CAD Tools

Introduction

CAD (Computer-Aided Design) tools are the core software for robot mechanical design. From concept modeling to detailed design, from structural simulation to manufacturing drawings, CAD is integral to the entire design workflow. This section compares mainstream CAD tools and introduces the URDF/MJCF export process specific to robotics.

Simulation and development toolchain: See Development Toolchain

Mainstream CAD Tool Comparison

Feature	SolidWorks	Fusion 360	OnShape	FreeCAD
Type	Parametric	Parametric + Direct	Parametric	Parametric
Platform	Windows	Windows/Mac/Web	Browser	All platforms
Price	~35,000+ RMB/yr	Free (personal) / ~4,000 RMB/yr	Free (education) / ~12,000 RMB/yr	Free open-source
Learning curve	Medium	Lower	Lower	Steeper
Assembly	Strong	Medium	Strong	Medium
Simulation	Strong (paid)	Medium	Weak	Medium (FEM)
Rendering	Strong	Strong	Weak	Weak
CAM	Paid plugin	Built-in	Paid plugin	Built-in (Path)
Collaboration	PDM (paid)	Cloud	Cloud real-time	Git
API/Scripting	VBA/C#	Python/JS	FeatureScript	Python
URDF export	Plugin	Plugin	Third-party	Plugin

SolidWorks

Features

SolidWorks is the most widely used parametric CAD software in industry:

Parametric modeling: Feature-based modeling; modifying parameters automatically updates the model
Assembly design: Mature constraint (Mate) system, supports large assemblies
Engineering drawings: Automatic generation of standards-compliant 2D drawings
Simulation: SolidWorks Simulation provides static/dynamic/thermal/fluid analysis
Ecosystem: Extensive plugin library and parts libraries (McMaster, Misumi)

URDF Export

SolidWorks has a dedicated ROS URDF export plugin sw2urdf:

Install the sw_urdf_exporter plugin
Define joint types and axes in the assembly
Define parent-child relationships for Links and Joints
Export URDF file + STL/DAE mesh files

# Install sw2urdf plugin
# Download from GitHub: ros/solidworks_urdf_exporter
# Run installer, activate plugin in SolidWorks

Suitable Scenarios

Professional robot companies' product development
Need to produce engineering drawings for machining
Large assemblies (>500 parts)
Team collaboration (using PDM)

Fusion 360

Features

Fusion 360 is Autodesk's cloud-based integrated CAD/CAM/CAE tool:

Free tier available: Free for personal non-commercial use
Multi-function integration: Modeling, assembly, simulation, rendering, CAM, PCB (Eagle integration)
Cloud storage: Projects automatically sync to the cloud
Collaboration: Shareable links for others to view/edit
CAM capability: Directly generates CNC toolpaths and G-code

URDF Export

Using the fusion2urdf or Fusion2PyBullet plugin:

# fusion2urdf installation steps:
# 1. Download fusion2urdf plugin
# 2. Fusion 360 → Scripts and Add-Ins → Add-Ins → +
# 3. Select plugin directory
# 4. Run plugin in assembly
# 5. Define Base Link and each Joint
# 6. Export URDF + mesh files

Suitable Scenarios

Individual / small team robot projects
Need for rapid prototyping design
Need for integrated CAM (CNC machining)
Students and hobbyists

OnShape

Features

OnShape is a fully browser-based CAD tool:

Runs in browser: No installation needed, usable on any device
Real-time collaboration: Like Google Docs, multiple people edit simultaneously
Version control: Git-like version management, branching/merging
FeatureScript: Scripting language for custom parametric features
Free education tier: Free for students and educational institutions

Unique Features

Standard Content: Rich standard parts library
Simultaneous Editing: Multiple people editing the same assembly in real time
Branching & Merging: Design version branch management
REST API: Complete API for programmatic access

URDF Export

Using the onshape-to-robot tool:

# Install
pip install onshape-to-robot

# Configuration (config.json)
{
    "documentId": "your_document_id",
    "outputFormat": "urdf",
    "robotName": "my_robot"
}

# Export
onshape-to-robot config.json

Suitable Scenarios

Team collaboration projects
No high-end computer (even Chromebooks work)
Open-source robot projects (free public sharing)
Educational environments

FreeCAD

Features

FreeCAD is an open-source parametric CAD software:

Completely free and open-source: GPL license
Cross-platform: Windows/Mac/Linux
Python scripting: Deep Python integration, programmable modeling
Modular: Extensible via Workbenches
FEM analysis: Built-in finite element analysis module

Main Workbenches

Workbench	Function
Part Design	Parametric solid modeling
Assembly	Assembly (A2plus/Assembly4)
FEM	Finite element analysis
Path	CAM/CNC toolpaths
TechDraw	2D engineering drawings
Robot	Robot simulation (basic)

URDF Export

Using the FreeCAD ROS Workbench or manual export:

# FreeCAD Python script to export STL
import FreeCAD
import Mesh

doc = FreeCAD.open("robot.FCStd")
for obj in doc.Objects:
    if obj.Shape:
        Mesh.export([obj], f"{obj.Label}.stl")

Combine with hand-written URDF files or use the freecad_to_urdf tool.

Suitable Scenarios

Budget-constrained projects
Linux users
Need for Python-scripted modeling
Introductory CAD learning

URDF/MJCF Model Export

URDF (Unified Robot Description Format)

URDF is the standard XML format for describing robot models in the ROS ecosystem:

<?xml version="1.0"?>
<robot name="my_robot">
  <!-- Base -->
  <link name="base_link">
    <visual>
      <geometry>
        <mesh filename="package://my_robot/meshes/base.stl"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <mesh filename="package://my_robot/meshes/base_collision.stl"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="2.0"/>
      <inertia ixx="0.01" ixy="0" ixz="0" iyy="0.01" iyz="0" izz="0.02"/>
    </inertial>
  </link>

  <!-- Joint -->
  <joint name="joint1" type="revolute">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 0.1" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-3.14" upper="3.14" effort="10" velocity="3.14"/>
  </joint>

  <!-- Link 1 -->
  <link name="link1">
    <!-- visual, collision, inertial -->
  </link>
</robot>

MJCF (MuJoCo XML Format)

MJCF is the model format for the MuJoCo physics engine, widely used in reinforcement learning simulation:

<mujoco model="my_robot">
  <worldbody>
    <body name="base" pos="0 0 0.5">
      <geom type="mesh" mesh="base"/>
      <joint name="joint1" type="hinge" axis="0 0 1"/>
      <body name="link1" pos="0 0 0.1">
        <geom type="mesh" mesh="link1"/>
      </body>
    </body>
  </worldbody>

  <asset>
    <mesh name="base" file="base.stl"/>
    <mesh name="link1" file="link1.stl"/>
  </asset>
</mujoco>

Export Workflow

graph LR
    CAD[CAD Model<br/>SolidWorks/Fusion360] --> EXPORT[Export Plugin]
    EXPORT --> URDF[URDF + STL meshes]
    URDF --> ROS[ROS/ROS2<br/>Gazebo Simulation]
    URDF --> MJCF_CONV[URDF→MJCF Conversion]
    MJCF_CONV --> MUJOCO[MuJoCo Simulation]

    CAD --> STL[Manual STL Export]
    STL --> MANUAL[Hand-written URDF/MJCF]
    MANUAL --> ROS
    MANUAL --> MUJOCO

Mesh Simplification

Collision meshes used in simulation should be simplified to improve simulation speed:

Tool	Method	Suitable For
MeshLab	Quadric edge collapse	General simplification
Blender	Decimate modifier	Visual simplification
V-HACD	Convex decomposition	Collision meshes
trimesh (Python)	Programmatic simplification	Batch processing

# Using trimesh for mesh simplification
pip install trimesh

python -c "
import trimesh
mesh = trimesh.load('robot_part.stl')
simplified = mesh.simplify_quadric_decimation(1000)  # Simplify to 1000 faces
simplified.export('robot_part_simple.stl')
"

Assembly Design Tips

Special Considerations for Robot Assemblies

Clear joint definitions: Clearly define rotation axis and range of motion for each joint
Interference checking: Ensure no collisions within the range of motion
Center of gravity calculation: Use CAD mass properties to compute CG
Inertia calculation: URDF requires accurate inertia matrices
Cable space: Model cable routing paths
Maintenance accessibility: Ensure screws and connectors are operable

Obtaining Inertia Matrices

CAD software can directly compute inertia matrices for parts/assemblies:

SolidWorks: Evaluate → Mass Properties
Fusion 360: Inspect → Physical Properties
OnShape: Right-click → Mass Properties

Output format is typically:

\[I = \begin{bmatrix} I_{xx} & I_{xy} & I_{xz} \\ I_{xy} & I_{yy} & I_{yz} \\ I_{xz} & I_{yz} & I_{zz} \end{bmatrix}\]

Recommended Workflows

Individual / Small Team

Fusion 360 (modeling) → fusion2urdf (export) → Gazebo (simulation) → 3D print/CNC (manufacturing)

Education / Open-Source Projects

OnShape (modeling + collaboration) → onshape-to-robot (export) → MuJoCo (simulation) → 3D print (manufacturing)

Professional Teams

SolidWorks (modeling) → sw2urdf (export) → Gazebo/Isaac Sim (simulation) → CNC + injection molding (manufacturing)

Budget-Constrained

FreeCAD (modeling) → Hand-written URDF + STL export → Gazebo (simulation) → 3D print (manufacturing)

References

SolidWorks URDF Exporter: GitHub - ros/solidworks_urdf_exporter
fusion2urdf: GitHub
onshape-to-robot: GitHub
FreeCAD Documentation: freecad.org
URDF Specification: wiki.ros.org/urdf
MJCF Documentation: mujoco.readthedocs.io

CAD Tools

Introduction

Mainstream CAD Tool Comparison

SolidWorks

Features

URDF Export

Suitable Scenarios

Fusion 360

Features

URDF Export

Suitable Scenarios

OnShape

Features

Unique Features

URDF Export

Suitable Scenarios

FreeCAD

Features

Main Workbenches

URDF Export

Suitable Scenarios

URDF/MJCF Model Export

URDF (Unified Robot Description Format)

MJCF (MuJoCo XML Format)

Export Workflow

Mesh Simplification

Assembly Design Tips

Special Considerations for Robot Assemblies

Obtaining Inertia Matrices

Recommended Workflows

Individual / Small Team

Education / Open-Source Projects

Professional Teams

Budget-Constrained

References

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