My first 3d-printed creation (and my 3d model that I printed it from). The photo below is three printings of the same design at different sizes:
The entire car prints, bottom to top, as a single run, and yes, those wheels actually turn. Each two wheel + axle combination is a single solid object, and the frame between the two actually has closed loops around the axles. So we have the magical-seeming trick of passing the solid axles through the loops - without needing any welding or gluing after the fact. Print it, unstick it, and roll it off the platform.
Playing with this has really connected a few physical-world concepts that didn't click for me before. For example, measurement tolerances are absolutes, not percentages. The printer I used is accurate to about 0.1mm. Previously, I had never cared about any distances less than a millimeter ("If it ain't on my ruler, it don't exist") but at this scale (the smallest car is only 1cm tall), it matters. To make the loops that wrap around the axle so they don't blur into the axle itself, I had to leave quite a bit of extra space. This produced a tight fit for the smallest car, but when we naively scale up the design linearly (the big one is about 3cm tall) that excess space leaves the axles pretty floppy. (Or we can call it 4-wheel steering, and then it's a feature.)
The other scaling-related lesson comes back to that old interview question: if somehow you were shrunk down to the size of a coin and put inside a blender, how would you escape?
The answer is that you'd jump out. Why? Because your body's mass scales down with n^3, but the strength in your muscles scales down much more slowly - let's say n^2. Relative to your size, you'd have super strength. That's why grasshoppers can easily leap to 100x their height, but nothing the size of a human can do so.
For the same reason, when you drop my big car on the floor, a wheel tends to break off. When you drop the little one on the floor, it stays intact. Why? Because the mass (and thus the force with which it hits the floor) has scaled up by 3^3 = 27, but the tensile strength is only about 3x higher. That's enough to break the rather weak connection between axle and wheel. That could presumably be fixed by better engineering, but I'm not really That Kind of Engineer.
In short, 3D printing is fun. But really it just makes me that much more impatient for nanobots. Ooh, micron-scale accuracy and toy cars made of diamonds!