Assembly Workbench (A2plus/Assembly4)

Definition & Context

FreeCAD supports multi-body assembly through several approaches: the built-in Assembly workbench (FreeCAD 1.0+), A2plus (mature addon), and Assembly4 (datum-based). Each uses different constraint philosophies. The built-in Assembly workbench uses a solver-based approach similar to SOLIDWORKS (coincident, tangent, distance constraints between faces/edges).

Assembly4 takes a unique coordinate-system approach: each part publishes Local Coordinate Systems (LCS) at its interface points, and assembly relationships map one LCS to another. This avoids traditional constraint solvers and is more predictable for complex mechanisms. A2plus uses a traditional constraint solver: Point-on-Point, Point-on-Line, Point-on-Plane, and rigid axis constraints.

Workflow: create parts in separate files → open an Assembly document → insert parts as links → define positional relationships → check for interferences. FreeCAD's Link system enables external file references without copying geometry into the assembly file.

Why It Matters

In FreeCAD the assembly method you pick is a real decision, because the built-in Assembly workbench, A2plus, and Assembly4 use different constraint philosophies that do not mix well in one model.

Understanding those differences lets you choose a stable approach up front — solver-based mates for familiar SOLIDWORKS-style workflows, or Assembly4's datum/coordinate-system method for reference-stable structures — instead of fighting broken links when parts change.

Best Practices

• Use the Link system (App::Link) for external file references — this keeps assembly files lightweight and enables concurrent editing
• Place datum LCS (Local Coordinate Systems) at all mating interfaces during part design to simplify assembly later
• For Assembly4: name LCS points semantically (e.g., "LCS_BoltHole_1") for self-documenting assembly structures
• For built-in Assembly: start by grounding one component, then constrain others sequentially to avoid over-determination
• Use the Interference Check tool to detect collisions before exporting to manufacturing or 3D printing

Common Pitfalls

• Mixing A2plus and Assembly4 constraints in the same project — choose one approach and stay consistent
• A2plus solver can fail to converge with over-constrained systems; reduce constraints to minimum necessary
• External file path breaks when moving project folders — use relative paths and a consistent project directory structure
• Large assemblies (200+ parts) may experience slow recomputation — use sub-assemblies to limit solver scope

References & Further Reading

• https://wiki.freecad.org/Assembly_Workbench
• https://wiki.freecad.org/A2plus_Workbench

Related Concepts

Explore more FreeCAD topics in our FreeCAD knowledge base, or browse the full terminology index.

For structured learning, see our FreeCAD learning path.

Enterprise Production Implementation & GstarCAD 2027 Workflow

• Database Serialization & Binary Fidelity: Ensure all entities and symbol table records associated with Assembly Workbench (A2plus/Assembly4) are authored on standardized ISO 13567 layers rather than Layer 0 or Defpoints, and verify lossless round-trip persistence in native DWG (AC1032) format.