Part Module/cn

FreeCAD 中的 CAD 功能以 OpenCasCade 为核心. 零件模块允许 FreeCAD 使用 OpenCasCade 的对象和函数. OpenCasCade 是一个专业级的 CAD 核心，它具有先进的三维几何操作和对象. 零件对象不同于 Mesh Module 对象，它比网格对象复杂的多，因此允许更加先进的操作，如相干布尔运算操作、修改历史和有关参数的操作.



FreeCAD中零件形状的例子

The tools
当你加载零件模块之后，将会出现“零件”菜单，零件模块工具就被放置在这个菜单中.

Boolean Operations


An example of union (Fuse), intersection (Common) and difference (Cut)

Explaining the concepts
In OpenCasCade terminology, we distinguish between geometric primitives and (topological) shapes. A geometric primitive can be a point, a line, a circle, a plane, etc. or even some more complex types like a B-Spline curve or surface. A shape can be a vertex, an edge, a wire, a face, a solid or a compound of other shapes. The geometric primitive are not made to be directly displayed on the 3D scene, but rather to be used as building geometry for shapes. For example, an edge can be constructed from a line or from a portion of a circle.

We could say, to resume, that geometry primitive are "shapeless" building blocks, and shapes are the real spatial geometry built on it.

To get a complete list of all of them refer to the OCC documentation and search for Geom_Geometry and TopoDS_Shape. There you can also read more about the differences between geometric objects and shapes. Please note that unfortunately the OCC documentation is not available online (you must download an archive) and is mostly aimed at programmers, not at end-users. But hopefully you'll find enough information to get started here.

The geometric types actually can be divided into two major groups: curves and surfaces. Out of the curves (line, circle, ...) you can directly build an edge, out of the surfaces (plane, cylinder, ...) a face can be built. For example, the geometric primitive line is unlimited, i.e. it is defined by a base vector and a direction vector while its shape representation must be something limited by a start and end point. And a box -- a solid -- can be created by six limited planes.

From an edge or face you can also go back to its geometric primitive counter part.

Thus, out of shapes you can build very complex parts or, the other way round, extract all sub-shape a more complex shape is made of.

Scripting
The main data structure used in the Part module is the BRep data type from OpenCascade. About all contents and object types of the Part module are now available to python scripting. This includes geometric primitives, such as Line and Circle (or Arc), and the whole range of TopoShapes, like Vertexes, Edges, Wires, Faces, Solids and Compounds. For each of those objects, several creations methods exist, and for some of them, especially the TopoShapes, advanced operations like booleans union/difference/intersection are also available. Explore the contents of the Part module, as described in the FreeCAD Scripting Basics page, to know more.

Examples
To create a line element switch to the Python console and type in: import Part,PartGui doc=App.newDocument l=Part.Line l.StartPoint=(0.0,0.0,0.0) l.EndPoint=(1.0,1.0,1.0) doc.addObject("Part::Feature","Line").Shape=l.toShape doc.recompute

Let's go through the above python example step by step: import Part,PartGui doc=App.newDocument loads the Part module and creates a new document l=Part.Line l.StartPoint=(0.0,0.0,0.0) l.EndPoint=(1.0,1.0,1.0) Line is actually a line segment, hence the start and endpoint. doc.addObject("Part::Feature","Line").Shape=l.toShape This adds a Part object type to the document and assigns the shape representation of the line segment to the 'Shape' property of the added object. It is important to understand here that we used a geometric primitive (the Part.line) to create a TopoShape out of it (the toShape method). Only Shapes can be added to the document. In FreeCAD, geometry primitives are used as "building structures" for Shapes. doc.recompute Updates the document. This also prepare the visual representation of the new part object.

Note that a Line can be created by specifiying its start and endpoint directly in the constructor, for ex. Part.Line(point1,point2) or we can create a default line and set its properties afterwards, like we did here.

A circle can be created in a similar way: import Part doc = App.activeDocument c = Part.Circle c.Radius=10.0 f = doc.addObject("Part::Feature", "Circle") f.Shape = c.toShape doc.recompute Note again, we used the circle (geometry primitive) to construct a shape out of it. We can of course still access our construction geometry afterwards, by doing: s = f.Shape e = s.Edges[0] c = e.Curve Here we take the shape of our object f, then we take its list of edges, in this case there will be only one because we made the whole shape out of a single circle, so we take only the first item of the Edges list, and we takes its curve. Every Edge has a Curve, which is the geometry primitive it is based on.

Head to the Topological data scripting page if you would like to know more.