Interference-free tool shaft direction acquisition method and tool path planning method

Abstract

The application discloses a non-interference tool axis direction obtaining method and a tool path planning method. The non-interference tool axis direction obtaining method comprises the following steps: 1, taking the normal vector direction of each tool position on the tool path as the initial tool axis direction; 2, judging whether the initial tool axis direction of each tool position on the tool path exists global interference; 3, determining the non-interference tool axis direction, taking the initial tool axis direction as the non-interference tool axis direction for the point without global interference; for the point with global interference, finally constructing the tool axis direction feasible region through the two adjacent points with the determined tool axis direction, and calculating the non-interference tool axis direction of the point with the undetermined tool axis direction between the two points. The application further discloses a tool path planning method. The application separates the global interference and the local overcut problem generated in the milling of the complex surface, greatly simplifies the calculation method, and has good universality and strong practicability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of numerical control milling, and in particular to a method for obtaining a non-interference tool axis direction and a tool path planning method. BACKGROUND

[0002] At present, in the manufacturing industry of aerospace, automobile and mold, there are many parts with complex curved surface shape, and the production and manufacturing of such parts are often the difficulty in the whole production process. With the development of computer technology, numerical control technology provides a better choice for the production and manufacturing of such parts. In numerical control machining, the quality of the machining program directly affects the machining effect and efficiency of the parts, and traditional manufacturing processes such as manual artificial numerical control programming and program trial cutting verification cannot meet the rapid development of industrial demand.

[0003] Computer Aided Manufacturing (CAM) technology is a key link of numerical control technology, and CAM software automatically plans tool path and generates machining program according to the geometric information of the part. For the numerical control machining of complex curved surface, CAM software must be used, therefore, CAM software is very important for the development of today's manufacturing industry.

[0004] In CAM technology, tool path planning is an important link. For four-axis or five-axis numerical control machining, due to the introduction of the rotary table, the tool can be variable-pose machined according to the surface shape of the workpiece surface to adapt to the complex curved surface of the workpiece, so it is more suitable for machining complex workpieces than three-axis machine tools.

[0005] However, in the variable-pose machining process of the tool, the tool may interfere with the workpiece surface. Interference includes local overcut and global interference, as shown in Figure 1A . Among them, the local overcut refers to the interference between the tool head and the workpiece surface in a local range; the global interference refers to the interference between the tool as a whole and the workpiece, fixture and worktable, etc. Compared with the global interference, the global interference is a global problem, and its detection and processing process is much more complex and difficult. Because of the inconsistency of the tool fixture and worktable and other equipment, the present application only considers the global interference between the tool and the workpiece surface.

[0006] The local overcut is caused by the existence of the radius of the tool, which is usually avoided in the tool position optimization process. As shown in Figure 1BAs shown, it is a local overcut schematic diagram of the tool and the workpiece surface. In order to determine the position of the tool in the workpiece coordinate system, a fixed position of the tool is usually selected as the driving point for controlling the movement of the tool, which is called the tool position point, and the trajectory of the tool position point of the tool is the tool trajectory. Taking the ball cutter commonly used in curved surface machining as an example, the ball cutter tip is taken as the tool position point. If the tool is directly driven along the tool trajectory for machining, due to the existence of the tool radius, the tool interferes with the workpiece surface at the point P position, which brings local overcut, thereby affecting the surface shape precision and quality of the workpiece surface.

[0007] In the prior art, in order to avoid local overcut and global interference, the tool angle is mainly corrected to solve the problem. For example: CN201310467081.1 discloses a "machining error uniform distribution method based on tool posture gradual change", and CN201710748234.8 discloses a "tool axis vector optimization method based on covariant field functional". However, the existing technical solutions usually adjust the tool posture to solve the two problems of avoiding local overcut and global interference, so the overall algorithm becomes complex and does not have universality.

[0008] Therefore, in order to ensure the quality of the machined workpiece surface and avoid local overcut or global interference of the tool and the workpiece surface, a simple calculation method, a good universal non-interference tool axis direction confirmation method and a tool trajectory planning method must be sought. SUMMARY

[0009] The technical problem to be solved by the present application is to provide a non-interference tool axis direction obtaining method with simple calculation method and good universality.

[0010] Another technical problem to be solved by the present application is to provide a tool trajectory planning method which simultaneously uses the non-interference tool axis direction obtaining method and can avoid tool overcut.

[0011] To solve the above technical problems, the technical solution of the present application is:

[0012] A non-interference tool axis direction obtaining method, comprising the following steps:

[0013] I. Determine the initial tool axis direction, and take the normal vector direction of each tool position point on the tool trajectory as the initial tool axis direction;

[0014] II. Determine whether the initial tool axis direction of each tool position point on the tool trajectory exists global interference;

[0015] III. Determination of non-interference tool axis direction, for the point determined in step II that there is no global interference, the initial tool axis direction is the non-interference tool axis direction; for the point that there is global interference, this part of points are the points whose non-interference tool axis direction is not determined, the tool axis direction of which is calculated by the following method: at one or several adjacent points whose non-interference tool axis direction is not determined, each has a point adjacent to it whose tool axis direction is determined, and the straight line where the tool axis directions of the two points lie has an intersection point g, the line connecting the intersection point g and each point whose non-interference tool axis direction is not determined is the non-interference tool axis direction of this part of points, and the direction points out of the workpiece surface.

[0016] Preferably, the method for determining whether there is global interference in step II is as follows: offset the tool path by a distance equal to the tool radius out of the workpiece surface to obtain an offset line, if the number of intersection points of the tool axis line and the offset line is equal to 1, it indicates that there is no interference between the tool and the workpiece surface, and if the number of intersection points is greater than 1, it is determined that there is interference.

[0017] Preferably, the method for determining whether there is global interference in step II includes the following steps:

[0018] (1) offset the tool path by a distance equal to the tool radius to obtain an offset line;

[0019] (2) calculate the normal vector direction of each point on the offset line, then calculate whether the ray in the normal vector direction of each point intersects with the offset line, and the point that does not intersect is a non-interference point;

[0020] (3) calculate the intersection point of the ray in the opposite direction of the normal vector of each non-interference point and the tool path, which is a tool position point on the tool path, and the normal vector direction is the non-interference tool axis direction of the tool position point on the tool path.

[0021] Preferably, before determining whether there is global interference, the tool path is first interpolated by a distance equal to the tool radius, and the interpolation points are the tool position points, and the specific steps are as follows:

[0022] (1) take the starting point of the tool path as the center and a set step length as the radius to make a circle, calculate the line connecting the two adjacent points on the tool path that intersect with the circle, and divide the tool path into two segments with the intersection point as the dividing point;

[0023] (2) take the starting point of the latter segment of the tool path as the center and continue to make a circle with a set step length as the radius, calculate the line connecting the two adjacent points on the tool path that intersect with the circle, then calculate the intersection coordinates of the circle and the line connecting the two points, and divide the tool path into two segments with the coordinates as the dividing point;

[0024] (3) repeat the above steps until there is no intersection between the line connecting any two points in the tool path and the circle with the center at the start point of the later tool path and the radius of the set step length.

[0025] A tool path planning method for avoiding tool overcut, comprising the following steps:

[0026] I. three-dimensional modeling of the workpiece surface to be machined;

[0027] II. slicing the three-dimensional model to obtain a slice profile as an initial tool path;

[0028] III. calculating the interference-free tool axis direction of each tool position on the initial tool path using the interference-free tool axis direction obtaining method;

[0029] IV. discretizing the three-dimensional model to obtain a plurality of discrete feature points on the surface of the workpiece surface to be machined;

[0030] V. establishing a tool model;

[0031] VI. optimizing the tool path based on the discrete data of the three-dimensional model of the machined surface to obtain an optimized tool path; that is, determining whether there are discrete feature points of the workpiece surface inside the tool at a certain position on the initial tool path, if there are, lifting the tool in the tool axis direction until there are no discrete feature points inside the tool; if there are no discrete feature points of the workpiece surface inside the tool, the original path is maintained; calculating all tool positions on the initial tool path to obtain an optimized tool path.

[0032] Preferably, in step VI, the tool lifting distance is calculated as follows: taking one of the discrete feature points inside the tool as the starting point, calculating the intersection point of the ray in the opposite direction of the tool axis and the tool bottom surface, and calculating the distance from the starting point of the ray to the intersection point, and lifting the tool in the tool axis direction by this distance to avoid overcutting of the discrete feature point; the lifting distance of other feature points inside the tool is calculated in the same way, and the maximum value is taken as the final lifting distance of the tool at this point.

[0033] Preferably, in step VI, the tool lifting distance is calculated as follows: first, rotate all feature points inside the tool around the tool axis to a fixed plane; then calculate the profile curve of the tool on the fixed plane; finally, calculate the intersection point of the ray in the opposite direction of the tool axis and the tool profile curve on the fixed plane, and calculate the distance from the starting point to the intersection point; the maximum value of the distances of all feature points is taken as the final lifting distance of the tool at this point.

[0034] Preferably, in step I, a three-dimensional modeling software is used for modeling, and the model is converted into an STL model format; the method for obtaining the slice profile in step II comprises the following steps:

[0035] (1)Firstly, each triangle facet in STL model and the two intersection points obtained by intersecting with the cutting plane form a small line segment, and the intersection of several triangle facets in STL model and the cutting plane can obtain several small line segments;

[0036] (2)Taking a small line segment obtained by intersecting with the cutting plane as a starting small line segment, and taking the non-common point end point of the starting small line segment as a common point, searching for another small line segment in which the common point is located in the remaining small line segments, which is the adjacent small line segment of the starting small line segment; then taking the non-common point end point of the second small line segment as a new common point, and continuing to search for another small line segment in which the new common point is located in the remaining small line segments; repeating this process can realize the ordering of the unordered small line segments, and further realize the ordering of the intersection points of the cutting plane section;

[0037] (3)Determining the starting small line segment, for the model with a closed two-dimensional contour, the starting small line segment can be any small line segment in it; for the model with a non-closed two-dimensional contour, the starting small line segment must be at one end point.

[0038] Preferably, the step one adopts a three-dimensional modeling software to model and convert into an STL model format; the step four adopts the following method for discrete processing of the STL model: firstly, determining the longest side of the three sides of the triangle facet, drawing a perpendicular line through the longest side, and dividing the original triangle facet into two right-angled triangle facets; then dispersing the right-angled side of the right-angled triangle facet at equal intervals, and offsetting the dispersed points of the right-angled side along the direction of the other right-angled side until the offset points are outside the right-angled triangle facet; the offset points in this process are the discrete feature points of the right-angled triangle facet, and the same method is adopted to disperse the other right-angled triangle facet; thus, the dispersion of one triangle facet is completed, and the above process is repeated for all triangle facets in the STL model, so that the dispersion of the entire STL model is completed.

[0039] Preferably, in the step two, the obtained tool path is subjected to equidistant interpolation, and the interpolation points are the tool position points, and the specific steps are as follows:

[0040] (1)Taking the starting point of the tool path as the center and a set step length as the radius to draw a circle, calculating the connecting line of the two adjacent points in the tool path which have intersection points with the circle, and dividing the tool path into two segments by taking the intersection point as a division point;

[0041] (2)Taking the starting point of the latter tool path as the center and continuing to draw a circle with a set step length as the radius, calculating the connecting line of the two adjacent points in the tool path which have intersection points with the circle, and then calculating the intersection point coordinates of the circle and the connecting line, and dividing the tool path into two segments by taking the coordinates as a division point;

[0042] (3) repeat the above steps until there is no intersection between the line connecting any two points in the tool path and the circle with the center at the start point of the tool path and the radius of the set step size.

[0043] After the above scheme is used, the tool axis direction without interference is obtained by the method of the present application as follows: firstly, the tool axis direction without interference is set as the initial tool axis direction, and then it is determined whether the initial tool axis direction has global interference, if not, the initial tool axis direction is taken as the tool axis direction without interference, if yes, the tool axis direction feasible region is constructed by using the points with the determined tool axis direction at both ends of the interference point, and the tool axis direction without interference is calculated for the points with the undetermined tool axis direction between the two points, and finally the tool axis direction without interference for all the tool position points is obtained. In the present application, the local overcut of the tool head is not considered at first, and only the tool axis direction without interference is calculated, so the calculation method can be greatly simplified, and the generality is good.

[0044] In addition, after the tool axis direction without interference is determined, the tool path is optimized based on this to avoid the local overcut. The idea of the tool path planning for avoiding the local overcut is that the workpiece surface is discretized to obtain the spatial discrete feature points which can express the geometric characteristics of the workpiece surface, at this time, the local feature of the workpiece surface which causes the overcut of the tool at a certain position can be simplified as the discrete feature points of the workpiece surface existing inside the tool at this position, and then the tool is lifted along the tool axis direction until there is no discrete feature point inside the tool, so the overcut of the tool at this position can be avoided, and the tool path is optimized.

[0045] In the present application, the global interference and the local overcut in the milling of the complex surface are handled separately, so the calculation method is greatly simplified, and the generality and practicability are good. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1A is a schematic diagram of the global interference and the local overcut of the tool and the workpiece surface in the numerical control milling of the surface;

[0047] Figure 1B is a schematic diagram of the local overcut of the tool and the workpiece surface in the numerical control milling of the surface;

[0048] Figure 2 is a flow chart of the method for obtaining the tool axis direction without interference according to the present application;

[0049] Figure 3 is a schematic diagram of the judgment of the global interference of the tool and the workpiece surface according to the present application;

[0050] Figure 4 is a schematic diagram of the point without interference of the equidistant line according to the present application;

[0051] Figure 5 is a schematic diagram of the calculation of the tool axis direction without interference in the tool path according to the present application;

[0052] Figure 6 is the feasible region calculation tool axis direction schematic diagram of the present application;

[0053] Figure 7 is the example schematic diagram of the interference-free tool axis direction obtaining method of the present application;

[0054] Figure 8 is the tool and workpiece surface relative position schematic diagram of the tool axis direction obtaining method example of the present application;

[0055] Figure 9 is the flow chart of the tool path planning method of the present application;

[0056] Figure 10 is the slice profile construction schematic diagram in the tool path planning method of the present application;

[0057] Figure 11 is the slice profile construction flow chart in the tool path planning method of the present application;

[0058] Figure 12 is the tool motion local overcut schematic diagram when the tool is too long between two adjacent points in the tool path;

[0059] Figure 13 is the equidistance interpolation schematic diagram of the tool path of the present application;

[0060] Figure 14 is the equidistance interpolation before and after comparison example schematic diagram of the tool path of the present application;

[0061] Figure 15 is the triangular facet discretization schematic diagram in the tool path planning method of the present application;

[0062] Figure 16 is the discretization method example schematic diagram of the present application;

[0063] Figure 17 is the commonly used tool model schematic diagram;

[0064] Figure 18 is the profile curve model schematic diagram of the commonly used tool;

[0065] Figure 19 is the tool local overcut simplified model schematic diagram of the present application;

[0066] Figure 20 is the feature point around tool axis rotation schematic diagram in the tool path planning method of the present application;

[0067] Figure 21 is the tool lifting to the no-overcut position schematic diagram in the tool path planning method of the present application;

[0068] Figure 22 (a) is the relative position relationship diagram of the optimized tool and the workpiece surface;

[0069] Figure 22 (b) is Figure 22 (a) is a partial enlarged view at A in (a);

[0070] Figure 23 (a) is the relative position relationship diagram of the optimized tool and the workpiece surface;

[0071] Figure 23 (b) is Figure 23 (a) is a partial enlarged view at A in (a). DETAILED DESCRIPTION

[0072] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0073] Disclosed is a method for obtaining a non-interference tool axis direction, as shown in Figure 2 which comprises the following steps:

[0074] I. Determine the initial tool axis direction.

[0075] In theory, the tool can have an infinite number of tool axis directions at a certain position on the tool path, and each tool axis direction is judged one by one. Obviously, the calculation amount will be very large. Therefore, in order to reduce the calculation amount and avoid the influence of the intersection of the tool head part tool axis line and the equidistant line (see later) on the final judgment result, the present application takes the normal direction of each tool position on the tool path as the initial tool axis direction.

[0076] It should be noted that the tool path here is known, and the tool path is generally calculated from the workpiece surface profile, so the tool path can represent the workpiece surface features.

[0077] II. Determine whether the initial tool axis direction of each tool position on the tool path exists global interference, that is, whether there is interference between the tool and the workpiece surface. For points without global interference, define them as points with determined non-interference tool axis direction; and for other points with global interference, define them as points with undetermined non-interference tool axis direction.

[0078] Because of the parameter determination of the initial tool axis direction, theoretically, it is simple to calculate whether there is interference with the workpiece in the initial tool axis direction. However, because of the existence of the tool radius, the interference judgment of the interference tool axis direction cannot be idealized as the intersection between the tool axis line and the workpiece surface. Therefore, the feature of the workpiece surface at this point (represented by the tool path at this point) is outwardly offset by an equal distance, and the offset distance is the tool radius, to obtain an equal distance line. At this time, the intersection point of the tool head part is not considered, and if there is no intersection point between the tool axis line and the equal distance line, it indicates that there is no interference between the tool and the workpiece surface at this point. Because in the tool head area, there is an intersection point U (as shown in Figure 3 ) between the tool axis line and the equal distance line located at the equal distance offset position, therefore, when calculating, the intersection point U needs to be removed, and when the number of intersection points between the tool axis line and the equal distance line is equal to 1, it is judged as no interference, and when the number of intersection points is greater than 1, it is judged as existing interference.

[0079] The specific description is as shown in Figure 3 . As shown in Figure 3 (a), P1 is a tool position point in the tool path, n1 is the initial tool axis direction (i.e. the normal vector direction of the point), the number of intersection points between the tool axis line and the equal distance line is equal to 1, indicating that there is no interference between the tool and the workpiece surface at this point. Similarly, as shown in Figure 3 (b), P2 is a point in the tool path, n2 is the tool axis direction, the number of intersection points between the tool axis line and the equal distance line is greater than 1, indicating that there is global interference between the tool and the workpiece surface at this point. In this way, the interference tool axis judgment problem is simplified to the position judgment between the tool axis line and the equal distance line.

[0080] In order to avoid the influence of the intersection point between the tool head part tool axis line and the equal distance line on the final judgment result, the present application can also take each point on the equal distance line as the calculation basis, and take the normal vector direction of each point on the equal distance line as the initial tool axis direction. First, calculate the normal vector direction of each point on the equal distance line; then calculate the position relationship between the rays in the normal vector direction of each point and the equal distance line, and screen out the points on the equal distance line whose normal vector direction rays have no intersection with the equal distance line to obtain the non-interference points; then calculate the intersection point of the rays in the reverse direction of each point in the non-interference points and the tool path, which is the tool position point of the interference tool axis direction on the tool path, and the normal vector direction of the non-interference point is taken as the non-interference tool axis direction of the intersection point in the tool path.

[0081] The specific steps of taking the normal vector direction of each point on the equal distance line as the initial tool axis direction and then screening out the non-interference normal vector as the tool axis direction are as follows:

[0082] (1) offset the known tool path by an equal distance, and the offset distance is the tool radius to obtain an equal distance line;

[0083] (2) calculate the normal vector direction of each point on the equal distance line, and then calculate the position relationship between the rays in the normal vector direction of each point and the equal distance line;

[0084] As shown in Figure 4 (a), Q1 is a point on the offset line, and n1 is its normal vector. If the ray in the direction of the normal vector does not intersect the offset line, it means that the tool will not interfere with the workpiece surface at this position and with the normal vector as the tool axis direction. As shown in Figure 4 (b), Q2 is another point on the offset line, and n2 is its normal vector. If the ray in the direction of the normal vector intersects the offset line, it means that the tool will interfere with the workpiece surface at this position and with the normal vector as the tool axis direction. By this method, points on the offset line whose normal vector direction (i.e. the initial tool axis direction) does not intersect the offset line can be screened out, and the non-interference points are obtained. The normal vector direction of these points is the determined non-interference tool axis direction.

[0085] (3) Calculate the intersection point of the ray in the opposite direction of the normal vector of the non-interference point with the tool path. This intersection point is a tool position point on the tool path, and the normal vector direction is the non-interference tool axis direction of the intersection point on the tool path.

[0086] As shown in Figure 5 , point Q is a non-interference point, and n is its normal vector direction. The intersection point P of the ray in the opposite direction of the normal vector with the tool path is calculated, and n is taken as the tool axis direction of point P, thereby ensuring that the tool does not interfere globally at point P and with the tool axis direction n.

[0087] III. Determination of non-interference tool axis direction.

[0088] The above method can determine the non-interference tool axis direction of part of the points on the tool path, while other points are defined as points whose non-interference tool axis direction is not determined because their initial tool axis direction interferes with the workpiece surface. For points whose non-interference tool axis direction has not been determined, the feasible region of the tool axis direction can be constructed by using the adjacent points whose non-interference tool axis direction has been determined, and the non-interference tool axis direction can be calculated. The specific method is as follows: at one or several adjacent points whose non-interference tool axis direction has not been determined, there is an adjacent point whose non-interference tool axis direction has been determined. The straight line formed by the tool axis directions of the two points has an intersection point g, and the line connecting the intersection point g and each point whose non-interference tool axis direction has not been determined is the non-interference tool axis direction of the point, pointing outward from the workpiece surface.

[0089] As shown in Figure 6As shown, P0, P1, P2, P3, P4 are consecutive tool position points on the tool path, the interference-free tool axis direction n0 of P0 and the interference-free tool axis direction n4 of P4 are known, the interference-free tool axis directions of P1, P2, P3 are not known (that is, P1, P2, P3 points have interference with the workpiece surface when their normal directions are the initial tool axis directions), at this time, the feasible region of the tool axis directions of P1, P2, P3 is constructed through the tool axis directions n0, n4 of P0, P4, and the tool axis direction calculation is performed.

[0090] As shown in Figure 6 (a), first, the intersection g of the straight line on which n0 is located and the straight line on which n4 is located is calculated, if the intersection point is located in the positive direction of n0 and n4, then the direction vector of P1, P2, P3 to g is taken as the tool axis direction thereof; if the intersection point is located in the opposite direction of n0 and n4, as shown in Figure 6 (b), then the direction vector of g to P1, P2, P3 is taken as the tool axis direction thereof.

[0091] Next, a set of examples is used to further illustrate the interference-free tool axis direction obtaining method, which includes the following steps:

[0092] (1) The feature of the workpiece surface at this point (actually the tool path) is offset outward by an equal distance, the offset distance is the tool radius to obtain an offset line, and the normal direction of each data point on the offset line is calculated, as shown in Figure 7 (a); then the points on the offset line for which the normal direction ray does not intersect the offset line are screened out to obtain interference-free points, as shown in Figure 7 (b).

[0093] (2) The intersection of the ray in the opposite direction of the normal vector of each point in the interference-free points and the tool path is calculated, and the normal direction is taken as the interference-free tool axis direction of the intersection point in the tool path, as shown in Figure 7 (c); then for the points for which the interference-free tool axis direction has not been determined, the tool axis directions of the two adjacent points for which the tool axis direction has been determined are taken as the tool axis direction feasible region, and the interference-free tool axis direction calculation is performed, as shown in Figure 7 (d).

[0094] After the interference-free tool axis directions of the points in the tool path are determined, as shown in Figure 8 the relative position relationship between the tool and the workpiece surface at each position of the tool path is expressed, and it can be seen that there is no global interference between the tool and the workpiece surface.

[0095] However, it can also be seen from Figure 8 that the interference-free tool axis direction obtaining method only avoids global interference, and cannot avoid local overcut of the tool head part, therefore, the tool path optimization method is also needed to avoid local overcut.

[0096] Therefore, to avoid global interference and local overcut at the same time, the present application also discloses a tool path planning method, which simultaneously uses the above-mentioned interference-free tool axis direction obtaining method, as shown in Figure 9 includes the following steps:

[0097] I. Three-dimensional modeling of the workpiece to be machined.

[0098] The three-dimensional modeling of the workpiece to be machined can be done in various ways. At present, the expression forms of complex surfaces mainly include parameter models and STL models. STL models use many small space triangles to approximate complex surfaces, have the advantages of flexible data format, easy storage, convenient reading and display, and high universality, and are widely used in 3D printing and numerical control machining tool path planning. The present application takes STL models as an example.

[0099] II. Slice processing of the three-dimensional model, taking the slice contour as the initial tool path.

[0100] The slice processing of the three-dimensional model is to intersect a series of parallel slicing planes with the three-dimensional model, and the intersection line obtained is the slice contour, which is taken as the initial tool path.

[0101] For STL models, STL model slicing is a process of intersecting a series of parallel slicing planes with the triangular facets in the model, and connecting the intersection points to form a slice contour that can express the geometric shape of the original model. Since the STL model is generated by triangulation processing of the model surface or from point cloud data, the triangular facets in it are unordered triangular facets, and after calculating the intersection points of the slicing planes and the triangular facets, the slice contour cannot be directly constructed. Therefore, an algorithm needs to be designed to obtain the slice contour. There are many methods for obtaining the slice contour in the prior art, such as CN202110100621.7, which discloses "a STL model 3D printing slicing method based on intersection edge mapping".

[0102] However, according to the characteristics that there is a common edge between adjacent triangular facets in the STL model, and there is a common point between the two small line segments obtained after intersection with the slicing plane, the present application embodiment sorts a number of unordered small line segments by searching for the common points between adjacent small line segments to obtain a slice contour that can express the geometric features of the original model. This method avoids the complexity of topological relationship reconstruction, and the specific steps are as follows:

[0103] (1) First, the two intersection points of each triangular facet and the slicing plane form a small line segment, and the intersection of a number of triangular facets in the STL model with the slicing plane can obtain a number of small line segments. Except for the case where the intersection is at a vertex of the triangular facet and at all three vertices; if it intersects at two vertices, it means that the slicing plane intersects the common edge of the two triangular facets at that point, and only one edge needs to be kept.

[0104] Since the STL model file stores three vertex coordinates of each triangular facet, the coordinates of the intersection point can be solved according to the intersection formula of a spatial straight line and a plane.

[0105] (2) A small line segment in the small line segments obtained by intersecting with the same plane is taken as a starting small line segment, and an end point of the starting small line segment is taken as a common point. Another small line segment in which the common point is located is searched in the remaining small line segments, and the small line segment is a neighboring small line segment of the starting small line segment. Then, a non-common point end point of the second small line segment is taken as a new common point, and another small line segment in which the new common point is located is continuously searched in the remaining small line segments. The process is repeated, so that the unordered small line segments are sorted, and the intersection points of the slice are sorted.

[0106] (3) The starting small line segment is determined. For a model with a closed two-dimensional contour, the starting small line segment can be any small line segment in the small line segments. For a model with a non-closed two-dimensional contour, the starting small line segment must be at one end point.

[0107] A specific embodiment is shown in Figure 10 In the figure, T1 and T2 are two adjacent triangular facets in the STL model, and the intersection points with the cutting plane form small line segments P1P2 and small line segments P3P4. The point P2 and the point P3 have the same coordinates, that is, they are common points of the two small line segments. Therefore, if the point P2 is taken as a common point, another small line segment in which the common point is located is searched in the remaining small line segments, and the neighboring small line segment P3P4 can be easily found. The connection order of the slice contour at this point is P1, P2 (P3), and P4. Then, the point P4 is taken as a new common point, and another small line segment in which the new common point is located is continuously searched in the remaining small line segments, so that the next neighboring small line segment is found. The process is repeated until the sorting of all small line segments is completed.

[0108] For a model with a closed two-dimensional contour, the starting small line segment can be arbitrary. For a model with a non-closed two-dimensional contour, the starting small line segment must be at one end point. The small line segment set before sorting is denoted as ls1, and the small line segment set after sorting is denoted as ls2. The small line segment sorting process is shown in Figure 11 .

[0109] Furthermore, due to the large number of triangular faces in the STL model, traversing the solution would place a significant burden on the computer's computational resources. Therefore, in the STL slicing algorithm, the triangular faces intersecting with each tangent plane in the model can be selected first to quickly obtain the set of small line segments before sorting, and then the coordinates of the intersection points can be solved based on this. For this reason, existing technologies have also adopted different slicing algorithms. For example, the method proposed in the following literature can be used: "A Fast Slicing Algorithm for Hierarchical Adjacency Sorting of STL Models" proposed by Wang Su, Liu Heng, and Zhu Xinxiong, published in the Journal of Computer-Aided Design & Graphics, 2011, 23(4):600-606.

[0110] Furthermore, in actual machining, the CNC system controls the tool to move point by point along the tool path; these points are called tool position points. Therefore, whether calculating the tool axis direction or planning the tool path, if the distance between adjacent points is too long, such as... Figure 12 As shown, after the calculation of the overcut tool path is completed, although the two adjacent points P1 and P2 in the tool path have been raised to the overcut position, when the tool moves to point P3 between the two, there is still a local overcut with the workpiece surface.

[0111] To achieve controllable distances between adjacent points in the tool path, this invention proposes an equidistant interpolation method. The main idea is as follows:

[0112] (1) Draw a circle with the starting point of the tool path as the center and the set step length as the radius. Calculate the line connecting two adjacent points in the tool path that intersect with the circle, and divide the tool path into two segments with the intersection point as the dividing point.

[0113] Specifically, such as Figure 13 As shown, the starting point of the tool path is point P1. First, draw a circle with point P1 as the center and the set step size r as the radius. Calculate the line connecting two adjacent points in the tool path that intersect with the circle, and use the coordinates of the intersection point as the dividing point to divide the tool path into two segments.

[0114] (2) Using the starting point of the next segment of the tool path as the center, continue to draw a circle with the set step size r as the radius. Calculate the line connecting two adjacent points in the tool path that intersect with the circle. Then calculate the coordinates of the intersection point between the circle and the line connecting the two points. Use the coordinates as the dividing point to divide the tool path into two segments.

[0115] (3) Repeat the above steps until, when a circle with a radius of a set step size r is drawn with the starting point of the next segment of the tool path as the center, there is no intersection between the line connecting two adjacent points in the tool path and the circle.

[0116] The intersection points in this process are the equidistant interpolation points of the tool path, i.e., the tool position points. Figure 14 The above examples of equidistant interpolation methods are shown in the comparison diagrams with tool trajectories without equidistant interpolation. Figure 14(a) is a schematic diagram of the original tool path, Figure 14 (b) is a schematic diagram of the equidistant interpolated tool path.

[0117] The points on the tool path used in the subsequent third and sixth steps can all be equidistantly interpolated points.

[0118] III. Obtain the interference-free tool axis direction of each tool position point on the original tool path. The method for obtaining the interference-free tool axis direction herein directly uses the above method, and will not be described in detail herein.

[0119] IV. Discretize the three-dimensional model to obtain a plurality of discrete feature points of the surface to be machined.

[0120] In order to facilitate the calculation of the local features on the workpiece surface that are cut by the tool, the STL model of the workpiece surface is discretized to express the geometric features of the original workpiece surface by a plurality of discrete feature points, and then the position relationship between the discrete feature points and the tool is determined to determine the local features that are cut. Therefore, the three-dimensional model of the machined workpiece needs to be discretized.

[0121] When determining the local spatial feature information of the STL model at a certain place, the triangular facet at the place needs to be determined first, and then the triangle is split, which is a relatively cumbersome process. Therefore, the present application proposes a new solution. The STL model is discretized into spatial discrete points that can express the geometric features of the original model, and the local spatial feature information of the original model at the place can be determined by directly calculating the spatial discrete points of the STL model at the place.

[0122] The STL model is composed of a plurality of triangular facets. Taking the discretization of a triangular facet in the STL model as an example, the method of the present application is introduced. First, the longest side among the three sides of the triangular facet is determined, a perpendicular line of the longest side is drawn through a point outside the longest side to divide the original triangular facet into two right-angled triangular facets; then one of the right-angled sides of the right-angled triangular facet is discretized at equal intervals, and the discrete points of the right-angled side are offset in the direction of the other right-angled side until the offset points are outside the right-angled triangular facet. The offset points in this process are the discrete feature points of the right-angled triangular facet, and the same method is used to discretize the other right-angled triangular facet. In this way, the discretization of one triangular facet is completed, and the discretization of the entire STL model can be completed by repeating the above process.

[0123] Specific illustration Figure 15 As shown in the figure, the coordinates of the three vertices of a triangular facet in the STL model are P0, P1 and P2. First, the longest side among the three sides of the triangular facet is determined, which is P0P1 in this case. Then, a perpendicular line of the longest side P0P1 is drawn through a point outside the longest side, i.e. point P2, and the foot of the perpendicular is P3. In this way, the original triangular facet can be divided into two right-angled triangular facets P0P3P2 and P3P1P2.

[0124] Taking the right triangle facet P0P3P2, after determining the discrete interval as s, the right angle side P0P3 is discretized with interval s to obtain discrete points O i , the discrete point O i coordinates are:

[0125]

[0126] In the formula, O i represents the i-th discrete point of the right angle side P0P3, and n is the direction from P0 to P3.

[0127] Then all the discrete points of the right angle side P0P3 are translated with interval s along the direction of the other right angle side P3P2 until the translated points are outside the triangle facet. Taking point O3 as an example, a perpendicular line of P0P3 is drawn through point O3, intersecting P0P2 at point P c , and |P c O3| is calculated by the following formula:

[0128]

[0129] O3 is translated along the direction of P3P2 to obtain the equidistant offset point O3, j coordinates are:

[0130]

[0131] In the formula, O 3,j represents the j-th equidistant offset point of O3, and n is the direction from P3 to P2.

[0132] The equidistant offset points of other discrete points of the right angle side P0P3 are calculated by this method, and all the offset points are the discrete feature points of the right triangle facet P0P3P2. The right triangle facet P3P1P2 is discretized by this method, and the discretization of one triangle facet in the STL model is completed.

[0133] All the triangle facets in the STL model are discretized by this method, and the overall discretization of the STL model is completed. The interval s controls the distance between adjacent discrete points, and thus can be used to represent the discretization precision of the model.

[0134] Figure 16 An example of discretizing the STL model into feature points is shown in FIG. 16, in which Figure 16 (a) is the original STL model, and (b) is the discrete feature points of the STL model.

[0135] Five, a tool model is established.

[0136] In NC machining, different machining scenarios will choose different tools to achieve better machining effect. For example, flat milling or roughing usually adopts flat bottom milling cutter, and curved surface machining or finishing usually adopts ball end milling cutter. Figure 17 Three kinds of commonly used milling cutter types and corresponding geometric parameters in the current engraving and milling operation are listed, including flat bottom milling cutter, ball end milling cutter and tapered ball end milling cutter.

[0137] As shown in Figure 17 (a) is a flat bottom milling cutter model: the flat bottom milling cutter is cylindrical as a whole, R is its radius, and L is its length. Because the load and cutting force in machining are very large, it can be used for rapid cutting of excess material on the blank in rough machining.

[0138] As shown in Figure 17 (b) is a ball end milling cutter model: the ball end milling cutter is composed of a hemispherical bottom and a cylindrical shank, R is its radius, L is its length, and the ball end milling cutter is generally used for semi-finishing or finishing of complex curved surfaces.

[0139] As shown in Figure 17 (c) is a tapered ball end milling cutter model: the tapered ball end milling cutter has a certain taper between the cylindrical and the bottom spherical, R is its shank radius, r is its head radius, α is its taper, and L is its length, which can be used for machining very small features.

[0140] In order to determine the local overcut feature between the workpiece surface and the tool, the tool can be modeled according to the type and geometric parameters of the tool. The above three commonly used milling cutters can be regarded as a solid of revolution formed by rotating a profile curve around the tool axis, so the profile curve represents the three-dimensional space model of the tool during work.

[0141] The flat bottom milling cutter can be simplified as a simple cylinder, and the profile curve is relatively simple, taking the center of the tool bottom as the coordinate origin, as shown in Figure 18 (a), the profile curve is composed of line segment OP1 and line segment P1P2, where P1 point coordinates are (R, 0), and P2 point coordinates are (R, L). The expression of the profile curve of the flat bottom milling cutter is:

[0142]

[0143] The ball end milling cutter is composed of a hemispherical bottom and a cylindrical shank, taking the center of the tool bottom as the coordinate origin, as shown in Figure 18 (b), the profile curve is composed of circular arc OP1 with P0 as the center and line segment P1P2, where P0 point coordinates are (0, R), P1 point coordinates are (R, R), and P2 point coordinates are (R, L). The expression of the profile curve of the ball end milling cutter is:

[0144]

[0145] The taper ball end mill is composed of a hemispherical bottom, a taper surface and a cylindrical shank, and the center of the tool bottom is taken as the coordinate origin, as shown in Fig. Figure 18 (c), the profile curve is composed of three parts of the circular arc OP1 with P0 as the center, the line segment P1P2 and the line segment P2P3, wherein the coordinates of the point P0 are (0, r), the coordinates of the point P3 are (R, L), and the coordinates of the point P1(x1, y2) are calculated by the following formula:

[0146]

[0147] After the coordinates of the point P1 are determined, the coordinates of the point P2(x1, y2) are calculated by the following formula:

[0148]

[0149] The expression of the final taper ball end mill profile curve is:

[0150]

[0151] Six, optimizing the tool path according to the discrete data of the machined curved surface three-dimensional model to obtain an optimized tool path.

[0152] After the workpiece surface is discretized, the local feature of the tool and the workpiece surface at a position of the tool can be simplified as the discrete feature point (as shown in Fig. Figure 19 ) of the workpiece surface existing in the tool at the position of the tool, and the tool path is optimized based on this. Specifically, it is judged whether the discrete feature point of the workpiece surface exists in the tool at a position on the initial tool path, if the discrete feature point exists, the tool is lifted in the direction of the tool axis until the discrete feature point does not exist in the tool, so that the overcut of the tool at this position can be avoided, if the discrete feature point of the workpiece surface does not exist in the tool, the original path is kept unchanged, and all the tool position points on the initial tool path are calculated to obtain the optimized tool path.

[0153] For the calculation of the lifting distance of the tool, a single discrete feature point in the tool is taken as an example, as shown in Fig. Figure 21 , first, the intersection of the ray with the tool bottom is calculated, the distance l from the starting point of the ray to the intersection point is calculated, and the tool is lifted in the direction of the tool axis by the distance l, so that the overcut of the tool on the point can be avoided. The lifting distance l of other feature points in the tool is calculated by this method, and the maximum value is taken as the final lifting distance of the tool at this position.

[0154] However, calculating the intersection of a ray along the opposite direction of the cutter axis with the bottom surface of the cutter is complex because the bottom surface of the cutter is a three-dimensional curved surface, and different types of cutters have different bottom surfaces. For example, the bottom surface of a flat end mill is a plane; the bottom surface of a ball end mill is a sphere; and the bottom surface of a tapered ball end mill is a combination of a sphere and a tapered surface. To simplify this process, we first rotate all feature points inside the cutter around the cutter axis to a fixed plane; then we calculate the profile curve of the cutter on this fixed plane; finally, we calculate the intersection point of the ray along the opposite direction of the cutter axis starting from the feature point and the profile curve of the cutter on this fixed plane, and calculate the distance from the starting point to the intersection point. This transforms the original calculation of the intersection of a line and a surface in three-dimensional space into the calculation of the intersection of a line and a curve in a two-dimensional plane.

[0155] Specifically, such as Figure 20 As shown, a tool coordinate system O is established at the tool position. T X T Y T Z T The tool tip is located at the origin of the coordinate system, Z. T Aligned with the tool axis direction. Using the Y-axis of the tool coordinate system... T O T Z T The plane is a fixed plane. First, the feature point Q inside the tool is... i Rotate around the tool axis to Y T O T Z T Point G is obtained in the plane i Then calculate the value at point G. i The ray originating from the tool axis in the opposite direction and the tool in the Y direction T O T Z T Find the intersection points of two-dimensional contour curves on a plane, and calculate the distance l between the starting point and the intersection points. i If the tool is raised along the tool axis by l i This can avoid Q i Overcutting, such as Figure 21 As shown.

[0156] Figure 22 , Figure 23 A comparison of toolpaths before and after optimization shows that the optimized toolpath effectively avoids local overcutting between the tool and the workpiece surface, further verifying the effectiveness of the algorithm. Figure 22 It is a diagram showing the relative positional relationship between the cutting tool and the workpiece surface before optimization. Figure 23 This is a diagram showing the relative positional relationship between the tool and the workpiece surface after optimization.

[0157] The above merely describes the preferred embodiments of the present application, and is not intended to limit the technical scope of the present application in any way. Any changes or modifications made according to the claims and the specification of the present application shall fall within the scope of the present application.

Claims

1. A method for obtaining tool axis direction without global interference, characterized in that... Includes the following steps:

1. Determine the initial tool axis direction, using the normal vector direction of each tool point on the tool trajectory as the initial tool axis direction; 2. Determine whether there is global interference in the initial tool axis direction at each tool position point on the tool path; The method for determining whether global interference exists includes the following steps: (1) Offset the tool path at equal intervals, with the offset distance being the tool radius, to obtain equidistant lines; (2) Calculate the normal vector direction of each point on the equidistant line, and then calculate whether there is an intersection between the ray and the equidistant line on the normal vector direction of each point. Points that do not intersect are points without global interference. (3) Calculate the intersection of the ray along the opposite direction of the normal vector at each point in the point without global interference with the tool trajectory. This intersection point is a tool position point on the tool trajectory, and the direction of the normal vector is taken as the direction of the tool axis without global interference at this intersection point in the tool trajectory. III. Determination of the non-global interference tool axis direction: For points where global interference is not determined through step two, their initial tool axis direction is the non-global interference tool axis direction. For points where global interference exists, these points are points where the non-global interference tool axis direction is not determined. Their tool axis direction is calculated as follows: At both ends of one or several adjacent points where the non-global interference tool axis direction is not determined, there is an adjacent point where the tool axis direction is determined. The straight lines containing the tool axis directions of the two points with determined tool axis directions intersect at a point g. The line connecting this intersection point g and each point where the non-global interference tool axis direction is not determined is the non-global interference tool axis direction of this part of the points, and the direction points outward from the workpiece surface.

2. The method for obtaining the tool axis direction without global interference according to claim 1, characterized in that: Before determining global interference, the tool path is first interpolated at equal intervals, with the interpolation points being the tool position points. The specific steps are as follows: (1) Draw a circle with the starting point of the tool path as the center and the set step size as the radius. Calculate the line connecting two adjacent points in the tool path that intersect with the circle, and divide the tool path into two segments with the intersection point as the dividing point. (2) Using the starting point of the next segment of the tool path as the center, continue to draw a circle with the set step size as the radius, calculate the line connecting two adjacent points in the tool path that intersect with the circle, then calculate the coordinates of the intersection point of the circle and the line connecting the two points, and use the coordinates as the dividing point to divide the tool path into two segments. (3) Repeat the above steps until when a circle with a radius of a set step length is drawn with the starting point of the next segment of the tool path as the center, there is no intersection between the line connecting two adjacent points in the tool path and the circle.

3. A tool path planning method to avoid overcutting, characterized in that... Includes the following steps:

1. Create a 3D model of the curved surface of the workpiece to be machined; 2. Slice the 3D model and use the slice outline as the initial tool path; 3. The global interference-free tool axis direction of each tool position on the initial tool trajectory is calculated using the global interference-free tool axis direction acquisition method as described in claim 1 or 2. Fourth, the three-dimensional model is discretized to obtain several discrete feature points on the surface to be processed. V. Establish the tool model; 6. Optimize the tool path based on the discrete data of the 3D model of the machining surface to obtain the optimized tool path; that is, determine whether there are discrete feature points on the workpiece surface inside the tool when the tool is at a certain position on the initial tool path. If there are, lift the tool along the tool axis until there are no discrete feature points inside the tool. If there are no discrete feature points on the workpiece surface inside the tool, keep the original path unchanged. Calculate all tool points on the initial tool path to obtain the optimized tool path.

4. The tool path planning method for avoiding tool overcutting according to claim 3, characterized in that... In step six, the method for calculating the tool lifting distance is as follows: taking one discrete feature point inside the tool as the starting point, the intersection of the ray along the opposite direction of the tool axis and the bottom surface of the tool is calculated, and the distance from the starting point of the ray to the intersection point is calculated. The tool is lifted by this distance along the tool axis to avoid overcutting of the discrete feature point. The lifting distance for other feature points inside the tool is calculated in the same way, and the maximum value is taken as the final lifting distance of the tool at this point.

5. The tool path planning method for avoiding overcutting according to claim 3, characterized in that... In step six, the method for calculating the tool lifting distance is as follows: First, rotate all feature points inside the tool around the tool axis to a fixed plane; then calculate the tool profile curve on this fixed plane; finally, calculate the intersection point of the ray starting from the feature point and moving in the opposite direction of the tool axis with the tool profile curve on the fixed plane, and calculate the distance from the starting point to the intersection point; the maximum value among all the distances of the feature points is taken as the final lifting distance of the tool at this point.

6. A tool path planning method for avoiding tool overcutting according to any one of claims 3-5, characterized in that: Step one involves using 3D modeling software to create a model and converting it into an STL model format. The method for obtaining the slice outline in step two includes the following steps: (1) First, form a small line segment by finding the two intersection points of each triangular facet in the STL model with the tangent plane. Then, a number of small line segments can be obtained by finding the intersection of several triangular facets with the tangent plane in the STL model. (2) Take one of the small line segments obtained by intersecting with the same tangent plane as the starting small line segment, take one endpoint of the starting small line segment as the common point, and search for another small line segment in the remaining small line segments where the common point is located. This small line segment is the adjacent small line segment of the starting small line segment. Then take the non-common point endpoint of the second small line segment as the new common point, and continue to search for another small line segment in the remaining small line segments where the new common point is located. Repeat this process to sort the unordered small line segments, and then sort the intersection points of the tangent plane slices. (3) Determine the starting small line segment. For models with a closed two-dimensional contour, the starting small line segment can be any one of the small line segments. For models with a non-closed two-dimensional contour, the starting small line segment must be at one of its endpoints.

7. A tool path planning method for avoiding tool overcutting according to any one of claims 3-5, characterized in that: Step one involves using 3D modeling software to create a model and converting it into an STL model format. The method for discretizing the STL model in step four is as follows: First, determine the longest side among the three sides of the triangular facet. Draw a perpendicular line from a point outside the longest side to the longest side, dividing the original triangular facet into two right-angled triangular facets. Then, discretize one right-angled side of the right-angled triangular facet at equal intervals. Offset the discretized point of the right-angled side along the direction of the other right-angled side at equal intervals until the offset point is outside the right-angled triangular facet. The offset point in this process is the discrete feature point of the right-angled triangular facet. Discretize the other right-angled triangular facet using the same method. This completes the discretization of one triangular facet. Repeat the above process for all triangular facets in the STL model to complete the discretization of the entire STL model.

8. A tool path planning method for avoiding tool overcutting according to any one of claims 3-5, characterized in that: In step two, the obtained tool trajectory is interpolated at equal intervals, with the interpolation points being the tool position points. The specific steps are as follows: (1) Draw a circle with the starting point of the tool path as the center and the set step size as the radius. Calculate the line connecting two adjacent points in the tool path that intersect with the circle, and divide the tool path into two segments with the intersection point as the dividing point. (2) Using the starting point of the next segment of the tool path as the center, continue to draw a circle with the set step size as the radius, calculate the line connecting two adjacent points in the tool path that intersect with the circle, then calculate the coordinates of the intersection point of the circle and the line connecting the two points, and use the coordinates as the dividing point to divide the tool path into two segments. (3) Repeat the above steps until when a circle with a radius of a set step length is drawn with the starting point of the next segment of the tool path as the center, there is no intersection between the line connecting two adjacent points in the tool path and the circle.

Images