Screws: Cold Headed and Thread Rolled

Most screws are made in three steps: heading, thread rolling and coating. Wire from a coil is cut into blanks, and each blank is cold headed: a punch squeezes the end into a die shaped like the head, so a flat head comes from a flat die, with almost no waste. The thread is then rolled, not cut: the blank is rolled between hardened dies that press the thread into it. Rolling makes no swarf, needs a blank 15–20 % lighter than cutting would, gives a 10–20 % stronger thread, and runs at about one piece a second. Most screws are then zinc plated or blackened against rust.