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Keep the Assembly Line Happy: 10 Design for Assembly Rules to Live By

Keep the Assembly Line Happy: 10 Design for Assembly Rules to Live By

“Perfection is achieved not when there Is nothing more to add, but when there is nothing left to take away”

– Antoine de Saint-Exupery

Your day isn’t going well when your boss quotes French novelists. In this case, my design was rejected under the accusation that it wasn’t “elegant”. What the heck was that supposed to mean?

Days later, having solved the puzzle, I returned with a design with half the parts. What was once captured in French prose became known as design for manufacture (DFM) and design for assembly (DFA) in the 1960s, a codification of ideas for creating products for large-scale manufacturing. So how can you make your design easier for your friends on the manufacturing floor and, well, more elegant?

Download this checklist as a PDF to keep as a handy reference!

1. Minimize Part Count

Look for ways to combine parts. For instance, many electronics housings use living hinges instead of a knuckle hinge. When routing wires, choose molded guide features, or use a heat-formed guide (like in old LazerTag guns). And speaking of minimizing part count…

2. Build in Fasteners

Whenever possible, build the assembly features directly into the parts, instead of using screws. Snap fits are often just as secure and require no tools to assemble. Sometimes screws are necessary, but use sparingly—fasteners can consume as much as 50% of assembly labor. One thing to note: Snap fits can increase the cost of injection-mold tooling, so be sure to design the parts to be injection-molding friendly.

3. Use COTS Parts

Being a product designer now is great. So many of our design problems are already solved! Our Victorian progenitors had to meticulously design each screw thread, but we have the option of hundreds of standard diameters and pitches.

And this extends way beyond basic nuts and bolts. COTS (commercial off the shelf) parts cover springs, pins, motors, microcontrollers, sensors, gears—most of the functional aspects of a design. This not only frees you to focus on unique challenges, it also means that the manufacturing team already has the tools and skills to assemble your design.

4. Use the Same Parts Throughout the Design and Product Family

A caveat on COTS parts: Just using standard screws isn’t enough. I once designed a robotic assembly with M5 x 10 mm socket head screws in one section and M4.5 x 12 mm hex head screws on another part of the design. Why? I plead Monday morning insanity.

I had to frequently switch between tools for assembly; it was easy to confuse which screw would go where—it was a very bad idea. Don’t follow my example: Standardize your parts across not just each assembly, but across the entire product line. Whenever possible, a single tool should be used for the whole assembly.

5. Use Modular Designs

A great application of both COTS parts and common parts is modularity—breaking designs down into smaller sub-assemblies, which can often be used in multiple products. Think of your first computer: You could combine a number of pre-assembled parts—motherboard, hard drive, graphics card—and it was easy. And as an added benefit, modular designs aren’t just great in the assembly line; they also help keep your products in the field longer by facilitating repairs and upgrades.

6. Make Connections Unique

Speaking of desktop computer assembly, another great example of design for assembly can be found both inside and outside computer cases from the 90s to the early 2000s—each connection is mechanically unique. The mouse cables can’t plug into the monitor port. The power cord port can’t be confused with with the keyboard cable.

Of course, I’m showing my age—today’s computers, where everything can be run using USB-C are even more DFA friendly (see rule 4).

7. Give Parts Clear Orientation

Related to the idea of unique connections is unique orientation: If there is a right and wrong orientation for parts to be assembled, make it obvious which way is correct. Even better, make it impossible to assemble the parts in the wrong way. For round parts, this may mean just having a notch, but with more complex shapes, this can provide an opportunity for some creative design. Just don’t get too creative…

8. Make Parts Easy to Manipulate

…because assembly these days is increasingly handled by robots. Automation costs are decreasing rapidly, and robots are appearing on more assembly lines. Design your components for easy grasping by robotic pincers, and avoid very small or very flexible parts when possible. This will also be appreciated by the humans on the assembly line—no one likes using a microscope to insert a screw.

9. Avoid Delicate Parts and No-Touch Areas

Similarly, help both the robots and humans on the assembly line by making parts durable during the assembly process. If parts are too delicate, or if they can easily be ruined by natural skin oils, rework will sky rocket. Think, if Bruce Banner were angry, could he still perform this assembly step in under 10 seconds? If the answer is no, redesign.

10. Design for Real-World Tolerances

Finally, make sure custom parts can be manufactured easily. Tolerances stack up, and a minor variation in each operation can add up to a big problem—especially if your design isn’t able to accommodate the variation.

Whenever possible, give your processes as much room for error as possible. Yes, the machinist can make your part fit to within one-ten-thousandth, but if that’s required you’ll unnecessarily introduce very expensive machining. And with 3D printed parts, consider the same: even laser-sintered have variation, and it’s worth considering how these tolerances will fit together.

Increasing Elegance

One of the key areas that differentiates great old engineers from wet-behind-the-ears grads is the simplicity of their designs. Cutting through the extra parts to a simple solution isn’t just “elegant”—it saves your company in a big way, both in manufacturing time and in reducing defects. For more on essential design for assembly lessons, check out our post on The Fundamentals of Hardware Assembly Design.