Remember, Dr. Wily made a robot out of wood. I'm sure Nitori can do way better.
Wood Man (ウッドマン Uddoman?) is a combat Robot Master created by Doctor Albert W. Wily using natural hinoki cypresses, his body being made almost completely out of wood, including some of his mechanisms. Because of this, Wood Man has great physical strength and resistance against physical blows, but is somewhat slow and highly vulnerable to fire and cutting weaponry.
However, it was mentioned in his CD data that his wooden body also has a coating of metal for extra protection.
Almost completely made of wood isn't completely made of wood. And you can still make gears and things out of wood.