The original RepRap design (Darwin) moved the print head in X and Y, and moved the build surface in Z only (downward as the print progresses). The Ultimaker ma

The original RepRap design (Darwin) moved the print head in X and Y, and moved the build surface in Z only (downward as the print progresses). The Ultimaker machine is based off the Darwin design, and works similarly. Ultimaker has a very lightweight printhead, so they can print incredibly fast. I haven't seen a moving-head deposition robot that moves only the head. I think this is because the simplest way to make a 3-axis robot involves two tools/surfaces, each moving in at least 1 axis. A tool/surface that moves in all 3 axes needs to be very stiff to avoid juddering around as the machine starts and stops line segments. It's much simpler to make a 2-axis robot and a 1-axis robot, than a 3-axis robot. :)