CN102866672A - Online detecting method of numerical control machining middle state of plane structural member - Google Patents

Abstract

The invention discloses an online detection method for the intermediate state of numerically controlled machining of aircraft structural parts. The method first constructs a theoretical surface of the intermediate processing state, and selects the surface to be detected according to the final state of the part CAD model for the surface of the three-axis for finishing. , offset a machining allowance value along the outer normal direction of the selected surface to form the theoretical surface of the intermediate processing state; for the surface of the five-axis finishing, use the surface formed by the tool sweep in the previous step as the intermediate processing state A series of points are discretized along the finishing tool path and projected to the theoretical surface of the intermediate state to form the intermediate state detection points. Finally, the detection path is planned and the NC program is formed for online detection. Calculate the theoretical thickness and theoretical distance according to the theoretical surface of the intermediate processing state, and form the basis for the evaluation of the detection results together with the theoretical position of the detection point. This method effectively solves the quality problems of the parts caused by the deformation and processing errors in the middle processing state of large parts, and ensures the processing quality.

Description

On-line detection method of intermediate state in NC machining of aircraft structural parts

technical field

The invention relates to a numerical control machining method for aircraft structural parts, especially a detection method for the intermediate processing state, specifically for online detection of the intermediate state before numerical control machining of aircraft structural parts, belonging to CAD (Computer Aided Design)/CAM (Computer Aided Manufacturing)/CAI (Computer Aided Inspection) technical field.

Background technique

For large parts such as aircraft structural parts, due to their complex structure, large size and thin-walled parts, they are easily deformed during processing. At the same time, machining errors may be caused by tool wear and chatter of the processing system during processing. Deformation or machining errors in the machining process will affect the subsequent machining process or machining operations. In serious cases, the workpiece will be scrapped. Since aircraft structural parts are high-value and high-value-added parts, the cost is very high. Therefore, in order to ensure Processing quality, reducing manufacturing costs, and testing the intermediate processing status of aircraft structural parts are very important. At the same time, due to the difficulty of clamping and other reasons, the best way to detect the intermediate state is online detection. However, the current detection methods are all detected after the parts are processed to the final state, so that the processing accidents caused by problems in the intermediate processing state cannot be avoided.

Retrieving existing technologies and literature found that Zhang Changxin's 2009 master's degree thesis "Research on Key Technologies of Online Inspection System of CNC Machining Center" studied the extraction method of inspection information and proposed an algorithm for matching the CAD model with the inspection object model. The above The methods are all aimed at the final CAD model;

Ding Yongfa, Nanjing University of Aeronautics and Astronautics' 2009 master's degree thesis "Automatic Generation and Analysis of On-line Inspection Data of Aircraft Structural Parts Based on Features" proposed the generation method of inspection points and the method of inspection path planning; the above methods are also aimed at the final CAD model.

The paper "STEP-NC enabled on-line inspection in support of closed-loopmachining" published by Fiona Zhao in the international academic journal "Robotics and Computer-Integrated Manufacturing" 2008(24), p200-216 gives a An integrated data model creation method, but the paper does not involve the construction method of the intermediate state model.

To sum up, regarding the online inspection of NC machining of large parts, the current technology does not have a detection method for the intermediate state, and there is no method for constructing the detection model of the intermediate state. At the same time, the above detection methods only only detect the coordinates of the detection points, without combining the thickness detection, and cannot provide sufficient data basis for the evaluation of the detection results.

Contents of the invention

The purpose of the present invention is to invent an online detection method for the intermediate state of CNC machining of aircraft structural parts in view of the lack of detection methods and the construction of intermediate state models for the detection of intermediate states of large-scale aircraft structural parts.

Technical scheme of the present invention is:

A method for detecting an intermediate state of numerical control machining of aircraft structural parts, characterized in that the method comprises the following steps:

Step 1, selecting the surface required for intermediate processing state detection based on the final state of the CAD model of the part;

Step 2. Determine whether the selected surface is a three-axis processing surface or a five-axis processing surface. If it is a three-axis processing surface, offset the surface by a machining allowance value along the outer normal direction of the surface to form a theory of an intermediate processing state surface; if it is a five-axis machining surface, use the sweeping surface formed by the tool’s feed movement along the tool track when processing the intermediate state of the surface as the theoretical surface of the intermediate processing state;

Step 3: Discretize a series of points along the finishing tool path according to the detection requirements, and project the discrete points to the theoretical surface of the intermediate processing state to form intermediate state detection points;

Step 4. According to the theoretical surface of the intermediate processing state, calculate the theoretical thickness of the intermediate state at the position of each intermediate state detection point and the theoretical distance between the intermediate states of two adjacent side milling surfaces;

Step 5, plan the detection path based on the intermediate state detection points and generate the NC program, and transmit it to the CNC machine tool for online detection;

Step 6, using an ultrasonic thickness gauge to measure the actual thickness of the intermediate state of each detection point after processing, using a contact infrared probe to measure the actual coordinates of the detection point after processing, and calculating the actual distance between the intermediate states of adjacent two sides according to the detection coordinates;

Step 7: Based on the theoretical position, theoretical thickness and theoretical distance of the detection point, evaluate the detection results and form a detection report, and then evaluate whether the intermediate processing state of the processing meets the requirements and adjust the strategy.

The generation method of the theoretical surface of the intermediate state of the five-axis machining surface is as follows:

Step 1, extracting and processing the surface to form an intermediate state processing operation;

Step 2, extracting the cutting tool path of the processing operation, and offsetting the cutting tool path by one tool radius along the normal direction of the five-axis machining surface to form a guide line;

Step 3: Extract the direction of the tool axis of the section of the tool path, and calculate the length of the five-axis machining surface along the direction of the tool axis, taking the end point of one end of the guide line as the end point, according to the direction of the tool axis and the length of the five-axis machining surface along the direction of the tool axis Make a straight line segment with the length as a straight generatrix;

Step 4. According to the guide line and straight generatrix formed in Step 2 and Step 3, make a ruled surface to form a theoretical surface in the middle state of the five-axis machining surface.

The generation principle of the detection point is as follows: the detection point is projected from the discrete point on the finishing tool track to the theoretical surface of the intermediate state, and the discrete point can be a tool position point, or an interpolation between two tool position points can be performed The density of interpolation depends on the accuracy requirements of the detection surface. The distance between discrete points ranges from 3mm to 20mm. The higher the accuracy requirement, the higher the density of interpolation points. In addition, according to the processing experience, the more easily deformed the position of the interpolation point is. The higher the density.

The calculation method of the theoretical thickness M_Th of the intermediate state is:

Step 1. First use F_Th to represent the final theoretical thickness of the detection surface, and Re to represent the machining allowance, then M_Th=F_Th+Re;

Step 2. The machining allowance depends on the processing situation. If the reverse side of the detection surface has not been processed in place, the machining allowance on the reverse side needs to be added; if the reverse side has been processed in place, then directly calculate according to the above formula;

表示检测面的上下公差，则中间状态的理论厚度为：

Step 3. In order to evaluate the test results, the theoretical thickness of the intermediate state also needs to consider the tolerance zone of the test surface, using

Indicates the upper and lower tolerances of the detection surface, then the theoretical thickness of the intermediate state is:

The calculation method of the theoretical distance is:

Step 1. Select a layer of detection points in the intermediate state of two adjacent surfaces, that is, detection points with the same Z coordinate value, to form point columns [Ltst1_Pt] and [Ltst2_Pt] respectively;

Step 2. The midpoints of the two point columns are respectively projected to the same horizontal plane in sequence, and the point columns formed respectively are [Ltst1_PPt] and [Ltst2_PPt];

Step 3. Traversing and looping the point columns as [Ltst1_PPt] and [Ltst2_PPt], find out the two closest points P1 and P2 in the two point columns, record the distance as Mtn_Dts, and Mtn_Dts is the theoretical distance;

Step 4. Find the original projection points of points P1 and P2 in the point columns [Ltst1_Pt] and [Ltst2_Pt] respectively. Since the projections are performed sequentially, you can find them according to the serial number, and use the found points as the detection points for calculating the actual distance .

The beneficial effects of the present invention are:

1. Provides a detection method for the intermediate state of the CNC machining process;

2. Provides the creation method of the intermediate state model in the NC machining process;

3. It can detect the deformation and processing errors in the intermediate processing process, prevent the influence on the subsequent processing procedures and processing operations, ensure the processing quality and reduce the processing cost.

Description of drawings

Figure 1 is the intermediate state model of the three-axis machining surface;

Figure 2 is the intermediate state model of the five-axis machining surface;

Fig. 3 is a schematic diagram of a method for generating a theoretical surface in an intermediate state of a five-axis machining surface;

Fig. 4 is a schematic diagram for calculating the theoretical distance between two adjacent surfaces.

The labels in the drawings are: 1 indicates the final theoretical state of the detection surface, 2 indicates the machining tool, 3 indicates the tool axis vector, 4 indicates the tool path, 5 indicates the guide line, 6 indicates the tool position point, 7 indicates the straight generatrix, 8 Indicates one of the two nearest detection points, 9 represents the second of the two closest detection points, 10 represents the machining allowance, and 11 represents the theoretical distance between the intermediate processing states of two adjacent surfaces.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

As shown in Figure 1 and Figure 2.

A method for detecting an intermediate state of numerical control machining of aircraft structural parts, the method comprising the following steps:

Step 1. Select the surface required for intermediate processing state detection based on the final state of the CAD model of the part;

Step 2. Determine whether the selected surface is a three-axis processing surface or a five-axis processing surface. If it is a three-axis processing surface, offset the surface by a machining allowance value along the outer normal direction of the surface to form a theory of an intermediate processing state Surface; if it is a five-axis machining surface, use the surface formed by the sweeping surface formed by the tool to move along the tool track when machining the intermediate state of the surface as the theoretical surface of the intermediate processing state;

Step 3. According to the detection requirements, a series of points are discretized along the finishing tool path, and the discrete points are projected to the theoretical surface of the intermediate state to form intermediate state detection points;

Step 4, according to the theoretical surface of the intermediate state, calculate the theoretical thickness of the intermediate state at the position of each detection point and the theoretical distance from the intermediate state of two adjacent side milling surfaces;

Step 5. Plan the detection path based on the detection points and generate the NC program, and transmit it to the CNC machine tool for online detection;

Step 6. Use an ultrasonic thickness gauge to measure the actual thickness of the intermediate state of each detection point, use a contact infrared probe to measure the actual coordinates of the detection point, and calculate the actual distance between the intermediate states of adjacent two sides according to the detection coordinates;

Step 7. According to the theoretical position, theoretical thickness and theoretical distance of the detection point, evaluate the detection result and form a detection report, and then evaluate whether the intermediate state of processing meets the requirements and adjust the strategy.

As shown in Figure 3, the generation method of the theoretical surface of the intermediate state of the five-axis machining surface is:

Step 1, extracting and processing the surface 1 to form a processing operation in an intermediate state;

Step 2, extracting the cutting tool path 4 of the processing operation, and offsetting the cutting tool path by one tool radius along the normal direction of the five-axis machining plane to form a guide line 5;

Step 3. Extract the direction of the tool axis of the section of the tool path, and calculate the length of the five-axis machining surface along the direction of the tool axis 3, taking the end point of one end of the guide line as the endpoint, according to the direction of the tool axis and the direction of the five-axis machining surface along the direction of the tool axis Make a straight line segment with the length of , as the straight bus 7;

Step 4. According to the guide line and straight generatrix formed in Step 2 and Step 3, make a ruled surface to form a theoretical surface in the middle state of the five-axis machining surface.

The generation principle of the detection point is as follows: the detection point is projected from the discrete point on the finishing tool track to the theoretical surface of the intermediate state, and the discrete point can be a tool position point, or an interpolation between two tool position points can be performed The density of interpolation depends on the accuracy requirements of the detection surface. The distance between discrete points ranges from 3mm to 20mm. The higher the accuracy requirement, the higher the density of interpolation points. In addition, according to the processing experience, the more easily deformed the position of the interpolation point is. The higher the density.

The calculation method of the theoretical thickness M_Th of the intermediate state is:

Step 1. First use F_Th to represent the final theoretical thickness of the detection surface, and Re to represent the machining allowance, then M_Th=F_Th+Re;

表示检测面的上下公差，则中间状态的理论厚度为：

Step 2. The processing allowance depends on the processing situation. If the reverse side of the detection surface has not been processed in place, it is necessary to add the processing allowance on the reverse side; if the reverse side has been processed in place, directly calculate according to the above formula; Step 3. In order to evaluate the test results, the theoretical thickness of the intermediate state also needs to consider the tolerance zone of the test surface, using

Indicates the upper and lower tolerances of the detection surface, then the theoretical thickness of the intermediate state is:

As shown in Figure 4, the calculation method of the theoretical distance is:

Step 1. Select a layer of detection points in the intermediate state of two adjacent surfaces, that is, detection points with the same Z coordinate value, to form point columns [Ltst1_Pt] and [Ltst2_Pt] respectively;

Step 2. The midpoints of the two point columns are respectively projected to the same horizontal plane in sequence, and the point columns formed respectively are [Ltst1_PPt] and [Ltst2_PPt];

Step 3. Traversing and looping the point columns as [Ltst1_PPt] and [Ltst2_PPt], find out the two closest points P1 and P2 in the two point columns, record the distance as Mtn_Dts, and Mtn_Dts is the theoretical distance 11;

Step 4. Find the original projection points of points P1 and P2 in the point columns [Ltst1_Pt] and [Ltst2_Pt] respectively. Since the projections are performed sequentially, you can find them according to the serial number, and use the found points as the detection points for calculating the actual distance 8,9.

Claims (5)

1. A method for detection of an intermediate state in CNC machining of aircraft structural parts, characterized in that the method may further comprise the steps: Step 1, selecting the surface required for intermediate processing state detection based on the final state of the CAD model of the part; Step 2. Determine whether the selected surface is a three-axis processing surface or a five-axis processing surface. If it is a three-axis processing surface, offset the surface by a machining allowance value along the outer normal direction of the surface to form a theory of an intermediate processing state surface; if it is a five-axis machining surface, use the sweeping surface formed by the tool’s feed movement along the tool track when processing the intermediate state of the surface as the theoretical surface of the intermediate processing state; Step 3: Discretize a series of points along the finishing tool path according to the detection requirements, and project the discrete points to the theoretical surface of the intermediate processing state to form intermediate state detection points; Step 4. According to the theoretical surface of the intermediate processing state, calculate the theoretical thickness of the intermediate state at the position of each intermediate state detection point and the theoretical distance between the intermediate states of two adjacent side milling surfaces; Step 5, plan the detection path based on the intermediate state detection points and generate the NC program, and transmit it to the CNC machine tool for online detection; Step 6, using an ultrasonic thickness gauge to measure the actual thickness of the intermediate state of each detection point after processing, using a contact infrared probe to measure the actual coordinates of the detection point after processing, and calculating the actual distance between the intermediate states of adjacent two sides according to the detection coordinates; Step 7: Based on the theoretical position, theoretical thickness and theoretical distance of the detection point, evaluate the detection results and form a detection report, and then evaluate whether the intermediate processing state of the processing meets the requirements and use this as the basis for tool path adjustment. 2. The method for detecting the intermediate state of CNC machining of aircraft structural parts according to claim 1, wherein the method for generating the theoretical surface of the intermediate state of the five-axis machining surface is: Step 1, extracting and processing the surface to form an intermediate state processing operation; Step 2, extracting the cutting tool path of the processing operation, and offsetting the cutting tool path by one tool radius along the normal direction of the five-axis machining plane to form a guide line; Step 3: Extract the tool axis direction of the section of the tool path, and calculate the length of the five-axis machining surface along the tool axis direction, take the end point of one end of the guide line as the end point, according to the tool axis direction and the five-axis machining surface along the tool axis direction Make a straight line segment with the length as a straight generatrix; Step 4: According to the guide line and straight generatrix formed in Step 2 and Step 3, make a ruled surface to form a theoretical surface in the middle state of the five-axis machining surface. 3. The intermediate state detection method of aircraft structural part CNC machining according to claim 1, characterized in that the generation principle of the intermediate state detection point is: the detection point is from the discrete point on the finishing tool track to the theoretical surface of the intermediate state It is formed by projection, and the discrete point can be the tool position point, or it can be interpolated between two tool position points. The interpolation density depends on the accuracy requirements of the detection surface. The distance between the discrete points ranges from 5mm to 20mm. The higher the requirement, the higher the density of interpolation points. In addition, according to the processing experience, the more easily deformed the higher the density of interpolation points. 4. The method for detecting the intermediate state of numerical control machining of aircraft structural parts according to claim 1, wherein the calculation method of the theoretical thickness M_Th of the intermediate state is: Step 1, first use F_Th to represent the final theoretical thickness of the detection surface, and Re to represent the machining allowance, then M_Th=F_Th+Re; Step 2, the processing allowance depends on the processing situation. If the reverse side of the detection surface has not been processed in place, it is necessary to add the processing allowance on the reverse side; if the reverse side has been processed in place, directly calculate according to the above formula; Step 3, in order to evaluate the test results, the theoretical thickness of the intermediate state also needs to consider the tolerance zone of the test surface, using Indicates the upper and lower tolerances of the detection surface, then the theoretical thickness of the intermediate state is:

5. A kind of method for detecting the intermediate state of CNC machining of aircraft structural parts according to claim 1, characterized in that the calculation method of the theoretical distance between the two adjacent side milling surfaces is: Step 1, respectively select a layer of detection points in the intermediate state of two adjacent surfaces, that is, detection points with the same Z coordinate value, to form point columns [Ltst1_Pt] and [Ltst2_Pt] respectively; Step 2, the midpoints of the two point columns are respectively projected to the same horizontal plane, and the point columns formed respectively are [Ltst1_PPt] and [Ltst2_PPt]; Step 3, traverse and loop the point columns as [Ltst1_PPt] and [Ltst2_PPt], find out the two closest points P1 and P2 in the two point columns, record the distance as Mtn_Dts, and Mtn_Dts is the theoretical distance; Step 4. Find the original projection points of points P1 and P2 in the point columns [Ltst1_Pt] and [Ltst2_Pt] respectively. Since the projections are performed sequentially, you can find them according to the serial number, and use the found points as the detection points for calculating the actual distance. .

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