FreeCAD Symmetry Constraint Between Two Lines

Applying a symmetry constraint between two lines in FreeCAD Sketcher ensures that the lines mirror each other across a designated axis, reference line, or center point. Because lines are defined by their endpoints, establishing full symmetry requires constraining the endpoints of both lines relative to a shared reference line or axis. This guide details the exact selection order and steps needed to achieve full symmetry between two line segments.

Understanding How Symmetry Works in Sketcher

In FreeCAD, the Symmetry constraint operates primarily on vertices (points). To make two complete lines symmetrical across a reference line (such as the horizontal or vertical sketch axis):

  1. The selection order is strictly: First item, Second item, and Symmetry reference last.
  2. Full symmetry of two lines requires applying the constraint to both pairs of corresponding endpoints, or applying symmetry to one pair of endpoints combined with an equality and angular constraint.

Step-by-Step Instructions

Method 1: Symmetrical Endpoints (Full Geometric Symmetry)

  1. Open the Sketch: Double-click your sketch in the tree view to enter the Sketcher workbench.
  2. Select the First Set of Endpoints: Click the starting vertex of the first line, then click the corresponding starting vertex of the second line.
  3. Select the Symmetry Reference: Click the reference line, construction line, or sketch axis (e.g., the horizontal or vertical axis) that will serve as the mirror plane.
  4. Apply the Constraint:
    • Click the Constrain symmetry tool in the toolbar (represented by two points mirrored across a line).
    • Alternatively, press the keyboard shortcut S.
  5. Repeat for the Opposite Endpoints:
    • Click the second endpoint of the first line.
    • Click the second endpoint of the second line.
    • Click the same symmetry reference line or axis.
    • Press S or click the Constrain symmetry icon.

Both lines are now fully symmetrical in length, position, and orientation across your reference.

Method 2: Single Point Symmetry with Geometric Constraints

If you want to define symmetry without applying two separate point-symmetry constraints:

  1. Constrain One Pair of Vertices: Select one endpoint on each line, select the mirror axis last, and press S.
  2. Apply Equal Length: Select both line segments and apply the Constrain equal length constraint (shortcut E).
  3. Constrain Angles: If the lines are not parallel to each other, use the Constrain angle tool to define the angle relative to the symmetry axis, ensuring both lines reflect each other's pitch.

Troubleshooting Common Issues

  • Selection Order Error: If the constraint fails or behaves unexpectedly, ensure the reference axis or line is selected last.
  • Redundant Constraints: If your sketch turns orange, you have over-constrained the geometry. Remove any existing angle, horizontal, or vertical constraints that conflict with the mirror orientation.
  • Selecting Line Bodies vs. Vertices: Selecting two line segments directly alongside an axis does not automatically mirror both ends in older FreeCAD versions. Always select the specific vertex pairs to guarantee predictable results.