A CFRP machining hole wall three-dimensional roughness prediction method

Abstract

This invention relates to the field of mechanical manufacturing and processing technology, and discloses a method for predicting the three-dimensional roughness of hole walls in CFRP machining. The invention first establishes a cutting force-cutting height equation through the cutting force equation to obtain the functional relationship between load and surface height. Then, through orthogonal cutting experiments, the coefficients in the functional expression are determined, ultimately yielding a complete calculation expression for predicting the three-dimensional roughness of the hole wall. Based on this expression, only the force signal or other signals that can be converted into force signals need to be monitored to predict the three-dimensional roughness of the hole wall in real time during machining, thereby achieving real-time monitoring of hole quality during machining, avoiding machining damage, and improving machining efficiency.

Description

Technical Field

[0001] This invention relates to the field of mechanical manufacturing and processing technology, and more specifically to a method for predicting the three-dimensional roughness of hole walls in CFRP machining. Background Technology

[0002] CFRP is widely used in aerospace, automotive, and marine industries due to its excellent performance. However, the anisotropic nature of CFRP and the unclear cutting mechanism limit the prediction of its surface roughness. Therefore, hole wall roughness is a key area of ​​research in CFRP machining.

[0003] Currently, existing inspection methods only allow for handheld roughness testers, which cannot provide real-time on-site monitoring. Therefore, even if the hole wall roughness is found to be non-compliant with specifications later, losses cannot be avoided. If on-site machining signals could be used for inspection, machining quality could be guaranteed and machining efficiency improved. Therefore, the prediction of hole wall roughness is of great significance.

[0004] For example, the invention patent CN116117211A discloses a method for predicting the surface roughness of whirl-milled threaded workpieces that takes into account the influence of cutting force. The objects involved in the above-mentioned prior art are homogeneous isotropic materials, and their surface roughness prediction methods cannot be used for CFRP with anisotropic characteristics. Summary of the Invention

[0005] To overcome the problems and shortcomings of existing technologies, this invention proposes a method for predicting the three-dimensional roughness of hole walls in CFRP machining. This invention establishes a cutting force-cutting height equation based on the cutting force equation, obtaining a functional relationship between load and surface height. Then, through orthogonal cutting experiments, the coefficients in the functional expression are determined, ultimately yielding a complete calculation expression for predicting the three-dimensional roughness of the hole wall. Based on this expression, by simply monitoring the force signal or other signals that can be converted into force signals, the three-dimensional roughness of the hole wall can be predicted in real time during machining. This enables real-time monitoring of hole quality during machining, thereby avoiding machining damage and improving machining efficiency.

[0006] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a method for predicting the three-dimensional roughness of the hole wall in CFRP machining, the method comprising the following steps:

[0008] Step S1. Establish the cutting force-cutting height equation through the cutting force equation to obtain the functional relationship between load and surface height;

[0009] Step S2. Determine the coefficients in the function expression of step S1 through orthogonal cutting experiments.

[0010] Furthermore, step S1 specifically includes the following steps:

[0011] Step S1.1. Combine the cutting force equation and determine the cutting depth of the first deformation cutting zone through inverse transformation;

[0012] Step S1.2. Combine the cutting force equation and determine the fiber fracture depth in the second cutting region through inverse transformation;

[0013] Step S1.3. Determine the fiber springback height in the third cutting zone using empirical formulas;

[0014] Step S1.4. Establish the final cutting force-roughness equation by using the partial differential equation relationship between the tool rake angle and clearance angle and the cutting force as a multivariate function, as well as the fiber fracture depth in the second cutting zone and the fiber springback height in the third cutting zone.

[0015] Furthermore, in step S1.1, the calculation expression for the cutting depth of the first cutting region is as follows:

[0016]

[0017] Where h is the material cutting width, τ1 is the shear strength of the material perpendicular to the fiber direction, and τ2 is the shear strength of the material along the fiber direction. The cutting angle that forms the chip, β is the rake face friction angle, γ0 is the tool cutting edge rake angle, and θ is the fiber direction angle; F y1 and F z1 These are the feed resistance and depth of cut resistance obtained after decomposing the cutting force of the tool in the first cutting region, respectively.

[0018] Furthermore, in step S1.2, the calculation expression for the fiber fracture depth in the second cutting region is as follows:

[0019] P·cosθ=L·d·ξ+C·4πL 2 υ+r e ·(1-Cd)·ζ Equation (3);

[0020] Furthermore, the depth of cut in the second cutting region is as follows:

[0021]

[0022] Where P is the resultant force in the second cutting region, L is the fiber fracture depth, C is the carbon fiber distribution density, ξ is the bond strength between the fiber and the matrix, υ is the fiber fracture strength, ζ is the matrix fracture strength; d is the diameter of the carbon fiber; r e Let be the radius of the tool's blunt circle.

[0023] Furthermore, in step S1.3, the depth-of-cut resistance and feed resistance within the third cutting region are as follows:

[0024]

[0025] Among them, k, H, r e E3 and E3 are the cutting coefficient, material hardness, cutting edge radius, and effective elastic modulus, respectively; a, α, and h are the contact length between the tool flank and the workpiece, the tool clearance angle, and the material cutting width in the third cutting region, respectively; F z3 and F y3 These are the depth-of-cut resistance and feed resistance within the third cutting region, respectively.

[0026] Furthermore, based on empirical formulas, the fiber springback height b within the third cutting region is calculated. c :

[0027]

[0028] Furthermore, in step S1.4, the calculation expression for the height of the cutting surface is as follows:

[0029] h c =|r e +Lb c | Equation (6);

[0030] Furthermore, based on the height of the cutting surface, the following cutting force-roughness equation is established:

[0031]

[0032] Where hc is the height of the cutting surface, h1 is the height of the initial plane from the machined surface at time t1, and h2 is the height of the initial plane from the machined surface at time t2.

[0033] Furthermore, the tool rake face and the chip form the first cutting area.

[0034] Furthermore, the cutting edge element is the second cutting region.

[0035] Furthermore, the flank face of the tool is the third cutting area.

[0036] The beneficial effects of this invention are:

[0037] The three-dimensional roughness prediction method for hole walls provided by this invention is applicable to various irregular materials. By establishing the relationship between cutting force and roughness, it is only necessary to monitor the force signal or other signals that can be converted into force signals to achieve real-time monitoring of hole quality during the processing, thereby avoiding processing damage and improving processing efficiency. Attached Figure Description

[0038] The foregoing and hereinafter detailed description of the invention becomes clearer when read in conjunction with the following drawings, in which:

[0039] Figure 1 This is a schematic diagram of the drilling process of the tool of the present invention (the angles in the figure represent four typical fiber direction angles);

[0040] Figure 2 This is the drilling deformation zone of the tool in this invention;

[0041] Figure 3 This is the cutting force model for the first cutting region of the present invention;

[0042] Figure 4 This is the cutting force model for the second cutting region of the present invention;

[0043] Figure 5 This is a schematic diagram of fiber breakage in the present invention;

[0044] Figure 6 This is the cutting force and springback model for the third shear deformation region of the present invention;

[0045] Figure 7 These are the forces acting on the cutting tool in three directions, measured by a force gauge during the drilling process.

[0046] Figure 8 and Figure 9 This is a comparison chart of the predicted and experimental values ​​of the hole wall roughness in an embodiment of the present invention. Detailed Implementation

[0047] To enable those skilled in the art to better understand the technical solutions of this invention, specific embodiments will be used to further illustrate the technical solutions for achieving the objectives of this invention. It should be noted that the technical solutions claimed by this invention include, but are not limited to, the following embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort should fall within the scope of protection of this invention.

[0048] CFRP is widely used in aerospace, automotive, and marine industries due to its excellent performance. However, the anisotropic nature of CFRP and the unclear cutting mechanism limit the prediction of its surface roughness. Therefore, hole wall roughness is a key area of ​​research in CFRP machining.

[0049] Currently, existing inspection methods only allow for handheld roughness testers, which cannot provide real-time on-site monitoring. Even if the hole wall roughness is found to be non-compliant with specifications later, losses cannot be avoided. If on-site machining signals could be used for inspection, machining quality could be guaranteed and machining efficiency improved. Therefore, predicting hole wall roughness is of great significance.

[0050] Based on this, embodiments of the present invention propose a three-dimensional roughness prediction method for CFRP machining hole walls. This method is based on the cutting force equation, establishes the cutting force-cutting height equation, obtains the functional relationship between load and surface height, thereby establishing the connection between macroscopic mechanical signals and microscopic roughness indicators. By simply monitoring the force signal or other signals that can be converted into force signals, the hole quality can be monitored, thereby avoiding machining damage and improving machining efficiency.

[0051] This embodiment discloses a method for predicting the three-dimensional roughness of hole walls in CFRP machining. The specific scheme adopted is as follows:

[0052] (1) During the drilling process, the outer edge turning point where the main cutting edge of the tool intersects with the back groove edge truly determines the hole wall morphology. Within a unit of time, the cutting of the material at this outer edge turning point can be considered orthogonal cutting. Since carbon fiber composites have a periodic ply structure, the hole wall roughness should also be periodic, even without considering the influence of machining defects during drilling. The three-dimensional profile roughness of the hole wall can be composed of two parts: one part is the surface roughness component S caused by the tool movement. a The other part is the residual error S caused by random factors. a_ε .

[0053] It is understandable that any simulation result will differ from the measured result; there is no completely identical simulation result. Therefore, this invention uses S to represent the residual error caused by random factors in the processing. a_ε This indicates that the residual error is used to correct the prediction results. Specifically, S... a_ε The value can be corrected and evaluated through subsequent experiments.

[0054] (2) From a microscopic perspective, the roughness of the hole wall is nothing more than affected by the tool movement and the material itself. (See attached image) Figure 2 As shown, the first cutting zone is the rake face of the tool and the chip. In this zone, the rake face of the tool exerts compression, bending and shearing deformation on the fiber and matrix to form chips. The second cutting zone is the cutting edge element. In the second cutting zone, the cutting edge element mainly exerts downward compression on the fiber and matrix. The third cutting zone occurs on the flank face of the tool. In this zone, the flank face of the tool exerts compression on the machined surface of the material. At the same time, when the flank face of the tool leaves the material, fiber springback occurs.

[0055] In the embodiments described in this invention, chips refer to the thickness of chips formed when the workpiece is cut by the cutting edge. However, unlike metal processing, in CFRP processing, chip formation can be captured by a high-speed camera. However, macroscopically, CFRP chips visible to the naked eye are powder and do not show obvious flakes as in metal processing.

[0056] By definition, material roughness S a It is the arithmetic mean of the height position changes within the evaluation area. If the initial machined surface is taken as the reference, then the cutting height is the result of the combined effect of the three cutting deformation zones.

[0057] (3) Refer to the instruction manual appendix Figure 3 As shown in the figure (AB represents the first cutting region, i.e., the theoretical shear plane formed by the material; F...), s F represents the shear force acting on a material. n This represents the normal force perpendicular to the shear plane. In the first cutting region, AB is the theoretical shear plane, AC is the fiber fracture cross-section, and CB is the direction of debonding between the fiber and the matrix. Therefore, the chip thickness (i.e., depth of cut) and cutting force in this region can be expressed as:

[0058]

[0059] Where h is the cutting width, τ1 is the shear strength of the material perpendicular to the fiber direction, and τ2 is the shear strength of the material along the fiber direction. The shear angle for chip formation, β is the rake face friction angle, γ0 is the tool cutting edge rake angle, and θ is the fiber direction angle; F y1 and F z1 These are the feed resistance and depth of cut resistance obtained after decomposing the cutting force of the tool in the first cutting region, respectively.

[0060] In the embodiments described in this invention, the cutting force model of the first cutting region belongs to the right-angle (orthogonal) cutting model. In right-angle cutting, it is assumed that the cutting edge of the tool is perpendicular to the cutting speed direction. Therefore, the decomposed cutting force consists only of the feed resistance along the cutting speed direction and the depth of cut resistance perpendicular to the material surface.

[0061] (4) Within the second cutting region, the cutting edge element exerts a squeezing effect on the fiber and matrix. Therefore, theoretically, the roughness component of the second cutting region is equal to the radius of the cutting edge element of the tool, such as... Figure 4As shown in the figure (b1 represents the cutting area near the rake face of the tool blunt circle, where the force mainly comes from the pressure exerted on the workpiece by the tool blunt circle cutting; b2 represents the cutting area near the flank face of the tool blunt circle, where the force mainly comes from the friction generated by the tool blunt circle contacting the workpiece; P1 and P2 represent the cutting forces experienced by the tool in these two areas, respectively). In the second cutting area, the fiber fracture depth L is mainly related to the tool blunt circle radius, fiber orientation angle, and cutting speed. However, the energy required for fiber fracture comes from the work done by the cutting force. According to the definition of mechanical work, without considering other losses, the work done by the cutting force is equal to the energy required for fiber fracture. Therefore, considering the fiber fracture depth under different orientation angles, this invention establishes the following equation based on the work-energy relationship.

[0062] P·cosθ=L·d·ξ+C·4πL 2 υ+r e ·(1-Cd)·ζ Equation (3);

[0063] Furthermore,

[0064]

[0065] Where P is the resultant force in the second cutting region, L is the fiber fracture depth, C is the carbon fiber distribution density, ξ is the bonding strength between the fiber and the matrix, υ is the fiber fracture strength, ζ is the matrix fracture strength, and d is the diameter of the carbon fiber.

[0066] In the embodiments described in this invention, L represents the fiber breakage depth, as shown in the appendix to the specification. Figure 5 As shown, taking a 90° fiber as an example, in this case, the fiber fracture depth is the same as the vertical distance from the actual fiber fracture surface to the cutting edge element.

[0067] Furthermore, due to the differences in fiber orientation angles in carbon fiber composites, the removal performance of the cutting element varies depending on the fiber orientation. Figure 5 As shown. Therefore, in the second cutting region, after the tool rounds off, the height of material removed in the second deformation cutting region is not the theoretical r. e In addition, the actual fracture depth of the fiber must also be added:

[0068]

[0069] Where, r e Let be the radius of the tool's blunt circle.

[0070] (5) Due to the compressive force exerted by the cutting tool on the material during the cutting process, the material will undergo elastic deformation to a certain extent after the tool has cut. During the machining process, the resultant force N in the third cutting region is:

[0071]

[0072] Furthermore, the contact length within the third cutting region can be approximately expressed as:

[0073]

[0074] Therefore, the resultant force N in the third cutting region can be obtained as:

[0075]

[0076] Furthermore, the resultant force N in the third cutting region is decomposed, yielding the following calculation expression:

[0077]

[0078] In the above expression, k, H, r e E3 and E3 are the cutting coefficient, material hardness, cutting edge radius, and effective elastic modulus, respectively. a, α, and h are the contact length, tool clearance angle, and material cutting width within the third cutting region, respectively. h can be taken as half of the tool's feed per revolution. F z3 and F y3 These are the depth-of-cut resistance and feed resistance within the third cutting region, respectively.

[0079] Furthermore, by combining empirical formulas, the fiber springback height b in the third cutting region is calculated. c The calculation expression is as follows:

[0080]

[0081] (6) The closer the tool is to the center of the drill bit, the smaller the rake angle and the larger the clearance angle. According to the differential equation, F of the cutting zone... z1 F z2 F z3 and F y1 F y2 F y3 The expression for is known, and according to the method of the present invention, as shown in the appendix. Figure 7 Based on the average cutting force collected in the orthogonal experiment and the tool angle combined with the material, the key parameters such as τ1, τ2, ξ, υ and ζ in the above formulas are determined, and finally a complete expression is formed. The calculation results are shown in Table 1 below.

[0082]

[0083] Among them, FS x and FS y These are the actual tangential forces measured during the experiment.

[0084] In the above partial differential equation, α refers to the tool clearance angle, and γ refers to the tool rake angle. For a drill bit, FS x and FS y It is the sum of the cutting forces of all cutting elements on the chisel edge and the main cutting edge of the drill bit. However, the rake angle and clearance angle of the cutting edge from the chisel edge to the outer edge inflection point are different and cannot be calculated according to fixed values. Therefore, integration is required.

[0085] In the embodiments described in this invention, by combining the measured tangential force obtained during the experiment, the key parameters in the formula, such as k (cutting coefficient), H (material hardness) and E3 (effective elastic modulus), can be solved by the method of undetermined coefficients. Specifically, different process parameters (rotation speed, feed) are used to measure the force and material surface roughness, and the results are calculated in Matlab.

[0086] Table 1 shows the values ​​of each parameter in the formula.

[0087]

[0088] (7) From the cutting trajectory of the tool during drilling, there are two factors that ultimately affect the roughness of the hole wall: one is related to the outer circle inflection point of the drill bit, and the other is the fiber angle (the fiber fracture depth L in the second deformation zone is mentioned). Based on the force measured during the experiment and the constant calculated in the differential equation, the force at the outer circle inflection point of the tool is determined. The cutting speed at the outer circle inflection point is the highest, the cutting conditions are relatively good, and a high cutting efficiency can be achieved. Therefore, the tangential force at the outer circle inflection point accounts for a relatively high proportion of the total cutting force. The radial force may have a smaller impact than the tangential force. The radial force accounts for about 50% of the total cutting force, while the radial load may account for a smaller proportion, about 20% to 30%. Based on the Fx and Fy collected during the drilling process, multiplying them by the corresponding coefficients gives the force at the outer circle inflection point. The cutting force distribution in the first cutting deformation zone generally accounts for 70% of the total cutting force, the second cutting deformation zone accounts for 20%, and the third cutting deformation zone accounts for 10%. Based on the above research, the cutting force distribution at the outer circle inflection point satisfies:

[0089]

[0090] Furthermore, the cutting surface height hc is calculated based on the distribution of Fx and Fy at the inflection point of the outer circle:

[0091] h c =|r e +Lb c | Equation (6);

[0092] Furthermore, combining equation (4) and equation (11), we can obtain:

[0093]

[0094] because Then we have:

[0095]

[0096] The layup angles of CFRP are mainly 0°, 45°, 90° and 135°. The fracture depth L is different at each angle, so theoretically, the surface height can be obtained at the four fiber angles.

[0097] 3D roughness S a The definition is the arithmetic mean of the heights at all points on the surface. Assuming that within a region with a period of four ply angles, S... a This is the arithmetic mean of the surface height under four fiber angles. However, in practice, CFRP often uses symmetrical layups, and the number of layers within its symmetrical period is not necessarily four. It depends on the specific material properties. Furthermore, the cutting force fluctuates during the cutting process, leading to different fracture depths and resulting in different surface morphologies, even at the same angle. Therefore, the following formula S... a This represents the number of ply passes traversed by the tool within a time interval ΔT.

[0098]

[0099] Where h1 is the height of the initial plane from the processed surface at time t1, and h2 is the height of the initial plane from the processed surface at time t2.

[0100] This invention establishes a functional relationship between the cutting force signal and the hole wall cutting height through theoretical derivation. The force signal at time t1 corresponds to the height h1, and the force signal at time t2 corresponds to the height h2. Based on roughness positioning, roughness S... a To evaluate the arithmetic mean of the heights of all points within the evaluation area, if it is necessary to evaluate the roughness between h1 and h2 (corresponding to time ΔT), the height values ​​within time ΔT should be linearly integrated and then averaged to obtain the final roughness prediction value.

[0101] (8) After determining the parameters in each formula, a twist drill bit specifically designed for composite materials was used to conduct hole-making tests on CFRP. A total of 80 holes were made in the tests, and the F values ​​during the drilling process were recorded and obtained. x F y F z The values ​​of the three forces, such as Figure 7 As shown.

[0102] (9) Based on the measured F x F y F zSubstituting the triaxial force values ​​into the formula, the three-dimensional roughness S of the hole wall is predicted. a。 After the experiment, the material was cleaned with alcohol. After cleaning, the hole wall was cut open, and the three-dimensional roughness S of the hole wall was measured using an optical measuring instrument. a The measurements were compared with the predicted values. The measurement errors at the drilling of the 20th, 40th, 60th, and 80th holes were 8.07%, 5.43%, 0.59%, and 1.10%, respectively. The results are shown in [Figure number missing]. Figure 8 and Figure 9 As shown, before the tool is fully worn, the error between the predicted hole wall roughness and the actual hole wall roughness is controlled within 10%.

[0103] In summary, the method of the present invention enables rapid prediction of the roughness of the material hole wall when the cutting force is known. As the number of holes made by the tool increases, the prediction accuracy also increases. The reason for this is that when the tool is worn appropriately, the resin coating phenomenon on the hole wall surface is reduced, thus improving the prediction accuracy.

[0104] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for predicting the three-dimensional roughness of hole walls in CFRP machining, characterized in that, Includes the following steps: Step S1. Establish the cutting force-cutting height equation through the cutting force equation to obtain the functional relationship between load and surface height; Step S2. Determine the coefficients in the function expression of step S1 through orthogonal cutting experiments; Step S1 specifically includes the following steps: Step S1.

1. Combine the cutting force equation and determine the cutting depth of the first deformation cutting zone through inverse transformation; Step S1.

2. Combine the cutting force equation and determine the fiber fracture depth in the second cutting region through inverse transformation; Step S1.

3. Determine the fiber springback height in the third cutting zone using empirical formulas; Step S1.

4. Establish the final cutting force-roughness equation by using the partial differential equation relationship between the tool rake angle and clearance angle and the cutting force, as well as the fiber fracture depth in the second cutting zone and the fiber springback height in the third cutting zone. In step S1.2, the calculation expression for the fiber fracture depth in the second cutting region is as follows: Equation (3); Furthermore, the depth of cut in the second cutting region is as follows: Equation (4); in, The resultant force in the second cutting region, L This represents the fiber fracture depth. The carbon fiber distribution density, The bonding strength between the fiber and the matrix. ζ represents the fiber breaking strength, and ζ represents the matrix breaking strength. The diameter of the carbon fiber; r e The radius of the tool's blunt circle; In step S1.3, the depth of cut resistance and feed resistance in the third cutting region are as follows: Equation (5); in, k , H , r e and E The three factors are the cutting coefficient, material hardness, cutting edge radius, and effective elastic modulus. a , α and h These are the contact length between the tool's flank face and the workpiece, the tool's clearance angle, and the material cutting width within the third cutting region; and F y3 These are the depth-of-cut resistance and feed resistance within the third cutting region, respectively. N The resultant force within the third cutting region; Furthermore, based on empirical formulas, the fiber springback height within the third cutting region is calculated. b c : Equation (11); In step S1.4, the calculation expression for the height of the cutting surface is as follows: Equation (6); Furthermore, based on the height of the cutting surface, the following cutting force-roughness equation is established: Equation (7); in, hc The height of the cutting surface. for The initial plane distance from the machined surface at any given time is the height of the distance between the two surfaces. for The initial plane distance is the height of the machined surface at any given time.

2. The method for predicting the three-dimensional roughness of CFRP machined hole walls according to claim 1, characterized in that, In step S1.1, the calculation expression for the cutting depth of the first cutting zone is as follows: Equation (1); Equation (2); in, h The material cutting width, τ 1 represents the shear strength of the material perpendicular to the fiber direction. τ 2 represents the shear strength of the material along the fiber direction. φ To form the cutting angle of the chip, β γ is the friction angle of the rake face, and γ0 is the rake angle of the cutting edge of the tool. θ It is the fiber orientation angle; F y1 and F z1 These are the feed resistance and depth of cut resistance obtained after decomposing the cutting force of the tool in the first cutting region, respectively.

3. The method for predicting the three-dimensional roughness of CFRP machined hole walls according to claim 1, characterized in that, The tool's rake face and the chip form the first cutting zone.

4. The method for predicting the three-dimensional roughness of CFRP machined hole walls according to claim 1, characterized in that, The cutting edge element is the second cutting zone.

5. The method for predicting the three-dimensional roughness of CFRP machined hole walls according to claim 1, characterized in that, The flank face of the tool is the third cutting zone.

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