An adaptive milling method for aircraft skin sink features based on in-flight measurement

Abstract

The application provides an adaptive milling method for a sinking feature of an aircraft skin based on in-machine measurement. First, the shape of the clamped skin blank is measured by an in-machine probe, and the thickness of the skin blank at each position is detected by an ultrasonic thickness gauge. Second, the sinking feature machining process is planned according to the actual skin blank shape and thickness, the cutter location point coordinates, the rotating shaft rotation angle and the corresponding cutting depth at the cutter location point are obtained, the machining program is generated and milling is performed. Then, after the machining is completed, the inside and outside of the sinking feature region are measured again by the in-machine probe to obtain the actual cutting depth and the skin surface sinking depth caused by clamping and cutting force. Finally, the compensated cutter location point coordinates and normal vector are calculated by superimposing the skin surface sinking depth, the cutting relief amount and the actual cutting depth, and the compensated machining program is generated to realize the compensation machining of the sinking feature. The method has the advantages of fast calculation efficiency, strong applicability, high automation degree and high machining precision.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of aircraft skin processing, and particularly relates to a self-adaptive milling method for aircraft skin sunken features based on in-machine measurement. BACKGROUND

[0002] The aircraft skin is an important structural component for bearing air power on the aircraft body, and has the characteristics of light weight, large size and weak rigidity. At present, the processing methods of the aircraft skin mainly include chemical milling and mirror image milling.

[0003] The chemical milling is to coat a corrosion-resistant coating on the surface of the area where the material is not needed to be removed, expose the surface of the area to be milled, and then put the workpiece in a chemical solution for corrosion, so as to realize the processing of the sunken features. This milling method has the following disadvantages: 1. The profile accuracy of the sunken features cannot be guaranteed; 2. For some aircraft skins made of new alloy materials, the surface of the aircraft skin is prone to produce pitting defects when this method is used, thereby reducing the surface quality of the aircraft skin; 3. The chemical milling does not conform to the concept of sustainable development and green manufacturing.

[0004] The second method is mirror image milling, which is to process the sunken features on the milling side with a cutter, and to realize support and thickness measurement on the support side with a thickness measuring device. The main shafts on both sides of the aircraft skin move synchronously and cooperatively to achieve synchronous mirror image milling. Although the mirror image milling can overcome the shortcomings of the chemical milling, it still has the following disadvantages: 1. The synchronous movement requires high control accuracy of the machine tool, and the corresponding special equipment is expensive and occupies a large area; 2. During the milling process of the sunken features, the local surface of the aircraft skin is prone to deformation, causing uneven milling depth of the sunken features and too large tool marks on the processed surface.

[0005] On the basis of the above two processing methods, researchers have also explored a method of milling the aircraft skin on a multi-axis numerical control machine tool, which is to use double swing head main shafts to process the aircraft skin adsorbed on a mold system. However, in the actual processing process, due to the reasons such as the deviation of the shape of the aircraft skin from the theoretical surface and the uneven thickness of the aircraft skin, there is a gap between the adsorbed aircraft skin and the surface of the mold, which causes the difficulty in controlling the thickness processing accuracy of the sunken features. SUMMARY

[0006] In view of the over-dimensioning problem of the existing aircraft skin sunken feature processing, to solve the defects in the above aircraft skin processing method and ensure the consistency of the thickness of the aircraft skin sunken features, the application provides a self-adaptive milling method for aircraft skin sunken features based on in-machine measurement.

[0007] The technical scheme of the application is as follows:

[0008] A self-adaptive milling method for aircraft skin sunken features based on in-machine measurement, comprising the following steps:

[0009] Step 1: Use the machine probe to measure the shape of the clamped skin blank and detect the thickness of the skin blank at various points, including the shape and thickness inside and outside the skin depression feature area;

[0010] Step 2: Based on the actual shape and thickness of the skin blank, plan the sink feature machining process, obtain the tool position coordinates, the rotation angle of the rotary axis and the cutting depth corresponding to the tool position, generate the machining program and perform one milling operation;

[0011] Step 3: After one milling operation, the shape inside and outside the sunken feature area is measured again using the in-machine probe to obtain the actual cutting depth and the skin surface sinking depth caused by clamping and cutting force.

[0012] Step 4: By superimposing the skin surface depression depth, cutting deflection amount and actual cutting depth, calculate the compensated tool position coordinates and normal vector and generate a compensation machining program to realize the compensation machining of the depression feature;

[0013] Step 5: Measure the shape inside and outside the sunken feature area again using the in-machine probe to obtain the actual cutting depth and the remaining thickness in the sunken feature area; repeat steps 3 and 4 until the remaining thickness in the sunken feature area is within the set range, so that the thickness of the sunken feature can meet the process requirements.

[0014] Furthermore, the specific process of step 1 is as follows:

[0015] Step 1.1: Set up an in-machine measurement range Π on the theoretical skin blank model that completely includes the depression feature; arrange n sampling points within the range Π, fit the n sampling points, and reconstruct the theoretical parametric surface S of the blank at the depression feature. n ;

[0016] Step 1.2: On the theoretically parameterized surface S n m measurement points P are arranged on the top i Given i = 1, 2, ..., m, obtain the coordinates and normal vectors of the corresponding measurement points, generate an in-machine measurement program, and import it into the CNC machine tool for measurement. During the measurement process, mark the positions of the measurement points on the blank. After the measurement is completed, obtain the actual measurement result P of the skin blank at the sunken feature. i a The actual measurement result P of the skin blank i a Including the results of k measurement points within the depression feature area Results of q measurement points outside the depression feature region

[0017] Step 1.3: Measure the thickness of the blank at the marked measurement point positions in step 1.2 using a thickness measuring device, to obtain the thickness of the blank at k measurement points in the sunken feature region as BT k , and the thickness of the blank at q measurement points outside the sunken feature region as BT q ;

[0018] Step 1.4: Fit the actual measurement results P i a obtained in step 1.2 to obtain the actual parametric surface S a of the blank at the sunken feature.

[0019] Further, in step 1.1, the boundary of the sunken feature is translated outward by a distance d on the theoretical skin blank model to obtain the in-machine measurement range Π of the sunken feature.

[0020] Further, in step 1.4, n r sampling points are arranged on the theoretical parametric surface S n of the skin blank, and then the closest points of the n r sampling points on the actual parametric surface S a are solved, and the actual parametric surface S f of the blank at the sunken feature is fitted again using the solved closest points to replace the surface S a .

[0021] Further, the specific process of step 2 is as follows:

[0022] Step 2.1: In the CAD / CAM software, plan the machining process according to the three-dimensional model of the theoretical skin sunken feature to obtain the planned p theoretical tool position coordinates and the theoretical AC axis rotation angle

[0023] Step 2.2: Calculate the closest point coordinates of the theoretical tool position coordinates on the theoretical parametric surface S n of the skin blank, and the UV parameters U j and V j corresponding to the closest point coordinates;

[0024] Step 2.3: Substitute the UV parameters U j and V j into the actual parametric surface of the blank at the sunken feature to calculate the actual tool position coordinates and the normal vector

[0025] Step 2.4: According to the thickness of the blank at k measurement points in the sunken feature region obtained in step 1.3, the thickness of the blank at the actual tool position coordinatesk For one milling, if the thickness of the sink feature is to be guaranteed 1 C d , the kth measurement point in the sink feature is processed with the cutting depth The calculation is as follows:

[0026]

[0027] Step 2.5: interpolate the coordinates of each tool position point with the cutting depth at the kth measurement point corresponding to the one to be cut depth, denoted as The actual tool position point coordinates of one milling are calculated as follows:

[0028]

[0029] The actual AC axis angle is obtained by post-processing the normal vector at the tool position point The numerical control program of one milling is generated using the actual tool position point coordinates and the actual AC axis angle , and is imported into the machine tool for processing.

[0030] Further, the specific process of step 3 is as follows:

[0031] Step 3.1: After one milling, the shape of the sink feature region is measured using the in-machine measuring head, where the measurement result of the sink feature outer region is denoted as The measurement result of the sink feature inner region is denoted as Combined with the previous measurement result of the sink feature outer region , the sinking depth of the skin at the qth measurement point of the sink feature outer region due to clamping and cutting force is obtained as

[0032]

[0033] Step 3.2: According to the sinking depth of the skin at the measurement point of the sink feature outer region , the sinking depth at the kth measurement point of the sink feature inner region is obtained by interpolation According to the in-machine measurement results before and after one milling of the sink feature and , the actual cutting depth after one milling at the kth measurement point is calculated as

[0034]

[0035] Step 3.3: According to the formula ​​​

[0036]

[0037] The relief amount at the kth measurement point in the subsidence feature area in the first milling is calculated

[0038] Further, the specific process of step 4 is as follows:

[0039] Step 4.1: To ensure that the thickness in the subsidence feature after the next milling is 2 C d , the theoretical depth of cutting required at the kth measurement point in the subsidence feature area in the next milling is :

[0040]

[0041] The compensation amount c of the next milling at the kth measurement point is k :

[0042]

[0043] Further, according to the compensation amounts at the k measurement points in the subsidence feature area, the corresponding compensation amounts at the tool position points in the subsidence feature area are obtained by interpolation, denoted as t c j ;

[0044] Step 4.2: According to the actual tool position coordinates and the normal vector , the compensation processing tool position coordinates are obtained by translating the compensation amount t c j in the opposite direction of the normal vector from the actual tool position coordinates , and the calculation is as follows:

[0045]

[0046] Step 4.3: Using the actual AC axis angle and the tool position coordinates , the compensation processing numerical control program is generated and imported into the machine tool for the next cutting.

[0047] Further, in step 5, the remaining thickness range [LT A -T ol , LT A +T ol ] in the subsidence feature area, where LT A is the final thickness to be retained in the subsidence feature, and [-T ol , T ol ] is the tolerance band.

[0048] Advantages

[0049] 1、The present application simultaneously considers the uneven thickness of the skin blank, the skin surface sinking in the processing and the tool relief, etc., adjusts the cutting depth at each place in the skin sinking feature region based on the in-machine measurement results, thereby ensuring the consistency of the cutting depth in the skin sinking feature region.

[0050] 2、The present application adopts the in-machine probe to detect the thickness and shape of the sinking feature in the milling of the aircraft skin sinking feature, greatly reduces the workload of the workers using the handheld ultrasonic thickness gauge for detection, reduces the uncertainty of the measurement results caused by human factors, and improves the automation degree of the aircraft skin sinking feature milling process.

[0051] 3、The sinking feature milling process method in the present application has fast calculation efficiency and strong applicability, and can be applied to the processing of aircraft skin sinking features with larger size and double curvature characteristics.

[0052] Additional aspects and advantages of the application will be in part apparent and in part pointed out hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0053] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the following description, taken in conjunction with the accompanying drawings, in which:

[0054] Figure 1 Flow chart of the method of the present application

[0055] Figure 2 Schematic diagram of the distribution of measurement points inside and outside the sinking feature region

[0056] Figure 3 Tool position and normal vector on the actual parametric surface

[0057] Figure 4 Processing of the cutting depth of the measurement points in the first milling

[0058] Figure 5 Schematic diagram of the calculation of the skin sinking depth, the actual cutting depth and the tool relief at the measurement points

[0059] Figure 6 Calculation principle of the compensation amount of the next milling at the measurement points

[0060] Figure 7 Shape and size of the sinking feature on the skin of a certain type of aircraft

[0061] Figure 8 Tool movement trajectory of the sinking feature milling

[0062] Figure 9Thickness distribution within the recessed feature after the first milling.

[0063] Figure 10 Compensation processing of G-code for depression features

[0064] Figure 11 Thickness distribution within the sunken feature after the second milling Detailed Implementation

[0065] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0066] This embodiment takes the machining of aircraft skin depression features on a gantry milling machine with an AC-axis double-swivel head structure as an example, and proposes the following implementation steps: Figure 1 As shown:

[0067] Step 1: Measure the shape of the clamped skin blank using the machine probe, and check the thickness of the skin blank at various points, including the shape and thickness inside and outside the skin depression feature area. Specifically, this includes the following steps:

[0068] Step 1.1: Set an in-machine measurement range Π that completely includes the depression feature on the theoretical skin blank model. For example, shift the boundary of the depression feature outwards by a distance d on the theoretical skin blank model to obtain the in-machine measurement range Π of the depression feature. Within the range Π, arrange n sampling points using the isoparametric sampling method, fit the n sampling points, and reconstruct the theoretical parametric surface S of the blank at the depression feature. n .

[0069] Step 1.2: On the theoretically parameterized surface S n The optimal parameters are used to plan and arrange m measurement points P. i Given i = 1, 2, ..., m, obtain the coordinates and normal vectors of the corresponding measurement points, generate an in-machine measurement program, and import it into the CNC machine tool for measurement. During the measurement process, use a marker to mark the positions of the measurement points on the blank, and the marked positions should be as consistent as possible with the measurement point positions. After the measurement is completed, obtain the actual measurement result P of the skin blank at the sunken feature. i a The actual measurement result P of the skin blank i a Including the results of k measurement points within the depression feature area Results of q measurement points outside the depression feature region A schematic diagram of the distribution of measurement points inside and outside the depression feature area is shown below. Figure 2 As shown.

[0070] Step 1.3: Use an ultrasonic thickness gauge to measure the blank thickness at the marked measurement points in Step 1.2, and obtain the blank thickness BT at k measurement points within the sunken feature area.k , the blank thickness at the q measurement points outside the sunken feature region is BT q .

[0071] Step 1.4: actual measurement P of the skin blank obtained in step 1.2 i a is fitted to obtain the actual parametric surface S of the blank at the sunken feature a .

[0072] Considering the detection efficiency of in-machine measurement, the number of measurement points in step 1.2 cannot be too large, but the problem introduced thereby is that the actual parametric surface fitted from a small number of measurement points may have local precision loss. To solve this problem, n n sampling points are arranged on the theoretical parametric surface S r of the skin blank, and then the nearest points of the n r sampling points on the actual parametric surface S a are solved by means of Newton iteration, denoted as , and the actual parametric surface S f of the blank at the sunken feature is fitted again using the solved nearest points to replace the surface S a . The specific solving method can refer to the paper [Hou Furu. Research on allowance optimization and tool position compensation of adaptive machining of aviation compressor blades. Northwestern Polytechnical University, 2017].

[0073] Step 2: according to the actual skin blank shape and thickness, the sunken feature machining process is planned to obtain the tool position point coordinates, the rotation angle of the AC axis and the corresponding cutting depth at the tool position point, the machining program is generated and once milling is carried out. Specifically, the following steps are included:

[0074] Step 2.1: in the CAD / CAM software, the machining process is planned according to the three-dimensional model of the theoretical skin sunken feature to obtain the planned p theoretical tool position point coordinates and the theoretical AC axis rotation angle

[0075] Step 2.2: calculate the nearest point coordinates of the theoretical tool position point coordinates on the theoretical parametric surface S n of the skin blank and the UV parameters U j and V j corresponding to the nearest point coordinates.

[0076] Step 2.3: substitute the UV parameters U j and V j into the actual parametric surface S f of the blank at the sunken feature to calculate the actual tool position point coordinates on the actual parametric surface S f . and Dharma Arrow like Figure 3 As shown. The actual parametric surface here has already adopted S... f Alternate surface S a If the number of measurement points in step 1.2 is sufficient to meet the accuracy requirements, the curved surface S can also be used directly. a .

[0077] Step 2.4: Since the measurement point positions were manually marked in Step 1.2, the blank thickness BT at k measurement points within the sunken feature area can be approximately obtained based on Step 1.3. k .

[0078] If the final thickness to be retained within the depression feature is LT A The tolerance zone is [-T] ol ,T ol The processing is divided into n p (n p >1) Completed in one operation. For the first milling operation, to ensure the thickness within the recessed feature is constant... 1 C d The cutting depth is processed at the k-th measurement point within the depression feature. The calculation is as follows:

[0079]

[0080] Calculation principle as follows Figure 4 As shown.

[0081] Step 2.5: Using the cutting depth at k measurement points The coordinates of each tool position point were calculated using cubic Hermitian interpolation. The corresponding first cutting depth is denoted as The actual tool position coordinates for the first milling operation The calculation is as follows:

[0082]

[0083] Then, the normal vector at the tool point is... Post-processing is performed to obtain the actual AC axis angle.

[0084]

[0085] Using actual tool position coordinates and the actual AC axis angle Generate the CNC program for the first milling operation and import it into the machine tool for machining.

[0086] Step 3: After the first milling, the shape of the inner and outer regions of the sink feature is measured again using the on-machine probe to obtain the actual cutting depth and the sink depth of the skin surface caused by clamping and cutting force. The specific process is as follows:

[0087] Step 3.1: After milling, the on-machine measurement program generated in step 1.2 is executed again to measure the inner region of the sink feature and the outer region of the sink feature using the on-machine probe. The measurement result of the outer region of the sink feature is denoted as The measurement result of the inner region of the sink feature is denoted as As shown in Figure 5 The sink depth of the skin at the qth measurement point outside the sink feature region caused by cutting force and adsorptive clamping is obtained by combining the measurement result of the outer region of the sink feature in step 1.2

[0088]

[0089] Step 3.2: According to the sink depth of the skin at the measurement point outside the sink feature region The sink depth of the kth measurement point inside the sink feature region is obtained by using cubic Hermite interpolation According to the on-machine measurement results before and after the first sink feature milling And The actual cutting depth of the kth measurement point after the first milling is calculated as

[0090]

[0091] Step 3.3: Due to the difference in stiffness between the tool and the skin material, and the existence of a cavity when the skin is adsorbed and clamped, there will be a tool relief during the milling of the skin. According to the formula

[0092]

[0093] The tool relief of the kth measurement point inside the sink feature region in the first milling is calculated as

[0094] Step 4: By superimposing the sink depth of the skin surface, the cutting tool relief, and the actual cutting depth, the compensated tool position coordinates and normal vector are calculated and the compensation machining program is generated to realize the compensation machining of the sink feature. The specific process is as follows:

[0095] Step 4.1: If the thickness of the inner region of the sink feature after the next milling is to be guaranteed 2 C d , the theoretical depth of the kth measurement point inside the sink feature region that needs to be cut in the next milling is .​​​

[0096]

[0097] the compensation amount c of the next milling at the kth measurement point k is:

[0098]

[0099] The principle is shown in Figure 6 . Then, according to the compensation amounts at the k measurement points in the sunken feature area, the cubic Hermite interpolation method is used to interpolate the corresponding compensation amounts at the tool position points in the sunken feature area, denoted as t c j .

[0100] Step 4.2: According to the actual tool position point coordinates and the normal vector , the compensation processing tool position point coordinates are obtained by translating the compensation amount t c j in the opposite direction of the normal vector , and the calculation is as follows:

[0101]

[0102] Step 4.3: Using the actual AC axis angle and the tool position point coordinates to generate a compensation processing NC program, and import it into the machine tool for the second cutting.

[0103] Step 5: After cutting, based on the method of step 3, the actual cutting depth after the current milling is calculated Then, using the blank thickness BT k , subtracting the actual cutting depth of twice, the remaining thickness R k in the sunken feature area is obtained:

[0104]

[0105] Then repeat steps 3 and 4 until the remaining thickness R k in the sunken feature area is within the range of [LT A -T ol , LT A +T ol ], so that the thickness of the sunken feature can meet the process requirements.

[0106] The equipment used in this embodiment mainly includes: AC axis gantry numerical control machine tool, RMP600 type trigger probe, ultrasonic thickness gauge and certain type of aircraft skin. ​

[0107] The dimensions of the recessed features on the skin of a certain aircraft model are 510×134mm. The theoretical blank thickness of the recessed feature is 2.6mm, and the final retained thickness is 2.0mm, with a tolerance of ±0.08mm. Figure 7 As shown. The skin is clamped onto the resin mold via vacuum adsorption, and its machining coordinate system and measurement coordinate system are set at the same reference hole on the mold. The theoretical skin blank model is translated 9mm around the depression feature to obtain the in-machine measurement range. Approximately 40,000 sampling points are arranged isoparametrically within this area. The theoretical parametric surface of the skin blank is obtained by fitting the sampling points. Six measurement points are arranged on each of the upper and lower sides outside the depression feature area, and eight measurement points are arranged along the UV direction within the depression feature area, resulting in a total of 76 measurement points. The coordinates of the 12 measurement points outside the depression feature area are used to calculate the skin's depression depth during machining, while the coordinates of the 64 measurement points within the area are used to calculate the actual cutting depth and tool deflection. The adaptive milling planning process for this depression feature is as follows:

[0108] First, 76 measurement points were measured using an in-machine probe and an ultrasonic thickness gauge to obtain the coordinates of the 76 measurement points and the thickness of the skin blank (the results showed that the blank thickness was within 2.59-2.61 mm). The in-machine measurement results were then fitted to obtain the actual parametric surface of the skin blank.

[0109] Secondly, the machining process of the recessed feature is planned using UG NX software to obtain the theoretical tool position point and AC axis rotation angle. Using the theoretical tool position point coordinates and steps 2.2-2.3, the actual tool position point coordinates and normal vector are calculated. The milling process is divided into two parts. After the first milling, the remaining thickness in the recessed feature is 2.2mm. The cutting depth at the 64 measurement points can be calculated using formula (1), which is within [0.39, 0.41]mm. Then, the cutting depth at each tool position point is interpolated. The actual tool position point coordinates of the first milling can be calculated using formula (2). During machining, the spindle speed is set to 12000r / min, the feed rate is 4000mm / min, and the generated tool motion trajectory is as follows. Figure 8 As shown.

[0110] Subsequently, the first milling program was executed on the machine tool. After machining, the on-machine probe was used again to inspect 76 measurement points. Based on the measurement results, the thickness distribution within the depression feature can be obtained, such as... Figure 9 As shown. Furthermore, the actual cutting depth and tool clearance at 64 measurement points can be calculated from the on-machine measurement results. When the remaining thickness within the recessed feature after the second milling is 2.0 mm, the compensation amount for the second milling at the 64 measurement points is calculated using step 4.1, and then the compensation amount corresponding to the tool position point within the recessed feature area is obtained through cubic Hermite interpolation.

[0111] Finally, the compensation amount is translated along the reverse direction of the normal vector from the actual tool position, and the compensated machining tool position coordinates are obtained, and the NC program for compensation machining is generated, part of the G code is shown as Figure 10 The spindle speed and feed speed are still 12000r / min and 4000mm / min. After machining, in-machine measurement is performed again, and the measurement results are obtained, and the thickness distribution in the sink feature is calculated, as shown in Figure 11 .

[0112] From the machining results of the thickness in the sink feature in the embodiment, it can be seen that the unevenness of the blank thickness is considered in the first milling, and the thickness in the sink feature after machining is in the range of [2.20, 2.23]mm, compared with the theoretical thickness 2.20mm, the deviation range is [0, 0.03]mm. In the second milling, the sinking depth and tool relief caused by cutting force and clamping during the machining process are considered, and the actual thickness in the sink feature region after machining is in the range of [2.00, 2.04]mm, the thickness at all measurement points is within the given tolerance band [1.92, 2.08]mm, and the thickness deviation range is 1 / 4 of the tolerance band, thus proving the effectiveness of the method in ensuring the machining consistency of the thickness in the sink feature.

[0113] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application without departing from the principles and purposes of the present application.

Claims

1. An adaptive milling method for aircraft skin sink features based on in-flight measurements, characterized by: Comprising the following steps: Step 1: Measure the shape of the clamped skin blank with the in-machine probe, and detect the thickness of the skin blank at all positions, including the shape and thickness inside and outside the sink feature area; Step 2: Plan the sink feature machining process according to the actual skin blank shape and thickness, obtain the cutter location point coordinates, rotation axis rotation angle and corresponding cutting depth at the cutter location point, generate the machining program and perform the first milling; The specific process is: Step 2.1: planning the machining process according to the three-dimensional model of the theoretical skin sag feature in CAD / CAM software to obtain the planned coordinates of the theoretical tool position points and the theoretical AC axis rotation angle ; Step 2.2: Calculate the theoretical tool position coordinates The closest point coordinates on the theoretical parameterized surface of the skin blank and the UV parameters corresponding to the closest point coordinates and ; Step 2.3: The UV parameters and are inserted into the actual parametric surface of the blank at the sink feature, and the actual tool position coordinates on the actual parametric surface are calculated and the normal vector ; Step 2.4: The thickness of the blank at the measurement point in the undercut feature region obtained according to step 1.3 is calculated as follows: For one milling, if the thickness in the undercut feature is to be ensured to be , the thickness of the blank at the th measurement point in the undercut feature is calculated as follows: ​​ Step 2.5: Using Cutting depth at each measurement point Interpolation was used to calculate the coordinates of each tool position point. The corresponding depth of cut is denoted as The actual tool position coordinates for one milling operation The calculation is as follows: Re-calculate the normal vector at the tool position Post-processing to get the actual AC axis angle ; Using actual tool position coordinates and actual AC axis angle Generate a one-time milling CNC program and import the machine tool for processing; Step 3: After the first milling is completed, measure the shape inside and outside the sink feature area again with the in-machine probe to obtain the actual cutting depth and the skin surface subsidence depth caused by clamping and cutting force; The specific process is: Step 3.1: After one milling operation, use the in-machine probe to measure the shape inside and outside the recessed feature area. The measurement result of the area outside the recessed feature is recorded as follows: The measurement results of the region within the depression feature are denoted as Combined with the previous measurement results outside the subsidence feature area The feature region outside the depression caused by clamping and cutting forces is obtained. The depth of skin subsidence at each measurement point for: Step 3.2: According to the sinking depth of the skin outside the sunken feature area , the sinking depth of the sunken feature area is obtained by interpolation at the measurement points ; and according to the in-machine measurement results before and after the first sunken feature milling and , the actual cutting depth after the first milling at the measurement points is calculated as follows : Step 3.3: According to the formula calculating a relief in the first measurement point in the subsidence feature region in the first milling ; calculating a relief in the first measurement point in the subsidence feature region in the first milling ; Step 4: By superimposing the skin surface subsidence depth, the actual cutting depth and the actual cutting depth, the compensated cutter location point coordinates and normal vector are calculated and the compensation machining program is generated to realize the compensation machining of the sink feature; The specific process is: Step 4.1: If the next milling is to ensure the thickness of the undercut feature is uniform after the next milling, then the theoretical depth of cut needed at the first measurement point in the next undercut feature region is: Depth of Cut = (Thickness of Undercut Feature - Current Depth of Cut) / 2 The compensation amount for the next milling at the first measurement point is: The compensation amount for the next milling at the first measurement point is: The compensation amount for the next milling at the first measurement point is: Further, according to the compensation amount at the measurement point in the sunken feature region , the corresponding compensation amount at the tool position in the sunken feature region is obtained by interpolation, denoted as ; Step 4.2: From the actual tool position coordinates and the normal vector , the compensated tool position coordinates are obtained by translating the actual tool position coordinates by the amount of compensation in the opposite direction of the normal vector , calculated as follows: Step 4.3: Utilizing the actual AC axis angle and tool position coordinates Compensated machining NC program is generated and imported into the machine tool for next cutting; Step 5: Measure the shape inside and outside the sink feature area again with the in-machine probe to obtain the actual cutting depth and the remaining thickness in the sink feature area; Repeat steps 3 and 4 until the remaining thickness in the sink feature area is within the set range, so that the thickness of the sink feature can meet the process requirements.

2. The adaptive milling method for aircraft skin sink feature based on in-flight measurement according to claim 1, characterized in that: The specific process of step 1 is: Step 1.1: Set an in-machine measurement range on the theoretical skin blank model that fully encompasses the depression features. ; within the scope Interior layout Each sampling point, for By fitting the sample points, the theoretical parameterized surface of the blank at the depression feature is reconstructed. ; Step 1.2: On the theoretically parameterized surface upper arrangement Measurement points The coordinates and normal vectors of the corresponding measurement points are obtained, an in-machine measurement program is generated and imported into the CNC machine tool for measurement, and the positions of the measurement points on the blank are marked during the measurement process; after the measurement is completed, the actual measurement results of the skin blank at the sunken feature are obtained. The actual measurement results of the skin blank Including the sunken feature area Results of measurement points Outside the depression feature area Results of measurement points ; Step 1.3: Use a thickness measuring device to measure the thickness of the blank at the measurement point marked in Step 1.2 to obtain the thickness within the sunken feature area. The blank thickness at each measurement point is Outside the sunken feature area The blank thickness at each measurement point is ; Step 1.4: Actual measurement of the skin blank obtained in step 1.2 Fitting is performed to obtain the actual parametric surface of the blank at the sink feature .

3. The adaptive milling method for aircraft skin droop feature based on in-flight measurement of claim 2, wherein: In step 1.1, the boundaries of the sink feature are translated a distance d around the theoretical skin blank model to obtain the in-machine measurement range of the sink feature .

4. The adaptive milling method for aircraft skin droop feature based on in-flight measurement of claim 2, wherein: In step 1.4, the theoretical parametric surface of the skin blank is re-fitted The upper arrangement The nearest point of the sampling points on the actual parametric surface is solved The nearest point of the sampling points on the actual parametric surface is solved The nearest point of the sampling points on the actual parametric surface is solved The nearest point of the sampling points on the actual parametric surface is solved .

5. The adaptive milling method for aircraft skin sink feature based on- board measurement according to claim 1, characterized in that: In step 5, the range of remaining thickness within the recessed feature region wherein is the final thickness to be retained within the recessed feature, is the tolerance band.

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