Comment by georgefrowny

Yep, "Harmonograph" is a similar device but uses gravity and strictly compounding (out of phase and magnitude) sinusoidal motions.

My dad and I each built one when I was up visiting him in Alaska 40 years ago or so. It became a kind of engineering competition between us. I stuck to the traditional harmonograph with just two axis: one pendulum rocking the "platen" that held the paper, the other pendulum driving a pen back and forth at 90 degrees relative to the first pendulum.

My dad threw money at his and purchased a couple of universal joints meant for RC cars. He then created a three-pendulum contraption. Two pendulums on the pen (here is where the universal joints came in) gave it movement in both X and Y. A single pendulum with the paper platen on it had on its own kind of gimbal mount that allowed it to swing around—not restricted to just one axis of travel.

My harmonograph always decayed. And the patterns it produced reflected it. My dad's, due to sometimes additive, sometimes subtractive motion in X and Y could appear to decay, visually, but then the pattern could expand again.

Clearly he won the competition that summer.