Point on Object Constraint¶

In one sentence: Force a point to lie somewhere on a line, arc, or circle — the point can slide along the edge but cannot leave it.

Overview¶

Workbench: Sketcher · Menu: Sketch → Sketcher constraints → Point-On-Object Constraint
Toolbar: Sketcher Constraints · Shortcut: O

Point on Object removes 1 degree of freedom: it forces a point onto an edge's curve but allows the point to slide along that curve. To fix the point at a specific location on the edge, add a second constraint (e.g. a distance from one endpoint, or a coincident with a crossing edge).

Intuition¶

Imagine threading a bead onto a wire: the bead cannot leave the wire, but it is free to slide back and forth. Point on Object is that wire constraint — the point is locked onto the edge but not pinned to a specific position on it.

When to use it¶

A construction point (centre of pattern, midpoint reference) must lie on a specific edge at an unspecified position.
You want a point to lie on a circle (the circle will pass through the point, wherever it is).
A tangent point between a line and a curve should be at the line's midpoint.
An external geometry projection should anchor your sketch geometry to a specific edge without fixing the exact position.

When NOT to use it¶

You want two vertices to share the same location — use Coincident.
You want a point at the midpoint of a line — apply Point on Object and then a Symmetric constraint about the line's midpoint, or add a Distance equal to half the line length.

Step-by-step walkthrough¶

Select a point and an edge — click the point vertex, then Ctrl-click the line or arc.
Press O or activate Sketch → Point-On-Object Constraint.
The point moves to the nearest location on the edge. It can now slide along the edge but not leave it.
Add a second constraint to fix where on the edge the point should lie.

Tip

If the unified Coincident tool (C) is used with a point-and-edge selection, it automatically applies Point on Object instead.

Parameters¶

No numeric parameters. Removes 1 DoF.

Common mistakes and pitfalls¶

Point slides along the edge unexpectedly when other geometry changes

Cause: Point on Object constrains the point to the edge but leaves 1 DoF remaining (position along the edge). If other constraints move the edge, the point stays on the edge but may shift to a different location on it.
Fix: Add a second constraint — a Distance from one endpoint, or a Coincident with a crossing edge — to fix the position along the edge.

Python API¶

Minimal example¶

import FreeCAD as App
import Sketcher
import Part

doc = App.activeDocument()
sketch = doc.getObject("Sketch")

line = sketch.addGeometry(
    Part.LineSegment(App.Vector(0, 0, 0), App.Vector(30, 0, 0)), False)
pt = sketch.addGeometry(Part.Point(App.Vector(15, 0, 0)), False)

# Force point to lie on the line
sketch.addConstraint(Sketcher.Constraint("PointOnObject", pt, 1, line))
doc.recompute()