Design tips, techniques for stamping BIW
Based on design architecture, you can mitigate some potential issues with fit-up during assembly by adding some margins into the design called design gaps. The strategy is more about manufacturing lessons learned than math.
How do you design a sheet metal part to deliver good body structures performance?
If you are looking for a single and magical response, sorry; it doesn't exist. But some design best practices often help to achieve the expected body structures performance and prevent potential problems.
No matter the vehicle architecture, body structure designers and engineers always encounter two “bad guys”: stress and strain. Whether you are an experienced or novice automotive designer, you are probably seeking ways to deal with stress and strain.
Some attribute problems can be managed by using different material grades and thicknesses. The shape also plays an important role in achieving a performance target and should be also part of solution—even more when you are working to deliver durability and safety performance.
Following are key design best practices to help you mitigate potential performance problems, avoiding stress risers and concentrators around your design:
Bear in mind that you may always have some correlation gaps when going from virtual to real phases. In other words, even if a problem isn't found during virtual analysis, that doesn't mean one won't appear during physical validations. That's why you need work in all ways to mitigate potential problems in your design.
It’s always best to avoid stress concentration points for all parts and joints in your design—from the first sketch. This will help save money and time.
Another term that needs attention, since it’s the initial sheet metal parts design and stamping process definition, is punch direction.
All the body-in-white (BIW) parts have a number of holes, each one with a specific function. The main functions are locating, clipping, gun access, joint clearance, anti-rotation, and weight relief.
After the piercing stages, the part gets some burrs around the holes opposite of where punch breaks through the material. This is a normal and expected result, inherent with the process.
Flutes design is a simple design strategy. By keeping some flanges off some surfaces to create gaps between mating surfaces, you reinforce part stiffness.
You always will get some burrs, but if they are not controlled, they will directly and negatively affect the clipping operation during assembly steps. The severity of hole quality affects clipping in the interface with the interior trim, exterior ornamentation, and wiring systems.
The best way to mitigate burring is to define the punch direction in your design, locating the punching and clipping installation in the same direction. But sometimes it is not possible to do this because of process constraints, and the hole tolerances have to handle the reverse directions. There is an acceptance criteria for burrs, including cut edges, and this also is part of quality inspection and BIW deliverables.
A design gap is a helpful strategy to support the BIW manufacturing phases.
Considering all geometric and process variations, one of the main challenges during BIW assembly steps is the parts’ marriage. Some constraints already are expected, based on part geometry and assembly tolerance stack-up previously identified at geometric dimensioning and tolerancing (GD&T) and virtual analysis. Still, managing the process variations in a physical environment is complex. This means that even when you do your homework very well, you may face some unexpected problems.
The last thing you want during a manufacturing rollout is to have a BIW assembly issue with no option to adjust the welding fixtures or parts location.
It’s true that sometimes you will have no option or other parts to assemble, and you need to keep going until you find the root cause. This is real life on the shop floor!
Fortunately, based on design architecture, you can predict and mitigate some potential issues during assembly steps by adding margins into the design as possible fixes. That’s called design gap strategy. The strategy is more about manufacturing lessons learned than math.
A simple example is a single part with more than three mating surfaces to match up. If you have worked already with GD&T, you may understand the potential concerns. On top of that, add the shape complexity and subassembly interfaces.
The point is, sometimes you will have no option to design the interface in another way, so you need to ensure that the part will fit properly during the assembly steps under the worst conditions. Plus, you still must have proper quality under control. The design gaps application has some limits and parameters and is part of manufacturing signoff before product release.
It's not just a matter of creating clearance among the parts. You still need for those surfaces to be welded without affecting the joint quality and appearance.
Escalope flanges are design features that reduce body weight by simply cutting off the material along the flanges not used for welding joints.
This is a simple example of product engineering working with eyes on the manufacturing process. This is the best way to design parts, define processes, and deliver proper manufacturing quality.
A flutes design is a simple strategy to support BIW deliverables without losing structural stiffness.
Never forget, the BIW assembly is designed to support functions and attributes. Every single hole, flange, form, and shape has a specific function to support the system interfaces and complete vehicle deliverables.
So, if you are working for body engineering, you must know your system very well—what it means, why, how, and when each detail is added into every single part.
Flutes design is a simple design strategy. Basically, you keep some flanges off some surfaces to create gaps between mating surfaces. It's a good strategy to keep the whole line flange from decreasing the part stiffness, which is the last thing you want to lose in BIW structure. This design strategy is used to support a few BIW deliverables. For example:
The main advantage of using the flutes design strategy is that you can keep the same part stiffness most of the time. You can keep the flanges with flutes instead of adding notches that weaken the systems.
So, whether you need to add an e-coat drainage feature, mitigate squeaks and rattles, or create a new welding joint, remember to first try adding flutes. Most of the time it works. But never forget to check stamping feasibility too.
This is a smart way to deliver the BIW weight target while retaining the part’s function.
Weight is one of the most important vehicle deliverables. In addition to being related to inertia class and emissions, it affects the vehicle performance and is part of body engineering achievements. Yes, the design has a weight target. The BIW is the biggest vehicle system, which means more weight contribution.
Weight targets are something that every BIW engineer faces, and with which they have troubles and challenges. Achieving a weight target is a complex task, and even more so when you need to improve the structural performance by adding parts or increasing the sections or gauges. It's a complex design trade with no option to fail, no matter how. The vehicle weight needs to be delivered.
Even under control, burrs are not without sharp edges. This is a critical condition that is an injury risk for operators. To avoid injury risks, simply design flanges for all hands-accessible slots.
To help with this task—considering the BIW is weighty—you can use a design strategy called escalope flanges to reduce weight.
This strategy consists of simply cutting off the material along the flanges not used for welding joints to reduce the final part shape.
It sounds simple but is not. Before you can apply this design strategy, some interfaces need considering. They include static sealing application, stamping feasibility, welding fusion area, and the overall system performance. Removing material often means reducing part stiffness. Adding the escalope also creates some triggers along the flanges, so this strategy often plays against the structural stiffness.
You can also use this design strategy to improve stamping feasibility by removing wrinkles or thinning areas from part design. This is not the best option, but sometimes is the only one you have. As always, the main focus is the final part shape working on the overall BIW system.Bear in mind that by using design escalope strategy, downgauging, or reducing part sections, the BIW weight target needs to be achieved. The same applies if the weight is under target.
A very important design rule that can't be forgotten during engineering phases and virtual manufacturing signoff is regarding sharp edges. Whether you’re an experienced body structures engineer or just starting out, one thing that you must know is that all external edges of sheet metal parts are as sharp as a samurai blade. Yes, anytime you need to grab a part with your hands, wear proper gloves and always do it with high attention. Never forget this!
Knowing that burrs are inherent to the piercing, cutting, and stamping process, you must follow some acceptance criteria to control the burrs.
It's not only about injuring hands; the burrs might affect parts’ marriage, corrosion protection, systems installation, and the attributes too.
Besides burr control, you should consider some engineering rules related to sharpening edges during the manufacturing phases—mainly for final manual assembly steps. Basically, you need to prevent sheet metal edges coming into direct contact with operator hands and arms in all open windows or slots at structures that may require hand access during systems installation.
Even under control, burrs are not without sharp edges. This is a critical condition that is an injury risk for operators. Safety first, always!
To avoid injuries, simply flange all hands-accessible slots into the BIW structure. This is the best way to prevent the operator from directly touching the sharp edges during assembly steps. All sheet metal part edges that pose injury risks to the operator must be avoided. As an added benefit, flanging the slots often helps to improve the parts' stiffness.
So, with one design feature you mitigate operator injury, reduce the BIW weight, and fortify BIW structural stiffness.As a final note, always try to use design challenges to bring advantages to your body structure. The vehicle requirements don’t change often, so it’s just a matter of finding the best way to deliver them.E-coat ingress and regress.Squeaks and rattles.Welding joint stack-up.