How screws are made

Screw threads are squeezed into the metal, not cut, which makes them stronger.

stations: 7
things upstream: 28
raw sources: 13
steps from the ground: 6

The short version

Screws are made from coils of steel wire in three main steps: heading, thread rolling and coating. A machine cuts a blank, squeezes a head onto it, then rolls a spiral thread into the shank between two hardened dies. A zinc or similar plating protects the finished screw from rust.

Cut blanks from wire

Wire is fed from a large coil and chopped into short pieces, one per screw. Bigger screws start from round bar instead of wire.

Goes in
Steel wire
Upset the head

Punches squeeze one end of each blank into a head at room temperature. A cold heading machine can turn out hundreds of parts a minute, and the grain of the steel flows around the head instead of being severed.

60 to 400 parts per minute
Polish the blanks

Before threading, the headed blanks are cleaned and polished so that the dies grip a smooth surface.
Roll the spiral

The shank is pressed between hardened dies that carry the thread pattern in reverse. The steel flows into the grooves, which makes rolled threads stronger than cut ones.

10 to 20 percent more tensile strength
Tumble with wood and leather

Screws spin in drums with wood and leather media that scrub off burrs and leave the surface bright.
Plate against rust

Most screws receive a coating, commonly a zinc electroplate, before packing.

Goes in
Zinc
Check and pack

Samples are gauged for thread fit and head size, then counted or weighed into boxes.

End of the lineScrew Next it goes into scissors, rubik's cube, ice skates and 10 more.

Worth repeating

More screw threads are produced each year than any other machine element.
The first screw factory in the United States was built in 1821 by Hardman Philips near Philipsburg.
Rolled threads can be 10 to 20 percent stronger in tension than cut ones.