Modeling Lead/Ball Screw Assemblies in Onshape

A Note on Modeling: Real-World Practice vs. CAD Skills

In an industrial setting, you would almost never model the complex internal workings of a standard component like a ball screw. The tiny recirculating balls and intricate raceways are computationally heavy and irrelevant for system-level integration. Instead, you would download a simplified 3D model from the manufacturer, just as we did for the linear guides.

However, knowing how to create custom screw threads from scratch is a fundamental CAD skill. It's essential for designing non-standard parts, custom adjustment screws, or even just representing threads accurately for visualization.

In this lesson, we will do both:

1. Integrate a Standard Part: We will follow the industry-standard workflow by sourcing and assembling a simplified model of our selected ball screw.
2. Build a Core Skill: We will then step aside from our main project to practice modeling a basic lead screw from scratch, using Onshape's Helix and Sweep tools.
3. Simulate the Motion: Finally, we'll learn how to make the parts move correctly in an assembly using a Screw Relation.

1. Sourcing and Assembling a Standard Ball Screw

Our project requires a 15 mm diameter, 10 mm lead ball screw. The assembly consists of the screw shaft itself, the ball nut that travels along it, and the end support bearings that hold the screw in place and manage the axial loads.

Just as with the linear guides, our first step is to source these components.

Your Task (Part 1):

1. Navigate to a supplier website that offers CAD models, such as McMaster-Carr, Misumi, or a manufacturer site like THK.
2. Search for a "Ball Screw Assembly".
3. Configure a set of components that match our specifications:
   • Ball Screw: ~15 mm diameter, 10 mm lead, length > 600mm.
   • Ball Nut: Matching the 15 mm screw.
   • End Supports: A set of fixed and simple supports for a 15 mm screw (often designated as BK/BF or FK/FF type supports).
4. Download the STEP files for the screw, nut, and both end supports.
5. In your Onshape project document, use the + menu and select Import to bring these components into your workspace.

Once imported, you will have a Part Studio for each component. For now, just confirm they are in your document. We will assemble them after learning about the specific mates required.

2. Modeling a Lead Screw from Scratch

Now, let's build a core CAD skill. We'll create a new Part Studio to model a simplified lead screw and nut. This will give you hands-on practice with the Helix and Sweep features, which are powerful tools for creating any kind of complex, path-based geometry.

The process involves creating a helical path and then "sweeping" a 2D profile along that path to create the 3D thread.

To see this process in action, the following video provides a concise demonstration.

Your Task (Part 2):
In a new Part Studio, try to replicate the process shown in the resources:

1. Create a cylindrical shaft.
2. Use the Helix tool on one of the circular faces. Set the definition to Pitch and enter a value (e.g., 2 mm).
3. Create a new Sketch on a plane that intersects the start point of the helix (e.g., the Front or Right plane).
4. Draw a small triangle or trapezoid for the thread profile. A key trick is to ensure the base of your profile is slightly smaller than the pitch to avoid errors.
5. Use a Coincident constraint to lock a point on your profile sketch to the end of the helix curve.
6. Use the Sweep tool, selecting your profile sketch as the 'Face' and the helix curve as the 'Sweep path'.

This exercise is purely for skill development, but it's a valuable one. Your experience with parametric modeling in other software will serve you well here.

3. Assembling and Simulating Screw Motion

Now, let's return to our downloaded ball screw components and assemble them. We need to create a relationship where rotating the screw causes the nut to translate linearly. This is achieved by combining a Cylindrical Mate with a Screw Relation.

First, let's understand what a Screw Relation does.

The process is to first allow both sliding and rotation, and then link them together. The following video demonstrates this sequence perfectly.

Your Task (Part 3):

1. Create a new Assembly tab in your Onshape document.
2. Insert the ball screw, ball nut, and end support components you downloaded.
3. Right-click the main screw shaft and select Fix. This will be our stationary reference. (In the final machine, one of the end supports would be fixed to the frame, but for now, this is simpler).
4. Select the Cylindrical Mate tool. Create a mate between the outer surface of the screw shaft and the inner cylindrical surface of the ball nut. At this point, you should be able to drag the nut up and down and also rotate it freely.
5. Now, select the Screw Relation tool from the toolbar.
6. In the dialog box, select the Cylindrical Mate you just created.
7. For Distance per revolution, enter 10 mm. This is the lead of our selected ball screw.
8. Click the green checkmark.

Now, try dragging the nut. You'll see that it rotates as it moves. Alternatively, right-click the nut and select "Animate." You should see the correct screw motion.

Conclusion

Excellent work. You've successfully modeled a critical assembly and learned how to simulate its unique motion, all while distinguishing between practical workflows and foundational skill-building.

Key Takeaways:

• For standard components like ball screws, the industry-standard workflow is to download and integrate supplier CAD models, not model them from scratch.
• Modeling custom threads is a valuable CAD skill accomplished using the Helix and Sweep features.
• To simulate screw motion in Onshape, you combine a Cylindrical Mate (which allows rotation and translation) with a Screw Relation that links them.
• The key parameter for a Screw Relation is the "Distance per revolution," which is equal to the screw's lead.

Preview of the Next Lesson:
We now have our primary motion components selected and modeled: the linear guides and the ball screw assembly. The next crucial step is to design the parts that connect everything. In the next lesson, you will design and model the custom mounting brackets needed to hold the motor and attach the ball screw assembly to our lift's structure. This is where we will start creating our own unique, project-specific parts.