness of the method of this invention, unoptimized stepped cutting tools (such as...) are selected. Figure 5 As shown in (a), the diameter of the first step is... D 1 is 4mm, the length of the first step L 1 is 3mm, the second step diameter D 2 is 6mm) and the optimized stepped tool (such as Figure 6 As shown in (a), the diameter of the first step is... D 1 is 4.5mm, the length of the first step L 1 is 3mm, the second step diameter D A comparative processing experiment was conducted using 2 (6mm) samples. The test results are as follows: Figure 5 (b) Figure 6 As shown in (b), the experimental results show that when machining with an unoptimized stepped tool, there are obvious chip intrusion and scratches at the interlayer interface. However, after machining with the stepped tool optimized by the present invention, the interlayer interface is intact and there is no obvious damage. The interlayer quality is significantly improved, which verifies the effectiveness of the method of the present invention in suppressing interlayer damage.

Claims

1. A method for optimizing the design of a stepped drilling tool with a weakly rigid stacked structure, characterized in that, Includes the following steps: Step 1: Determine the basic parameters of the laminated structure and the machining system, including the material properties of the laminated structure, the combination form of the laminated structure, the material thickness of each layer of the laminated structure, the overall geometric dimensions of the laminated structure, the clamping boundary conditions of the laminated structure, the drilling position, the target hole diameter, the machining speed, and the feed parameters. Step 2: The structural parameters of the stepped tool include the diameter of the first step. D 1. Length of the first step L 1 and the second step diameter D 2, where the diameter of the second step is... D 2 equals the target hole diameter; three drilling modes are preset for the stacked structure to be processed, and the requirements for the relationship between the first step length of the stepped tool and the thickness of the stacked structure under different drilling modes are determined; the three drilling modes include sequential drilling mode, layered synchronous drilling mode, and same-layer synchronous drilling mode; for different drilling modes, steps three to six need to be performed sequentially to obtain the preferred combination of structural parameters of the stepped tool under different drilling modes; Step 3: Analyze the drilling deformation behavior of the laminated structure under the current drilling mode through experiments or experience, analyze the stress on each layer of material and the change in interlayer gap during the drilling process, identify the key stage in the drilling process that generates the maximum interlayer gap and the relative positional relationship between the stepped tool and the laminated structure when the maximum interlayer gap is generated. Step 4: Using the pre-constructed coupled model of drilling axial force and workpiece deformation, input the structural parameters of the stepped tool, the basic parameters of the machining system, and the basic parameters of the stacked structure to calculate the drilling axial force and workpiece deformation when the maximum interlayer gap is generated under the current drilling mode and different first step diameters. Using the workpiece deformation, the deformation difference between the upper and lower layers of material is obtained. The deformation difference is the value of the maximum interlayer gap, thereby establishing a quantitative mapping relationship between the first step diameter of the stepped tool, the maximum drilling axial force, and the maximum interlayer gap. Step 5: Establish the critical gap criterion for interlayer damage; through experimental data fitting or empirical models, determine the critical interlayer gap threshold that induces interlayer chip intrusion and interface scratches under different first step diameters, and establish the mapping relationship between the critical interlayer gap threshold and the first step diameter of the stepped tool. Step 6: Using the critical interlayer clearance threshold from Step 5 as a constraint condition for the optimized design of the first step diameter of the stepped tool, the quantitative mapping relationship between the first step diameter and the maximum interlayer clearance obtained in Step 4 is compared with the mapping relationship between the critical interlayer clearance threshold and the first step diameter of the stepped tool obtained in Step 5. The goal is to ensure that the maximum interlayer clearance generated throughout the drilling process is always less than the critical interlayer clearance threshold, thereby avoiding interlayer damage and optimizing the first step diameter parameter of the stepped tool. The optimized first step diameter of the stepped tool is then combined with the first step length of the stepped tool that satisfies the current drilling mode to obtain the structural parameter combination of the stepped tool. If there is no case where the first step diameter is less than the critical interlayer clearance threshold, the current drilling mode is discarded. Step 7: For the structural parameter combinations of stepped tools obtained under different drilling modes, iterative screening is carried out through multi-faceted engineering rationality judgment to finally optimize the structural parameters of the stepped tools. The effectiveness of the optimized structural parameters of the stepped tools in suppressing interlayer damage is verified through experiments, thus completing the optimized design of the stepped tools.

2. The method for optimizing the design of a stepped drilling tool for a weakly rigid stacked structure according to claim 1, characterized in that: In the second step: Sequential drilling mode refers to each step of the stepped tool drilling the stacked structure sequentially. The sequential drilling mode requires that the length of the first step of the stepped tool is greater than the sum of the thickness of the stacked structure and the maximum interlayer gap. Layered synchronous drilling mode refers to the two steps of the stepped tool drilling the upper and lower layers of material simultaneously. The layered synchronous drilling mode requires that the sum of the thickness of the stacked structure and the maximum interlayer gap is greater than the length of the first step of the stepped tool, which is greater than the thickness of the lower layer. Same-layer synchronous drilling mode refers to the two steps of the stepped tool drilling the same layer of material simultaneously. The same-layer synchronous drilling mode requires that the length of the first step of the stepped tool is less than the thickness of the lower layer.

3. The method for optimizing the design of a stepped drilling tool for a weakly rigid stacked structure according to claim 1, characterized in that: The results of the third step of identification differ depending on the drilling mode. Specifically: when the current drilling mode is sequential drilling, the maximum interlayer gap occurs when the first step is fully drilled into the lower layer material; when the current drilling mode is synchronous drilling in the same layer, the influence of the superposition of the drilling axial forces of the two steps on the deformation needs to be considered, and the maximum interlayer gap occurs when the two steps are drilling the lower layer material at the same time; when the current drilling mode is layered synchronous drilling, the value of the maximum interlayer gap can be directly determined, and the value of the maximum interlayer gap is the maximum value of the difference in deformation between the lower layer material and the upper layer material.

4. The method for optimizing the design of a stepped drilling tool for a weakly rigid stacked structure according to claim 1, characterized in that: In the fourth step, the drilling axial force-workpiece deformation coupling model is used to describe the dynamic coupling relationship between the drilling axial force and the workpiece deformation during the drilling process, laying a theoretical foundation for the subsequent structural optimization of the stepped tool. The drilling axial force-workpiece deformation coupling model includes, but is not limited to, analytical models, numerical simulation models, deep learning models, or finite element models. The drilling axial force-workpiece deformation coupling model takes the structural parameters of the stepped tool, the basic parameters of the machining system, and the basic parameters of the stacked structure as inputs, and takes the drilling axial force and workpiece deformation during the drilling process as outputs.

5. The method for optimizing the design of a stepped drilling tool for a weakly rigid stacked structure according to claim 1, characterized in that: In the seventh step, the engineering rationality judgment needs to consider the structural strength of the stepped tool, the forming and manufacturing process, the rationality of the chip removal space, and whether the functional structure interferes with each other. The structural parameter optimization of the stepped tool is achieved through parameter traversal, successive approximation method or intelligent optimization algorithm, and the engineering rationality judgment is made in combination with the structural strength, stiffness, manufacturability and chip removal space of the stepped tool.

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