A nonlinear error control method by changing the machining speed of a five-axis machine tool

Abstract

A nonlinear error control method for a five-axis machine tool, relating to the field of precision machining technology, is disclosed. The method involves setting a nonlinear error threshold; deriving forward and inverse kinematics based on the machine tool structure; acquiring tool position information at the beginning and end of the toolpath, calculating the corresponding movement of each axis, setting the starting point, deceleration times, and desired feed rate; calculating the fine interpolation points between the two axes; calculating the nonlinear error; if the error is not specified, then setting the feed rate and recalculating; if the error is not specified, then updating the speed between the two axes, setting the feed rate, and resetting the feed rate. This process is repeated until the end of the operation. For five-axis machine tools with RTCP functionality, by setting an error threshold, adjusting the machining speed between tool positions, and controlling the number and distance between interpolation points, the nonlinear error can be effectively controlled while ensuring machining efficiency.

Description

Technical Field

[0001] This invention relates to the field of precision machining technology, specifically a nonlinear error control method that changes the machining speed of a five-axis machine tool. Background Technology

[0002] Multi-axis linkage machine tools can adjust the tool tilt angle in real time during machining and complete multiple processes on a single machine, adapting to various machining scenarios. They have significant advantages, especially when machining complex parts with free-form surfaces such as impeller blades. With technological advancements, the requirements for machining accuracy are becoming increasingly stringent. The nonlinear error of five-axis linkage machine tools is an indispensable key factor when considering machining accuracy, and it has a significant impact on machining quality.

[0003] Five-axis linkage machine tools add two rotary axes (A and C axes) to the three linear axes. However, when the machine tool performs interpolation, it only considers the three linear axes X, Y, and Z. This causes the actual trajectory of the tool tip to deviate from the ideal trajectory due to the movement of the rotary axes during the interpolation process, resulting in nonlinear errors.

[0004] In this situation, machine tools need to compensate for nonlinear errors to avoid overcutting and undercutting that would reduce machining quality. Currently, economical machine tools mainly use post-processing methods to reduce nonlinear errors, while high-end machine tools use the RTCP (Interpolation-based Processing) function. Compared to post-processing, RTCP reduces nonlinear errors by interpolating the cycle time, making it more effective. However, current RTCP and post-processing functions only reduce nonlinear errors during machining without considering their magnitude. Therefore, controlling the magnitude of nonlinear errors during machining is crucial.

[0005] Currently, there are methods for calculating nonlinear errors and identifying the factors that influence them, allowing for an approximation of the magnitude of nonlinear errors during machining. One crucial issue in machining is how to control nonlinear errors through machining parameters.

[0006] Chinese invention patent CN108983704B, authorized on February 9, 2021, entitled "Online Nonlinear Error Compensation Method Based on Five-Axis Dual Rotary Table," discloses a nonlinear error method characterized by calculating the maximum nonlinear error of a path segment using two known programming points. If the error exceeds a set nonlinear error threshold, an interpolation point located between these two points is added, and the nonlinear error between the first point and this interpolation point is calculated. This process is repeated until an interpolation point less than the threshold is obtained, before proceeding to the next segment. This invention is stable and does not rely on a maximum error vector, but it does not address nonlinear error control on machine tools with RTCP functionality.

[0007] Chinese invention patent CN119369174A, published on January 28, 2025, entitled "A Nonlinear Error Compensation Device and Method for a Five-Axis Machine Tool," discloses a nonlinear error compensation method characterized by an X-axis compensation component mounted on a bottom fixed plate, a Y-axis compensation component mounted on an X-axis slider, a Z-axis compensation component mounted on a Y-axis slider, and a worktable mounted on a Z-axis slider. It calculates nonlinear errors for five-axis machine tools with AB, BC, AC double-swivel head configurations, AC, BC double-rotary table configurations, and A-swivel head C-rotary table and B-swivel head C-rotary table configurations. This invention also compensates for nonlinear errors in machine tools without RTCP functionality, but it does not address the control of nonlinear errors in machine tools with RTCP functionality.

[0008] In summary, for five-axis machine tools with RTCP functionality, there is an urgent need for a control method that can reduce nonlinear errors by adjusting machining parameters, in order to achieve maximum machining efficiency while meeting nonlinear error requirements. Summary of the Invention

[0009] To address the shortcomings of the prior art, this invention provides a nonlinear error control method by changing the machining speed of a five-axis machine tool. Specifically designed for five-axis machine tools with RTCP functionality, this method adjusts the machining speed between tool positions by setting an error threshold, thereby controlling the number of interpolation points and the distance between them, thus ensuring machining efficiency while effectively controlling nonlinear errors.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: a nonlinear error control method by changing the machining speed of a five-axis machine tool, comprising the following steps:

[0011] S1. For five-axis machine tools with RTCP functionality, set the maximum allowable nonlinear error threshold during machining. ;

[0012] S2. Perform structural analysis on the five-axis machine tool, establish the machine tool kinematic chain, derive the forward and inverse kinematic models, and define the tool position information. Coordinates of the knife tip and tool axis vector combination ;

[0013] S3. For the part to be processed, the first step is to obtain the result through CAM software. Tool position information at the beginning and end of the tool path and , The data is then imported into a five-axis machine tool to calculate the movement of each axis corresponding to the initial and final positions based on the inverse kinematics model. and Set the starting point Used to record The coordinates of the knife tip point Set the number of decelerations. , set the first Duan Daolu Expected feed rate ;

[0014] S4. Based on the interpolation period and the Feed rate of the toolpath , Indicates the desired feed rate slow down The size after that, if This is the desired feed rate. Then calculate the starting point. and the end point The first fine interpolation point coordinates ;

[0015] S5, Calculation Corresponding movement of each axis Turntable corner , combined coordinates Obtained through inverse kinematics model The corresponding three linear axis displacements This allows us to obtain the actual motion of each axis of a five-axis machine tool over time. Substituting this into the forward kinematics model, we obtain the actual trajectory of the blade tip over time. ,Will The tool tip point is set at the location of the maximum nonlinear error, and the result is obtained. The actual tip of the blade coordinates Calculate its relationship with the line. The distance between them is the nonlinear error. ;

[0016] S6, will and If a comparison is made, If so, proceed to step S7; Then proceed to step S8;

[0017] S7, Order ,like Adjust the feed rate to And return to step S4; if Adjust the feed rate to and return to step S4;

[0018] S8, will and The speed between is set to ,make Reset ,like Then return to step S4 to process the next toolpath segment; if Then the execution will end.

[0019] Furthermore, in step S2, the forward and inverse kinematics models are derived as follows:

[0020] The derivation results of the forward kinematics model are as follows:

[0021]

[0022]

[0023] The derivation results of the inverse kinematics model are as follows:

[0024]

[0025]

[0026] In the formula, X, Y, Z, A, and C represent the movement of each axis of a five-axis machine tool. This represents the offset of the origin of the workpiece coordinate system relative to the machine tool coordinate system. This represents the offset of the program origin relative to the machine tool coordinate system. This is the distance from the common perpendicular to the AC axis.

[0027] Furthermore, in step S4, fine interpolation points are... The formula for calculating the coordinates is as follows:

[0028]

[0029] In the formula, End point The coordinates.

[0030] Furthermore, in step S5, the turntable angle... The calculation formula is: .

[0031] Furthermore, in step S5, the nonlinear error The expression is as follows:

[0032]

[0033] In the formula, , , .

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows: On a five-axis precision machine tool with RTCP function, the present invention obtains a nonlinear error calculation and compensation method by performing kinematic modeling analysis on the machine tool structure. Based on this, by setting an error threshold, the machining speed between tool positions can be adjusted, the number of interpolation points and the distance between interpolation points can be controlled, thereby controlling the nonlinear error. By modifying the machining speed, the magnitude of the nonlinear error can be controlled, thereby improving machining efficiency while ensuring effective control of the nonlinear error. Attached Figure Description

[0035] Figure 1 This is a flowchart of the method of the present invention;

[0036] Figure 2 This is a schematic diagram of a five-axis machine tool structure model;

[0037] Figure 3 This is a schematic diagram of the motion chain of a five-axis machine tool. Detailed Implementation

[0038] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0039] Nonlinear error control methods are mainly used to address the deviation between the actual and theoretical tool tip paths during multi-axis machining due to the presence of rotary axes. These deviations constitute the control methods for nonlinear errors. In precision and ultra-precision machining, nonlinear errors have a significant impact on machining quality, potentially leading to overcutting or undercutting, resulting in scrap parts that fail to meet machining specifications. In traditional three-axis machine tool machining, all three linear axes are linear, and nonlinear errors do not exist during real-time interpolation. However, in five-axis machine tool machining, CAM software typically generates tool position information in the workpiece coordinate system. If the machine tool lacks RTCP functionality, the tool position information is post-processed to obtain the actual machine tool commands; while machine tools with RTCP functionality directly input the tool position information, which is automatically interpreted into the movement of each axis. Both methods involve nonlinear errors, leading to overcutting or undercutting and affecting machining quality. Currently, most research focuses on controlling and studying nonlinear errors in machine tools without RTCP functionality. Therefore, this invention proposes a method for reducing nonlinear errors in AC rotary table-type five-axis machine tools with RTCP functionality by adjusting machining parameters.

[0040] like Figures 1-3As shown, a nonlinear error control method for five-axis machine tools by changing the machining speed is described, and its process is combined with... Figure 1 As shown, it includes the following steps:

[0041] S1. For five-axis machine tools with RTCP functionality, set the maximum allowable nonlinear error threshold during machining. ;

[0042] S2. Analyze the five-axis machine tool, targeting... Figure 2 The five-axis machine tool structure model shown is established. Figure 3 The five-axis machine tool kinematic chain shown is defined as follows: Located on the A-axis, Located on the C-axis, Let be the common perpendicular of the two axes AC; This point is the origin of the machine tool coordinate system (MCS). In the initial state of the machine tool, this point is... coincide, , and These are the X, Y, and Z axes of the machine tool coordinate system (MCS), and the positive directions of each axis are shown in the figure. Let WCS be the origin of the workpiece coordinate system. , and The X, Y, and Z axes of the workpiece coordinate system WCS are shown in the figure; the positive directions of each axis are shown in the figure. The movement of each axis of the five-axis machine tool are X, Y, Z, A, and C, and the positive directions of each axis are shown in the figure. The vector in the machine tool coordinate system at the initial state of the machine tool. , represented as , for The distance between; The origin of the workpiece coordinate system WCS The offset relative to the machine tool coordinate system (MCS) is expressed as ; The offset of the program origin relative to the machine tool coordinate system (MCS) is expressed as: ; for and A combined vector.

[0043] Next, the forward and inverse kinematics are derived, including:

[0044] The derivation results of the forward kinematics model are as follows:

[0045] (1)

[0046] (2)

[0047] The derivation results of the inverse kinematics model are as follows:

[0048] (3)

[0049] (4)

[0050] Define tool position information Coordinates of the knife tip and tool axis vector combination ;

[0051] S3. For the part to be processed, the first step is to obtain the result through CAM software. Tool position information at the beginning and end of the tool path and , The data is then imported into a five-axis machine tool and the movement of each axis corresponding to the first and last positions is automatically calculated according to formulas (3) and (4). and Set the starting point. Used to record The coordinates of the knife tip point ,set up Record the number of decelerations, and set the number of decelerations. Duan Daolu Expected feed rate ;

[0052] S4. Based on the forward and inverse kinematics models, the interpolation period is known. and the Feed rate of the toolpath ( Indicates the desired feed rate slow down The size after that, if This is the desired feed rate. ), calculate the processing time The starting point of the segmented knife path coordinates and the end point coordinates fine interpolation points between The coordinates.

[0053] In actual machining, a five-axis machine tool will be in the first... The starting point of the segmented knife path and the end point Linear interpolation is performed between these points to obtain a series of fine interpolation points. These points are the actual points of motion for each axis of the five-axis machine tool, but there will be nonlinear errors in the paths between adjacent points. Based on the starting point... coordinates and the end point coordinates Interpolation period and the Feed rate of the toolpath Then the first fine interpolation point The following relationship is approximately satisfied:

[0054] (5)

[0055] The calculation formula is further obtained as follows:

[0056] (6)

[0057] Thus, the first fine interpolation point is obtained. coordinates ;

[0058] S5, Calculate the... Starting point of the segmented knife path With the first fine interpolation point Nonlinear error between .

[0059] Given the starting point coordinates ,end coordinates , Turntable corner as well as Turntable corner , to obtain the first The first precise insertion point in the Duan Daolu route Corresponding movement of each axis Turntable corner The calculation formula is as follows:

[0060] (7)

[0061] The first fine interpolation point Turntable corner and coordinates Substituting into formula (4) yields the corresponding movement of each axis. The displacement of the three linear axes .

[0062] Actual motion of each axis of a five-axis machine tool over time The calculation formula is as follows:

[0063] (8)

[0064] Because of the existence of the rotation axis, the actual motion of each axis will be calculated. Substituting into formula (1) yields the actual trajectory of the tool tip changing over time. for:

[0065] (9)

[0066] Because the point of maximum nonlinear error is difficult to find, the time is set to half the interpolation period. The tool tip point is approximately set at the location of the maximum nonlinear error. The result is obtained according to formula (9). The actual tip of the blade coordinates Calculate its relationship with the line. The distance between them is the nonlinear error. The expression is as follows:

[0067] (10)

[0068] In the formula, , , .

[0069] Because the interpolation points between adjacent tool tip points are very dense, and the changes in adjacent tool position information are small, to avoid excessive calculations, the first... Duan Daolu The nonlinear error is determined as obtained. ;

[0070] S6. Calculate the nonlinear error With the set nonlinear error threshold Comparison:

[0071] like Then proceed to step S7;

[0072] like Then proceed to step S8;

[0073] S7, Number of decelerations ;

[0074] like Based on the calculated nonlinear error With the set nonlinear error threshold Adjust the feed rate to Then proceed to step S4;

[0075] like To avoid excessive calculations, the feed rate was adjusted to... Then proceed to step S4;

[0076] After adjustment The distance is shortened, thereby reducing nonlinear errors;

[0077] S8, Starting Point and the end point Interval speed set ;

[0078] Pick Reset ,like If so, proceed to step S4 to calculate the nonlinear error of the next toolpath segment; if Then the execution will end.

[0079] In summary, this invention calculates nonlinear errors on a five-axis machine tool with RTCP functionality. By knowing the information between two path points during machining, the nonlinear error can be controlled by adjusting the machining speed to reduce the distance between interpolation points.

[0080] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0081] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A nonlinear error control method by changing the machining speed of a five-axis machine tool, characterized in that: Includes the following steps: S1. For five-axis machine tools with RTCP functionality, set the maximum allowable nonlinear error threshold during machining. ; S2. Perform structural analysis on the five-axis machine tool, establish the machine tool kinematic chain, derive the forward and inverse kinematic models, and define the tool position information. Coordinates of the knife tip and tool axis vector combination ; S3. For the part to be processed, the first step is to obtain the result through CAM software. Tool position information at the beginning and end of the tool path and , The data is then imported into a five-axis machine tool to calculate the movement of each axis corresponding to the initial and final positions based on the inverse kinematics model. and Set the starting point Used to record The coordinates of the knife tip Set the number of decelerations. , set the first Duan Daolu Expected feed rate ; S4. Based on the interpolation period and the feed rate of the toolpath , Indicates the desired feed rate slow down The size after that, if This is the desired feed rate. Then calculate the starting point. and the end point The first fine interpolation point coordinates ; S5, Calculation Corresponding movement of each axis Turntable corner , combined coordinates Obtained through inverse kinematics model The corresponding three linear axis displacements This allows us to obtain the actual motion of each axis of a five-axis machine tool over time. Substituting this into the forward kinematics model, we obtain the actual trajectory of the blade tip over time. ,Will The tool tip point is set at the location of the maximum nonlinear error, and the result is obtained. The actual tip of the blade coordinates Calculate its relationship with the line. The distance between them is the nonlinear error. ; S6, will and If a comparison is made, If so, proceed to step S7; Then proceed to step S8; S7, Order ,like Adjust the feed rate to And return to step S4; if Adjust the feed rate to and return to step S4; S8, will and The speed between is set to ,make Reset ,like If so, return to step S4 to process the next toolpath segment; like Then the execution will end.

2. The nonlinear error control method for changing the machining speed of a five-axis machine tool according to claim 1, characterized in that: In step S2, the forward and inverse kinematics models are derived as follows: The derivation results of the forward kinematics model are as follows: The derivation results of the inverse kinematics model are as follows: In the formula, X, Y, Z, A, and C represent the movement of each axis of a five-axis machine tool. This represents the offset of the origin of the workpiece coordinate system relative to the machine tool coordinate system. This represents the offset of the program origin relative to the machine tool coordinate system. This is the distance from the common perpendicular to the AC axis.

3. The nonlinear error control method for changing the machining speed of a five-axis machine tool according to claim 2, characterized in that: In step S4, fine interpolation is performed. The formula for calculating the coordinates is as follows: In the formula, End point The coordinates.

4. The nonlinear error control method for changing the machining speed of a five-axis machine tool according to claim 3, characterized in that: In step S5, the turntable angle... The calculation formula is: 。 5. The nonlinear error control method for changing the machining speed of a five-axis machine tool according to claim 4, characterized in that: In step S5, the nonlinear error The expression is as follows: In the formula, , , .

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