Design for assembly

"Design for assembly" is an idea in engineering design that is meant to prevent you from designing an amazing, beautiful machine that is difficult or impossible to build.

I'm running into that problem occasionally with the wall and steps I'm building in the backyard—probably because I'm not an engineer, I'm a systems engineer, so I've never really had to design anything myself. The wall, as seen from the model, looks simple. (And it is simple, I suppose, but I'm inexperienced.) As I'm building the steps up to the garage into one leg of the wall, I'm finding little puzzles to solve during assembly that I didn't think of before.

The main problem is: a simple retaining wall is built with a layer of ~1-inch rocks behind it to help with drainage, so I cut the dirt so that there was room for that rock on both the deck wall and the wall under the steps. However, the wall under the steps will—obviously—have to support the steps, so it's not a good idea to put that chunky drainage rock there because it will be a poor support for the steps. Now I find myself making some slight adjustments to the design during assembly—filling in that space with the base rock (limestone, with large pieces and fine pieces that can all be compacted into a fairly solid mass).

This should have been obvious from the design, but I only modeled the large things like the retaining wall blocks, not the other bits that go around them such as size and shape of the base rock, or how the dirt would need to be cut away so that things could be built. In my mind assembly was just a simple shovel problem. In reality it's a moderately hard shovel problem, and each layer of steps takes extra time now as I have to imagine what that layer will look like—the dug hole, the base rock, the retaining wall blocks—before starting.

Assembly is an unloved aspect of engineering, and it should get more respect than it does, but it is only noticed when it doesn't work. Perhaps that's because much of assembly work is tacit knowledge—knowledge gained through experience, often unwritten, unspoken, known but unknown. Design is thought of as the Real Work. Assembly just happens. Don't think about it like that. Assembly is dark magic, and if complex parts and subassemblies and assemblies can be put together the way they are supposed to, that's because someone did the job right.


A reasonably good paper that explains DFX, or "design for x", where x = assembly, manufacturing, test, etc.: Kuo, Tsai-C., Samuel H. Huang, and Hong-C. Zhang. "Design for manufacture and design for ‘X’: concepts, applications, and perspectives." Computers & industrial engineering 41.3 (2001): 241-260. (pdf)

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