I was having a conversation with a friend at work last week and I needed to refer to some my old notes. I generally keep an A4 pad to scribble notes, make calculations and draw diagrams for whatever I’m working on. It occurred to me that some of my more recent diagrams, which I’ve done using the editor are ‘missing’ from my notes. My notebook will contain some related calculations, or even a rough sketch of the diagram, but not the proper version. I have printed off a few of them, and kept them on a separate folder, but it’s not the same.

Growing up as the son of a Civil Engineer, I was fortunate to have access to a proper HP pen plotter. That thing was AMAZING!

A surge of nostalgia and a drawer full of various bit of electronics, Arduino and motor parts and I was quickly putting together a pen plotter design.

There are many DIY plotter builds out there, ranging from those that reuse part of old disk drives to some really professional looking designs. I’m quite keen on building my own. It will be a fun, learning experience and I also have some particular requirements:

– It needs to fit fully inside an A4 page. That’s so that I can plot on any notepad I have, including those with spiral binding.

– It needs to have multiple pens! My diagrams use colors. I need to reproduce those colors!

The first requirement isn’t too bad. Most of the parts are quite small so I’m sure I can fit everything inside an A4. The print are will probably be quite small. I was hoping for A5 (half an A4), but i think it will be smaller.

The multiple pen caddy is a nice problem to solve! I had already started designing a 3 pen version when I realized that it’s not enough. I was thinking of red, green and blue, but I’ll also need black. Moving to a 4 pen design make things a bit more difficult, things are taking up more space, but it’s still doable. The bigger problem is how to switch from one pen to another.

I’m planning on using a spinning disk to lift/lower each pen in turn. As the disk turns it will lower one pen while lifting the next. At any one time only one pen will be down. The disk will be rotated using a servo motor, which allows precise control of its angular position. This would also let me do a half rotation, which would effectively lift all the pens off the paper to allow for repositioning.

Alas, moving to a 4 pen design creates a design flaw. With 3 pens you can move from any one pen to any other without ‘passing by’ another pen first. So, let’s say you have red, green and blue pens, you can move from the red to the green without ever lowering the blue one.

With 4 pens you can’t do that. To move from pen 1 to pen 3 you have to lower pen 2 first! This will end up putting a dot on the paper when there shouldn’t be any!

I’m now working on adding a proper z axis motor, to lift the whole caddy up/down. This means that I won’t have to program in a half turn to keep all pens off the paper and it will allow switching from one pen to another without any extra ink marks.

I’ve already gone through a few iterations of the design, but it’s not finished yet. Kudos to Fusion 360 for being such brilliant software to use!

Initial design with a massive caddy. This was too big, it would reduce the print are down to a post stamp!

A smaller design, but still not quite happy with it.

This is the current favourite!

More pictures and build progress notes to follow!