Making a Straight Rod Through a Curved Hole Demo
I made a compact version of this classic science museum demonstration, the rotating rod appears to magically pass through a curved slot. At first glance the motion seems impossible, however after a close look you can get some intuition of why this can work.
My version has a few extra features I find helpful to fully understanding what's going on here. I added the option to rotate the rod in 30 degree increments, so it's easier to observe how the cross section looks in different rod angles.
If you wish to build one your own, the exact dimensions including the curve templates can be downloaded here:
The models for the 3D printed angle adjustment mechanism can be found on my thingiverse page:
If you want one, but not into building this your own, there're a couple of options out there:
A unique clock that uses these mathematical concepts is sold by the guys over at maths gear
A beautifully designed 3D printed version from Devin Montes