by Jenny Knodell, IQS Editor
In the simplest of terms, fasteners are connective mechanisms used to hold 2 objects securely together. Sound familiar? It should, because I’m talking about screws, those simple little pieces of metal (or sometimes plastic) that keep our world in place. Your house’s construction is full of them, as is your stove, bed frame, car, computer desk, and on and on. They are similar to nails in design and function, but have one main defining characteristic—the threading. Those spiral-shaped raised ridges that wrap around the shaft of the screw at an angle keep them in place. Most screws are considered threaded fasteners and must be rotated in order to move in or out of a hole, while nails that have a smooth shaft may be easily pulled out of place.
The first screw was used in Germany by a clockmaker in the early 1500s, which means their 500th birthday is a mere 6 years away! It was handmade, most likely out of brass, and the threading was cut by a metal file. It also had a blunt end, instead of the standard tapered point seen on most screws today. This early version had a couple problems—mainly with standardization. During that time, each screw manufacturer had their own threading sizes and styles, which were not interchangeable with screws made by other metal smiths. They were also un-uniform, with differing angles and threading sizes on a single screw. If your screw manufacturer went out of business, moved or died, you were plum out of luck if you needed a replacement. The invention of the lathe in the late 1700s brought about the modern screw fabrication process. Creating uniform threading with high precision became much easier by cold cutting with a revolving spindle that had the inverse thread pattern.
Old handmade screws with filed threading.
Even after the lathe was invented, there was still no uniform threading, and each screw manufacturer used their own. It wasn’t until 1841 that an Englishman named Whitworth proposed the idea of screw threading standardization. By collecting threaded fasteners and studying which were most successful, he came up with the best threading dimension—a 55º angle between threads, and 0.1373 times the pitch as the measurement from the top to the bottom of the thread. At the same time, Americans were experiencing similar problems. William Sellers, a mechanic, proposed a thread with a 60º angle, the top to bottom measured 1/8th the pitch and the crest was flattened instead of rounded; similar, but not exactly the same as Whitworth’s idea. Not much later, the ASME got involved, officially deeming Seller’s threading the national dimensions for American screw threading. To this day Britain and the U.S. have different threading styles, and standardization is an ongoing struggle.
With the industrial revolution and vehicle production in the early 1900s, screw manufacturing vastly improved. The methods were now standardized, and mass production by rolling instead of cutting ensured exact replication. The rolling method allowed for harder steel, making screws more durable and longer lasting. Today, rolling machinery can thread thousands of screws per minute. The process conditions are at around room temperature, and the metal isn’t heated. The plain screw is placed between 2 dies, and the threaded profile is transferred under high pressure.
Photo courtesy of Ford Fasteners, Inc.