CN111215519A - Process method for compensating mesh deformation process in incremental forming - Google Patents

Abstract

The invention discloses a process method for mesh hole deformation process compensation in incremental forming, which belongs to the field of sheet metal numerical control incremental forming. The main strain ε _m of sheet metal forming is used to establish a mesh deformation prediction model; according to the mesh deformation prediction model, the initial mesh of the workpiece is designed. The invention provides a process method for compensating the mesh hole deformation process in the incremental forming, which can make the shape and size of the mesh hole of the workpiece after forming meet the requirements for the use of the workpiece.

Description

A process method for process compensation of mesh hole deformation in incremental forming

technical field

The invention relates to a process method for mesh hole deformation process compensation in incremental forming, and belongs to the field of sheet metal numerical control incremental forming.

Background technique

Incremental Sheet Forming (ISF) is a moldless flexible forming technology proposed by Leszak of the United States in the 1960s. In the 1990s, the technology was further studied by Shigeo Matsubara of Japan, which gradually attracted the attention of various countries. scholars' attention. Incremental forming technology is an advanced manufacturing technology based on computer technology, numerical control technology and plastic forming technology. It adopts the idea of "layered manufacturing" of rapid prototyping manufacturing technology to discretize the three-dimensional model into a series of two-dimensional contour shapes. It is a flexible dieless forming technology that obtains the overall shape of the part through the accumulation of local plastic forming. It has the characteristics of high flexibility, low cost, high efficiency, small required forming force, low equipment energy consumption, low vibration and low noise. It belongs to green At the same time, it can greatly improve the forming limit of the plate, and it is an advanced plate forming technology that has developed rapidly in recent years.

Due to the special processing method of incremental forming, the forming tool head thins and draws the original sheet under the action of the forming force, and the material in the forming area is sheared and flowed along the axial direction of the workpiece under the action of the tool head. The thickness will be reduced. Research shows that the law of thinning in incremental forming follows the cosine law δ=δ ₀ cosθ (where δ ₀ is the original thickness of the sheet, θ is the incremental forming angle, and δ is the theoretical thickness at the position of the forming angle θ after forming) , the thickness of the sheet will change with the change of the incremental forming angle θ during the forming process. Because the sheet follows the principle of constant volume during forming, the mesh size of the mesh plate will also follow the incremental forming angle during forming. changes with the change of θ.

At present, Tan Fuxing et al. proposed the research on the incremental forming of the mesh plate with circular holes. Since the axial force on the mesh in the incremental forming is much greater than the tangential force, the initial circular hole will gradually become in the incremental forming process. elliptical hole, and experiments show that the long axis of the elliptical hole will be much larger than the diameter of the initial circular mesh due to the large axial force, while the short axis is only slightly larger than the initial circular mesh due to its small tangential force after forming diameter of the mesh. The aforementioned studies did not consider the deformation of the mesh during incremental forming, which resulted in the deviation of the mesh shape and size after the workpiece was formed. In practical application processing, for example, when a metal mesh plate is made into a skull restoration by incremental forming, since the round hole becomes an irregular oval hole after incremental forming, it will cause uneven attachment and growth of human tissue on the restoration. The condition directly affects the tissue repair and growth of the patient site, and increases the pain of the patient.

SUMMARY OF THE INVENTION

The invention provides a process method for compensating the mesh hole deformation process in the incremental forming, which can make the shape and size of the mesh hole of the workpiece after forming meet the requirements for the use of the workpiece.

In order to achieve the above object, the technical scheme adopted in the present invention is: a process method for mesh hole deformation process compensation in incremental forming, characterized in that it comprises the following steps: according to the incremental forming main direction, processing track, According to the forming force of the mesh plate and the main strain ε _m of sheet metal forming, a prediction model of mesh deformation is established; according to the prediction model of mesh deformation, the initial mesh of the workpiece is designed.

Further, the main direction of the incremental forming of the mesh plate workpiece is determined by using the triangular face off-chip normal vector in the STL file format.

Further, the determination of the processing track of the mesh plate workpiece includes the following steps: according to the main forming direction of the mesh plate workpiece, the mechanical properties and processing parameters of the plate are generated into NC codes, and the NC code is used to determine the mesh plate workpiece. machining track.

Further, the determination of the forming force of the mesh plate includes the following steps: selecting a mesh plate of the same material, and performing incremental forming according to the main forming direction of the mesh plate workpiece and the processing track; Obtain the forming force P of each position of the mesh sheet during the forming process.

Further, the principal strain ε _m of the sheet metal forming is calculated by formula (1):

r₁为被加工板料的中性层处的接触区域半径，

h为板料厚度，Δh为层进给量，r为工具球头半径。Among them, μ is the friction coefficient between the sheet and the surface of the tool head, θ is the incremental forming angle, E is the elastic modulus of the sheet, P is the forming force, S is the area of the contact between the tool head and the sheet projected in the vertical direction,

r ₁ is the radius of the contact area at the neutral layer of the processed sheet,

h is the thickness of the sheet, Δh is the layer feed, and r is the radius of the tool ball head.

Further, the initial mesh hole of the workpiece is an elliptical hole, and designing the initial mesh hole specifically includes the following steps: extracting the data of ε _m in the mesh hole deformation prediction model, calculating the initial mesh hole short axis b; extracting the mesh hole deformation amount. Predict the machining trajectory in the model, and arrange the two end points of the initial mesh long axis a on the machining trajectory.

Further, the initial mesh short axis b is calculated by formula (2):

b=D/(ε _m +1) (2)

Among them, ε _m is the principal strain of sheet metal forming, and D is the diameter of the target circular hole.

Since the design of the existing mesh plate does not consider the size change of the mesh after the sheet is formed, the invention makes up for the traditional mesh design only considering its size from the circle and square, and proposes an efficient mesh size deformation process. The compensation method adopts the establishment of a mechanical model, predicts the strain amount of the mesh after forming in advance, and designs the initial mesh size according to the strain amount. The initial mesh hole is finally formed into a round hole after being formed at different incremental forming angles θ, which is beneficial to the promotion and application of the incremental forming technology in the processing of metal mesh plates.

Description of drawings

1 is a schematic diagram of a method for compensating for a mesh deformation process in an incremental forming process provided in an embodiment of the present invention;

2 is a schematic diagram of the principle of a mesh deformation prediction model in an embodiment of the present invention;

Fig. 3 is the schematic diagram of the force of the unit body in the embodiment of the present invention;

FIG. 4 is a schematic diagram of the processing parameters of the contact area between the sheet material and the tool head in the embodiment of the present invention.

Among them: 1-sheet, 2-tool head, 3-first endpoint, 4-second endpoint, 5-processing track, 6-target round hole.

Detailed ways

In order to better understand the essence of the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings.

The invention discloses a process method for mesh hole deformation process compensation in incremental forming, which is especially suitable for mesh hole design of metal mesh plates, and specifically includes the following steps:

Step 1, establish a mesh deformation prediction model.

1) Determine the main direction of incremental forming of the mesh plate workpiece.

Using the triangular face off-chip normal vector in the STL file format, the main direction of the incremental forming of the mesh plate workpiece is determined.

The surface of the mesh plate workpiece model is discretized into triangular patches in STL format, and the outer normal vector of the position of the triangular patch is obtained by the vertex coordinates of each triangular patch in the STL file. The angle between the outer normal vector of the triangular facet and the positive angle of the Z axis is used as the incremental forming angle of the corresponding position of the surface of the mesh plate workpiece. The appropriate processing position is selected by adjusting the incremental forming angle θ of each position through vector rotation, so as to ensure that the incremental forming angle of each position of the mesh plate workpiece does not exceed the forming limit angle θ _max of the plate. The processing position where the mesh plate workpiece model meets the above conditions is the main direction of the incremental forming of the mesh plate workpiece. As shown in Figure 2, the main direction of the incremental forming of the mesh plate workpiece is the Z-axis direction.

2) Determination of the machining track 5 of the mesh plate workpiece.

According to the main direction of the incremental forming of the mesh plate workpiece, the mechanical properties and processing parameters of the plate are selected, and the NC code is generated, and the processing trajectory 5 of the mesh plate part is determined by the NC code, as shown in Figure 1. Processing parameters include: tool ball nose radius r, layer feed Δh, sheet thickness h, incremental forming angle θ, as shown in Figure 4.

3) The determination of the mesh forming force includes the following steps:

Select the mesh plate of the same material, place it according to the main direction of the incremental forming of the mesh plate workpiece in 1), and place the dynamometer under the sheet. Incremental forming is performed according to the machining trajectory in 2). The forming force P of each position of the mesh plate during the forming process is measured by the force gauge under the mesh plate.

4) Establish a mesh deformation prediction model. According to the main direction of incremental forming, processing trajectory, mesh forming force, main strain ε _m of sheet metal forming and processing parameters of the mesh plate workpiece, a prediction model of mesh deformation is established.

(1) The force of the local forming area in the incremental forming of the workpiece can be simplified as the force of the unit body ΔABC. Both θ in Fig. 2 and θ in Fig. 3 are incremental forming angles.

As shown in Figure 3, let the vertical feed direction of the tool head 2 be the m direction, the normal of the die support surface is the f direction, the total axial force of the unit body in the m direction ∑F _m =0 and the unit body in the f direction The total axial force ∑F _f = 0 can be obtained:

∑F _m =σ _m S _AC -(τ ₀ sinθ)S _AB +(σ _x cosθ)S _AB + μσ _f S _BC =0 (1)

∑F _f =σ _f S _BC -(τ ₀ cosθ)S _AB -(σ _x sinθ)S _AB =0 (2)

Among them: σ _m is the stress along the wall on the AC surface of the unit body; σ _x is the radial stress on the shear plane AB of the unit body; σ _f is the stress generated by the back pressure of the supporting die on the BC surface of the unit body; μ is the friction coefficient between the surface of the sheet 1 and the tool head 2; S _AC is the sheet surface area corresponding to the AC side, S _AB is the sheet surface area corresponding to the AB side, S _BC is the surface area corresponding to the BC side; θ is the incremental forming angle ;τ ₀ is the shear stress on the shear plane, obtained from formula (3):

τ ₀ S=Psinθ (3)

r₁为被加工板料的中性层处的接触区域半径，

h为板料厚度，Δh为层进给量，r为工具球头半径。Among them, P is the forming force, S is the area of the contact between the tool head and the sheet projected in the vertical direction,

r ₁ is the radius of the contact area at the neutral layer of the processed sheet,

h is the thickness of the sheet, Δh is the layer feed, and r is the radius of the tool ball head.

According to the principle of plastic deformation volume invariance, we can get:

ε _m +ε _y +ε _f =0 (4)

Among them, ε _m is the principal strain of sheet forming, that is, the principal strain in the m direction in Fig. 3; ε _y is the sheet circumferential strain, that is, the principal strain in the y direction in the three-dimensional coordinate system xyz in Fig. 2; ε _f is The principal strain of the plate thickness, that is, the principal strain in the f direction in Figure 3.

From the shear deformation assumption, it can be seen that the incremental forming does not undergo circumferential deformation after the sheet is formed:

ε _y = 0 (5)

From the stress-strain formula σ=Eε and the elastic modulus of the mesh plate is unchanged, it can be obtained:

σ _m +σ _f =0 (6)

According to formula (1) to formula (6), the principal strain of sheet metal forming can be obtained:

Among them, E is the elastic modulus of the plate.

Step 2: Design the initial mesh of the workpiece according to the mesh deformation prediction model.

1) Extract the data of ε _m in the mesh deformation prediction model, and calculate the initial mesh short axis b.

b=D/(ε _m +1) (8)

Among them, ε _m is the main strain of sheet metal forming, and D is the diameter of the target circular hole 6 .

2) Extract the processing trajectory in the mesh deformation prediction model, design the workpiece to be formed in UG software and generate the processing trajectory, so that the processing trajectory is projected on a sheet, and the two end points of the initial mesh long axis a are arranged. on the machining path.

a=kD (9)

Among them, k is a constant, and its value ranges from 0.8 to 0.9.

As shown in Figure 1, the designed elliptical holes are arranged on the processing track 5 generated in steps 1 and 2), so that the first endpoint 3 and the second endpoint 4 of the long axis a of the initial mesh are arranged on the processing track 5, to ensure that the short axis b is deformed along the m-axis direction, and finally the target circular hole 6 is formed.

It should be noted that although the present invention has been described in terms of the above-described embodiments, the present invention may have other various embodiments. Without departing from the spirit and scope of the present invention, those skilled in the art can obviously make various corresponding changes and modifications to the present invention, but these changes and modifications should belong to the appended claims of the present invention and their equivalents the scope of protection.

Claims (7)

1. a process method for mesh deformation process compensation in incremental forming, is characterized in that, comprises the following steps: According to the main direction of the incremental forming of the mesh plate workpiece, the processing trajectory, the forming force of the mesh plate and the main strain ε _m of the sheet metal forming, a prediction model of the mesh deformation is established; According to the mesh deformation prediction model, the initial mesh of the workpiece is designed. 2. The process method for mesh deformation process compensation in incremental forming according to claim 1, characterized in that: the main direction of the incremental forming of the mesh plate workpiece is determined by using the triangular face off-chip normal vector in the STL file format. 3. the process method of mesh hole deformation process compensation in the described incremental forming of claim 1, is characterized in that: the determination of the machining track of described mesh plate workpiece comprises the following steps: According to the main forming direction of the mesh plate workpiece, the mechanical properties and processing parameters of the plate are generated into NC codes, and the processing track of the mesh plate workpiece is determined by the NC code. 4. according to the process method of mesh deformation process compensation in the described incremental forming of claim 1, it is characterized in that: the determination of described mesh plate forming force comprises the following steps: Select the mesh plate of the same material, and perform incremental forming according to the main forming direction and processing track of the mesh plate workpiece; The forming force P of each position of the mesh plate during the forming process is measured by the force gauge under the mesh plate. 5. according to the process method of mesh deformation process compensation in the incremental forming of claim 1, it is characterized in that: the main strain ε _m of described sheet metal forming is calculated by formula (1):

Among them, μ is the friction coefficient between the sheet and the surface of the tool head, θ is the incremental forming angle, E is the elastic modulus of the sheet, P is the forming force, S is the area of the contact between the tool head and the sheet projected in the vertical direction,

r ₁ is the radius of the contact area at the neutral layer of the processed sheet,

h is the thickness of the sheet, Δh is the layer feed, and r is the radius of the tool ball head. 6. according to the process method of mesh hole deformation process compensation in the described incremental forming of claim 5, it is characterized in that: described workpiece initial mesh hole is oval hole, and designing described initial mesh hole specifically comprises the following steps: Extract the data of ε _m in the mesh deformation prediction model, and calculate the initial mesh short axis b; Extract the processing trajectory in the mesh deformation prediction model, and arrange the two end points of the initial mesh long axis a on the processing trajectory. 7. The process method for mesh deformation process compensation in the described incremental forming of claim 6, wherein: the initial mesh short axis b is calculated by formula (2): b=D/(ε _m +1) (2) Among them, ε _m is the principal strain of sheet metal forming, and D is the diameter of the target circular hole.

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