This is a knurling tool. Its purpose is to press patterns onto metal to improve grip or surface texture. A straight knurl makes parallel lines, often used on knobs and handles. But when you switch to a toothed rolling tool, the metal can be pressed along a thread path to form the actual threads on bolts — a process called thread rolling, which makes the threads stronger and more durable than cutting. | Great leader

Knurling on a Lathe