Designing Sheet Metal Assemblies with Spot Welds in CATIA

In our previous lesson, we dissected the formal Engineering Change process and interpreted an ECO for a major BIW redesign. That exercise gave you a senior-level perspective on the "why" behind a design change. Today, we shift our focus to execution. You are the engineer tasked with implementing a BIW modification, and you need to build the new assembly in CAD.

This lesson directly addresses our learning outcome: to model a multi-part sheet metal assembly in CATIA, including spot weld flanges and joint features. We will explore the critical design rules for creating weldable geometry and then apply CATIA's specialized Body in White workbench to model the joints connecting the components. This is where DFM principles meet practical CAD modeling.

1. Fundamentals of BIW Assembly and Joining

A vehicle's Body-in-White (BIW) is the core structural shell, an intricate assembly of stamped sheet metal panels. Before we can model it, we must understand how it's held together.

The dominant joining technology in BIW is Resistance Spot Welding (RSW). In RSW, two or more sheets of metal are clamped together by copper electrodes, a high current is passed through them, and the resistance of the metal generates intense heat, melting the material locally to form a small, fused "nugget."

As a design engineer, your primary responsibility is to create the geometry that makes this process possible, reliable, and cost-effective. This means designing a proper weld flange. A weld flange is the overlapping area of two parts specifically designed to accommodate spot welds.

2. Golden Rules for Spot Weld Flange Design

Designing a robust and manufacturable weld flange is not arbitrary. It's governed by a set of "golden rules" derived from the physics of the welding process and the physical constraints of the manufacturing equipment.

Please review the following document, which provides an excellent overview of spot welding and the associated design rules.

Based on your reading, let's codify the essential rules for a typical 2-layer (2T) mild steel joint with a thickness of 1.0 mm:

Parameter	Rule	Rationale (Why is this important?)
Edge Distance	Min 6.5 mm from weld center to flange edge.	Prevents the weld nugget from being too close to the edge, which can cause material blowout, weak welds, and cosmetic defects ("spatter").
Weld Pitch	Min 25 mm between weld centers.	Prevents "shunting," where current from a new weld bypasses the weld interface and flows through a previously created, nearby weld, resulting in an undersized or cold weld.
Flange Width	Min 16 mm (approx. 2 x Edge Distance + margin).	Ensures enough material exists to safely place the weld and accommodate minor positioning variations in the assembly plant.
Accessibility	Ensure clear access for both weld electrodes.	The C-frame of the weld gun must be able to reach the flange without colliding with other geometry. This is a critical DFM check.

These rules are not just suggestions; they are fundamental constraints that you must build into your CAD models. A part that violates these rules cannot be manufactured correctly.

3. Modeling a Multi-Part Assembly in CATIA

Now, let's translate these rules into a CATIA modeling workflow. While you'll be using Onshape, the logic is based on the CATIA V5/V6 Automotive Body in White Fastening workbench. This specialized toolset is designed to manage the complexity of BIW joints.

The process involves creating logical "joint" features within the assembly, which contain the geometric spot or curve fasteners. This approach separates the part geometry (the flanges) from the joining features (the welds), which is crucial for downstream analysis and documentation.

The resource below is a technical user manual for this workbench. It is dense, but we will focus on the core workflow for creating a simple spot-welded joint.

4. Your Task: Model a BIW Reinforcement Assembly

Your assignment is to execute a portion of our hypothetical ECO. You will model a two-part assembly representing a reinforcement being added to an A-pillar.

Objective: Create a two-part sheet metal assembly joined by five spot welds, adhering to the golden rules of weld flange design.

Step 1: Model the Components (CATIA Part Design / Sheet Metal Logic)

1. A-Pillar Inner (APillar-Inner.CATPart):

   • Create a simple, curved sheet metal part representing a section of the A-pillar inner panel. You can start with a surface from GSD and use the Thicken command in Part Design or use the Wall command in the Sheet Metal Design workbench.
   • The part should be approximately 200 mm long and 80 mm wide.
   • Crucially, design a flat weld flange along one of its long edges with a width of 16 mm.
   • Assign a thickness of 1.2 mm.

2. Pillar Reinforcement (Pillar-Reinf.CATPart):

   • Create a second sheet metal part that will mate with the A-Pillar Inner.
   • This part should also have a corresponding 16 mm wide weld flange.
   • Add a feature to this part, such as a lightening hole (Pocket) or a small stamped feature (Stiffening Rib or Surface Stamp). The image below shows an example of a Surface Stamp feature being created in CATIA.

Step 2: Create the Assembly and Joints (CATIA Assembly Design / BIW Fastening Logic)

1. Create the Assembly (BIW-Assy.CATProduct):

   • Create a new Product file.
   • Insert both APillar-Inner.CATPart and Pillar-Reinf.CATPart.
   • Use assembly constraints to position the parts so their weld flanges are in contact (coincident surfaces).

2. Define the Joining Features:

   • Follow the logic from the CATIA BIW Fastening guide (LINK):
   • Create a BiW Joint: Select both parts to include them in the joint definition.
   • Create a Joint Body: Select the contact zones (the weld flanges) to define where the parts are joined.
   • Create BiW Welding SpotPoint features: Now, place five spot welds on the Joint Body.
     • Use the On Surface location method.
     • Define a spot diameter of 5.5 mm (referencing the table in LINK for a ~1.2mm thickness).
     • Place the first spot weld. Question: What is the minimum distance from the center of this spot to the edge of your 16 mm flange?
     • Place the remaining four spot welds. Question: Based on the golden rules, what is a suitable pitch (center-to-center distance) you should use between these welds?

Your final assembly should show the two parts correctly constrained, with five spot weld features joining them along the flange.

Conclusion

In this lesson, we bridged the gap between engineering theory and CAD execution. We established that for a BIW assembly, the design of the joining features is as important as the design of the parts themselves.

Key Takeaways:

• BIW structures are predominantly joined by Resistance Spot Welding (RSW).
• The weld flange is the critical geometric feature that enables RSW, and its design is governed by rules for width, edge distance, and weld pitch to ensure manufacturability and joint quality.
• Specialized CATIA workbenches allow the engineer to model not just the parts, but also the logical Joints and geometric Fasteners that define the assembly method.

In our next lesson, we will continue to analyze the impact of the ECO. Having modeled the new design, we must now quantify its effect on material usage by comparing flat patterns and perform a Design for Assembly (DFA) review to assess how the change impacts the assembly line.