build123d

build123d is a Python-based, parametric (BREP) modeling framework for 2D and 3D CAD. It is built on the Open Cascade geometric kernel and is designed for creating precise models for 3D printing, CNC machining, and laser cutting.

Key features include:

• Algebraic Modeling: Uses operator-driven design (e.g., obj += sub_obj) for readable and composable logic.
• Explicit Geometry Classes: Well-defined 1D, 2D, and 3D geometry classes.
• Pythonic Integration: Selectors behave like lists, locations are iterables, and supports natural conversions (e.g., Solid(shell), tuple(Vector)).
• Extensibility: Supports subclassing and functional composition without monkey patching.
• Interoperability: Models can be exported to CAD tools like FreeCAD and SolidWorks.

Surface modeling in build123d involves the direct creation and manipulation of the skin (bounding faces) of a 3D object. Unlike volumetric modeling (extruding or revolving), surface modeling focuses on building individual curved or planar faces.

In the BREP (Boundary Representation) system used by build123d, solids are defined by a hierarchy of faces, edges, and vertices. To create a solid, you must ensure that adjacent faces share edges consistently to form a continuous, manifold topology.Shell. A shell that is properly oriented and encloses a finite region of space becomes a solid. For robust results, it is recommended to reuse the same topology.Edge objects across adjacent faces to ensure the final shell is watertight.

build123d provides several stateful contexts for managing geometric construction workflows:

• 1D Construction: BuildLine
• 2D Sketching: BuildSketch
• 3D Part Building: BuildPart
• Location Helpers: GridLocations, HexLocations, Locations, and PolarLocations

build123d supports parameterized modeling, which allows you to define designs using variables and constraints rather than fixed geometry.

Benefits of using parameterized models:

• Reusability: Modify designs by changing parameters (e.g., length, width) instead of manual geometry editing.
• Design Exploration: Quickly visualize different design options by adjusting parameters.
• Constraints and Relationships: Define relationships between parameters to ensure the model remains valid during changes.
• Automation: Automate repetitive tasks like generating parts lists or detailed drawings.
• Collaboration: Maintain consistency across different development stages and team members.

build123d uses Boundary Representation (BREP) for 3D modeling. Unlike mesh-based systems (like Blender or OpenSCAD) that use triangles, BREP uses mathematical representations to define shapes.

Key advantages of the BREP approach used in build123d:

• Precision: Mathematical definitions allow for highly accurate modeling of complex shapes.
• Topology: Maintains explicit information about edges, faces, and vertices, enabling robust Boolean operations.
• Analytical Modeling: Supports operations like collision detection, mass property calculations, and finite element analysis via topological data.
• Feature-based Modeling: Models are built by creating and modifying individual features (holes, fillets, chamfers), which facilitates parametric design.
• Efficient Storage: Uses a compact mathematical representation rather than large triangle datasets.

Topology in build123d refers to the hierarchical structure of geometric elements and their relationships. All topological objects inherit from the Shape base class. The hierarchy typically follows this structure:

• Shape: The base class for all topological elements.
• Compound: A container for grouping multiple geometric shapes (vertices, edges, faces, etc.).
• Solid: A bounded, watertight volume suitable for Boolean operations.
• Shell: A collection of Face objects defining a connected volume.
• Face: A 2D surface.
• Wire: A connected sequence of Edge objects (loops).
• Edge: A 1D curve.
• Vertex: A 0D point.

You can inspect this hierarchy using the show_topology() method on any shape.

To place text on a face:

1. Use BuildSketch on a target face.
2. Use build_enums.Align to control text placement.
3. To Emboss (raise): Place the sketch on the top face.
4. To Deboss (indent): Place the sketch on the same face but use a negative extrusion or a different operation.

Tip: When performing multiple operations on the same face, avoid re-selecting the face using a generic selector like faces().sort_by(Axis.Z)[-1] if the first operation changed the face topology; instead, store the face in a variable.

Instead of manually calculating positions for fillets or chamfers (as required in OpenSCAD), use build123d's selector system to query topological features. You can filter edges, vertices, or faces of an existing object to apply operations precisely.

For example, to fillet only the interior edges of a part, use .edges().filter_by(lambda e: e.is_interior).

A CylindricalJoint allows for motion both around and along an axis, mimicking the behavior of a screw.

• Axis of Motion: Defines the direction of movement.
• Position: Controls the linear movement along the axis. For example, a position of 0 might align a screw head with a surface, while positive values move it further along the axis.
• Angular Range: You can set an angular range (e.g., (0, 360)) to allow full rotation.