A project that I've been desperate to try for a while: convert Eggbot from Evil Mad Scientist Laboratories into a plotter.

This isn't a plotter that works with X-Y axes, rather it's a plotter that works by hanging a pen assembly from a string passing over two motor spindles.  I have no idea what this kind of plotter is called if in fact it has a name...

The hardware from Eggbot worked perfectly once I de-mounted it from the eggbot chassis.  I made some really rough mounting plates for the motors and the control board and a couple of iterations produced a working pen holder.

The pen holder gave me some trouble, it needs to hold the pen at constant alignment to the paper, any wobble makes the lines drawn really imprecise, so I settled on using 4 bolts as guides (with 2 heavier ones at the bottom), the suspension cotton is just run through two holes in centre of the holder.

The cotton is wrapped around the motor spindles a few times, a counterweight on each side (more bolts) keeps it in tension and reduces the load on the motor.  I was expecting some slip but I saw nothing significant, even on a really long plot.  I was also prepared for the flat spots on the spindles to make the plots wobble but I couldn't see anything on the ellipses I drew.

I had a happy accident with the mounting for the board, I put it on the Ikea painting/drawing easel the kids use and it turned out to be exactly the right height, width and angle, it should even be possible to mount the plotter directly on the easel & use it as a dedicated free-standing piece...

Theres a video (YouTube link) of the plotter in action embedded below, it's not fast so I think it might need larger spindles...

It would be very possible to make up some laser-cut mounts for the motors, control board and pen holder - in fact I'm working on that now & when I'm finished I'll make them available for download.

The next step is software, the geometry of this plotter means that the rotational movements of the motors don't neatly break down into linear X or Y movements and so the control software will need some tweaks to allow un-distorted shapes to be drawn.