one issue is that pens need to have some compliance to the surface (versus a 3d printer or CNC, which don't) to get good lines on my CNC machine, I had to make

one issue is that pens need to have some compliance to the surface (versus a 3d printer or CNC, which don't) to get good lines on my CNC machine, I had to make a slide on the z axis with a spring to apply force downward with the pen.