Some notes about the options:

• Rotator: this controls the amount of rotation. Closer makes the increment between points smaller.
• Box Size: this controls how large the boxes that the sliders move in are.
• Separation: this is the distance between the center of the sliders.
• Speed: this controls how fast the sliders move.
• Length: this is the length of the "bar" that extends out from the center. On physical do-nothing grinders this is a handle that you can turn the grinder with.
• Show Lines: this toggles the lines on and off.
• Toggle Text: this toggles the text off and on.
• Clear: this clears the lines and dots, but keeps the sliders moving.
• Restart: this restarts the action with the current parameters. Use the browser refresh button to completely start over.
• Stop: this stops the simulation, keeping the screen as it is.

A do-nothing grinder has a platform with grooves that allow sliders to freely move in the grooves. In this case the grooves are at 90 degrees to each other. The sliders are rigidly connected at the centers along a rod that extends out to a handle that can be used to turn the grinder. You can make grinders with more grooves, but this simulation only models the 2-groove version. Here are a couple of websites that show and explain more about these do-nothing grinders: Secrets of the Nothing Grinder and The Trammel of Archimedes

I find this thing facinating. I tell my students that motion along the x-axis can't influence the motion along the perpendicular y-axis. That is how we solve 2-D physics problems, separate into two easier 1-D problems and then combine them at the end.

This thing forces the x-axis and the perpendicular y-axis, which are usually independent, to work together!

And it results in these circles and beautiful ellipses.

Change the options to get different shapes, etc.