STL (stereolithography) was founded by 3D system software company. It was initially used in the early stereolithography machine to determine the closed polyhedron with different Z values. It is a file format representing triangular mesh. Now it is widely used in rapid prototyping process and computer-aided manufacturing. The essence of this method is to use triangle patches to approximate the model. Each triangle edge is just a part of the second triangle, and there is no intersection between them. The definition of each triangle patch includes the three-dimensional coordinates of each fixed point of the triangle and the normal vector of the triangle patch, which replaces the original model.
(b) Chord tolerance 0.003, triangular patch 814858
STL format has inherent disadvantages
(1) The contour approximated by triangle patch can’t fully express the actual surface of the model, so the size error can’t be avoided.
(2) In STL format, in order to achieve more accurate model fitting effect, smaller triangular patches are needed, which increases the amount of calculation data.
(3) In the STL format process, when the curvature of the surface changes greatly, the triangular patches can not approach or even overlap, which makes the jagged concave Kang appear.
STL format files are mainly stored in two ways, one is ASCII format, the other is binary data format. In terms of storage space, the length of binary data format file is much smaller than the former, which is only 1 / 6 of the former, so the appropriate storage mode is selected according to the size of the model. In the process of transforming STL file format by 3D software, the setting of specific parameters (chord tolerance, error, angle tolerance, etc.) also has an impact on the accuracy of the model. According to the generation principle of STL format, when the numerical value is within the set range, the smaller the numerical value is, the higher the corresponding accuracy value will be, and the more triangles will be divided, the closer the triangular patch will be to the actual size of the model, and the more accurate the corresponding model will be, as shown in the figure. Therefore, for different parts on the size accuracy and rapid prototyping equipment on STL data size requirements, generally set in the case of meeting the basic requirements.