Variation in chip formation is investigated. For chip control, it is important to clarify how chip flows and is broken. But sufficient answer has not been obtained. For one thing, so many a factor govern chip behavior, and for another, chip does not always flow stably and sometimes shows instability. The latter is the major subject of this paper. This instability and the factors governing chip behavior are discussed referring to photographs of produced chips taken with a camera fixed on the employed lathe above its tool post.