There is a huge leap in complexity between simply making a machine that will exist on its own and making a part to work inside an existing machine. At St. Raymond our Mechatronics classes are decidedly "non-robot" and "non-car". We want students of all interests to pick up Mechatronics as a skill, so our classroom projects don't look like the stereotypical robots and cars of other programs. We design and build things like microprocessor controlled water drinking bugs and wooden speakers.
Our after-school Makerspace is very different. Students can walk in with any interest and simply build what is in their mind's eye. Projects are anywhere from a few hours long to 30 hours long (like in this case). We had a student who is totally into Radio Control race cars and wanted to design a part to improve on the design suspension of a factory product. This meant 1) Understanding the Geometry involved in the suspension system, 2) Identifying the current and modified performance characteristics they want to see and 3) Design and building a part with very, very tight tolerances to meet the stated goals for performance characteristics.
The student that embarked on this project is 9 years old. The student spent every available hour in the after school Makerspace at St. Raymond learning Tinkercad and how to set up, use and maintain the Dremel Idea Builder 3D printer. Thirty+ hours, and a many iterations later, the student installed a successful prototype part into the suspension system that is more than 20% lighter and modifies the car's suspension geometry to the desired 1.4° of camber angle. The part had to fit with the other parts from the factory perfectly. It did. We don't have a camera lens big enough to capture the sense of pride-in-craft that the student showed while driving the car around those initial laps with their newly minted parts successfully installed and an engineering problem solved!