FreeCAD Point on Object Constraint Explained

In FreeCAD's Sketcher workbench, the point-on-object constraint is a fundamental tool used to attach a point or vertex to a continuous geometric element, such as a line, arc, circle, or spline. This article provides a clear overview of what the constraint does, how it affects the degrees of freedom in a 2D parametric sketch, the exact steps to apply it, and common practical use cases for effective 3D modeling workflows.

What the Constraint Does

The point-on-object constraint forces a selected vertex or point to lie directly on the path of a target line, curve, or axis. While it locks the point to the object, it does not lock the point's specific position along that object. The point remains free to slide back and forth along the length of the line or perimeter of the curve, meaning it reduces only one degree of freedom (DOF) from the sketch. To lock the point into a fixed position, additional constraints—such as a distance dimension or a symmetry constraint—are required.

How to Apply It in FreeCAD

Applying the point-on-object constraint is straightforward:

  1. Open an active sketch in the Sketcher Workbench.
  2. Select the point or vertex you wish to constrain (it can be an endpoint of a line, a center point of an arc/circle, or a standalone point).
  3. Select the edge or curve you want the point to lie on (such as a straight line, an arc, an ellipse, or the sketch's horizontal/vertical axes).
  4. Click the Constrain point onto object tool in the toolbar, or press the default keyboard shortcut O.

Once applied, FreeCAD will display a distinct constraint symbol near the selected point and the edge will act as a guide path for that vertex.

Difference Between Coincident and Point-on-Object Constraints

While both constraints restrict movement, they serve different purposes:

  • Coincident Constraint: Locks two points together into a single shared location.
  • Point-on-Object Constraint: Locks a point to a continuous line or curve, allowing it to slide along that path.

If you attempt to apply a point-on-object constraint by selecting two points, FreeCAD will either fail or automatically default to a coincident constraint instead.

Common Use Cases

  • Centering Elements on Axes: Attaching the center point of a circle or rectangle directly to the horizontal or vertical sketch axis to maintain alignment without hardcoding coordinates.
  • T-Junctions and Intersections: Connecting the endpoint of one line to the middle of another line without breaking the continuous edge into two separate segments.
  • Constraining to External Geometry: Anchoring sketch elements to edges projected from existing 3D features using the "External Geometry" tool.
  • Guiding Mechanisms: Defining dynamic paths for geometric shapes where a component must follow a contour or curved trajectory.