Posted November 6, 2017
4 Design For Assembly Considerations
Although proper Design for Manufacturing requires the utmost care be taken during the design of your product, manufacturing is not the only step that should be on your mind when designing: assembly is critical as well. Whether you assemble the product yourself or someone else does, it’s important to anticipate what will unfold on the assembly line and how your design can affect the process. Here are a few tips for how to best design your product with assembly (and disassembly) in mind.
Tools, Fixtures & Fasteners
Consider all of the tools needed to perform assembly, and how the technician will use them. Try to change your design to minimize the number of tools required. For example, if a driver is required for a single bolt of one type, change the design so a more common bolt and tool can be used. When it comes to fasteners, consider the time it takes to install and the tool needed to do so. Using fewer tools and types of fasteners simplifies both the purchasing and assembly processes. The best solution, though, is to design fasteners out altogether by utilizing fastening features such as internal clips.
Milled plastic clips for holding a wire in place.
For the tools that are required, note their physical limitations and size: will the technician be able to fit them into the space they’re needed to do their job? When considering fastening screws on the inside of a product, ensure the tool can access the fastener at all. Adding a tool access hole into your design may remove the need for technicians or customers to disassemble a significant portion of the product. Also ensure your technicians can see what they’re doing at all times, as a fastening step will take much longer and be far more challenging if they can’t see anything.
Without a tool access hole (left), the bolt is inaccessible and cannot be installed.
Also consider if any custom tools will have to be purchased or fabricated. These can be simple jigs for aligning components, a special attachment for an off-the-shelf tool, or tooling for installing press-in fasteners. It may be possible to change the design so these aren’t necessary, or utilize unmodified off-the-shelf tools. Don’t forget that you’ll likely need multiple of each tool, once you move past a certain number of production units, so keeping cost low is key!
Also, ask yourself if precision interfaces and locations are present in your product. Instead of building tight tolerances into the parts of the product, consider the possibility of building one or two fixtures for precise assembly. By doing this, you remove the burden of dealing with tight tolerances from your contract manufacturers, therefore dropping the price of your parts. Instead, focus on the design and manufacture of the precision fixture, which can lead to the same end result, but relies on precision in assembly rather than precision in manufacturing. In the same vein, if precise location isn’t required, consider designing in self-locating features to your parts to make it difficult, or impossible, to assemble incorrectly.
Safety & Ergonomics
The safety and comfort of your technicians is important. If they aren’t comfortable while doing their work, the end product will suffer as a result. It’s likely that the design of your product can be changed to reduce the physical effort needed to put it together. Safety is paramount, of course: consider beforehand if a particular assembly step requires a loud operation. If so, use safety protection and require others nearby to do the same.
If the assembly environment contains any potential airborne projectiles, such as clipped ends of wires or splattering liquid, safety glasses should be worn. Gloves are also fantastic for protecting both the technician and the product: hand oils can quicken the spread of rust on an unprotected steel surface, for example. The chance of this occurring during assembly can be reduced with a coating or material choice: the operator could first coat the steel with oil to protect the surface, or aluminum could be selected in the design phase, for an even greater rust-resistant part.
Also consider how your technicians will interact with the product or part they’re assembling: excessive movements of a heavy part can be taxing. However, a clever design choice can improve ergonomics for your technicians. For example, placing many of the fasteners along a face that can be accessed from the same orientation reduces burden on the technician. If you can’t design in hand-holds or features that make it easy to carry the product, consider designing a custom fixture that makes it easier to move the product around. Fixtures that keep your product stationary on the line are always helpful, too.
One of the biggest challenges is the ability to foresee issues with wiring, as it’s time-consuming to model the exact path it will follow in a 3D model and therefore sometimes neglected in CAD. As a result, it’s important to consider the requirements of your wiring: the size of holes needed for connectors to pass through, the path the wiring will travel, and opportunities to provide strain relief. It’s important to walk through the assembly steps before wrapping up the design, and ask yourself if the product can even be assembled as-is. If your wiring harnesses are being made elsewhere during production, it’s important to either receive a sample or make one yourself, to test the fit. If you’re not careful, you may design an impossible-to-assemble product by removing access to wiring before it’s connected to a PCB, for example.
Designing out fasteners by using clever techniques such as internal clips is just as possible as removing the need for cable ties and other wire retention products by designing in wiring channels or other fastening features.
Channels can be added for routing wiring.
The way a product is disassembled largely affects the speed with which you turn around test results or support requests. Consider the inevitable occurrence of a used product showing up on your bench again, whether it’s at the end of a lot of testing, returning from a review or from an unhappy customer. The product will have to be disassembled and inspected quickly. This is where designing in an unnecessarily rigid seal, maybe one that’s glued instead of fastened with a more temporary method, can add significant time to your process. Similarly, don’t use a permanent fastening method like rivets in an area that may need disassembly.
If the customer may end up doing some disassembly themselves, you can reduce the burden on your support team by incorporating features to make certain components obviously disassemble-able. Consider using a different finish or color for fasteners that can be removed by customers, and include the tools required for them to do so. When it comes to applying threadlocker, use the super strong stuff on any fasteners that you never want touched, but use slightly less strong liquid for anything the customer may interact with, and instruct them on how to deal with it.
In general, there are a few key takeaways when it comes to Designing for Assembly: First, reduce overall part and tool count. Next, reduce the complexity of your mass-produced parts, both for the sake of easy assembly and lower part costs. Carefully consider how your wiring will be routed throughout your product, and design your parts to be assembled without fail.
The disk part can be installed in four orientations, increasing the risk of incorrect placement.
Now, with a flat edge, the disk part can only be installed in one orientation.
Ideally, your parts are symmetric and can be installed in a variety of orientations, but if they must be asymmetric, make it blatant in your design. And finally, consider the health of the workers on the assembly line, and how they’re interacting with the product.
In the end, though, it’s all about consistent improvement through many iterations: you’re going to miss some things from the start, but by keeping a close eye on how your product is assembled (and trying it for yourself!), you can rapidly improve the design to make everyone’s lives easier, and more profitable.