In the world of robotics and many other maker activities, a linear actuator is an important piece of the plan. These devices take the rotation of a simple motor and convert that to a linear motion, often with gearing to improve the force.
Micheal Rectin wanted to tackle the idea of building one of these using 3D Printing. Right off the bat, he acknowledges that you can buy them for less than $40, but designing something is a fun project all in itself.
While the value or utility of this specific example may be debatable, now Michael has a firm comprehension of how these work, and how to design them. That means if he needs a specific size, shape, or even angle, he can produce his own to fit the needs.