How to Make a Line Tangent to an Arc in FreeCAD

Applying a tangent constraint between a line and an arc in FreeCAD is a fundamental skill in the Sketcher workbench that ensures smooth, continuous transitions between straight edges and curves. This guide provides a clear, step-by-step walkthrough on how to select your geometric elements, apply the constraint using toolbar tools or keyboard shortcuts, and resolve common constraint conflicts that may arise during the modeling process.

Step-by-Step Guide to Applying the Tangent Constraint

  1. Open Your Sketch Open FreeCAD, navigate to the Sketcher Workbench (or Part Design Workbench), and open the sketch containing the line and the arc you want to constrain.

  2. Select the Geometry

    • Click on the line segment so that it turns green (default selection color).
    • Hold the Ctrl key (or Cmd on macOS) and click on the arc so both geometries are selected simultaneously.
    • Note: Ensure you are selecting the bodies of the line and arc, not just their endpoints, unless you specifically want the tangency to occur precisely at a shared vertex.

  3. Apply the Tangent Constraint

    • Click the Constrain tangent or collinear icon in the Sketcher constraints toolbar (represented by two circles touching or a line tangent to a circle).
    • Alternatively, press the default keyboard shortcut: T.

The line will instantly adjust its angle and position to remain tangent to the curvature of the arc.

Creating a Tangent Connection at a Shared Endpoint

If you want the end of the line to touch the end of the arc smoothly:

  1. Click the endpoint of the line.
  2. Hold Ctrl (or Cmd) and click the endpoint of the arc.
  3. Press T or click the Constrain tangent icon.

FreeCAD will automatically merge the points (coincident constraint) and enforce tangency at that shared joint simultaneously.

Troubleshooting Tangent Constraints

  • Over-constrained Sketch: If the solver turns orange or red, the line or arc may already have conflicting constraints, such as fixed angles, horizontal/vertical locks, or distance dimensions that prevent tangency. Remove the conflicting constraint to resolve the issue.
  • Opposite Curvature: If the line snaps to the wrong side of the arc, undo the action (Ctrl + Z), drag the line closer to your desired position, and reapply the constraint. FreeCAD solves constraints based on the elements' current proximity.