Characteristics and application of three traditional CAD modeling models.

1. wireframe model: wireframe model is a modeling method consisting of a series of spatial lines, arcs and points, which is used to describe the outline of products.

Features are:

1) is simple in structure, small in data storage and easy to generate models.

2) It is easy to generate three views and perspective views.

3) When the shape of a part is complex, there are many wireframes, and the graphics are difficult to identify, which will lead to ambiguity.

4) It is difficult to calculate the geometric characteristics of objects (such as weight, center of gravity and moment of inertia, etc.). )

2. Surface model: A surface model is a modeling method that defines the faces between wireframes of a wireframe model.

Advantages of surface model: 1) cross section can be generated and hidden; 2) Information needed for NC machining can be obtained.

Disadvantages of surface model: it is not clear which side of the boundary the solid part is on.

3. Solid model: A solid model consists of basic voxels with a certain volume. It can completely describe the geometric characteristics of objects and has certainty.

1 boundary representation mode: a shape is divided into some bounded subsets called "faces" according to its boundaries, and each face can be represented by its edges and vertices.

Advantages: convenient deformation operation

Disadvantages: the definition of entity is not easy to change.

2. Construct the representation mode of solid geometric tree: This mode can be regarded as an ordered binary tree, and the internal nodes of the tree are gradually synthesized by voxel set operation and position transformation.

1) This representation method has few voxels, and it is easy to change the defined entity and calculate the geometric parameters such as the volume and surface area of the object.

2) It is the most important representation of three-dimensional finite element analysis.

3. Sweep-and-scrape representation mode: this mode represents an entity through the trajectory swept by a surface or entity in spatial motion.

1) transforms the problem of representing three-dimensional objects into the problem of representing two-dimensional sets.

2) Translating and scraping to generate planar parts.

3) Rotary scraping generates rotary parts.

4) Boundary representation is used as the input means of two scanning transformation forms.