As sempaiscuba says, the big problem with the question is that there is no provided definition of what a robot is.
Without a definition of a robot, it's not really possible to answer whether a 3D printer is or is not a robot. Clearly there is disagreement on everyone's personal opinions of what constitutes a robot.
I believe that, if the OP of that question could clearly define what is or is not a robot, then there would not be a need to ask if a 3D printer is a robot; it should be immediately evident by whether it does or does not meet the definition of a robot.
So then, in my opinion, the essence of the question reduces to, "What differentiates a robot from a machine?" If the OP there could define the difference between robot and machine, then again it becomes evident whether a 3D printer is a robot or "just" a machine.
This is why I marked the question as a duplicate of "What is the difference between a robot and a machine?" The printer question asks if a particular object is a machine. It's a more specific version of the broader "what's the difference between a robot and machine?" question.
If you can answer the broader question (it has an accepted answer) then the more narrow question should be answered by default.