The two largest problems with the first prototype were rigidity and speed. The leadscrew drew at a crazy high resolution of 4800 steps per inch, but a rate of about 30 seconds per inch. We abandoned the leadscrew for a rack and pinion system for both the horizontal and vertical car. This brought the resolution down to about 100 steps per inch, and the speed up to about one inch per second.To improve rigidity, the horizontal carriage rolls on bearings along an OpenBeam rail.
We moved the electronics to the horizontal car, and dramatically simplified the laser-cut construction. The stepper motor drives a gear along an laser-cut acrylic gear-track attached to the back of the open beam. The laser-cut gear is simply presure fit to stepper's D-shaft, this has been surprisingly reliable.
The vertical carriage rides on two pan-head M3 screws slotted into the OpenBeam. The screws are adjusted so the carriage can slide but doesn't wobble too much. We tried a small, light stepper first, and then a more powerful but heaveir one. The steppers partially, but struggled moving up and occassionally stalled and missed steps. WD-40 helped quite a bit but vertical motion was still not reliable enough.
The first prototype couldn't lift the pen at all. For the second prototype, mounted the pen to a micro-servo.
For this round we started building the front-end control software with Processing. The version had no GUI, but was able to send a series of hand coded instructions to the Arduino, or to parse an .svg using the geomerative library.