It took almost two years before I was ready to start building actual Bobs. The speed at which a printer could produce an item was dependent partly on the size of the item, but also very much on the level of detail required. 3D printers delivered individual atoms using a number of tuned carbon nanotubes, each sized for specific elements. Building something like itself required the maximum level of detail and precision, as you had to place individual carbon atoms, one after another, with zero defects. This made 3D printers one of the most complex items that a 3D printer could be asked to build. Only something biological would be harder.