How coil springs are made

Squeeze a coil spring and its wire twists along its length, which is where the push comes from.

stations: 7
things upstream: 62
raw sources: 21
steps from the ground: 10

The short version

A coil spring is spring-steel wire wound around a rotating mandrel into a helix. Small springs can be wound from wire that is already hard, while bigger ones are wound soft and hardened afterward. The ends are ground flat, and the finished spring is tested for the force it gives at a set length.

Feed the wire

Spring steel wire is pulled from a spool through straightening guides into the winding machine.

Goes in
Steel wire
Wind the helix

The wire is fed against a pin that forces it to curl as it wraps around a rotating mandrel. The spacing between turns sets how stiff the spring is.
Heat treat the coil

Coils go straight onto a conveyor through an oven. Wire that was already hard only needs its winding stress relieved, so it keeps its shape and springs back. Springs wound from soft steel are hardened and then tempered.
Grind the ends flat

A dish carries compression springs slowly between two horizontal wheels, which grind the last coils flat so they stand upright and push evenly.
Blast with steel shot

Round shot is fired at the wire, leaving a skin of tiny dimples held in compression. That skin resists the cracks that end a spring's life.
Coat against rust

Springs are plated, painted or coated in oil, depending on how they will be used.

Goes in
Paint
Test the force

Machines compress each batch sample to a set length and record how hard it pushes back.

End of the lineCoil spring Next it goes into padlock.

Worth repeating

Coiled springs first appeared in the early 15th century, in door locks.
When you compress or stretch a coil spring, its wire twists, so the spring's stiffness depends on the shear modulus of the steel.
A freshly coiled spring needs a trip through an oven, because without that stress relief it would not hold the shape it springs back to.