Part Module

From FreeCAD Documentation
Jump to: navigation, search
This page is a translated version of the page Part Module and the translation is 4% complete.

Outdated translations are marked like this.
Other languages:
čeština • ‎Deutsch • ‎English • ‎español • ‎français • ‎hrvatski • ‎Bahasa Indonesia • ‎italiano • ‎日本語 • ‎polski • ‎português • ‎português do Brasil • ‎română • ‎русский • ‎svenska • ‎Türkçe • ‎українська • ‎中文 • ‎中文(中国大陆)‎ • ‎中文(台灣)‎

Workbench Part.svg

Introduction

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 is a layer sitting on top of the OCCT libraries, that gives the user access to OCCT geometric primitives and functions. Essentially all 2D and 3D drawing functions in every workbench (Draft, Sketcher, PartDesign, etc.), are based on these functions exposed by the Part Workbench. Therefore, the Part Workbench is considered the core component of the modelling capabilities of FreeCAD.

The objects created with the Part Workbench are relatively simple; they are intended to be used with boolean operations (unions and cuts) in order to build more complex shapes. This modelling paradigm is known as the constructive solid geometry (CSG) workflow, and it was the traditional methodology used in early CAD systems. On the other hand, the PartDesign Workbench provides a more modern workflow to constructing shapes: it uses a parametrically defined sketch, that is extruded to form a basic solid body, which is then modified by parametric transformations (feature editing), until the final object is obtained.

Part objects are more complex than mesh objects created with the Mesh Workbench, as they permit more advanced operations like coherent boolean operations, modifications history, and parametric behaviour.

Part example.jpg

Part Workbench relationships.svg

The Part Workbench is the basic layer that exposes the OCCT drawing functions to all workbenches in FreeCAD


Tools

The tools are all located in the Part menu.

Primitives

These are tools for creating primitive objects.

  • Part Box.png Box: Draws a box 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 Cone.png Cone: Draws a cone 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 Extrude.png Extrude: Extrudes planar faces of an object
  • Part Revolve.png Revolve: Creates a solid by revolving another object (not solid) around an axis
  • Part Mirror.png Mirror: Mirrors the selected object on a given mirror plane
  • Part Fillet.png Fillet: Fillets (rounds) edges of an object
  • Part Chamfer.png Chamfer: Chamfers edges of an object
  • Part RuledSurface.png Ruled Surface:
  • Part Loft.png Loft: Lofts from one profile to another
  • Part Sweep.png Sweep: Sweeps one or more profiles along a path
  • Part Thickness.png Thickness: Hollows out a solid, leaving openings next to select faces.
  • Part Booleans.png Booleans: Performs boolean operations on objects
  • Part Union.png Union: 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

Other tools

Preferences

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.

Scripting

See also: Part 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 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.

Examples

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.

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.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.

doc.recompute()

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[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.

Tutorials