Difference between revisions of "Part Module"
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Revision as of 09:06, 6 December 2018
The solid modelling capabilities of FreeCAD are based on the Open Cascade Technology (OCCT) kernel, a professional-grade CAD system that features advanced 3D geometry creation and manipulation.
The Part Workbench allows the user to access and use the OCCT objects and functions. Part objects, unlike Mesh objects, are more complex, and therefore permit more advanced operations like coherent boolean operations, modifications history, and parametric behaviour.
The tools are all located in the Part menu.
An example of fusion (union), intersection (common) and difference (cut) of solid shapes
OCCT geometric 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 a surface. A shape can be a vertex, an edge, a wire, a face, a solid or a compound of other shapes. The geometric primitives 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.
In summary, geometry primitives are "shapeless" building blocks, while topological shapes are the real objects built on them.
A complete list of all primitives and shapes refer to the OCC documentation (Alternative: sourcearchive.com) and search for Geom_* (for geometric primitives) and TopoDS_* (for shapes). There you can also read more about the differences between them. Please note that the official 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 counterpart.
Thus, out of shapes you can build very complex parts or, the other way round, extract all sub-shapes a more complex shape is made of.
The main data structure used in the Part module is the BRep data type from OpenCascade. Almost all contents and object types of the Part module are available by 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 creation methods exist, and for some of them, especially the TopoShapes, advanced operations like boolean union/difference/intersection are also available. Explore the contents of the Part module, as described in the FreeCAD Scripting Basics page, to know more.
To create a line element switch to the Python console and type in:
import Part,PartGui doc=App.newDocument() l=Part.LineSegment() 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.LineSegment() 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.
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.LineSegment) 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.
Updates the document. This also prepares the visual representation of the new part object.
Note that a Line Segment can be created by specifying its start and endpoint directly in the constructor, for example Part.LineSegment(point1,point2), or we can create a default line and set its properties afterwards, as 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 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.