How to Create a Bezier Curve in FreeCAD Sketcher

Creating a Bezier curve in FreeCAD allows you to design smooth, organic, and complex profiles within the Sketcher Workbench. While FreeCAD primarily categorizes these curves as B-splines, you can easily generate and manipulate standard Bezier curves using control polygons and knot vectors. This guide explains how to locate the B-spline tools, draw a curve using control points, adjust its curvature, and apply constraints for precise parametric modeling.

1. Open the Sketcher Workbench

  1. Open an existing FreeCAD document or create a new one.
  2. Switch to the Part Design or Sketcher workbench.
  3. Create a new sketch on your desired plane (e.g., XY-Plane) or open an existing sketch.

2. Select the B-Spline Tool

FreeCAD implements Bezier functionality through its B-spline tools. In the top toolbar, locate the B-spline dropdown menu:

  • B-spline by control points: This is the direct equivalent of drawing a standard Bezier curve. The curve is defined and pulled toward a framework of control points (the control polygon).
  • B-spline by knots: This method forces the curve to pass directly through every point you click on screen.

For a traditional Bezier workflow, select Create a B-spline by control points.

3. Draw the Curve

  1. Click in the 3D view to place the starting point of the curve.
  2. Move the cursor and click to place intermediate control points. Each point generates a segment of the control polygon that influences the curve's tangency and curvature.
  3. Finish the curve:
    • Open curve: Right-click anywhere in the viewport or press Esc to finish at the last placed point.
    • Closed curve: Click back onto the initial starting point to close the loop.

4. Adjust the Curve and Control Polygon

Once drawn, you can fine-tune the shape of the curve:

  • Move Points: Click and drag any of the control points (represented by small dots connected by dashed lines) to reshape the curve dynamically.
  • Toggle Information: Use the Sketcher B-spline tools to show or hide the control polygon, curvature comb, or degree representations to visualize how smooth the curve is.
  • Increase/Decrease Degree: Select the curve and use the Increase B-spline degree or Decrease B-spline degree tools to add or remove flexibility without manually redrawing the curve.

5. Constrain the Curve

To fully define your sketch, apply constraints to the B-spline:

  • Points: Apply standard dimensional constraints (horizontal, vertical, or distance) directly to the control points or endpoints.
  • Tangency: Select the endpoint of the B-spline and an adjacent line or arc, then apply the Tangent constraint (T) to ensure smooth transitions between geometries.