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Fixed Joint

In one sentence: Lock two parts together with no relative motion — the equivalent of welding or gluing them.

Overview

Workbench: Assembly
Menu: Assembly → Joints → Fixed
Shortcut: F
DOF removed: 6 | DOF remaining: 0

A Fixed joint locks two parts together completely. All six degrees of freedom (3 translations + 3 rotations) between the two parts are removed. The solver moves Part 2 so its selected geometry aligns with Part 1's selected geometry, then freezes all relative motion.

Intuition

Think of a Fixed joint as a weld bead or a structural adhesive bond. Once applied, the two parts become a single rigid unit — they move together as if they were one piece. No sliding, no spinning, no tilting.

The solver uses the selected geometry to determine the alignment before locking: if you select two coplanar faces, the parts are brought into contact and locked. If you select two vertices, the vertices are brought to the same point and locked.

When to use it

  • Bolted flanges, press-fit components, or glued surfaces that must not move.
  • Sub-assemblies that are internally rigid — fix the sub-assembly to its mounting interface.
  • Temporarily locking a part during assembly setup to simplify the solver while adding other joints.
  • Any component that is permanently attached and has no relative motion.

When NOT to use it

  • Don't use Fixed when parts have relative motion — use Revolute for rotation, Slider for translation, etc.
  • Don't use Fixed in addition to another joint between the same pair of parts — the Fixed joint already removes all 6 DOF, making any other joint between those parts redundant and over-constraining the system.

Step-by-step

  1. Press F or choose Assembly → Joints → Fixed.
  2. Click a face, edge, or vertex on Part 1 (the reference part).
  3. Click the corresponding feature on Part 2 (the part to align and fix).
  4. Click OK in the task panel.

The solver moves Part 2 so its selected feature aligns with Part 1's feature, then removes all relative motion. Run Solve Assembly to apply.

Parameters

Parameter Type Description
Reference 1 Link Geometry on Part 1 (face, edge, or vertex)
Reference 2 Link Geometry on Part 2

Fixed joints have no offset or rotation parameters — the parts are locked at their alignment position without any additional offset.

Python API

Minimal example

import FreeCAD as App

doc = App.ActiveDocument
asm = doc.getObject("Assembly")

joint = asm.Joints.newObject("Assembly::JointObject", "Fixed")
joint.JointType = "Fixed"
joint.Reference1 = (doc.getObject("Plate"), ["Face1"])
joint.Reference2 = (doc.getObject("Bracket"), ["Face3"])
doc.recompute()

Resulting object properties

Property Type Description
JointType App::PropertyString Always "Fixed"
Reference1 App::PropertyXLinkSub Geometry on Part 1
Reference2 App::PropertyXLinkSub Geometry on Part 2

Common mistakes and pitfalls

Fixed + another joint between the same parts over-constrains

A Fixed joint removes all 6 DOF. Adding any other joint (Revolute, Slider, Parallel, …) between the same two parts attempts to remove DOF that are already removed. The solver will report a conflict. Use only one joint type per part pair when Fixed is involved.

Using Fixed for a 'not-yet-jointed' part

It is tempting to Fix a newly inserted part in place before deciding on the right joint type. That is fine for setup, but remember to delete the temporary Fixed joint before adding the correct kinematic joint — two joints between the same parts over-constrain the system.

See also