How to Fix Flipped Normal in a FreeCAD Sketch

A flipped normal orientation in a FreeCAD sketch occurs when the local Z-axis of the sketch points in the opposite direction of your intended design, causing features like pads, pockets, and extrusions to generate backwards. This guide outlines how to identify inverted sketch normals and provides step-by-step methods to fix them using the Attachment properties, the Attachment Offset settings, or sketch reorientation tools without breaking existing geometry.

Understanding the Problem

Every sketch in FreeCAD contains a local coordinate system with its own normal vector (the local Z-axis). When a sketch is mapped to a face or a datum plane, FreeCAD determines which side is the "front" based on the underlying surface's normal. If the surface normal is inverted or referenced inconsistently, the sketch's normal will flip, causing operations like Pad to push into the model instead of out of it, or making sketch constraints behave unpredictably.

The cleanest way to fix an inverted normal is through the Attachment task panel, which recalculates how the sketch binds to its support plane.

  1. Select the affected sketch in the Tree view.
  2. In the Data tab of the Properties panel, locate the Map Mode property under the Attachment group.
  3. Click the ellipsis button (...) next to Map Mode to open the Sketch Attachment task panel.
  4. Locate the Flip sides checkbox at the bottom of the Attachment dialog.
  5. Check (or uncheck) Flip sides to reverse the normal vector.
  6. Click OK to apply the change.

Method 2: Adjust Attachment Offset

If the "Flip sides" option does not achieve the desired orientation, you can manually rotate the sketch around its local axis.

  1. Select the sketch in the Tree view.
  2. Go to the Property view and select the Data tab.
  3. Expand the Attachment Offset property.
  4. Expand the Rotation sub-property.
  5. Set the rotation angle to 180° along either the X or Y axis (e.g., set the X-axis to 1 and the Angle to 180°).
  6. Press Enter and recompute the model by clicking the Refresh button or pressing F5.

Method 3: Reorient the Sketch

If the sketch is attached to an ambiguous reference or is completely unmapped, you can reorient it from the menu.

  1. Select the sketch in the Tree view.
  2. In the top menu, go to Part Design (or Sketch) > Reorient sketch.
  3. A prompt may warn that reorienting will detach the sketch from its support; confirm to proceed if your sketch is not strictly dependent on face references.
  4. Choose the base plane (e.g., XY-Plane, XZ-Plane, or YZ-Plane) and toggle the Reverse direction option if prompted.
  5. Click OK to commit the orientation.

Workaround for Downstream Features

If changing the sketch normal breaks downstream constraints or referenced geometry:

  1. Select the dependent 3D feature (such as a Pad or Pocket) in the Tree view.
  2. In the Data tab, set the Reversed property to True (or toggle it back to False).
  3. This forces the feature to extrude in the opposite direction without altering the sketch's underlying coordinates.