Construction vs Normal Geometry in FreeCAD Sketches

In FreeCAD's Sketcher workbench, geometry is divided into two primary types: normal geometry and construction geometry. While normal geometry forms the actual physical boundaries used to generate 3D solid features like pads, pockets, and revolves, construction geometry acts solely as an internal reference framework to help align, position, and dimension the sketch. Understanding the functional differences between these two modes is essential for creating parametric, error-free models.

Normal Geometry

Normal geometry, often referred to as standard or real geometry, represents the definitive outlines of a part. By default, it appears as white lines (or green when fully constrained) inside the sketcher interface.

  • 3D Feature Generation: When you close a sketch to perform an operation in the Part Design or Part workbenches—such as a Pad (extrude) or a Pocket (cut)—FreeCAD only processes normal geometry.
  • Profile Requirements: Normal geometry must adhere to topological rules depending on the intended 3D operation. For example, a Pad typically requires normal geometry to form a closed, non-self-intersecting loop.

Construction Geometry

Construction geometry consists of auxiliary lines, arcs, circles, and curves used strictly for layout, alignment, and mathematical guidance. By default, it appears as blue dashed or solid lines within the Sketcher.

  • Ignored by 3D Operations: FreeCAD completely ignores construction geometry when generating 3D features. A sketch containing open construction lines can still be padded without triggering "open profile" errors, provided the normal geometry forms a valid closed loop.
  • Parametric Framework: It is primarily used to position normal geometry using constraints. For instance, a construction circle can be used as a bolt-hole circle (pitch circle diameter) to symmetrically space normal circles representing holes, or a construction line can establish a line of symmetry between features.

Key Differences at a Glance

  • Purpose: Normal geometry defines the finished profile of the model; construction geometry provides the reference structure to position that profile.
  • Visibility Outside Sketcher: Normal geometry produces visible 3D edges and faces upon extrusion; construction geometry disappears completely once the sketch is closed.
  • Color Coding: In the default FreeCAD color scheme, normal geometry is white (unconstrained) or green (fully constrained), whereas construction geometry is blue.
  • Constraint Interaction: Constraints such as dimensions, tangency, and symmetry apply identically to both types. You can dimension a normal line against a construction line without converting either.

Toggling Between Modes

FreeCAD allows you to switch between normal and construction modes at any time:

  1. Before Drawing: Click the Toggle Construction Geometry tool on the Sketcher toolbar (or press the corresponding shortcut) to change the active drawing mode.
  2. After Drawing: Select any existing element in the sketch and click the Toggle Construction Geometry tool to convert it from normal to construction, or vice versa.