2026-teamH-wiki-entry CAD to URDF

Author: kodikarthik21

assets/images/simulation/export.png

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+---
+date: 2026-04-23
+title: Creating a URDF from a CAD Model using OnShape
+published: true
+---
+This tutorial covers the process of taking a robot CAD model and exporting it 
+as a URDF (Unified Robot Description Format) file. You will learn what a URDF 
+is, how to bring a SolidWorks assembly into OnShape, how to set up joints and 
+links correctly, and how to export the final URDF. The steps use SolidWorks 
+as the starting CAD tool and OnShape for the export pipeline, but the 
+concepts apply to other CAD tools as well. By the end, you will have a URDF 
+file ready to load into a simulator like Isaac Sim or Gazebo, or into a ROS 2 
+pipeline for real robot control.
+
+## What is a URDF?
+
+A URDF is an XML file that describes a robot's physical structure. Simulators 
+and robot software like ROS use it to understand how a robot is built, what 
+parts it has, how they connect, and how they move relative to each other.
+
+A URDF describes the robot as a tree of links and joints:
+
+- **Link**: a rigid body such as a base plate, arm segment, or gripper finger. 
+  Each link has geometry (the mesh shape), mass, and inertia properties.
+- **Joint**: the connection between two links. It defines how one link moves 
+  relative to another. Common types are:
+  - `fixed` — no motion, the two links are rigidly attached
+  - `revolute` — rotation about a single axis (e.g., a hinge or motor)
+  - `prismatic` — sliding along a single axis (e.g., a linear actuator)
+- **Parent/Child**: joints connect a parent link to a child link. The whole 
+  robot forms a tree starting from a single root, usually called `base_link`.
+
+A simple two-link arm would look like this in structure:
+
+```
+base_link
+└── shoulder_joint (revolute)
+    └── upper_arm_link
+        └── elbow_joint (revolute)
+            └── forearm_link
+```
+
+Inside the URDF file, a single joint looks like this:
+
+```
+<joint name="shoulder_joint" type="revolute">
+  <parent link="base_link"/>
+  <child link="upper_arm_link"/>
+  <axis xyz="0 0 1"/>
+  <limit lower="-1.57" upper="1.57" effort="10" velocity="1.0"/>
+</joint>
+```
+
+The `axis` field defines which direction the joint rotates around. The `limit` 
+field sets the minimum and maximum angle in radians, along with effort and 
+velocity limits. Getting these values right is important for simulation — a 
+joint with wrong limits will either not move or move past its physical range.
+
+## Step 1: Prepare Your CAD Assembly
+
+Before importing into OnShape, make sure your CAD assembly is clean 
+and well-organized. This will save significant time during the joint 
+assignment step. A messy assembly with undefined mates or floating parts will 
+cause problems after import.
+
+- Each moving part should be a separate component in the assembly
+- Parts that move together as one rigid body should be grouped or mated as 
+  fixed in the CAD
+- Suppress any cosmetic or non-structural parts such as bolts, labels, and 
+  fasteners that you do not need in simulation. These add unnecessary 
+  complexity to the URDF and slow down the simulator.
+
+It is also worth sketching out your intended link tree on paper before 
+starting. Knowing which parts form each link, and which joints connect them, 
+makes the OnShape setup much faster.
+
+## Step 2: Import Your CAD File into OnShape 
+
+(Note that this is commmon to all CAD softwares. Solidworks is just chosen as an example)
+
+OnShape supports two main import approaches for SolidWorks assemblies.
+
+### Option A: Import SolidWorks Files Directly
+
+OnShape can import SolidWorks native files (`.sldprt` and `.sldasm`).
+
+1. Go to <https://cad.onshape.com> and log in
+2. Click **Create** in the top left corner
+3. Select **Import**
+4. Upload your `.sldasm` file along with all referenced `.sldprt` part files
+5. OnShape will convert the assembly and open it as a new document
+
+> Import all part files together with the assembly file in one upload. 
+> OnShape needs all referenced parts to reconstruct the assembly correctly.
+> If you upload only the `.sldasm` without the part files, parts will appear 
+> missing or as empty shells.
+
+This approach preserves your existing SolidWorks mate structure, which saves 
+time in the joint assignment step. However, it can sometimes produce broken 
+references or missing geometry if the SolidWorks file uses external references 
+or complex configurations.
+
+### Option B: Export as STEP and Import (Recommended)
+
+A more reliable approach is to export your SolidWorks assembly as a STEP file 
+(`.step` or `.stp`) and import that into OnShape instead. STEP is a universal 
+CAD exchange format that OnShape handles cleanly without broken references.
+
+To export from SolidWorks:
+1. Go to **File** > **Save As**
+2. Set the file type to **STEP AP214** or **STEP AP203**
+3. Save the file
+
+Then import into OnShape the same way — **Create** > **Import** > select the 
+`.step` file.
+
+The tradeoff is that STEP files do not preserve mate or assembly structure. 
+All parts will appear as independent bodies in OnShape, and you will need to 
+manually group parts that belong to the same rigid link before assigning mates. 
+For most robot assemblies this is straightforward since the grouping follows 
+the physical structure of the robot.
+
+Once imported, check that all parts appear correctly. Cosmetic issues like 
+color or appearance can be ignored, but make sure no parts are missing. 
+Rotate the model and inspect it from multiple angles.
+
+OnShape may change the appearance of some parts during conversion. This does 
+not affect the URDF export — only geometry and mate structure matter.
+
+## Step 3: Name Your Parts
+
+The OnShape URDF exporter reads part names to build the link names in the 
+URDF. If your parts are named `Part1`, `Part2`, and so on, your URDF will be 
+very hard to read and debug.
+
+In the parts list on the left panel:
+- Rename each part to match the link name you want in the URDF
+- Use underscores and no spaces (e.g., `base_link`, `upper_arm`, `wrist_link`)
+- Keep names short and descriptive
+- Parts that will be combined into one rigid link can share a name or be 
+  merged before export
+
+![Image of the links on onshape, labeled clearly](/assets/images/simulation/onshape_link_names.png)
+## Step 4: Assign Joints Using Mates
+
+In OnShape, joints in the URDF come from mates between parts. You need to 
+define a mate for each joint in your robot.
+
+### Mate Connectors
+
+Before creating mates, you should define **Mate Connectors** on each part. 
+A mate connector is a coordinate frame you place on a part to define exactly 
+where and in what orientation a mate will be created. Think of it as marking 
+the precise joint location on the geometry.
+
+To add a mate connector:
+1. Right click on a part in the assembly
+2. Select **Add mate connector** (or just click on the mate connector button near the "Insert" button on the toolbar at the top)
+3. Place it at the center of the joint — for example, at the center of a 
+   rotating shaft or hinge pin
+4. Orient the Z axis of the connector to match the intended axis of rotation 
+   or translation. The Z axis of the mate connector becomes the joint axis 
+   in the URDF.
+
+> Taking time to place mate connectors accurately is worth it. A poorly 
+> placed connector will result in the wrong joint origin in the URDF, which 
+> causes the robot geometry to appear offset or rotated incorrectly in 
+> simulation.
+
+The image below shows a mate connector in the figure as well as the menu on the left (highlighted)
+![Mate connector](/assets/images/simulation/onshape_mate_connector.png)
+
+### Creating Mates
+
+Once mate connectors are placed, create mates between them:
+![alt text](/assets/images/simulation/onshape_revolute_mate_menu.png)
+1. For each joint, choose the correct mate type (look at the image above to see the place where the mates will be):
+   - **Fastened mate** becomes a `fixed` joint in the URDF
+   - **Revolute mate** becomes a `revolute` joint
+   - **Slider mate** becomes a `prismatic` joint
+2. Select the mate connector on the parent part and the mate connector on 
+   the child part
+3. Name each mate clearly (e.g., `shoulder_revolute`, `elbow_revolute`)
+
+> Every joint must connect exactly one parent part to one child part. 
+> The assembly must form a tree with no loops. If part A connects to part B 
+> and part B connects back to part A through a different mate, the URDF will 
+> be invalid.
+
+The axis direction of the mate connector is particularly important. In the 
+URDF, the joint axis is defined by the Z axis of the mate connector on the 
+child part. If you orient the connector wrong in OnShape, the joint will 
+rotate around the wrong direction in simulation. After creating each mate, 
+animate it in OnShape to confirm the motion direction matches what you expect 
+before moving on.
+
+The image below shows OnShape mate dialog showing a revolute mate being defined between two mate connectors
+![Revolute joint](/assets/images/simulation/onshape_revolute_joint.png)
+
+### Setting Joint Limits
+
+To set joint limits, click on the mate in the feature tree, open its 
+properties, and enter the minimum and maximum angle or distance. The 
+OnShape URDF exporter reads these directly and writes them into the URDF 
+limit tag.
+
+If you skip this step, the exported URDF will have no limits on those joints. 
+In simulation this means the joint can rotate freely to any angle, which 
+usually causes the robot to collapse or behave unrealistically.
+
+
+## Step 5: Export the URDF
+
+Once your parts are named and all mates are defined, you can export the URDF 
+directly from OnShape.
+
+Right click on the file name in the tab bar at the bottom of the screen. 
+Click **Export**. In the export dialog, select **URDF** as the target file 
+format and then click **Export**. It will take a few minutes for the export 
+to process and download depending on the complexity of your assembly.
+
+![Export](/assets/images/simulation/export.png)
+
+This will generate a zip file containing:
+- `robot.urdf` — the URDF file describing your robot's structure
+- A folder of mesh files (`.stl` or `.obj`) for each link's geometry
+
+Extract the zip file and keep the mesh folder in the same directory as the 
+URDF file. The URDF references the meshes using relative paths, so moving 
+them apart will cause the robot geometry to not load.
+
+## Step 6: Validate the URDF
+
+You can use online URDF viewers such as 
+<https://viewer.robotsfan.com/> to visually inspect the robot 
+without needing a full simulator installed. Load the entire folder generated by the onshape (urdf and the meshes) and check that all links appear in the right positions and 
+orientations.
+
+In the image below, you can see the entire folder (urdf + meshes loaded), which then generates the robot structure, the joint-link tree, etc. The viewer displays four key panels:
+
+Files panel (left) — shows the loaded package with the meshes/ folder and urdf/autolab.urdf file, confirming the relative path structure is intact and the viewer resolved all references correctly.
+3D Viewport (center) — renders the full robot geometry using the loaded meshes, allowing you to visually confirm that all links appear in the correct positions and orientations.
+Joints panel (center) — lists every joint (arm_1_0 through arm_6_0) with interactive sliders so you can manually drive each joint and verify that motion direction and limits match your expectations.
+Structure panel (right) — displays the full parent-child link tree, from arm_shoulder_4 at the base up through wrist_2_p4 at the tip, confirming the tree has no loops and the hierarchy exported correctly.
+![URDF Validation](/assets/images/simulation/urdf_validation.png)
+
+## Common Issues and Fixes
+
+**Parts are missing after import into OnShape**
+Make sure you uploaded all `.sldprt` files together with the `.sldasm` file 
+in one import batch. OnShape cannot resolve external references if the part 
+files are missing.
+
+**Joints move in the wrong direction**
+Flip the axis in the OnShape mate properties, re-export, and re-validate. 
+You can also edit the axis line directly in the URDF file as a quicker fix.
+
+**Joint limits are wrong or missing**
+Set limits in the mate properties in OnShape before exporting. The exporter 
+reads them directly from the mate. If limits are missing the joint will be 
+unconstrained in simulation.
+
+**Part masses are zero**
+Assign a material to each part in OnShape so the exporter can compute mass 
+from density. Alternatively, edit the inertial blocks in the URDF file 
+manually after export.
+
+**check_urdf reports a loop**
+Your assembly has a part connected to more than one parent. Find and remove 
+the extra mate causing the loop.
+
+**Meshes do not appear in the simulator**
+Check that the mesh folder is in the same directory as the URDF. The URDF 
+uses relative paths like `meshes/base_link.stl` — if the mesh folder is 
+moved or renamed the simulator cannot find the geometry.
+
+## Opening Your URDF in Isaac Sim
+
+1. Enable the `isaacsim.asset.importer.urdf` extension if not already active:
+   **Window** > **Extensions** > search for `isaacsim.asset.importer.urdf` and enable it
+2. Go to **File** > **Import** and select your `robot.urdf` file
+3. In the import settings:
+   - Set **USD Output** to your desired output location
+   - Check **Static Base** if your robot has a fixed base (e.g. a mounted arm)
+   - Enable **Allow Self-Collision** under the Colliders section
+4. Click **Import** — the robot will appear in the stage
+
+> Keep the `meshes/` folder in the same directory as the `.urdf` file before 
+> importing, or Isaac Sim will not resolve the geometry paths correctly.
+
+
+## Summary
+
+To generate a URDF from a SolidWorks CAD model: clean up your assembly, 
+import it into OnShape, rename parts to match your intended link names, assign 
+mates for each joint with correct axis directions and limits, then export as 
+URDF. Validate the output with check_urdf or an online viewer before using 
+it in a simulator or ROS package. Getting the assembly structure and mate 
+setup right before export avoids most common errors downstream.
+
+## See Also
+- [ROS URDF Documentation](http://wiki.ros.org/urdf)
+
+## Further Reading
+- URDF XML specification: <http://wiki.ros.org/urdf/XML>
+- OnShape Learning Center: <https://learn.onshape.com>
+- Online URDF Viewer: <https://myrobotics.eu/urdftools/viewer/>
+
+## References
+- Open Robotics, "URDF XML Specification," ROS Wiki, 2021. [Online]. 
+  Available: <http://wiki.ros.org/urdf/XML>
+- OnShape, "Importing and Exporting Files," OnShape Help, 2024. [Online]. 
+  Available: <https://cad.onshape.com/help>