Like the screw, the bolt occupies an integral position in both industrial and everyday life. In fact, bolts and screws are used more than any other type of mechanical fastener, and they can be found in nearly every simple or complex machine. Although there is no absolute distinction, the difference between screws and bolts can be broadly defined as one of thread size and tapering. Bolts are generally larger and do not have tapered ends. In standard usage, a fastener that is torqued with a nut is usually considered a bolt.
Without bolts, we would not be able to hold together the frames of cars or the arms and backs of chairs. A device as common as a pair of scissors or as sophisticated as a particle accelerator would be rendered inoperable. The self-evident utility of the modern bolt makes it all the more interesting to discover how this object came to be so crucial to our way of life. In the United States alone, the bolt has undergone several distinct stages of development.
The Origins of Bolt Production
Bolt usage can be traced back to ancient irrigation systems and construction projects, but metal bolts and screws did not become the standard until the early modern era. The first machines used to produce metal bolts resembled cutting lathes and were invented in France in the mid-sixteenth century. However, it wasn't until the nineteenth century and the beginning of mass production that bolts became the norm in industrial manufacturing.
In the United States, the first systematic bolt manufacturing operation was founded by Micah Rugg in 1818. Rugg was a Connecticut blacksmith who developed a process of cutting and heating square iron bars into bolt-sized pieces. These workpieces were then smoothed along an anvil, and a die-cutting press was used to shape the bolt's head and threads. Using machine tooling processes, such as drop hammering and die trimming, proved to be both time- and cost-efficient. By 1840, Rugg had sold several thousand bolts and expanded his operation to produce nearly 500 bolts a day.
Following the success of Rugg's pioneering bolt production methods, other manufacturers began developing new technologies and techniques to capitalize on the burgeoning fastener market. William Clark, another manufacturer from Connecticut, is credited with designing the first bolts and dies made from round, rather than square, iron in the 1860s. Clark also streamlined the bolt head formation process by using die compression to create both the head and the angled neck in the same operation. His pinched and concave neck bolts proved highly cost-efficient and reduced the risk of splitting wood when driving the bolts into a workpiece.
Some of the other new bolt configurations that emerged over the next thirty-year period included:
• Star Bolt: This was a pinched neck bolt similar to Clark's first design that eventually fell under his patent.
• Bastard Neck Bolt: The bastard neck configuration had a thin bolt head and a short rectangular shank.
• Fin-Head Bolt: This bolt was designed with narrow lugs underneath the head that helped keep it steady while a nut was being tightened or removed.
By 1905, there were over five hundred factories in the United States specializing in bolt and nut production. Part of the skyrocketing demand for bolts in the later half of the nineteenth century was driven by the spreading utility of new bolt designs.
Current Production Methods
The twentieth century saw the development of our present-day bolt manufacturing methods, particularly through the advances and armaments engendered by the two world wars. Although these techniques greatly expanded previous production capabilities, they were similar in principle to the original processes established in the 1800s. For example, the cold-forging technique used today hearkens back to the cold-forged fin-head bolts first developed in 1890.
The majority of current bolt manufacturing methods employ cold-forged heading to shape a steel workpiece. A gripping die holds the metal stock in place while a concave compression punch forms the bolt's angled round head. The bolt's shaft is then deformed through the thread rolling process, which uses cutting dies to shape threads into the metal shaft. The bolt is then usually coated with anti-corrosive substances to strengthen its durability. Hot or cold blackening and galvanization may be used to chemically bond a sealant, such as oil, onto the bolt in order to extend its working life. While these methods are more cost-efficient, boast higher production rates, and create less waste than the older methods of the nineteenth century, the modern-day bolt still owes its design and central attributes to the pioneering efforts of early manufacturers.