I. Introduction
Because the computer 3D technology can realistically simulate the virtual reality model, the stereoscopic, luminescent, and colored vivid images express the design results of the products in the brain, which is more in line with the human thinking habits and visual habits than the traditional two-dimensional design drawings. Therefore, the three-dimensional design method has attracted great attention in the technical field, and replacing the two-dimensional design will become the future development trend. The 3D modeling technology has evolved from the original 3D CAD to the current 3D software based on feature modeling. Commonly used are SolidWorks, SolidEdge, MDT, Pro/E and so on. More and more people are starting to learn this kind of software, and a large number of books are also published to learn such software. However, these books generally have such characteristics as the introduction of a variety of commands for a certain software, or for an instance. Specifically, the operation steps do not explain the origin of the operation plan. It seems to be the instruction manual of a certain software, which objectively serves as a guide for beginners to spend a lot of time and energy to learn and adapt to software commands from the beginning. Leading to modeling practice is often blind and empirical. Because of the difficulty in remembering the many commands, it restricts the speed of people's introduction of modern design methods to a certain extent, and this learning method also affects the improvement of modeling skills. Drawing on the learning of traditional cartography, with the guidance of the theory of descriptive geometry, a way to easily learn different drawings such as mechanical drawing and architectural drawing has been formed. Therefore, it is necessary to explore the graphical theory of computer 3D maps and common thinking modes to cope with the learning of different software, to achieve quick start and effective improvement of modeling skills.
Second, the connotation of CAD three-dimensional modeling
The Sino-Japanese Graphic Education Seminar held in July 2001 pointed out that computers are only tools for design and painting, not only to train practical design and drawing skills, but more importantly to cultivate design feelings and spatial feelings. Professor ThruIHARA of Japan also pointed out: "There is a doubt that beginners use computer software to create from the beginning, especially under the bad skill operation, which will cause the lack of geometric feeling and space feeling. This is a focus that must be discussed. "Like two-dimensional drawing, the movement of pencils and compasses is performed according to the instructions issued by the brain when the brain is mature in the concept of graphic drawing. Therefore, the design model generated in the brain is the "psychological model" through computer software. Transformed into a computer virtual model, first in the brain to have a psychological modeling process for the mental model. In the brain, using the theory of graphs, the composition of the mental model and the formation of the form are systematically and accurately described by geometric terms, so that the order of the modeling is the "building blocks". This process can be called the psychological modeling language system, computer interface elements. (menu commands, icons, cursors) leave a memory system in the brain. These two systems are in parallel. When the two systems in the brain achieve correct conversion, the correct operation of the computer can be achieved. Three-dimensional modeling is a process of transforming mental modeling language and computer interface elements. The current study pays attention to the memory of computer interface elements, ignoring the mental modeling ability. Practice has shown that a person's drawing speed and correctness depend on his clarity on the shape of the model being painted in the brain. The relevant 3D educational research in the United States also points out that the most important thing is the activities that occur during the planning and expression phases.
Third, three-dimensional configuration graphics theory
3D software is based on feature-based solid modeling, so it is different from the traditional projection theory of guiding two-dimensional images. The three-dimensional modeling mainly adopts the theory of constructive solid geometry (CSG) and the formation and analysis of geometric features. CSG is an analysis of the overall formation of an entity. Any complex entity can be regarded as a combination of simple unit bodies, which are called voxels. Boolean operations (collection, difference, intersection) are generally used to achieve this combination. Formation analysis is a specific analysis of the formation of each voxel geometry, so the mindset is full three-dimensional and is a three-dimensional simulation of the dynamic psychological activities of the physical processing in the objective world.
Fourth, three-dimensional modeling framework
According to the theory of three-dimensional configuration graphics, there should be a mental framework for mental modeling before using computers.
V. Voxel decomposition
Traditional manual 2D drawings or 2D CAD drawings are drawn with various lines. No matter how the graphics can always be drawn, the importance of this order is not very prominent. The computer solid modeling is a collection of geometric features. The order of the modeling is especially important. It is similar to the processing order of the parts in the objective world. If the parts are not arranged properly, the generation process is too complicated. Conversely, generating parts is simple and convenient. For this purpose, it can be processed in a modular manner to perform voxel decomposition on the shaped body. The principle of decomposition is divided into aspects such as reflecting the obvious degree of the main features of the body and the size of the total volume and its main functions. Generally, it can be divided into the basic feature voxel series, the auxiliary feature voxel series, and the additional characteristic voxel series. Subdivide within each series. The decomposition steps are as follows:
(1) Dividing the basic characteristics of voxel series
The partial combination of the voxels reflects the main physical features and main functions of the entity and accounts for a relatively large proportion. In the series, a number of single voxels are further divided according to the primary and secondary. The most important first individual element that is divided should be the basic characteristic voxel of the configuration, that is, the reference body that generates other voxels.
(2) Division of auxiliary features voxel series
This part of the voxel is added to the basic characteristic voxels and does not play a major role in function, such as ribs, bosses and the like. A separate voxel is then divided within the series. 3 additional feature voxel series. Such voxels have features that cannot exist independently and must be added to the above two voxel series, such as holes, cavities, grooves, and the like. It belongs to the cut and the difference set. The above series are superpositions of voxels, ie, unions. Figure 1 is a voxel exploded view of the gear reduction case. According to this orderly voxel decomposition, the order of "building blocks" is gradually established in the brain. Therefore, the thinking process is the basis for planning the order in which the geometric features of the part are created. That is, the basic feature voxels are determined in the basic feature voxel series, and then the other voxels in the basic feature voxel series are constructed in turn by the union of Boolean operations, and then each of the auxiliary feature voxel series is constructed. The voxels are then subtracted from the voxels in the additional feature voxel series by the difference set operation on the basis of the above construction.
Figure 1 Voxel decomposition diagram of gear reduction box
(III) Analysis of the formation of voxel geometric features
The creation of voxels is an important part of the styling. As long as the voxel features are created successfully, the styling can be completed by building in the above order. The trajectory of the point is the line, the trajectory of the line is the face, and the trajectory of the face is the body. According to this geometric description, most voxels can be regarded as a scanning body formed by the movement of the initial face of a certain geometry along a certain trajectory. For example, the common cuboids and cylinders can be seen as rectangles and circles formed by linear motion, but most of the trajectories are complex. The concept of scanning is shown by the formation of the simple body of Fig. 2, and its formation is divided into initial planes. Formation, formation of scanning trajectories, and determination of scanning direction.
Figure 2 Schematic diagram of the scanning of the shape
Through the above three processes, the formation of geometric features of a voxel can be fully and accurately described. Each individual voxel drawing that is decomposed can be regarded as a local feature creation process. The above detailed thinking steps provide a basis for the planning of the local drawing sequence.
(4) Spatial location analysis of voxels
To create a part feature, you also need to analyze the mutual position of the unit cells that make up the part, that is, the spatial positioning of the voxels. Since each voxel feature is created by using a face of the previous voxel as the datum, it is determined that the datum of the voxel is located and the starting point for drawing the voxel "straw cross section" is equivalent to the reference datum. The geometric positioning of the geometric elements of the face. The thinking process provides the basis for the operation sequence planning in the 3D modeling to determine the starting point for creating each individual elementary geometric feature.
Sixth, the conclusion
Some special 3D modeling softwares that are commonly used at present, although their interface elements are different in form arrangement, operation method and some functions, they are basically the same in the overall design of the modeling, and follow the human-shaped geometric features. The law of understanding. Therefore, it is necessary to explore the thinking mode of 3D modeling, strengthen the systematic training in this aspect, and be able to cope with the learning of different software calmly, and more effectively memorize software commands under the guidance of correct thinking rules, to achieve quick entry and effective improvement of modeling skills. .
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