Milling cutter mounting error online identification method based on milling force signal

Abstract

A kind of milling cutter installation error online identification method based on milling force signal, a series of angular microelement is obtained by discretization to the rotation angle under the rotation period of milling cutter, and the tool shape and machining parameters of milling cutter are discretized along the axial direction to obtain a series of axial microelement;After calculating the instantaneous cutting thickness of each axial microelement of each tooth at the discrete time and the total milling thickness of the corresponding tooth in each tooth period in turn, the theoretical value of the total milling thickness corresponding to different tooth periods is determined, and then the milling force integral ratio corresponding to different teeth is obtained;According to the actual measured milling force in the milling process, the integral of milling force curve corresponding to each tooth with respect to time is calculated, then the milling force integral ratio corresponding to different tooth periods is obtained, and an optimization function about milling force integral ratio and milling force integral ratio is established, the PSO algorithm is used to optimize to reach minimum, and then the parameters of tool installation error are obtained.The present application can significantly improve the installation accuracy without knowing the milling force coefficient of workpiece-tool pair in advance.

Description

Technical Field

[0001] This invention relates to a technology in the field of machining, specifically a method for online identification of milling cutter installation errors based on milling force signals. Background Technology

[0002] Tool installation errors are common in machining processes using tool holders. These errors lead to variations in cutting loads across different cutting teeth, accelerating tool wear or breakage. Furthermore, tool installation errors increase surface roughness and shape defects. While tool installation errors can be measured using a dial indicator before machining, this method has several drawbacks: 1) Measurement is difficult because the helical cutting edge of the milling cutter prevents continuous smooth contact between the dial indicator probe and the cutting edge; 2) Tool installation errors change after each tool change, requiring remeasurement, which is time-consuming; 3) During machining, the actual runout of the tool under stress differs from the runout measured statically. Therefore, there is an urgent need for an online, automated method to identify tool runout during milling. Summary of the Invention

[0003] This invention addresses the problem that existing tools installation errors affect instantaneous milling forces and that current technologies cannot solve the problem of online identification of milling cutter installation errors. It proposes an online identification method for milling cutter installation errors based on milling force signals. A new theoretical model is established to quantitatively describe the influence mechanism of tool installation errors on instantaneous milling forces. Based on the theoretical model and the milling forces measured online during the machining process, online identification of tool installation errors is performed, significantly improving installation accuracy.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to an online identification method for milling cutter installation errors based on milling force signals. The method discretizes the rotation angle of the milling cutter during one rotation cycle to obtain a series of angular micro-elements, and discretizes the cutter shape and machining parameters along the axial direction to obtain a series of axial micro-elements. The instantaneous cutting thickness of each axial micro-element of each tooth at discrete moments and the total milling thickness of that tooth within each tooth cycle are calculated sequentially to determine the theoretical value of the total milling thickness corresponding to different tooth cycles, thereby obtaining the milling force integral ratio for different teeth. Then, based on the actual measured milling force during the milling process, the integral of the milling force curve for each tooth with respect to time is calculated. The milling force integral ratio for different tooth cycles is then obtained, and an optimization function for the milling force integral ratio and the milling force integral ratio is established. The PSO algorithm is used to optimize and minimize the function, thereby obtaining the parameters of the tool installation error.

[0006] This invention relates to a system for implementing the above-mentioned method, comprising: a milling cutter discretization unit, a total milling thickness calculation unit for different cutter teeth, a measured milling force integration unit, and a tool installation error identification unit, wherein: the milling cutter discretization unit performs discretization processing based on the geometric information and machining parameters of the milling cutter to obtain the axial micro-element of the milling cutter at each simulation time step; the total milling thickness calculation unit for different cutter teeth calculates the instantaneous cutting thickness corresponding to each axial micro-element of each cutter tooth based on the axial micro-element of the milling cutter, and then accumulates the cutting thickness of each axial micro-element of each cutter tooth to obtain the total cutting thickness of each cutter tooth; the measured milling force integration unit integrates the milling force collected by the force measuring instrument during the experiment. The information is obtained by integrating the milling force of each cutting tooth (i.e., the different peak values ​​of the milling force curve) over time to obtain the total milling force of each cutting tooth during the cutting process. The tool installation error identification unit calculates the ratio of the total milling thickness of different cutting teeth based on the total milling thickness. After calculating the ratio of the integral of the milling force of different cutting teeth based on the integral value of the milling force of each cutting tooth, the tool installation error identification unit identifies the optimal tool installation error based on the PSO algorithm, which minimizes the difference between the ratio of the total milling thickness of the cutting teeth and the ratio of the integral of the milling force of different cutting teeth, and finally obtains the identified tool installation error.

[0007] The geometric information includes: number of cutting teeth, helix angle, and diameter; the machining parameters include: depth of cut and spindle speed.

[0008] Technical effect

[0009] This invention discretizes the milling cutter and calculates the ratio of total milling thickness for different cutter teeth; then, it calculates the integral ratio of milling force for different cutter teeth based on the measured milling force; and finally, it identifies tool installation errors online based on these two ratios. This achieves accurate online identification of milling force for tool installation errors. Attached Figure Description

[0010] Figure 1 This is a flowchart of the present invention;

[0011] Figure 2 This is a schematic diagram of tool discretization;

[0012] Figure 3 This is a schematic diagram of tool runout;

[0013] Figure 4 This is a schematic diagram illustrating the integral of the milling force for different cutter teeth in actual milling force measurements.

[0014] Figure 5 This is a schematic diagram of the measured milling forces Fx and Fy for one cycle.

[0015] Figure 6 A schematic diagram showing the transformation of milling forces Fx and Fy into tangential force Ft and radial force Fr;

[0016] Figure 7 A schematic diagram illustrating the relationship between particle fitness and the number of iterations in the PSO algorithm's search for optimal tool mounting error parameters ρ and λ. Detailed Implementation

[0017] like Figure 1 As shown in the figure, this embodiment relates to an online identification method for milling cutter installation errors based on milling force signals, which includes the following steps:

[0018] Step 1) Discretize the rotation angle of the milling cutter in one rotation cycle to obtain a series of angular micro-elements, and discretize the tool shape and machining parameters of the milling cutter along the axial direction to obtain a series of axial micro-elements. Specifically, discretize one tool rotation cycle into N. s For each time increment, the angle dφ of each angular element is 2π / N. s Then the milling cutter is discretized along the axial direction into N... a There are several axial infinitesimal elements, and the height of each axial infinitesimal element is dz = a. p / N a , where: a p Let i be the axial depth of cut; then, through discretization, a series of milling cutter axial micro-elements (i,j,k) are obtained, where i is the micro-element number in the circumferential direction. j is an axial infinitesimal element. k is the number of the cutting teeth. N t This represents the total number of cutting teeth.

[0019] Step 2) Calculate the instantaneous cutting thickness h(i,j,k) of each axial micro-element of each cutter tooth at discrete time i, specifically including:

[0020] 2.1) Calculate the instantaneous immersion angle of the infinitesimal element (i,j,k). Where: n is the main spindle speed, φ p For the tooth tip angle, φ p =2π / N t β is the helix angle, R is the nominal radius of the tool, R = D / 2, and D is the nominal diameter of the tool.

[0021] 2.2) Calculate the actual rotation radius of each axial micro-element of each cutter tooth due to tool installation error. This actual rotation radius is a function of the cutter tooth number and the axial micro-element number, specifically: Where: ρ is the radial deviation length of the tool caused by the tool installation error, λ is the radial deviation angle of the tool caused by the tool installation error, and the radial offset caused by the tool installation error is as follows: Figure 3 As shown.

[0022] 2.3) Calculate the instantaneous cutting thickness Where: f t The feed per tooth (mm / tooth / rev) means that the material being cut by the current cutting tooth k is the material left after the previous m cutting teeth (the kth cutting tooth) have cut.

[0023] In this embodiment, when h(i,j,k) is less than 0, it should be forcibly set to 0, indicating that no material is cut.

[0024] Step 3) Calculate the total milling thickness of the cutter tooth within each cutter tooth cycle, then determine the theoretical value of the total milling thickness corresponding to different cutter tooth cycles, and thus obtain the milling force integral ratio corresponding to different cutter teeth, specifically including:

[0025] 3.1) Calculate the tangential cutting force F within all tooth cycles. t Or radial cutting force F r The integral over the rotation angle is equal to the sum of the integrals of the axial cutting elements of each participating cutter tooth over the rotation angle, specifically:

[0026] Where: the subscript q represents t or r, indicating tangential milling force or radial milling force; Kqc and Kqe are the shear force coefficient and tillage force coefficient, respectively; K tc K rc These are the tangential and radial cutting force coefficients, K. te and K re These are the tangential and radial tillage force coefficients, respectively, and are unit step functions used to determine whether the tool element participates in cutting. Where: φ st and φ ex Indicates the entry and exit angles;

[0027] The entry and exit angles in the climb milling stage are respectively The entry and exit angles during the reverse milling stage are as follows: Where: a e This represents the radial cutting width.

[0028] 3.2) Calculate the difference in the integral of the cutting force with respect to time within different cutter tooth cycles, and then calculate the ratio of the milling force integrals corresponding to different cutter teeth, specifically:

[0029] , where m≠n, p≠q, and m,n and p,q are not respectively equal.

[0030] Because a linear cutting force model is used, the difference between any two cutting force integrals can eliminate the contribution of the plowing force to the cutting force. Then, dividing by the difference in the areas of another pair of cutting force integrals eliminates the cutting force coefficient. Thus, the milling force integral ratio simplifies to: Wherein: T ratio (ρ,λ)

[0031] These are the tool runout parameters and machining parameters.

[0032] Step 4) Based on the actual measured milling force during the milling process, calculate the integral of the milling force curve of each cutter tooth with respect to time, and then obtain the ratio of the milling force integral corresponding to different cutter tooth cycles. Specifically, perform coordinate transformation on the milling force to convert the milling force F in the feed direction (x direction) and the perpendicular feed direction (y direction) into coordinates. x and F y Transformed into tangential and radial milling forces F t and F r Specifically: After obtaining the measured radial and tangential milling forces, the numerical integral of the milling force with respect to time for each cutter tooth cycle is calculated to obtain the ratio of the milling force integrals for different cutter tooth cycles. Where m≠n, p≠q, and m,n and p,q are not respectively equal.

[0033] Step 5) Establish the ratio of milling force integral T ratio The ratio of the integral of the milling force S ratio After optimizing the objective function f(ρ,λ), the PSO algorithm is used to further optimize the objective function to minimize f(ρ,λ). At this point, ρ and λ represent the parameters of the tool installation error.

[0034] include:

[0035] 5.1) Based on the theoretical value T of the integral ratio ratio Compared with experimental value S ratio Set the objective function

[0036]

[0037] 5.2) Set the initial value ρ for the iteration. (r) =0,λ (r) =0, where r is the number of iterations.

[0038] 5.3) For a given cutter tooth and axial infinitesimal element, calculate the instantaneous cutting thickness h(i,j,k) of each angular infinitesimal element.

[0039] 5.4) Repeat step 5.3) for each cutter tooth and each axial infinitesimal element until all cutter teeth and all axial infinitesimal elements have been completely accumulated at the m-th axial infinitesimal element. th The sum of the cutting thicknesses within the range of each tooth tip angle.

[0040] 5.5) Calculate the theoretical value T of the ratio of the integrals of the milling forces corresponding to different cutter teeth. ratio .

[0041] 5.6) Calculate the value of the objective function f(ρ,λ) and record the tool installation error (ρ,λ) at this time.

[0042] 5.7) Repeat steps 5.2)-5.6) until all tool runout parameters have been traversed, and obtain the minimum value of the objective function f(ρ,λ) for all traversal times. The corresponding tool installation error is the final identification result.

[0043] In the online tool installation error identification method based on measured milling force signals, the optimization algorithm for tool installation error (ρ, λ) uses the particle swarm optimization (PSO) algorithm. The search range for ρ is 0-30 μm, and the search range for λ is 0-359°. The population size is 20.

[0044] Through practical experiments, dry milling experiments were conducted on a HURCO VMX42 machining center. The cutting tool was a solid carbide end mill with the following parameters: diameter D = 12mm, number of teeth Nt = 4. The machining parameters were as follows: axial depth of cut ap = 5mm, radial width of cut ae = 1.2mm. The workpiece material was aluminum alloy. During the milling process, a Kistler milling force measurement system was used to measure the milling force. The Kistler milling force measurement system included: a Kistler 9272 piezoelectric sensor, a Kistler 5697A charge amplifier, and a Kistler 9233 data acquisition card. The measured milling force signals Fx and Fy are as follows: Figure 5 As shown, the milling forces Fx and Fy are then transformed into radial and tangential directions to obtain the tangential force Ft and the radial force Fr, as follows. Figure 6 As shown in the figure, the relationship between the optimal fitness of a particle and the number of iterations during the PSO algorithm search for the optimal ρ and λ is as follows. Figure 7 As shown, convergence is achieved in the 51st iteration. According to this method, the milling cutter installation error parameters are identified as ρ = 10μm and λ = 26°; while the tool installation error parameters measured by the laser displacement sensor are ρ = 12μm and λ = 35°. The error in radial offset length is only 2μm, and the error in radial offset angle is only 9°, indicating that the method proposed in this patent is correct.

[0045] This method can identify tool installation errors without prior knowledge of the workpiece-tool pair milling force coefficient. In contrast, existing methods require prior knowledge of the workpiece-tool pair milling force coefficient, which is a time-consuming and complex process involving a series of variable feed experiments. This method, without requiring prior knowledge of the milling force coefficient, can effectively optimize the milling process, control surface quality, predict tool wear, and improve machining stability.

[0046] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A method for online identification of milling cutter installation error based on milling force signal, characterized in that, include: Step 1) Discretize the rotation angle of the milling cutter in one rotation cycle to obtain a series of angular micro-elements, and discretize the tool shape and machining parameters of the milling cutter along the axial direction to obtain a series of axial micro-elements; Step 2) Calculate the instantaneous cutting thickness of each axial micro-element of each cutter tooth at discrete moments; Step 3) After calculating the total milling thickness of the cutter tooth in each cutter tooth cycle, determine the theoretical value of the total milling thickness corresponding to different cutter tooth cycles, and then obtain the milling force integral ratio corresponding to different cutter teeth. ; Step 4) Based on the actual measured milling force during the milling process, calculate the integral of the milling force curve with respect to time for each cutter tooth, and then obtain the ratio of the milling force integral for different cutter tooth cycles. ; Step 5) Establish the ratio of milling force integrals Ratio of milling force integral The optimization function is optimized using the PSO algorithm to reach its minimum value, thereby obtaining the parameters of the tool installation error; Step 3) specifically includes: 3.1) Calculate the tangential cutting force F within all tooth cycles. t Or radial cutting force F r The integral over the rotation angle is equal to the sum of the integrals of the axial cutting elements of each participating cutter tooth over the rotation angle, specifically: Where: the subscript q represents t or r, indicating tangential milling force or radial milling force; Kqc and Kqe are the shear force coefficient and tillage force coefficient, respectively; K tc K rc These are the tangential and radial cutting force coefficients, K. te and K re These are the tangential and radial tillage force coefficients, respectively, and are unit step functions used to determine whether the tool element participates in cutting. ,in: and Indicates the entry and exit angles; the entry and exit angles during climb milling are respectively The entry and exit angles during the reverse milling stage are as follows: , where: a e Radial cutting width; 3.2) Calculate the difference in the integral of the cutting force with respect to time within different cutter tooth cycles, and then calculate the ratio of the milling force integrals corresponding to different cutter teeth, specifically: Where: m≠n, p≠q, and m, n and p, q are not respectively equal; The milling force integral ratio is simplified as follows: , where: T ratio (ρ, λ) represent the tool runout parameters and machining parameters; Step 4) specifically involves: performing a coordinate transformation on the milling force, converting the milling force F in the feed direction and perpendicular to the feed direction... x and F y Transformed into tangential and radial milling forces F t and F r Specifically: After obtaining the measured radial and tangential milling forces, the numerical integral of the milling force with respect to time for each cutter tooth cycle is calculated to obtain the ratio of the milling force integrals for different cutter tooth cycles. Where m≠n, p≠q, and m, n and p, q are not respectively equal.

2. The online identification method for milling cutter installation error based on milling force signal according to claim 1, characterized in that, Step 1) specifically involves discretizing a tool rotation cycle into N. s For each time increment, the angle of each angular element is... ; Then the milling cutter is discretized along the axial direction into N. a There are several axial infinitesimal elements, and the height of each axial infinitesimal element is dz=a. p / N a , where: a p Let i be the axial depth of cut; then, through discretization, a series of milling cutter axial micro-elements (i, j, k) are obtained, where i is the micro-element number in the circumferential direction. j is an axial infinitesimal element. k is the number of the cutting teeth. N t This represents the total number of cutting teeth.

3. The online identification method for milling cutter installation error based on milling force signal according to claim 1, characterized in that, Step 2) specifically includes: 2.1) Calculate the instantaneous immersion angle of the infinitesimal element (i, j, k). Where: n is the spindle speed, For the tooth tip angle, β is the helix angle, R is the nominal radius of the tool, R=D / 2, D is the nominal diameter of the tool; 2.2) Calculate the actual rotation radius of each axial micro-element of each tooth due to tool installation error. This actual rotation radius is a function of the tooth number and the axial micro-element number, specifically: Where: ρ is the radial deviation length of the tool caused by the tool installation error, and λ is the radial deviation angle of the tool caused by the tool installation error; 2.3) Calculate the instantaneous cutting thickness , where: f t The feed per tooth is measured in mm / tooth / rev. The material cut off by the current cutting tooth k is the material left after the first m cutting teeth, i.e., the material left after the km-th cutting tooth cuts.

4. The online identification method for milling cutter installation error based on milling force signal according to claim 3, characterized in that, When h(i, j, k) is less than 0, it should be forced to be equal to 0, indicating that no material should be cut.

5. The online identification method for milling cutter installation error based on milling force signal according to claim 1, characterized in that, Step 5) specifically includes: 5.1) Based on the theoretical value T of the integral ratio ratio Compared with experimental value S ratio Set the objective function , 5.2) Set the initial value ρ for the iteration. (r) =0, λ (r) =0, where: r is the number of iterations; 5.3) For a given cutter tooth and axial infinitesimal element, calculate the instantaneous cutting thickness h(i,j,k) of each angular infinitesimal element; 5.4) Repeat step 5.3) for each cutter tooth and each axial infinitesimal element until all cutter teeth and all axial infinitesimal elements have been completely accumulated at the m-th axial infinitesimal element. th The sum of the cutting thickness within the range of each tooth tip angle; 5.5) Calculate the theoretical value T of the ratio of the integrals of the milling forces corresponding to different cutter teeth. ratio ; 5.6) Calculate the value of the objective function f(ρ, λ) and record the tool installation error (ρ, λ) at this time; 5.7) Repeat steps 5.2)-5.6) until all tool runout parameters have been traversed, and obtain the minimum value of the objective function f(ρ, λ) for all traversal times. The corresponding tool installation error is the final identification result.

6. The online identification method for milling cutter installation error based on milling force signal according to claim 1, characterized in that, The optimization algorithm for the tool installation error (ρ, λ) uses the particle swarm optimization algorithm, with the search range of ρ being 0-30μm, the search range of λ being 0-359°, and the population size being 20.

7. A milling cutter installation error online identification system for implementing the method of any one of claims 1-6, characterized in that, include: The system comprises a milling cutter discretization unit, a total milling thickness calculation unit for different cutter teeth, a measured milling force integration unit, and a tool installation error identification unit. Specifically: the milling cutter discretization unit discretizes the milling cutter based on its geometric information and machining parameters to obtain the axial micro-element of the milling cutter at each simulation time step; the total milling thickness calculation unit calculates the instantaneous cutting thickness corresponding to each axial micro-element of each cutter tooth based on the axial micro-element of the milling cutter, and then sums the cutting thicknesses of each axial micro-element of each cutter tooth to obtain the total cutting thickness of each cutter tooth; the measured milling force integration unit calculates the total cutting thickness of each cutter tooth based on the milling force information collected by the force gauge during the experiment, according to the milling force of each cutter tooth... The milling force, i.e., the different peak values ​​of the milling force curve, is integrated over time to obtain the total milling force of each cutting tooth during the cutting process. The tool installation error identification unit calculates the ratio of the total milling thickness of different cutting teeth based on the total milling thickness. After calculating the ratio of the milling force integral of different cutting teeth based on the integrated value of the milling force of each cutting tooth, the tool installation error identification unit identifies the optimal tool installation error based on the PSO algorithm, which minimizes the difference between the ratio of the total milling thickness of the cutting teeth and the ratio of the milling force integral of different cutting teeth, and finally obtains the identified tool installation error.

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