Here is an old cnc project I made when I was just getting started with all of this. I had a broken XYZ DaVinci Mini W printer that was missing several key parts. I decided to turn it into a pen plotter.
I made a new pen holding tool head that fit with the built in quick release of the original head. To make the pen have some give to it for a more natural line I took my pen apart and moved the retract spring from the bottom of the pen to the top so it provides some downward pressure and has a quarter inch of give.
On the control side I removed the original board and replaced it with a grbl shield and an uno. I made a small daughter board that lets me use the optical limit switches already built into the machine. I don't remember the details anymore but I had to wire in some sort of a transistor to make this work.
Anyway it worked fine for what it was, and it was a good way to get some CNC building experience before setting out to make my MultiBot. I have no idea what I will do with this but I thought you may be interested in a bit of the history behind this project.