5 practical methods to improve the machining accuracy of thin-walled parts

Aerospace industry products are commonly used in thin-walled parts, complex structures, and high-precision requirements. Due to poor stiffness, processing deformation caused by cutting force is easy to produce, resulting in wall thickness on the thick, thin size over the phenomenon.

The standard method used in the factory is to cut several times without feed after the last tool travels in finishing and is combined with manual grinding. Although this method can remove most of the residual material, the adult increased machining hours, and machining surface roughness will be significantly reduced [1].

For thin-walled parts processing wall thickness accuracy control problems, Russia uses a particular angle of conical tool processing but can not completely solve the problem; the limitations’ application is also extensive. The current development of high-speed cutting technology can solve the problem of thin-walled parts processing but requires high-speed cutting machine tools or machining centers.

This paper proposes the application of the finite element method to analyze the thin-walled parts due to the deformation caused by the cutting force and calculated machining error, based on which a CNC compensation method: through the overcutting compensation to let the knife error, finishing a tool to achieve the machining accuracy requirements.

1. thin-walled parts machining error

Figure 1 Factors affecting the machining accuracy of parts

CNC machining of thin-walled parts, the workpiece due to lack of rigidity caused by machining deformation has become the principal contradiction affecting dimensional accuracy [2]. Thin-walled frame parts machining simplified schematic diagram in Figure 2.

Figure 2 Thin-walled parts machining schematic

Using an end milling cutter, milling thin-walled surface AB should be removed from the shaded part ABDC, but due to the role of cutting force, so that the thin-walled parts of the elastic deformation, A, C two points were moved to the A’, C’ two points, assuming that the tool stiffness is much greater than the stiffness of the thin-walled part of the tool only to remove the part of the A’BDC material.

After the thin-walled elastic recovery, the residual CDC part of the material is not removed, resulting in a wall thickness processing error. The wall thickness processing overshoot in thin-walled parts is mainly due to the knife cutting a piece of material.

Thus, a new idea of deformation control of thin-walled parts is proposed: the deformation of thin-walled parts is calculated by finite element analysis, and then the residual part of the material is removed using CNC machining compensation.

After the side wall’s elastic recovery, the workpiece’s wall thickness just reaches the tolerance requirement. Therefore, the key to the problem is to solve certain processing conditions of thin-walled parts using deformation analysis and calculation methods.

2. thin-walled parts machining deformation analysis

2.1 Establishment of force model

To solve the problem of controlling the accuracy of machining of thin-walled parts, the force model, deformation model, and CNC compensation model need to be established. The keyword is the establishment of an accurate force model. Based on analysis and verification, this paper establishes a spiral end mill force model in the existing literature.

Figure 3 for the end milling milling force analysis map

The size of the cutting force is related to the cutting thickness [3,4], to facilitate the analysis, the total cutting area is divided into the illustrated loading unit through the calculation of all in the cutting area of each Figure 3 vertical milling force analysis chart unit load, you can get the spatial distribution of force state.

The unit cutting force can be decomposed into tangential and radial cutting forces [5] and has the following relationship.

DFT is the unit tangential cutting force, DFR is the unit radial cutting force, DZ is the axial width of the load unit, and tc is the unit thickness: KT and KR are constants derived from experiments. It can be approximated as a function of tK for the average unit thickness tc; for a particular cutting parameter, KT, KR is constant.

According to the experimental data, it is assumed that the relationship between KT, KR, and the feed per tooth fz, axial depth of cut αρ, and radial depth of cut αe can be described by a polynomial model.

This paper solves the polynomial model coefficients by average cutting force tests. The test is carried out on a Mikron ucp710 machining center; the tool is a cemented carbide monoblock end mill, and the material is aluminum alloy LY12CZ. All coefficients can be solved by regression analysis.

2.2 Typical thin-walled structure

To illustrate the problem, this paper selects the thin-walled box as the typical structure of thin-walled parts (see Figure 4). Workpiece material 7075-T6, Young’s modulus fz– 77 GPa, density P- 2.78×10°kg/m, Poisson’s ratio γ= 0.33

2.3 Some basic assumptions

(1) To simplify the model and calculation, assuming that the milling thickness is significantly smaller than the wall thickness of the processed material, the modeling can be ignored when the wall thickness difference between the surface of the processed surface and the surface to be processed, according to the calculation of equal wall thickness.

Figure 4 Typical structure and size

(2) Assuming that the entire process is in the elastic range of the material

(3) Assume that the material is completely isotropic.

2.4 Working conditions and analysis

As shown in Fig. 5, a distributed load along the helix direction is applied to the frame wall at a particular instant when machining the frame wall with a spiral end mill. Because the tool speed is much larger than the feed speed, it can be assumed that the tool axis is fixed in a particular feed position. The cutter teeth move from the bottom up along the A direction until the workpiece is cut out, forming the machined surface at the axis [6,7].

Fig. 5 Spatial distribution of instantaneous radial force

In actual machining, because the radial depth of cut is small, the distribution of loads in the cut-in region of Fig. 5 can be approximated to the mesh nodes, and the deformation of each part can be easily and quickly calculated using finite element analysis software. For illustration, the following scheme is taken as an example.

Scenario 1 analyzes the deformation of different wall thicknesses in the x-direction under the same loading conditions. The specific conditions are a radial depth of cut of 0.4 mm, an axial depth of cut of 24 mm, a rotational speed of 6000 r/m, feed of 0.04 mm per tooth, and wall thicknesses of 1.5 mm, 2 mm, and 3.0 mm, respectively.

Scenario 2 analyzes the frame’s deformation at different positions (x=25, x=50) under the same loading conditions and wall thickness. The loading conditions are the same as in Scheme I. The wall thickness is chosen to be 3 mm.

3. Computational analysis

Analyzing thin-walled parts with ANSYS 5.7 is equivalent to simulating the deformation of the workpiece during the milling process on a computer. Vacuum suction cups clamp the thin-walled box and end mills machine on four sides. According to the typical thin-walled parts of the structure to establish a geometric model of the frame, select the type of unit for Elastic 8node93 type, which will be used in the program of machining conditions into the established force model, calculate the force of each node, based on the loading. After pre-analysis, the horizontal force on the deformation of the thickness direction is negligible. For Workpiece mesh division and marking, see Figure 6.

Figure 6 Workpiece mesh map

In Program 1, when the milling cutter is in the x—50 position, Figure 7 shows the workpiece height direction z to 1/2 of the horizontal cross-section of the instantaneous deformation curve (3 wall thickness).

Fig. 7 Displacement of the center of the frame at each node in the x-direction.

It can be seen that the thin wall z direction 1/2 at the midpoint of the most extensive deformation, while at the two ends of the connected sidewall constraints, the deformation is zero. The deformation curves of the middle part of the frame at x-50 (three wall thicknesses) are shown in Fig. 8.

Fig. 8 Deformation of each node along the z-direction at x = 50 (Scheme 1)

The figure can be seen in the same load under the deformation of thin-walled parts and its wall thickness is inversely proportional to the wall thickness, that is, the thinner the wall thickness of the parts in the processing the more likely to produce deformation. As can be seen from the figure, the upper part of the workpiece wall plate deformation, while the bottom of the workpiece is fixed by the vacuum suction cup in the processing of almost no deformation. At the top of the thin-walled parts, the cutting area is sharply reduced to zero because of the tool helix angle, so the deformation is close to zero.

Fig. 9 shows the deformation of each node in the z direction in Scheme II. The deformation at the midpoint (x-50) is larger than that at other positions (x-25).

Fig. 9 Deformation of each node along the z direction (Scheme II)

4. Numerical control compensation scheme

According to the finite element calculation results of the above analysis program, it can be found that, in the milling of a thin-walled frame, due to the action of the cutting force, the frame wall will be elastic deformation, the amount of deformation along the height of the direction of the parabola gradually increases, the amount of deformation in a specific limit can be approximated as a straight line: the deformation along the length of the frame wall at the midpoint of the length direction is greater than the other positions (x-25).

The deformation along the length of the frame wall is more significant at the midpoint but almost zero at the two endpoints.

Under certain machining conditions, the thinner the wall thickness, the more serious the deformation, the greater the error generated. Due to the deformation of the knife phenomenon, resulting in a part of the material being left behind, the formation of the thin thickness of the upper and lower thickness of the thin over difference is fully consistent with the actual processing results.

Finite element analysis can guide the selection of reasonable machining dosage in the finishing process if the tool in the original CNC programming according to the degree of deformation in the trajectory of the tool with an additional deflection to compensate for the deformation of the amount of letting the tool, can be eliminated letting the tool error. Thin-walled parts CNC compensation processing and control of machining accuracy of the process are shown in Figure 10.

Figure 10 Thin-walled parts CNC compensation machining flow chart

5. Conclusion and outlook

(1) machining deformation and machining surface thickness are inversely proportional to the thickness of the other three surfaces because the thinness of the influence is very small; (2) machining deformation and machining surface thickness are inversely proportional to the thickness of the other three surfaces.

(2) Processing deformation along the height direction of the frame wall is a parabolic change. When the amount of deformation is limited, the maximum amount of deformation at the wall opening is similar to a straight line.

(3) Processing deformation in the length of the frame wall at the midpoint of the maximum, the two endpoints at almost zero, and the three-dimensional deformation of the frame wall saddle-shaped.

(4) Different load distributions will affect the distribution of deformation. When automation, intelligent processing, and the direction of development toward virtual manufacturing are the main components, the finite element calculation method is used to estimate the processing error.

The above preliminary conclusions provide a guiding opinion for the control of wall thickness machining accuracy of thin-walled parts. However, accurate deformation analysis relies on precise load modeling, so it is necessary to establish an accurate milling force model through cutting force experiments; the

Based on an accurate iterative analysis of machining deformation, CNC machining offset compensation can remove most of the residual material of the letting tool.

Further accurate analysis and practical application, but also need to consider the tool deformation, workpiece clamping, cutting temperature, tool wear,, and other factors, through CNC compensation, in the finishing of a go can ensure that the wall thickness accuracy of thin-walled parts to achieve the purpose of high efficiency, economy, and quality processing of thin-walled parts.

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