Here is interesting proof of concept for a single arm 3d printer. It's name is Single Arm Printer Experiment (SAPE) and it uses extremely few vitamins and has only two bearings for movement. The arm looks like its made from simple PVC pipes.
Solidus Labs are well known for their GDR (grounded delta robot) 3d printer which is a platform for SAPE.
Here is SAPE doing the first print, there is still work to be done on it:
