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About

Build123d is a Python-based, parametric (BREP) modeling framework for 2D and 3D CAD. Built on the Open Cascade geometric kernel, it provides a clean, fully Pythonic interface for creating precise models suitable for 3D printing, CNC machining, laser cutting, and other manufacturing processes. Models can be exported to popular CAD tools such as FreeCAD and SolidWorks.

Designed for modern, maintainable CAD-as-code, build123d combines clear architecture with expressive, algebraic modeling. It offers:

  • Minimal or no internal state depending on mode

  • Explicit 1D, 2D, and 3D geometry classes with well-defined operations

  • Extensibility through subclassing and functional composition—no monkey patching

  • Standards-compliant code (PEP 8, mypy, pylint) with rich pylance type hints

  • Deep Python integration—selectors as lists, locations as iterables, and natural conversions (Solid(shell), tuple(Vector))

  • Operator-driven modeling (obj += sub_obj, Plane.XZ * Pos(X=5) * Rectangle(1, 1)) for algebraic, readable, and composable design logic

The result is a framework that feels native to Python while providing the full power of OpenCascade geometry underneath.

With build123d, intricate parametric models can be created in just a few lines of readable Python code—as demonstrated by the tea cup example below.

Teacup Example

from build123d import *
from ocp_vscode import show

wall_thickness = 3 * MM
fillet_radius = wall_thickness * 0.49

with BuildPart() as tea_cup:
    # Create the bowl of the cup as a revolved cross section
    with BuildSketch(Plane.XZ) as bowl_section:
        with BuildLine():
            # Start & end points with control tangents
            s = Spline(
                (30 * MM, 10 * MM),
                (69 * MM, 105 * MM),
                tangents=((1, 0.5), (0.7, 1)),
                tangent_scalars=(1.75, 1),
            )
            # Lines to finish creating ½ the bowl shape
            Polyline(s @ 0, s @ 0 + (10 * MM, -10 * MM), (0, 0), (0, (s @ 1).Y), s @ 1)
        make_face()  # Create a filled 2D shape
    revolve(axis=Axis.Z)
    # Hollow out the bowl with openings on the top and bottom
    offset(amount=-wall_thickness, openings=tea_cup.faces().filter_by(GeomType.PLANE))
    # Add a bottom to the bowl
    with Locations((0, 0, (s @ 0).Y)):
        Cylinder(radius=(s @ 0).X, height=wall_thickness)
    # Smooth out all the edges
    fillet(tea_cup.edges(), radius=fillet_radius)

    # Determine where the handle contacts the bowl
    handle_intersections = [
        tea_cup.part.find_intersection_points(
            Axis(origin=(0, 0, vertical_offset), direction=(1, 0, 0))
        )[-1][0]
        for vertical_offset in [35 * MM, 80 * MM]
    ]
    # Create a path for handle creation
    with BuildLine(Plane.XZ) as handle_path:
        Spline(
            handle_intersections[0] - (wall_thickness / 2, 0),
            handle_intersections[0] + (35 * MM, 30 * MM),
            handle_intersections[0] + (40 * MM, 60 * MM),
            handle_intersections[1] - (wall_thickness / 2, 0),
            tangents=((1, 1.25), (-0.2, -1)),
        )
    # Align the cross section to the beginning of the path
    with BuildSketch(handle_path.line ^ 0) as handle_cross_section:
        RectangleRounded(wall_thickness, 8 * MM, fillet_radius)
    sweep()  # Sweep handle cross section along path

assert abs(tea_cup.part.volume - 130326) < 1

show(tea_cup, names=["tea cup"])

Note

This documentation is available in pdf and epub formats for reference while offline.

Note

There is a Discord server (shared with CadQuery) where you can ask for help in the build123d channel.

Table Of Contents

Indices and tables