Mechanism Housing Design for DFM

1. The "Why" Before the "How": An Introduction to DFM

Before we jump into CAD, it's essential to understand the mindset behind Design for Manufacturing (DFM). In industry, a design is only successful if it can be produced reliably, on time, and within budget. DFM is the practice of designing products with the manufacturing process in mind from the very beginning.

Your experience with FMEA has already introduced you to thinking about failure, and DFM is a similar proactive approach, but focused on production.

To start, let's watch a brief clip that powerfully illustrates the financial impact of good DFM.

Now, let's explore the fundamental principles that guide DFM. The following reading provides a concise, structured overview.

2. Designing for Stiffness: Using Ribs Effectively

Our housing needs to be rigid enough to maintain the alignment of the shafts under the tension of the belt drive. A common mistake is to simply make the walls thicker, which adds significant weight and material cost. A much more efficient method is to add ribs.

Ribs are thin, wall-like features that dramatically increase a part's stiffness with minimal material addition. However, they must be designed correctly to be effective and manufacturable, especially for processes like injection molding or casting.

The image below shows a housing that uses ribs (the internal cross-braces) to add stiffness.

Let's dive into the specific design rules for creating effective ribs.

Here is a helpful visual summary of these DFM rules for ribs and other common features.

3. Designing for Manufacturability: Process Constraints

To apply DFM, we must have a manufacturing process in mind. While our one-off project could be CNC machined, designing it as if it were an injection-molded plastic part is an excellent exercise for your portfolio. This is a very common scenario for producing housings in high volumes.

Let's learn the key constraints of injection molding and CNC machining, as these will directly influence our CAD model.

4. Your Task: Modeling the Housing in Onshape

Now it's time to apply these principles. Your task is to model a two-part housing that encloses the belt drive, using the support plate from the last lesson as the central structure.

1. Set up the Assembly Context:

• Open the assembly you created in the last lesson, which contains the two shafts, pulleys, and your support plate.
• If you haven't already, add a Belt feature to connect the two pulleys. You can find this in the assembly toolbar. This will give you a visual reference for the enclosure.

2. Model the Housing Base (In Context):

• Start a "Create Part Studio in Context" from your assembly.
• The goal is to build the "bottom" half of the enclosure.
• Create a sketch on the front face of your existing support plate.
• Use the Extrude tool to build up the outer walls around the belt and pulleys. Make them tall enough to enclose about half of the mechanism.
• Use the Shell command to hollow out the part, creating a uniform wall thickness (e.g., 3 mm). This is a critical DFM step.
• Use the Draft command to apply a small draft angle (1-2 degrees) to the inside and outside vertical walls. This is essential for mold release.
• Use the Fillet command to add radii to all the sharp internal and external corners. This helps with stress distribution and manufacturability.

3. Add Stiffness with Ribs:

• Now, let's reinforce the large, flat walls of the housing.
• Watch this short Onshape tutorial to understand how the Rib tool works. It's a powerful feature that automates the creation of these complex shapes.

• Create a new sketch on a plane that bisects your housing (e.g., the Top Plane).
• Draw lines where you want to add ribs. A simple cross or parallel pattern on the largest faces is a good start.
• Use the Rib command, selecting your sketch lines. Set the thickness according to the DFM rules (e.g., 50-70% of your 3 mm wall, so ~1.5-2 mm).

4. Model the Housing Cover (Optional, if time permits):

• Create a second new part that will act as the cover. You can model this in the same Part Studio.
• The cover should be a simpler part that mates to the top of the housing base. You can add a lip/groove feature to help align the two halves.
• Add clearance holes for fasteners that will hold the two halves together.

Conclusion

Great work today. You've tackled a task that is at the heart of mechanical product design: creating a functional part that is also designed for efficient manufacturing and assembly. This is exactly the kind of practical, process-oriented skill that makes a strong portfolio piece.

Key Takeaways:

• Design for Manufacturing (DFM) is a proactive mindset that integrates production constraints into the design process to reduce cost and improve quality.
• Ribs are a structurally efficient way to add stiffness to a part without adding excessive mass or material cost.
• DFM rules like uniform wall thickness, draft angles, and corner radii are driven by the constraints of specific manufacturing processes like injection molding and CNC machining.
• Onshape tools like Shell, Draft, and Rib are designed to help you efficiently implement these DFM principles in your models.

Preview of the Next Lesson:
We now have all the major custom components for our rotary mechanism: the shafts, the support plate, and the housing. In the next lesson, we will integrate all parts, including bearings and fasteners, into a complete assembly. This will involve organizing our final top-level assembly, adding standard hardware from Onshape's library, and ensuring everything fits together as intended.