The Manufacture of Screws
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The Manufacture of Screws I
The concept of the screw, both as a fastener and as a device to move material, has been known since ancient Greek times. Not until the age of mechanization, though, were screws produced with enough precision and in sufficient quantities to become crucial components of the development of technology. Screws can be made of iron, steel, copper, bronze, brass, aluminum, wood, or hard plastics. Mechanically, a screw is a continuous inclined plane wrapped around a cylinder. Several names are often mentioned in connection with the invention of the screw. The Pythagorean philosopher Archytas of Tarentum is alleged to be the actual inventor of the screw in the fifth century B.C., though the date of the screw's first appearance as a useful mechanical object is not clear. Archimedes (third century B.C.) is often credited with the invention of the water screw, or "screw of Archimedes," though similar devices appear to have been used for irrigation in Egypt much earlier than this. The Greek scientist Hero, living in Alexandria in the first century A.D., invented a screw press for squeezing olives and grapes to produce olive oil and wine. Turning the shaft of the screw press applied great and constant pressure to the fruit, producing much more liquid than other simple crush presses could. The Romans used the screw press for pressing clothes. A drawing of such a clothes press survived in the ruins of Pompeii from 79 A.D. External screw threads could be made by tediously marking the diagonal channels on a wooden rod, then sawing, and finishing by filing. But cutting the internal thread of a nut or a pipe proved to be problematic. By the first century A.D., taps for cutting or grinding internal threads were also in use. Many centuries later, as technology advanced into the age of mechanization, screws took a more prominent role. For instance, England's large Salisbury clock (made in 1380, and perhaps the oldest clock still operating in the world) was constructed without a single screw; craftsmen used wedges or rivets to hold its iron frame together. The delicate pieces of smaller precision clocks created in later years, though, could not obviously be hammered together, but required small metal screws for assembly. Metal screws and nuts came into use in 68
the 15th century. A hexagonal or square head was turned with a box wrench. A century later, the screws used in armor featured nicks or slots in their round heads, possibly for the use of a screwdriver. Centuries later, James Watt (17361819), best known for his invention of the practical steam engine, also designed a screw printing press: A letter written in special ink was screwed up against moist copy sheets for reproduction. In his notebooks, Leonardo da Vinci (1452-1519) sketched a machine for cutting screws as well as a screw-operated printing press. The wood screw, which deforms the wood itself into accompanying nut threads, is not mentioned until the mid16th century. A different variation, the metal screw, is designed to hold sheets of metal that
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