The CAD capabilities of FreeCAD are based on the OpenCasCade kernel. The Part module allows FreeCAD to access and use the OpenCasCade objects and functions. OpenCascade is a professional-level CAD kernel, that features advanced 3D geometry manipulation and objects. The Part objects, unlike Mesh Module objects, are much more complex, and therefore permit much more advanced operations, like coherent boolean operations, modifications history and parametric behaviour.

Part example.jpg

Example of Part shapes in FreeCAD

The tools

The Part module tools are all located in the Part menu that appears when you load the Part module.

Primitives

These are tools for creating primitive objects.

  • Part Box.png Box: Draws a box by specifying its dimensions
  • Part Cone.png Cone: Draws a cone by specifying its dimensions
  • Part Cylinder.png Cylinder: Draws a cylinder by specifying its dimensions
  • Part Sphere.png Sphere: Draws a sphere by specifying its dimensions
  • Part Torus.png Torus: Draws a torus (ring) by specifying its dimensions
  • Part CreatePrimitives.png CreatePrimitives: A tool to create various parametric geometric primitives
  • Part Shapebuilder.png Shapebuilder: A tool to create more complex shapes from various parametric geometric primitives

Modifying objects

These are tools for modifying existing objects. They will allow you to choose which object to modify.

  • Part Booleans.png Booleans: Performs boolean operations on objects
  • Part Fuse.png Fuse: Fuses (unions) two objects
  • Part Common.png Common: Extracts the common (intersection) part of two objects
  • Part Cut.png Cut: Cuts (subtracts) one object from another
  • Part JoinConnect.svg Join features: smart booleans for walled objects (e.g., pipes) (v0.16)
    • Part JoinConnect.svg Connect: Connects interiors of objects (v0.16)
    • Part JoinEmbed.svg Embed: Embeds a walled object into another walled object (v0.16)
    • Part JoinCutout.svg Cutout: Creates a cutout in a wall of an object for another walled object (v0.16)
  • Part Extrude.png Extrude: Extrudes planar faces of an object
  • Part Fillet.png Fillet: Fillets (rounds) edges of an object
  • Part Revolve.png Revolve: Creates a solid by revolving another object (not solid) around an axis
  • Part Section.png Section: Creates a section by intersecting an object with a section plane
  • Part SectionCross.png Cross sections...:
  • Part Chamfer.png Chamfer: Chamfers edges of an object
  • Part Mirror.png Mirror: Mirrors the selected object on a given mirror plane
  • Part RuledSurface.png Ruled Surface:
  • Part Sweep.png Sweep: Sweeps one or more profiles along a path
  • Part Loft.png Loft: Lofts from one profile to another
  • Part Offset.png Offset: Creates a scaled copy of the original object.
  • Part Thickness.png Thickness: Assign a thickness to the faces of a shape.

Other tools

  • Part ImportCAD.png Import CAD: This tool allows you to add a file *.IGES, *.STEP, *.BREP to the current document.
  • Part ExportCAD.png Export CAD: This tool allows you to export a part object in a *.IGES, *.STEP, *.BREP file.
  • Part ShapeFromMesh.png Shape from Mesh: Creates a shape object from a mesh object.
  • Convert to solid: Converts a shape object to a solid object.
  • Reverse shapes: Flips the normals of all faces of the selected object.
  • Create simple copy: Creates a simple copy of the selected object.
  • Make compound: Creates a compound from the selected objects.
  • Part RefineShape.png Refine shape: Cleans faces by removing unnecessary lines.
  • Part CheckGeometry.png Check geometry: Checks the geometry of selected objects for errors.
  • Measure: Allows linear and angular measurement between points/edges/faces.


Boolean Operations

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


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 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.

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 (Alternative: sourcearchive.com) and search for Geom_* (for geometry) and TopoDS_* (for shapes). There you can also read more about the differences between geometric objects and shapes. Please note that unfortunately 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 counter part.

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.

Scripting

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 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 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.

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 prepares the visual representation of the new part object.

Note that a Line can be created by specifying its start and endpoint directly in the constructor, for example Part.Line(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[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.

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