Mirror Sketch Elements Across an Axis in FreeCAD

Mirroring sketch elements across an axis in FreeCAD allows you to create symmetrical 2D geometry efficiently, ensuring balanced designs while cutting design time in half. This guide walks you through the exact steps required to use the Sketcher Workbench’s symmetry tool, whether you are reflecting lines, arcs, or points across the standard horizontal and vertical axes or across a custom construction line.

How FreeCAD's Sketcher Symmetry Works

In FreeCAD's Sketcher Workbench, the symmetry operation duplicates selected geometries and automatically applies a symmetry constraint relative to a specified reference axis or line. The most critical aspect of this tool is the order of selection: the axis of symmetry must always be selected last.

Step-by-Step Guide to Mirroring Elements

  1. Open the Sketch: Double-click your sketch in the tree view to enter the Sketcher Workbench edit mode.
  2. Select the Elements to Mirror: Click on the points, lines, arcs, or other geometry that you want to duplicate. You can select multiple items by clicking them individually or by dragging a selection box around them.
  3. Select the Mirror Axis Last: While holding Ctrl (or simply clicking, depending on your selection preferences), select the reference line you want to mirror across. This can be:
    • The sketch's native Horizontal Axis
    • The sketch's native Vertical Axis
    • Any drawn standard line or Construction Line
  4. Apply the Symmetry Tool: Click the Symmetrical (Mirror) icon on the Sketcher toolbar (represented by two mirrored shapes separated by a line) or use the keyboard shortcut (default is usually Z).

FreeCAD will immediately generate mirrored copies of your geometry on the opposite side of the chosen axis, linked to the original elements via symmetry constraints.

Tips for Best Results

  • Using Construction Lines: If you need to mirror across an angle or an offset location, draw a normal line, select it, and toggle it to a construction line (blue) using the Toggle Construction Geometry tool. Then use it as your last-selected mirror reference.
  • Constrained Movement: Because the generated elements are bound by symmetry constraints, altering the position, length, or angle of the original elements will automatically update the mirrored elements.
  • Selection Order Errors: If the symmetry tool is grayed out or results in an unexpected error, press Esc to deselect everything and repeat the process, taking care to click the axis of reflection strictly as the final selection.