This is my first model I've made, so it's far from perfect, but it's meant to be a teaching tool for explaining instantaneous rates of change to students.

To assemble, you simply push the small roller bottoms through the hole in the main body, then press-fit (and super glue if necessary) the larger roller tops onto them. The line with dy/dx can then be slid through the corresponding square holes in the roller tops. Finally, the square cap for the line can be affixed to the end of the line with super glue. This prevents it from sliding out of the roller tops.

The main body is printed without supports (as removing them from the channel is a nightmare), so there will be a little bit of stringing if you use the default settings. It's easily removable, however, so that's not a big issue.

Originally this build was designed to take up almost the entire 256x256 build surface, but I scaled it all to 95 percent (with the exception of the line and line cap, which are scaled to 93 percent). Even though the line is printed to 93 percent scale, it will still need some light sanding to move freely through the roller tops.

Feel free to provide suggestions for future models, as this is only the first version. I have included all of the associated STL files as well, if anyone wishes to modify the design.