How bolts are made

Before Joseph Whitworth's 1841 proposal for a standard, each nut was made by hand to match one bolt.

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

The short version

Most bolts are cold-formed from steel wire or bar. A machine cuts a slug, squeezes one end into a hexagonal head, and rolls a thread onto the shank between hardened dies. Strong grades are then heat treated, and many bolts get a zinc coating so they survive rain and rust.

Cut a slug of wire

Steel wire or bar is sheared into pieces, each long enough to make one bolt. Because the metal is only moved around later, the slug carries exactly the volume of steel the finished bolt needs.

Goes in
Steel wire
Upset the head

A punch hammers one end of the slug into a head inside a die, without heating the steel first. The grain of the steel flows into the head, which is stronger than a head carved out of a bigger bar.
Roll the thread

The shank is pressed between two threaded dies and the steel flows into the grooves. Rolled threads are stronger than cut ones.
Harden the strong grades

High-strength bolts are heated and quenched, then tempered. The strength class stamped on the head records the result.
Coat in zinc

For outdoor use, bolts are galvanized or electroplated with a layer of zinc that corrodes before the steel does.

Goes in
Zinc
Sort and gauge

Samples are checked against thread gauges and pulled in test machines, then the batch is counted into boxes.

End of the lineBolt Next it goes into roller skates, wheelbarrow.

Worth repeating

Joseph Whitworth proposed a thread standard in 1841 because nuts and bolts had been made in matched pairs that did not fit any others.
The Crystal Palace of 1851 had to go up in 190 days, and interchangeable nuts and bolts worked so well there that Whitworth's standard spread.