I had to change out a flat tire on my bike this weekend, and I took the opportunity to take a closer look at the tire levers I was using. These tire levers from Park Tool has nifty feature where they snap together. This is very useful in keeping them together in a messy tube change kit.
The way they attach to each other is through a Lego-esque snap-fit feature molded into the part. Unlike Lego, however, the boss on the bottom lever only makes contact with the mating lever at the two cylindrical faces. And therein lies the reason why these levers don’t align perfectly every time, unlike Lego. As we saw a few weeks ago, Lego bricks make use of elastically averaged interfaces to achieve amazing performance (at least for an injection-molded plastic brick). Like Lego bricks, these levers are held together by local elastic deformation; but unlike Lego bricks, these only make contact at a small number of points (2 in this case). Obviously, averaging over a small number of contact points results in significantly lower repeatability.
Another issue with this interface is that it has a large amount of wiggle in it. That is, not only do the levers not line up the same way every time you snap them together, but they also rotate appreciably (~0.5 mm) relative to each other while engaged. This is a result of using only 2 contact points. As we know, 3 points of contact are necessary to fully constrain a planar part (the local elastic deformation provides the preload in this case). Unfortunately, this wiggle does detract slightly from the sense of quality you otherwise get from using these otherwise excellent tools.
Well, if anyone from Park Tool is reading this: when you next get a new mold, include a third point of contact!