Why would a tinkerer break his hand?

The protagonist's adoptive father was changed from his father's hands, and he chopped off his hand because the emperor needed a human tinker's left hand, and this emperor was a human tinker's right hand and heart's blood, so the protagonist chopped off his hand to save his life.

The Human Tinker is a novel serialized by the author Fong Sugar on Zhihu, the background is that in this world there are painters, carpenters, masons, and human tinkers. The craftsmanship of a man maker is a rare craft. Not exquisite and pure to the extreme, the fire old way of people, is never dare to mention their own human craftsman's name.