CN105160133B - Progressive molding supporter method of formation based on STL triangular plate vertex iso-metric offset - Google Patents

Abstract

The invention discloses a progressive forming support body generation method based on equidistant offset of vertices of STL triangular pieces. The STL model of the outer surface of the part is used to generate the curved surface of the support body model. The direction is equidistantly offset by an original plate thickness, which provides a basis for the rapid and automatic generation of a reasonable support surface during incremental forming. The beneficial effects achieved by the present invention: a reasonable support body surface model can be obtained according to the thickness change of the part, which can effectively ensure the processing quality of the part; the present invention ensures the reasonable gap between the tool head and the support body at different forming angle positions; The STL file and all Z coordinates are equidistantly offset, which is easy to operate and easy to use; reasonable support body surface design can improve the processing quality of incremental forming parts, and is conducive to the popularization and use of incremental forming technology.

Description

Generation method of incremental forming support based on equidistant offset of vertices of STL triangular sheet

technical field

The invention relates to a method for automatically generating a support body model required for progressive forming of a sheet metal part, in particular to a method for generating a support body for progressive forming based on equidistant offset of vertices of STL triangular pieces.

Background technique

Sheet metal incremental forming is a moldless flexible forming technology proposed by Leszak of the United States in the 1960s. In the 1990s, Matsubara Shigeo of Japan conducted further research on this technology, which gradually attracted the attention of scholars from all over the world. Incremental forming technology is an advanced manufacturing technology based on computer technology, numerical control technology and plastic forming technology. It uses the idea of "layered manufacturing" of rapid prototyping technology to discretize the three-dimensional model into a series of two-dimensional contour shapes. A flexible moldless 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, and high efficiency. The required forming force is small, the equipment has low energy consumption, low vibration, and low noise. It is green. It is a sheet forming technology that has developed rapidly in recent years.

The overall flow of metal materials in progressive forming is not obvious, and the change of material thickness follows the principle of cosine law thinning, that is, t=t ₀ *cosθ (forming angle θ is the angle between the normal vector of the processing position and the Z axis), so when the surface of the workpiece When it is a curved surface, the parts with different forming angles have different thicknesses. As shown in Figure 1, in positive forming processing, assuming that the gap between the tool head and the support body is Δ, and the wall thickness of the workpiece is Δs, then when Δ<Δs, the tool head and the support body will Strong extrusion results in excessive thinning of the sheet metal, excess material is extruded from around the tool head and accumulated in front of the forming tool, causing severe scratches on the surface of the workpiece; when Δ>Δs, the sheet material and the support body cannot Fitting, the part is prone to demoulding, shaking and instability, which will cause local wrinkles; only when Δ=Δs, the sheet metal and the support body are closely bonded, and the outer periphery of the blank may not be squeezed. Therefore, a reasonable determination of the gap Δ will be It becomes the key to improving the surface quality of workpieces.

Regarding the determination of the contour size of the surface of the support in incremental forming, Zhu Hu and other scholars proposed an algorithm for offsetting the STL model of the part by unequal distances. and offset distances are grouped according to their differences; then in each group, the weighted calculation method is used to calculate the offset vector and offset distance of the vertices, and the vertices are biased towards the offset direction of each group; finally, the triangular network is used to The grid fills the gaps generated by the multi-directional offset of vertices. Although this algorithm has some innovations, there is a lot of uncertainty in the grouping of triangle slices. In addition, a weighted algorithm is used in each group to obtain the vertex offset vector. The weighted value The size is not easy to determine; finally, because the vertices are biased in multiple directions, the topological relationship of the triangular mesh vertices will be changed when the triangular mesh is used to fill the gap caused by the bias in the later stage, and there is a big difference in surface fitting. Certainty, the author's own experiments have also proved that there is a large error between the actual value and the theoretical value of the support body gap obtained according to this algorithm; at the same time, the algorithm is relatively complicated, and it is very difficult to operate in practical applications.

Hirt, Zhou Ru, Mao Feng and many other scholars believe that progressive forming is mainly shear deformation, and the change of workpiece thickness follows the cosine law δ=δ ₀ cosθ(θ is the forming angle, that is, the normal vector at this position and the Z-axis clamp angle; δ ₀ is the original thickness of the plate). As shown in Fig. 2, the ABCD area of the original sheet is sheared and deformed by the tool head into the A ₁ B ₁ C ₁ D ₁ area of the workpiece, although the thickness of the sheet in the deformed area changes from δ ₀ to is δ, but according to the principle of shear deformation, the thickness δ ₀ of each position in the deformation zone along the Z-axis direction does not change, so the support body can be regarded as offset by the upper surface of the part model along the negative direction of the Z-axis by one The original plate thickness δ ₀ is obtained.

The STL file format is an interface protocol formulated by 3D SYSTEMS in 1988. It is a three-dimensional graphics file format serving rapid prototyping technology. It is now widely used in scientific computing visualization, animation, virtual reality and rapid prototyping technologies. middle. The STL file is a collection of triangle faces to represent the outline model of the object, and the definition of each triangle face includes the three-dimensional coordinates of the vertices and the normal vector of the triangle face.

Contents of the invention

In order to solve the deficiencies in the prior art, the object of the present invention is to provide a progressive forming support body generation method based on the equidistant offset of the vertices of the STL triangular piece, which adopts the STL model of the outer surface of the part to generate the support body curved surface model, and uses each The vertices of the triangular facets are equidistantly offset by an original plate thickness along the negative direction of the Z axis, which provides a basis for the rapid and automatic generation of a reasonable support surface during incremental forming.

In order to achieve the above object, the present invention adopts the following technical solutions:

A kind of progressive forming support body generation method based on STL triangular sheet vertex equidistant offset, it is characterized in that, comprises the steps:

1) Determine the main direction of the progressive forming of the part, so that the forming angle θ of each position does not exceed the forming limit angle θ _max of the plate;

2) Extract the upper surface of the part model and generate the corresponding STL format file;

3) Simplify and merge the redundant vertex data and redundant edges of the STL file;

4) Check the validity of the STL model data and the closure of the STL model;

5) According to the generated STL file, read the vertex coordinates (x _i1 , y _i1 , z _i1 ), (x _i2 , y _i2 , z _i2 ), (x _i3 , y _i3 , z _i3 ) of each triangle surface, and keep The _x and y coordinate values of the vertices of the triangular sheet remain _unchanged , and the z coordinate value is offset along the negative direction of the Z axis by an original plate thickness _{δ 0} . ₀ ), (x _i2 ,y _i2 ,z _i2 -δ ₀ ), (x _i3 ,y _i3 ,z _i3 -δ ₀ );

6) According to step 5), the surface fitted by the STL file generated by biasing is used as the upper surface of the support body model, and the support body solid model is generated, and the support body is processed.

The aforesaid incremental forming support body generation method based on the equidistant offset of the vertices of the STL triangular piece is characterized in that, in the step 2), the STL format file is generated on the CAD platform with a certain triangle tolerance and adjacent tolerance precision; the selected The triangular and adjacent tolerances are selected based on empirical values.

The aforementioned incremental forming support body generation method based on the equidistant offset of the vertices of the STL triangular piece is characterized in that, in the step 4), the validity inspection content includes whether there are cracks, geometric defects of the isolated edge in the model; The content includes whether all STL triangles form an inner and outer closed geometry.

The beneficial effects achieved by the present invention: a reasonable support body surface model can be obtained according to the thickness change of the part, which can effectively ensure the processing quality of the part; the present invention ensures a reasonable gap between the tool head and the support body at different forming angle positions; The STL file and all Z coordinates are equidistantly offset, which is easy to operate and easy to use; reasonable support surface design can improve the processing quality of incremental forming parts, and is helpful for the promotion and use of incremental forming technology.

Description of drawings

Fig. 1 is a schematic diagram of a support body;

Fig. 2 is a schematic diagram of progressive forming thickness shear deformation;

Figure 3 is a schematic diagram of the apex offset of the triangular sheet.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings. The following examples are only used to illustrate the technical solution of the present invention more clearly, but not to limit the protection scope of the present invention.

The invention relates to a method for generating a progressively formed support body based on the equidistant offset of the vertices of the STL triangular piece, comprising the following steps:

1) Determine the main direction of the progressive forming of the part, so that the forming angle θ of each position does not exceed the forming limit angle θ _max of the sheet.

2) Extract the upper surface of the part model and generate the corresponding STL format file; the STL format file is generated on the CAD platform with a certain triangular tolerance and adjacent tolerance accuracy; the selected triangular tolerance and adjacent tolerance are selected based on empirical values.

3) Simplify and merge the redundant vertex data and redundant edges of the STL file.

4) Check the validity of the STL model data and the closure of the STL model; the validity inspection includes whether there are cracks in the model and geometric defects of isolated edges; the closure inspection includes whether all STL triangles form a closed geometry inside and outside .

5) According to the generated STL file, read the vertex coordinates (x _i1 , y _i1 , z _i1 ), (x _i2 , y _i2 , z _i2 ), (x _i3 , y _i3 , z _i3 ) of each triangle surface, and keep The _x and y coordinate values of the vertices of the triangular sheet remain _unchanged , and the z coordinate value is offset along the negative direction of the Z axis by an original plate thickness _{δ 0} . ₀ ), (x _i2 ,y _i2 ,z _i2 -δ ₀ ), (x _i3 ,y _i3 ,z _i3 -δ ₀ ). After the calculation in this step is completed, the topological relationship between the biased triangles remains unchanged.

6) According to step 5), the surface fitted by the STL file generated by biasing is used as the upper surface of the support body model, and the support body solid model is generated, and the support body is processed.

An example of an embodiment is shown in Figure 3: assuming that the external normal vector of any triangular piece is n _i , and its three-way components in x, y, and _z are respectively n _ii , nij , and ni _ik , then according to the right-handed spiral rule, the coordinates of the vertices of the triangular piece There is the following relationship with the external normal vector component:

It can be seen that when any triangular surface is equidistantly offset along the negative direction of the Z axis, v _ix and v _iy (i=1, 2, 3) in the coordinates of the vertices of the triangular surface remain unchanged, and only v _iz (i=1 , 2, 3) are unified into (ν _iz -δ ₀ ), it can be seen that the normal vector of the triangular piece after offset will remain unchanged, so it can also be regarded as the whole Equidistant offset, if the angle between the normal vector of the triangular piece and the positive direction of the Z axis is regarded as the forming angle of the position, then the offset distance along the direction of the normal vector of the triangular piece is δ=δ ₀ cosθ, just in line with Incremental forming thickness cosine law. In this way, the equidistant offset of the vertices of each triangular sheet along the Z axis can be used to obtain thickness gaps at different forming angle positions.

The above is only a preferred embodiment of the present invention, and it should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present invention, some improvements and modifications can also be made. It should also be regarded as the protection scope of the present invention.

Claims (3)

1. based on the incremental forming support body generation method of STL triangular plate apex equidistant offset, it is characterized in that, comprises the steps: (1) Determine the main direction of the progressive forming of the part, so that the forming angle θ of each position does not exceed the forming limit angle θ _max of the plate; (2) Extract the upper surface of the part model and generate the corresponding STL format file; (3) Carry out redundant vertex data and redundant edges to the STL file to simplify and merge; (4) Check the validity of the STL model data and the closure of the STL model; (5) According to the generated STL file, read the vertex coordinates (x _i1 , y _i1 , z _i1 ), (x _i2 , y _i2 , z _i2 ), (x _i3 , y _i3 , z _i3 ) of each triangle surface, Keep the x and y coordinate values of the triangular sheet vertices unchanged, and the z coordinate value is offset along the negative direction of the Z axis by an original plate thickness δ ₀ , and the coordinates of any triangular sheet vertices after offset are (x _i1 , y _i1 , z _i1 - δ ₀ ), (x _i2 ,y _i2 ,z _i2 -δ ₀ ), (x _i3 ,y _i3 ,z _i3 -δ ₀ ); (6) The surface fitted by the STL file offset generated in step (5) is used as the upper surface of the support body model to generate a support body solid model and process the support body. 2. the incremental forming support body generation method based on the equidistant offset of the STL triangular plate vertices according to claim 1, it is characterized in that, in the described step (2), the STL format file is on the CAD platform with a certain triangular tolerance, The adjacent tolerance accuracy is generated; the selected triangular tolerance and adjacent tolerance are selected based on empirical values. 3. the incremental forming support body generation method based on the equidistant offset of the vertices of the STL triangular sheet according to claim 1, wherein in the step (4), the validity check content includes whether there are cracks, isolated edges in the model The geometrical defects; the content of the closedness check includes whether all STL triangles form a closed geometry inside and outside.