OpenSCAD Integration for URDF Skill

This skill guides the creation and use of OpenSCAD files for custom 3D geometry in URDF robot descriptions.

When to Use This Skill

Use OpenSCAD when:

•Basic URDF geometry (box, cylinder, sphere) is insufficient
•You need parametric, programmable 3D shapes
•You want to leverage existing OpenSCAD libraries
•You need maintainable, version-control-friendly geometry (vs binary STL)

⚠️ Important: Always Verify with Screenshots

CRITICAL: Always take a screenshot after creating or modifying OpenSCAD files to verify the generated geometry renders correctly. OpenSCAD syntax errors or invalid geometry will cause rendering failures. Use the MCP take_screenshot tool after save to confirm:

•The .scad file compiles without errors
•The geometry matches expectations
•The STL conversion succeeded

If the screenshot times out or shows errors, check the OpenSCAD code for syntax issues.

⚠️ Important: Always Ask Permission

Before creating OpenSCAD files, ask the user:

•Creating .scad files adds complexity
•User may prefer different CAD workflow
•May require understanding OpenSCAD syntax

Always ask: "I can create an OpenSCAD file to generate this geometry. It will be automatically converted to STL. Would you like me to do that?"

OpenSCAD Basics

What is OpenSCAD?

OpenSCAD is a script-based 3D CAD modeler:

•Write code to describe geometry
•Parametric (easy to modify dimensions)
•Text-based (works great with version control)
•Rich ecosystem of libraries

How the Extension Handles OpenSCAD

•You create a .scad file in your project
•Extension automatically converts it to .stl when saved
•Reference the .stl file in your URDF
•Preview updates automatically when .scad changes

File watching: The extension monitors .scad files and regenerates STL on save.

Basic OpenSCAD Syntax

Primitive Shapes

scad

// Cube
cube([width, depth, height]);
cube([10, 20, 30]);  // 10mm × 20mm × 30mm box

// Sphere
sphere(radius);
sphere(r=15);  // 15mm radius sphere

// Cylinder
cylinder(h=height, r=radius);
cylinder(h=50, r=10);  // 50mm tall, 10mm radius

// Cylinder with different top/bottom radii (cone)
cylinder(h=30, r1=15, r2=5);

Transformations

scad

// Translate (move)
translate([x, y, z])
  cube([10, 10, 10]);

// Rotate (in degrees)
rotate([rx, ry, rz])
  cylinder(h=20, r=5);

// Scale
scale([sx, sy, sz])
  sphere(r=10);

// Combine transformations
translate([0, 0, 10])
  rotate([0, 90, 0])
    cylinder(h=30, r=5);

Boolean Operations

scad

// Union (combine shapes) - default behavior
union() {
  cube([20, 20, 20]);
  translate([10, 10, 10])
    sphere(r=12);
}

// Difference (subtract)
difference() {
  cube([30, 30, 30]);  // Main shape
  translate([15, 15, 0])  // Subtract this
    cylinder(h=30, r=8);
}

// Intersection (only overlapping parts)
intersection() {
  cube([20, 20, 20]);
  sphere(r=15);
}

Modules (Functions)

scad

// Define a reusable module
module bracket(width, height, thickness) {
  difference() {
    cube([width, height, thickness]);
    // Mounting holes
    translate([5, height/2, 0])
      cylinder(h=thickness, r=2);
    translate([width-5, height/2, 0])
      cylinder(h=thickness, r=2);
  }
}

// Use the module
bracket(50, 30, 5);
bracket(width=60, height=40, thickness=8);

Common Patterns for Robotics

Pattern 1: Wheel with Tread

scad

// wheel_tread.scad
module wheel_with_tread(radius=50, width=30, tread_depth=3, tread_count=12) {
  difference() {
    // Main wheel body
    cylinder(h=width, r=radius, center=true);
    
    // Tread grooves
    for (i = [0:tread_count-1]) {
      rotate([0, 0, i * 360/tread_count])
        translate([radius - tread_depth/2, 0, 0])
          cube([tread_depth, 3, width+1], center=true);
    }
  }
}

// Generate wheel
wheel_with_tread(radius=100, width=40, tread_depth=5, tread_count=16);

URDF reference:

xml

<link name="wheel">
  <visual>
    <geometry>
      <!-- Extension auto-converts wheel_tread.scad to wheel_tread.stl -->
      <mesh filename="package://my_robot/meshes/wheel_tread.stl" scale="0.001 0.001 0.001"/>
    </geometry>
  </visual>
</link>

Pattern 2: Gripper Finger

scad

// gripper_finger.scad
module gripper_finger(length=60, width=15, thickness=8) {
  difference() {
    // Main finger body
    hull() {
      cube([thickness, width, thickness]);
      translate([length-10, 0, 0])
        cube([10, width, thickness]);
    }
    
    // Grip texture (small bumps)
    for (i = [10:10:length-15]) {
      translate([i, width/2, thickness])
        sphere(r=2);
    }
  }
}

gripper_finger(length=70, width=18, thickness=10);

Pattern 3: Sensor Housing

scad

// sensor_housing.scad
module sensor_housing(sensor_diameter=20, wall_thickness=3) {
  difference() {
    // Outer shell
    cylinder(h=30, r=sensor_diameter/2 + wall_thickness);
    
    // Inner cavity for sensor
    translate([0, 0, wall_thickness])
      cylinder(h=25, r=sensor_diameter/2);
    
    // Mounting holes
    for (angle = [0:90:270]) {
      rotate([0, 0, angle])
        translate([sensor_diameter/2 + wall_thickness/2, 0, 15])
          rotate([90, 0, 0])
            cylinder(h=wall_thickness+2, r=2);
    }
  }
}

sensor_housing(sensor_diameter=25, wall_thickness=4);

Pattern 4: Custom Bracket

scad

// mounting_bracket.scad
module l_bracket(width=50, height=40, depth=30, thickness=5) {
  union() {
    // Vertical plate
    cube([thickness, depth, height]);
    
    // Horizontal plate
    cube([width, depth, thickness]);
    
    // Reinforcement gusset
    hull() {
      translate([0, depth/2, 0])
        cube([thickness, 1, thickness]);
      translate([0, depth/2, height-thickness])
        cube([thickness, 1, 1]);
    }
  }
}

l_bracket(width=60, height=50, depth=35, thickness=6);

Using OpenSCAD Libraries

Checking Available Libraries

Before writing custom code, check what's available:

•Run: Command Palette → "URDF: Generate OpenSCAD Libraries Documentation"
•Review: Generated markdown shows all available modules/functions
•Use: Include or use the library in your .scad file

Library Locations

The extension automatically loads libraries from:

•Workspace root: Any .scad files in your project
•SCAD file directory: Same directory as your .scad file (highest priority)
•
OS-specific defaults:
- •Linux: ~/.local/share/OpenSCAD/libraries
- •macOS: ~/Documents/OpenSCAD/libraries
- •Windows: %USERPROFILE%\Documents\OpenSCAD\libraries
•Custom paths: Configured in urdf-editor.OpenSCADLibraryPaths setting

Common Libraries

MCAD (Mechanical CAD)

scad

include <MCAD/motors.scad>
use <MCAD/gears.scad>
use <MCAD/bearing.scad>

// Standard stepper motor
stepper_motor_mount(28);

// Spur gear
gear(number_of_teeth=20, 
     circular_pitch=5,
     gear_thickness=5,
     rim_thickness=5);

BOSL2 (Belfry OpenSCAD Library 2)

scad

include <BOSL2/std.scad>

// Rounded box
cuboid([50, 30, 20], rounding=5);

// Threaded rod
threaded_rod(d=10, l=50, pitch=2);

// Grid of holes
grid_copies(spacing=20, n=3)
  cylinder(h=10, r=3);

Include vs Use

scad

// include: Makes everything available (variables, modules)
include <MCAD/motors.scad>

// use: Only makes modules available (not variables)
use <MCAD/gears.scad>

Best practice: Use use unless you need the library's variables.

Integration with URDF

File Organization

Recommended structure:

code

my_robot/
├── urdf/
│   └── robot.urdf.xacro
├── meshes/
│   ├── wheel.scad          ← OpenSCAD source
│   ├── wheel.stl           ← Auto-generated
│   ├── gripper.scad
│   └── gripper.stl
└── scad/                   ← Optional: keep .scad separate
    └── library.scad

Workflow

•Create .scad file in meshes/ directory
•Save file → Extension auto-generates .stl
•IMMEDIATELY take screenshot to verify geometry renders correctly
•Reference .stl in URDF:

xml

<link name="custom_part">
  <visual>
    <geometry>
      <mesh filename="package://my_robot/meshes/wheel.stl"/>
    </geometry>
  </visual>
</link>

•Preview URDF → See the generated geometry
•Modify .scad → Auto-regenerates STL on save, take another screenshot to verify

Units and Scaling

Important: OpenSCAD typically uses millimeters, URDF uses meters.

xml

<!-- Scale from mm to m (divide by 1000) -->
<mesh filename="package://my_robot/meshes/part.stl" 
      scale="0.001 0.001 0.001"/>

Tip: Document units in your .scad file:

scad

// Units: millimeters
// This will be scaled to meters in URDF (scale="0.001 0.001 0.001")

module wheel(diameter=100) {  // 100mm = 0.1m
  cylinder(h=40, r=diameter/2);
}

Advanced Techniques

Parametric Design

Create families of related parts:

scad

// parametric_wheel.scad
// Parameters can be overridden when including this file

wheel_diameter = 100;  // mm
wheel_width = 40;      // mm
hub_diameter = 20;     // mm
spoke_count = 5;

module parametric_wheel() {
  difference() {
    // Rim
    cylinder(h=wheel_width, r=wheel_diameter/2, center=true);
    
    // Hub cavity
    cylinder(h=wheel_width+2, r=hub_diameter/2, center=true);
    
    // Spokes (material removal between spokes)
    for (i = [0:spoke_count-1]) {
      rotate([0, 0, i * 360/spoke_count + 360/(2*spoke_count)])
        translate([hub_diameter/2 + (wheel_diameter-hub_diameter)/4, 0, 0])
          cube([wheel_diameter/2, 5, wheel_width+2], center=true);
    }
  }
}

parametric_wheel();

Animation and Testing

Use animation to verify moving parts:

scad

// Animate to test range of motion
$fn = 50;  // Resolution

module articulated_part(angle=0) {
  // Base
  cube([50, 30, 10]);
  
  // Moving part
  translate([25, 15, 10])
    rotate([angle, 0, 0])
      translate([0, 0, 20])
        cube([20, 10, 40], center=true);
}

// Use $t for animation (0 to 1)
articulated_part(angle=$t * 180 - 90);

Resolution Control

scad

// $fn controls circle resolution
$fn = 100;  // High quality (slower)
// $fn = 30;   // Medium (default-ish)
// $fn = 12;   // Low quality (faster)

cylinder(h=10, r=5);  // Uses $fn setting

// Override for specific shapes
cylinder(h=10, r=5, $fn=6);  // Hexagon

Best Practices

DO:

✅ Take screenshots immediately after creating/modifying .scad files to verify rendering ✅ Ask permission before creating OpenSCAD files ✅ Use descriptive module names and parameters ✅ Add comments explaining what the code does ✅ Define dimensions as variables/parameters ✅ Document units (mm vs m) in comments ✅ Check for available libraries before writing from scratch ✅ Test with preview after creating/modifying ✅ Keep .scad files in version control ✅ Use MCP take_screenshot tool to validate geometry after every change

DON'T:

❌ Skip screenshot verification - geometry errors won't be caught ❌ Create overly complex geometry (affects preview performance) ❌ Forget to scale from mm to m in URDF ❌ Hardcode dimensions (use parameters instead) ❌ Create .scad files for simple shapes (use URDF primitives) ❌ Ignore available libraries (don't reinvent the wheel) ❌ Mix .scad logic with URDF structure

Troubleshooting

STL not generating?

•Check OpenSCAD syntax (look for errors in output)
•Verify file saved properly
•Check extension output channel for conversion errors

Preview not updating?

•Save the .scad file again
•Refresh the URDF preview
•Check that .stl file was regenerated (timestamp)

Library not found?

•Run "Generate OpenSCAD Libraries Documentation" to see available libraries
•Check library paths in settings: urdf-editor.OpenSCADLibraryPaths
•Verify library is in one of the standard locations

Geometry looks wrong?

•Check units (mm in OpenSCAD, m in URDF)
•Verify scale attribute in URDF mesh tag
•Review $fn resolution setting
•Test .scad file in standalone OpenSCAD application

Example Workflow

User: "I need a wheel with 12 spokes for my robot"

Agent:

code

Basic cylinder geometry won't give you spokes. I can create an 
OpenSCAD file to generate a wheel with spokes. This will:
- Create a parametric wheel design
- Auto-generate STL on save
- Allow easy dimension changes

Would you like me to create this?

After approval:

scad

// meshes/spoke_wheel.scad
// Units: millimeters (scale by 0.001 in URDF)

module spoke_wheel(diameter=100, width=30, hub=20, spokes=12, spoke_thickness=3) {
  difference() {
    union() {
      // Rim
      cylinder(h=width, r=diameter/2, center=true);
      
      // Hub
      cylinder(h=width, r=hub/2, center=true);
      
      // Spokes
      for (i = [0:spokes-1]) {
        rotate([0, 0, i * 360/spokes])
          translate([hub/2, 0, 0])
            cube([(diameter-hub)/2, spoke_thickness, width], center=true);
      }
    }
    
    // Center hole
    cylinder(h=width+2, r=hub/4, center=true);
  }
}

spoke_wheel(diameter=100, width=30, hub=20, spokes=12);

xml

<!-- In URDF -->
<link name="wheel">
  <visual>
    <geometry>
      <mesh filename="package://my_robot/meshes/spoke_wheel.stl" 
            scale="0.001 0.001 0.001"/>
    </geometry>
    <material name="wheel_color">
      <color rgba="0.2 0.2 0.2 1"/>
    </material>
  </visual>
</link>

Summary

OpenSCAD integration provides:

•Parametric, maintainable 3D geometry
•Automatic STL generation
•Access to rich library ecosystem
•Version control friendly workflow
•Perfect for custom robot parts

Remember: Always ask before creating OpenSCAD files, check for existing libraries first, and test with preview!