Lock Position Constraint¶

In one sentence: Fix a vertex at absolute X and Y coordinates in the sketch, removing both its translational degrees of freedom in one step.

Overview¶

Workbench: Sketcher · Menu: Sketch → Sketcher constraints → Lock Position
Toolbar: Sketcher Constraints · Shortcut: K, L

Lock Position applies a horizontal distance and a vertical distance constraint from the sketch origin to the selected vertex simultaneously. The result is that the vertex is pinned to a specific (X, Y) coordinate. It is equivalent to manually adding DistanceX and DistanceY constraints with the same values.

Intuition¶

Think of pressing a pin through a drawing into the board at an exact measured position: X mm from the left edge, Y mm from the bottom. Lock Position is that pin — the vertex cannot drift because both its horizontal and vertical positions are specified absolutely.

When to use it¶

Anchoring one corner of a profile to the sketch origin (set X=0, Y=0).
Pinning a bolt-hole centre to a precisely known coordinate.
Fully constraining a standalone point used as a layout reference.
Fixing the position of one key feature before constraining others relative to it.

When NOT to use it¶

You want the vertex to follow another vertex's position — use Coincident instead.
You want to fix only one coordinate (X or Y) but leave the other free — use a single Horizontal Distance or Vertical Distance constraint.
You want the geometry frozen regardless of its coordinates — use Block for a quick non-parametric freeze.

Step-by-step walkthrough¶

Select a vertex (a line endpoint, arc endpoint, or standalone point).
Press K, L or activate Sketch → Lock Position.
A value dialog appears with the current X and Y coordinates pre-filled.
Edit the X and Y values to the desired absolute coordinates.
Click OK. Two constraints appear in the Constraints panel: a horizontal distance from the origin and a vertical distance from the origin.

Tip

To anchor a vertex to the sketch origin exactly, select it and apply Lock Position, then set both X and Y to 0. Or use Coincident with the origin point — it is equivalent and uses fewer constraints.

Parameters¶

Parameter	Type	Default	Description
X	Distance	current X	Horizontal distance from the sketch origin to the vertex. May be negative.
Y	Distance	current Y	Vertical distance from the sketch origin to the vertex. May be negative.

Common mistakes and pitfalls¶

Lock Position creates redundant constraints

Cause: The vertex was already constrained by other means (e.g. Coincident with the origin, or two Distance constraints from other elements).
Fix: Resolve the redundancy by deleting one of the competing constraints. The solver highlights them in red.

Lock Position on an interior vertex of a polyline over-constrains it

Cause: Interior vertices of a polyline are already constrained by Coincident with the adjacent edges; pinning them with Lock Position on top is redundant.
Fix: Apply Lock Position only to free vertices (endpoints, standalone points). Constrain interior vertices with Distance constraints relative to adjacent geometry.

Python API¶

Minimal example¶

import FreeCAD as App
import Sketcher
import Part

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

idx = sketch.addGeometry(
    Part.LineSegment(App.Vector(0, 0, 0), App.Vector(20, 10, 0)), False)

# Fix the start point at (5, 3)
sketch.addConstraint(Sketcher.Constraint("DistanceX", idx, 1, -1, 1, 5.0))
sketch.addConstraint(Sketcher.Constraint("DistanceY", idx, 1, -1, 1, 3.0))
doc.recompute()

Method signatures¶

# DistanceX from origin to vertex (idx, vertex_num) — positive = rightward
sketch.addConstraint(Sketcher.Constraint("DistanceX", geoId, vertex, -1, 1, x_value))

# DistanceY from origin to vertex — positive = upward
sketch.addConstraint(Sketcher.Constraint("DistanceY", geoId, vertex, -1, 1, y_value))