Bolts are one of the most basic components of engineering and construction, yet their production has become an advanced, high-tech process with multiple steps. Find out how raw steel is transformed into highly specified and exact metal implements
BOLTS can come in a wide range of different sizes and shapes, but the basic production process generally remains the same. It starts by cold forging steel wire into the right shape, followed by heat treating to improve strength and surface treating to improve durability, before being packed for shipment. However, for more advanced bolt designs, the production process can expand by a number of additional steps.
COLD FORGING STARTS with large steel wire rods, which are uncoiled and cut to length. The grade of steel is standardised across the industry, according to the requirements of ISO 898-1. Using special tooling, the wire is then cold forged into the right shape. This is basically where the steel is moulded, while at room temperature, by forcing it through a series of dies at high pressure. The tooling itself can be quite complex, containing up to 200 different parts with tolerances of hundredths of a millimetre. Once perfected, cold forging ensures bolts can be produced quickly, in large volumes, and with high uniformity.
For more complex bolt designs, which cannot be contoured through cold forging alone, some additional turning or drilling may be needed. Turning involves spinning the bolt at high speed, while steel is cut away to achieve the desired shape and design. Drilling can be used to make holes through the bolt. If required, some bolts may also have washers attached at this stage of the process.
HEAT TREATMENT IS a standard process for all bolts, which involves exposing the bolt to extreme temperatures in order to harden the steel. Threading is usually applied before heat treatment, either by rolling or cutting, when the steel is softer. Rolling works much like cold forging, and involves running the bolt through a die to shape and mold the steel into threads. Cutting involves forming threads by cutting and removing steel.
Since heat treatment will change the properties of the steel to make it harder, it is easier and more cost-effective to apply threading beforehand. However, threading after heat treatment will mean better fatigue performance.
For long bolts, where the length is more than ten times the bolt’s diameter, the heat treatment can have the effect of making the steel revert to the round shape of the original steel wire. Therefore, a process of straightening often needs to be applied.
WHEN CORROSION RESISTANCE is not an issue – such as inside an engine or an application that is regularly exposed to oil – using phosphate is a more cost-effective option. Once the surface treatment has been applied, standard bolts are typically ready to be packaged. However, more advanced designs may require some additional assembly, such as brackets. Other bolts will also require some form of patching, either a locking patch or a liquid patch. A locking patch consists of a thick nylon layer over the threads, which helps improve grip. A liquid patch will help improve thread-forming torque.
Uncoiled, straightened and cut to length.
Moulding the steel into the right shape at room temperature.
Progressively formed by forcing the steel into various dies at high pressure.
Threads are formed by rolling or cutting.
The bolt is exposed to extreme heat to harden steel.
Depends on the application. Zinc-plating is common to increase corrosion resistance.
After quality control to ensure uniformity and consistency, the bolts are packaged.