Hello! Welcome back to the course.
In our last lesson, you learned how to strategically add stiffness to a component using ribs and bosses, focusing on how geometry is often more important than sheer mass. This is a key principle in designing efficient, lightweight parts.
Today, we'll continue exploring methods for creating strong, lightweight structures by focusing on another fundamental manufacturing process: sheet metal fabrication. Our learning outcome is to design sheet metal brackets for mechanical systems using OnShape's tools.
Brackets are ubiquitous in mechanical design, and sheet metal is one of the most common and cost-effective ways to produce them. You'll learn the core concepts of sheet metal design and the specific workflow and tools within Onshape to create your own parts. This is a critical skill for any design engineer and will be a great addition to your portfolio projects.
1. The Fundamentals of Sheet Metal Design
Before we jump into Onshape, let's establish the "why" and "what" of sheet metal design. The core idea is that you design a 3D part that is manufactured by cutting a 2D flat sheet of metal and then bending it into its final shape.
A key advantage of modern CAD systems like Onshape is the ability to work with the 3D folded model and the 2D flat pattern simultaneously. This is crucial because the flat pattern is what the fabrication shop will use to cut the part, while the 3D model is needed for assembly and analysis.
This simultaneous view allows you to see instantly how a change in the 3D model affects the flat pattern, helping you catch design issues early.
How Sheet Metal Parts are Started
There are three main ways to initiate a sheet metal model in Onshape. Understanding them helps you choose the right tool for the job.
Unfolding Sheet Metal Design from CAD Model to Fabrication
The Onshape blog article, "Unfolding Sheet Metal Design from CAD Model to Fabrication," provides a great overview of these methods.
Please read sections 2 and 3, titled "Embrace Simultaneous Sheet Metal Views" and "Understand Extrude, Convert, or Thicken."
As the article explains:
- Extrude: Creates a sheet metal part from a 2D sketch of lines and arcs. This is the most common method for designing a new bracket from scratch.
- Convert: Transforms an existing solid 3D model into a sheet metal part. This is useful if you've already modeled a part's overall shape and want to see if it can be made from sheet metal.
- Thicken: Creates a sheet metal part by giving thickness to a set of surfaces. This is powerful for more complex, freeform shapes.
For today's lesson on designing brackets, we will be focusing on the Extrude method.
2. Key Manufacturing Parameters
When you create a sheet metal part, you'll immediately be asked for a few key parameters that directly relate to the manufacturing process.
- Material Thickness: The uniform thickness of the metal sheet. This is the first and most fundamental parameter.
- Bend Radius: The inside radius of a bend. This is critical. If the radius is too small for the material type and thickness, the metal can crack on the outside of the bend. A common rule of thumb is to have a bend radius at least equal to the material thickness (e.g., 1.5mm thickness, 1.5mm bend radius).
- K-Factor: This is a value that represents how the material stretches during bending. It's used to precisely calculate the required length of the material in the flat pattern. For now, you can rely on the default values in Onshape (typically 0.45-0.5), but it's important to know this parameter exists as it's a key input for accurate fabrication.
- Bend Relief: When a bend doesn't run along the entire edge of a part, a small cut is needed at the end of the bend to prevent the material from tearing and deforming. Onshape provides several standard relief shapes, like
Tear,Rectangle, andObround.
3. Guided Practice: Modeling a Sheet Metal Bracket
Now it's time to apply these concepts. The best way to learn is by doing. You will follow a video tutorial to model a complete angle bracket in Onshape. This exercise will introduce you to the core features and workflow.
Sheet Metal Beginner Tutorial (Angle Bracket)
This video, "Sheet Metal Beginner Tutorial," from Onshape's official channel is an excellent, clear tutorial. You'll model a bracket from a drawing, which simulates a common real-world task. I encourage you to open Onshape and follow along step-by-step.
Watch from 00:32 to 10:34. The goal is to build the part along with the instructor. Pause the video as needed to complete each step in Onshape. Pay close attention to the following key operations: Initial Part Creation (00:32 - 05:00): Starting with a 2D sketch and using the Sheet Metal Model > Extrude command. Note how thickness, bend radius, and material direction are set. Adding a Flange (05:00 - 05:55): Using the Flange command, one of the most-used sheet metal tools. Cuts and Flat Pattern (05:55 - 07:15): Creating cutouts and, importantly, opening the Sheet Metal Table and Flat View to check your work. Completing the Part (07:15 - 10:34): Using a partial flange, adding holes, and mirroring the part to finish the design.
After completing this exercise, you will have used the fundamental tools to create a complete, manufacturable sheet metal part. Make sure to save this part, as we may use it in our next lesson on FEA.
Test your understanding!
In the video (around 03:45), the instructor points out that the bend radius should generally be greater than the wall thickness. Based on your engineering background, why is this a good design practice for sheet metal? What physical phenomenon is this rule trying to prevent?
Show answer
This rule is designed to prevent cracking on the outer surface of the bend. When metal is bent, the inner surface is compressed, and the outer surface is stretched (put under tension). If the bend radius is too tight relative to the material's thickness and ductility, the tensile strain on the outer surface can exceed the material's limit, causing it to fracture. A larger bend radius reduces this strain.
4. Expanding Your Sheet Metal Toolkit
The Extrude and Flange commands are the workhorses of sheet metal design. However, there are a few other tools that are very useful for creating more complex brackets.
The Tab Feature
While a Flange adds material by bending from an existing straight edge, a Tab adds a flat profile to a planar face. This is useful for adding mounting tabs or complex side features.
BEGINNERS Onshape SHEET METAL Tutorial - Step by Step guide
Let's look at another excellent tutorial from Too Tall Toby. We won't model this entire part, but we will watch a specific section to understand how the Tab feature works.
Watch from 06:42 to 11:20. The instructor first creates a detailed sketch on the face of the sheet metal part and then uses the Tab command to add that profile as a new feature. Notice how this differs from creating a flange.
Other Useful Features
Onshape has a full suite of sheet metal tools. You can explore these as you encounter more complex design needs. Here are a few to be aware of, which you can find in the Onshape Help documentation.
- Hem: Used to fold an edge over on itself to create a rounded, safe, and stiff edge.
- Corner: Allows you to modify how corners come together, for example, by creating closed or welded corners.
- Make Joint: Converts the intersection of two overlapping walls into a proper bend or rip feature.
You can get an overview of all these tools by reviewing the official documentation.
The Onshape Help document "Sheet Metal Model" contains a list of all available tools. It's a good resource to bookmark for future reference.
Briefly scan the section titled "Additional sheet metal tools" to familiarize yourself with the names and icons of other features available.
Conclusion
In this lesson, you've taken a significant step into practical, manufacturable design. You've learned how to think like a sheet metal fabricator while using the powerful and intuitive tools within Onshape.
Key Takeaways:
- Sheet metal design is a balance between the 3D folded part and the 2D flat pattern required for manufacturing.
- Onshape's simultaneous folded, flat, and table views are essential for efficient and error-free design.
- The primary workflow for creating new brackets involves starting with
Sheet Metal Model > Extrudeand adding features withFlangeandTab. - Considering manufacturability from the start by setting appropriate
thickness,bend radius, andreliefis crucial for a successful design.
Preview of the Next Lesson:
Now that you've designed a structural component, the next logical step in the engineering process is to verify its performance. In our next lesson, you will learn how to set up a basic stress analysis in Onshape, including applying materials, loads, and constraints. We will use a bracket similar to the one you designed today to see if it holds up under its expected load.