Building a Full 3D Assembly in OnShape

1. Planning for Change: The Core of Professional CAD

Before creating a single solid feature, it's essential to plan your model's architecture. In a real-world project, requirements change. A professional engineer designs models that can adapt to these changes without catastrophic failures or requiring a complete rebuild. This is known as designing with design intent.

The Onshape blog article, "Flexible Parametric CAD Models," provides an excellent overview of the philosophy and techniques used by industry experts to build such models.

As you read, you'll see a recurring theme: structure and planning are paramount. Two key takeaways for your project are:

• Skeleton-Driven Layouts: Using one or more master sketches to define the critical dimensions and positions of multiple parts. If a dimension in the skeleton sketch changes, all dependent parts update automatically.
• Multi-Part Part Studios: Modeling parts that are designed to work together within the same Part Studio. This allows you to create parametric relationships between parts, not just within a single part.

2. In-Context Design: The Multi-Part Part Studio

The Multi-Part Part Studio is one of Onshape's most powerful features for implementing a top-down design strategy. It allows you to model components relative to each other, ensuring they always fit and update together. This is a significant departure from traditional CAD workflows where each part is modeled in isolation.

Let's watch a short video from Onshape that demonstrates this "in-context" workflow.

This in-context approach is incredibly powerful. For your project, this means you can create a "skeleton" sketch defining your mechanism's layout, then build your links, brackets, and other components directly referencing that master layout, all within one organized space.

3. Creating the Assembly and Defining Motion

Once you have modeled your parts (whether in single or multi-part studios), the next step is to bring them into an Assembly tab. This is where you define the kinematic relationships—how the parts move relative to each other.

You've done this in previous projects, so this should be a refresher. Mates like Fastened, Revolute, and Slider are the building blocks of any mechanism.

Remember to always test your mechanism's motion after adding mates by clicking and dragging components. This is the quickest way to verify that your assembly behaves as you intended.

Your Task: Develop Your Concept Assembly (30 minutes)

Now it's time to apply this professional workflow to your independent project. You will begin translating your chosen sketch into a robust, parametrically-driven 3D assembly. You may not finish the entire assembly in this session, but the goal is to establish the core structure and model the key components.

Follow these steps:

1. Plan Your Structure:

   • Before you model, think about the hierarchy. What are the main structural parts that define the overall size and shape? What are the moving parts?
   • Review the "4 Things to Consider Before You Start Modeling" (Orientation, Symmetry, Patterns, Complexity) in the Mastering 3D Modeling guide (LINK, p. 15-20) to inform your plan.

2. Create a Layout/Skeleton Studio:

   • In a new Onshape document for your project, create a Part Studio. Rename it "Layout" or "Skeleton".
   • Use the Variables tool to define key global parameters (e.g., #overall_length = 300mm, #pivot_distance = 75mm).
   • Create one or more master sketches (your "skeleton"). Dimension these sketches using your variables. This skeleton should define the key anchor points, pivot locations, axis lines, and overall envelope of your design. Do not model detailed parts here; this is just the framework.

3. Model Components In-Context:

   • Create a new Part Studio for your first set of related parts (e.g., "Frame and Mounts").
   • Use the Derived feature to bring in the skeleton sketch from your Layout studio.
   • Begin modeling your parts, using the derived skeleton as the primary reference for your sketches.
   • Use the "New" part option in your extrude/revolve features to create multiple parts within this studio, just as shown in the "Designing Parts Together" video.

4. Create the Top-Level Assembly:

   • Insert a new Assembly tab.
   • Insert the components you have modeled from your Part Studios.
   • Use a Fastened mate to fix one of your main structural components (like the base frame) to the assembly's origin. This provides a stable anchor for the rest of the mechanism.
   • Apply Revolute, Slider, or other appropriate mates to define the intended motion of your mechanism.

5. Test and Verify:

   • As you add mates, click and drag the moving parts to ensure the motion is correct. Use the Animate mate feature to see the pure motion.
   • If something doesn't move as expected, review your mates. Are they constrained correctly? Are you missing a degree of freedom?

Keep the "Do's and Don'ts for parametric modeling" list (LINK, p. 14) handy as you work. Building good habits now will save you immense time later.

Conclusion

Fantastic work today. You've begun the detailed design phase by applying a powerful, professional CAD methodology. By prioritizing planning and using a top-down approach with skeleton sketches and multi-part studios, you are building a design that is not only functional but also robust and adaptable.

Key Takeaways:

• Top-Down Design is a professional workflow that starts with a high-level layout (a "skeleton") to drive the geometry of detailed parts.
• Multi-Part Part Studios are Onshape's primary tool for designing interconnected parts in-context, ensuring they always fit and update together.
• Design Intent is captured by creating parametric relationships between a master layout and the components, making the entire assembly resilient to changes.
• Assemblies are used to bring parts together and define their kinematic motion using Mates.

Preview of the Next Lesson:
Now that you have the initial 3D geometry of your assembly, the next step is to ensure it's strong enough to work. In our next lesson, you will perform targeted hand calculations and FEA to validate critical components. We will identify the most highly stressed parts in your design and begin the process of analyzing them to verify their performance.