SchetsjesJS: assembling a basic drawing board

I’m getting somewhere with my Raspberry Pi/NodeJS plotter! Step 5 of my planning was to build the hardware-side of the plotter. It’s starting to look professional!

(Step 5:) Assemble a basic drawing board

Step 5 of my ‘Let’s build a Raspberry Pi Plotter’-plan was making a basic drawing board. On this I can attach the motors, a sheet of paper, etc. Essentially it’s the back bone of the whole plotter. I needed to buy some screws and stuff but I found some plates of thin wood at my attic which I attached to each other. After that I attached the motors, and some guiding nails to guide the string. The nails weren’t in my original plan but I noticed the wires attached to the stepper motors weren’t long enough to reach the sides. The nails serve to extend the reach. The result so far:

The nails causing the string to make a wider ‘V shape’, hopefully offering more precise control

The gondola

I’m not even sure if this is the right name, but what I mean is the hanging pen. I’ve seen a lot of plotters online already, most of them have custom 3D printed gondolas. I have no idea where to get my hands on a 3D printer, or a model of a gondola. I thought I’d be smart and make a very simplistic gondola using 2 types of paper clips. That didn’t work. The pen was the only thing touching the surface causing it to go it’s own way:

I now know that you need a lot more to get a stable gondola. Multiple contacts to the surface, a weight to make it more precise, a way to attach the pen, … It took me a lot of time figuring this out (and going through my trash and toolbox) but I’ve made something that seems to work:

The diagonal lines are the effect of one of the motors pulling the string. You see the curved effect it has on the line. A straight vertical line is easy: both motors need to pull the wires at the same speed. Going in a straight line horizontally will be a whole different problem, I’m currently not sure how. Goal 1 of this project is to draw a rectangle. I chose this shape on purpose: drawing straight lines is essential: it proves precision and a solution to the horizontal problem. But, at this moment I have a problem that could ruin it all, even with vertical straight lines:

Halp! Deviation!

I decided to test the amount of steps needed to go to the far left, far right, and center top of the paper. Everything is neatly centered: the sheet of paper, the motors, the nails guiding the string, etc. But still: after testing I found an issue: the same amount of steps doesn’t mean the same amount of distance drawn on paper.

An example using the simplest of tests, a vertical line upwards. I decided to do batches of 800 steps per try, and found out that the amount of steps needed to go all the way to the top is 2.400. But, I’ve found a problem. The first 800 steps account for 8 centimeters on the paper. The second 800 steps result in 9.7cm, and the third 800 steps result in 10.5cm. Dividing the amount of steps by the amount of centimeters we get the following: 100 steps/cm, 165 steps/cm, 229 steps/cm. On the image above you see that left and right also have an issue.

At this moment there are more issues than there are solutions I can think of. One cause of the deviation could be that the string is wound multiple times, increasing the diameter by a tiny bit, and causing it to deviate.

Here we’ve only got one ‘layer’ or string. It does wind up more, causing the diameter to increase. But is that enough to be the cause of the results I’m getting?

Next steps

There’s a couple of problems I need to fix but I’m not sure how yet. Therefore I’ll focus on the software side a bit in the upcoming period. Stay tuned!