Designing Structural Supports for Guide Rails and Assemblies

1. Why the Frame is a Precision Component

Before we jump into CAD, it's crucial to establish the design requirements. For a linear motion system, the primary function of the structural support is to maintain the alignment of all moving components under load. Misalignment is a major cause of premature wear, binding, and inconsistent performance.

This document from Tolomatic, a leader in linear actuators, provides an excellent overview of the critical alignment principles.

As the guide illustrates, our frame must ensure that the two linear guide rails are perfectly parallel in multiple planes. Any deviation will put unfavorable bending moments on the bearing blocks, leading to the problems mentioned.

Therefore, our design goals for the frame are:

• Stiffness: It must resist deflection under the weight of the components and the operational forces from the actuator.
• Parallelism & Flatness: It must provide mounting surfaces for the guide rails that are coplanar and parallel to a high degree of accuracy.
• Modularity & Manufacturability: It should be designed using common components and methods.

A very common and effective way to build such frames in industrial machinery is by using standard aluminum extrusions, like the popular 80/20 system.

2. Introduction to Onshape Frames

Instead of modeling these extrusions as individual parts and painstakingly mating them, Onshape provides a dedicated Frames toolset. This allows you to create complex structures quickly from a library of standard profiles, and it automatically manages joints and generates a cut list for manufacturing. This is the professional workflow for this type of design task.

First, let's get a quick overview of what the Frames toolset offers.

Now, let's see it in action. The following video provides a thorough demonstration of the frame design workflow.

3. Practical Application: Modeling Our Project Frame

We will now apply this knowledge to model the frame for our vertical linear actuator. A robust and simple design will consist of two parallel rails (for mounting the linear guides) connected by a few cross-members.

A highly effective method for creating frames is to first model a "skeleton" part. This is a simple solid body whose edges define the path for the frame members. This makes the design fully parametric; if you need to change the frame's size, you just edit the simple skeleton part, and the entire complex frame updates automatically.

Your Task: Model the Structural Frame

In your Onshape project, open a new Part Studio to create the frame.

1. Create the Skeleton:

   • Model a simple solid block that represents the overall volume your guide rails will mount to.
   • For dimensions, a good starting point would be a length slightly longer than your ball screw (e.g., if your screw is 500mm, make the skeleton 550mm long). The width should accommodate your linear guide spacing, and the height can be a standard profile size like 40mm or 80mm.
   • A simple rectangular Extrude is all you need.

2. Generate the Frame:

   • Activate the Frames tool.
   • Profile: From the Onshape-provided library, select a standard like 8020, T-Slotted Extrusions. A good, sturdy profile to start with is the 40 Series, 40-4040.
   • Selections: Select the four long edges of your skeleton block. You should see four frame members appear.
   • Alignment: Use the on-screen manipulator to ensure the profiles are aligned as you want them (e.g., flush with the top surface of your skeleton).

3. Add Cross-Members:

   • Create a new Frames feature.
   • Select the same profile.
   • Select the short edges at the ends of your skeleton to create two cross-members.
   • In the Frames dialog, under "End condition", you'll see the members are automatically trimmed to the existing long rails.

4. Review the Cut List:

   • On the right side of the Onshape window, find the Cut list table icon (it looks like a small table).
   • Open it and observe how Onshape has automatically created a list of all frame members, their lengths, and end-cut angles. This table can be placed directly onto a drawing later.

Conclusion

Excellent work today. You have now designed the structural foundation of our machine using a professional, industry-standard workflow. By starting with the engineering requirements for alignment and then using Onshape's specialized Frames tool, you've created a support structure that is not only easy to model but also stiff, manufacturable, and parametrically robust.

Key Takeaways:

• Structural supports in motion systems are precision components whose primary role is to ensure alignment and stiffness.
• Onshape's Frames toolset is the ideal way to model structures made from standard extruded profiles.
• The "skeleton modeling" technique creates robust, easily modifiable frame designs.
• Professional CAD tools automate tedious documentation, like generating a Cut List for manufacturing.

Preview of the Next Lesson:
We have now modeled all the major sub-systems: the drive (motor and ball screw), the guidance (linear rails), the custom interfaces (brackets), and the main structure (frame). In our next lesson, we will finally bring it all together. You will integrate all parts into a top-level assembly with appropriate mates, and for the first time, you will see your complete linear actuator system take shape.