One Good Turn: A Natural History of the Screwdriver and the Screw (9 page)

—

For some reason, the potential of the
Housebook
lathe was not immediately recognized. Perhaps the unknown inventor did not publicize his lathe; as far as we know, the
Housebook
existed in only one copy, and medieval craftsmen
were often possessive about their work. Yet it appears that Leonardo da Vinci, at least, was aware of the innovative lathe, for in the early
1500
s he designed a number of screw-making machines, one of which bears a striking resemblance to the earlier machine. Characteristically, Leonardo made improvements. Instead of advancing the blank through the cutter, he made the cutter move along the rotating blank, as it does in a modern lathe. Further, by using different interchangeable gears (four are shown in his sketch), he could make the cutter advance at different rates. Since the blank was turning at a constant rate, if the cutter moved more slowly, the pitch (the distance between the threads) of the resulting screw was smaller; if the cutter moved more quickly, the pitch was larger. Thus, the same machine could make screws with four different pitches. As with so many of Leonardo’s inventions, it is unclear if this remarkable machine was actually built.

Leonardo da Vinci’s screw-cutting machine, c.
1500
.

Jacques Besson’s screw-cutting machine,
1579
.

Although Agostino Ramelli worked in France, Leonardo’s actual successor as engineer to the French court was Jacques Besson, who designed several screw-cutting lathes.
⁵
Besson’s lathes were extremely elaborate, turned not by means of a crank but by pulling on counterweighted cords. This produced the old-fashioned
alternating rotation and also resulted in slippage and a loss of power. But efficiency was not uppermost in Besson’s mind, for his machines were not intended for industrial workers but for hobbyists. Turning had become the gentleman’s equivalent of needlepoint and remained in vogue as a pastime until the end of the eighteenth century. “It is an established fact that in present-day Europe this art is the most serious occupation of people of intelligence and merit,” wrote Fr. Charles Plumier, who published the first treatise on the lathe,
L’art de tourner,
in
1701
, “and, between amusements and reasonable pleasures, the one most highly regarded by those who seek in some honest exercise the means of avoiding those faults caused by excessive idleness.”
⁶
Hobbyists turned a variety of materials, not only wood, but also horn, copper, silver, and gold. Although the products of their labors were purely decorative, they took their machines seriously. The lathes could be simple bench models driven by a treadle, or complex machines with cams and other devices for making elaborate forms, including ornamental screws. The so-called guilloching lathe was capable of tracing complex intertwining curves onto flat disks such as watch cases and medallions. Louis XVI owned a guilloching lathe equipped with a mahogany bench, a gilded iron regulating device, and a tool-holding carriage of gilt bronze inset with the royal coat of arms.
⁷

Aristocrats used lathes to fill their idle hours, but for others, precision lathework was a purposeful occupation. In
1762
, a London instrument-maker named Jesse Ramsden began a project that would revolutionize the lathe. Ramsden, born in Yorkshire in
1735
, had originally been apprenticed to a cloth-maker. When he was twenty-three, he unexpectedly quit his trade and went to London to work for a maker of mathematical instruments. Four years later, he opened his own business. Now, to make his mark, he set out to solve a problem that plagued instrument-making: graduated scales. Linear scales, subdivided into standard measures, were a key ingredient of sextants, theodolites, and instruments used in astronomical observation. Graduated scales were traditionally made by hand and hence lacked accuracy. Ramsden designed a scale-dividing machine that was capable of engraving scales with great precision.

The machine incorporated long, fine-threaded regulating screws of microscopic accuracy. Regulating screws, the refined relatives of ordinary screws, convert rotation into minute horizontal movement by means of tracking pins or nuts. To ensure accuracy, the threads must meet stringent requirements: pitch must be constant; the cores must be exactly parallel and concentric; and the friction against the adjusting nut must be minimal but steady. In other words, these screws must be perfect.

Since screws of such precision were not available,
Ramsden set out to make them himself. It was a daunting problem: how to produce a perfect screw using a lathe with an imperfect lead screw. Patiently, he produced a succession of screws of increasingly greater accuracy. In the process, he made important improvements to the lathe. At a time when most instrument-makers still used wooden pole lathes, he built bench lathes entirely of steel. He invented a triangular slide bar, which gave more accuracy, and he was also the first to use diamond-tipped cutting tools. He finally was able to produce a screw with a claimed accuracy of one four-thousandth of an inch. In all, it took him eleven years to build his dividing machine.

Jesse Ramsden’s precision screw-cutting lathe,
1777
.

Ramsden’s achievement had enormous implications. Accurate regulating screws were incorporated into a variety of precision instruments and opened up new worlds to science as they facilitated the work not only of astronomers, but also of physicists, who depended on
accurate regulating screws in microscopes. Other new worlds were opened up, too. The navigation instrument most affected by Ramsden’s work was the sextant. A sextant incorporates an arclike graduated scale that spans sixty degrees (one-sixth of a circle, whence the Latin root
sextus
), a movable radial arm, and a fixed telescope. The navigator “shooting the sun” lines up the horizon in the telescope, then adjusts a mirror attached to the radial arm until he sees a reflection of the sun. The angle between the mirror and the telescope, which is read off the graduated scale, corresponds to the angle of the sun above the horizon. With this information, and the aid of published tables, the exact latitude can be computed. Thanks to Ramsden’s scale-dividing machine, it was possible to know a ship’s position to within ten seconds of latitude, or about a thousand feet. Such accuracy facilitated the feats of navigation and great voyages of discovery of explorers like Captain Cook.

—

Ramsden was working on his screw-cutting lathes in London at the same time the Wyatt brothers were organizing their screw factory in Staffordshire. The scientific instrument-maker’s precision machines and the crude factory lathes both used regulating screws, but they existed in two different realms. These realms were soon to meet, thanks to the inventive genius of Henry Maudslay. Maudslay was born in
1771
in humble circumstances
and was apprenticed to a blacksmith in the Royal Arsenal at Woolwich, near London. Unusually gifted as a metalworker, Maudslay came to the attention of Joseph Bramah, a prominent manufacturer and inventor. Bramah was looking for someone to make a prototype of his latest invention, an unpickable bank lock. The design, which incorporated numerous tumblers, was so complicated that it had confounded his own experienced craftsmen. Maudslay, then only eighteen, not only successfully built the prototype, but also designed and built the tools and machines needed to commercialize its production.
^II

The burly young blacksmith was a mechanical prodigy. Just as some people have a natural aptitude for chess or playing the violin, Maudslay could shape metal with a dexterity and precision that amazed his contemporaries. Moreover, he was able to intuit solutions to mechanical problems. For example, while he was building a lathe for Bramah, he invented the slide rest, a perfectly straight bar that supported a movable tool holder. Although a similar device had been suggested by Leonardo, it had never been implemented. The importance of the slide rest cannot be understated. In previous
lathes, the turner guided the cutter by hand. The slide rest allowed the cutting tool to move smoothly and precisely along the length of the revolving workpiece. At first the device was greeted with scorn and nicknamed Maudslay’s Go-Cart, but it proved so successful that it was soon widely copied (Maudslay rarely patented his inventions).

Henry Maudslay’s first screw-cutting lathe, c.
1797
.

After eight years with Bramah, and having risen to the position of foreman, Maudslay struck out on his own. While filling orders for customers—his first commission was a metal easel for an artist—he continued to tinker with precision lathes. His first breakthrough—in
1797
—was a lathe for cutting long screws that incorporated a three-foot-long regulating screw. In a later version,
following once more in Leonardo’s footsteps, he added interchangeable gears to produce screws of different diameter and pitch.

Thanks to a precision regulating screw displayed in his shopwindow, Maudslay met an extraordinary Frenchman, Marc Isambard Brunel. A royalist, Brunel had fled the French Revolution for America, had worked in New York as an engineer and architect, and was now settled in London. A prolific inventor—and ex–naval officer—he had a plan for manufacturing wooden ships’ pulleys for the British navy. He needed someone who could build the prototype machines to demonstrate the practicality of his process. With Maudslay’s help, Brunel won the contract. The factory in Portsmouth, with forty-four of Maudslay’s machines, took six years to build. It was the world’s first example of a fully mechanized production line. Ten men produced
160
,
000
pulleys a year, the navy’s entire annual requirement.

Maudslay and Brunel collaborated on another venture. In
1825
, Brunel received the commission to build a twelve-hundred-foot tunnel under the Thames River. Previously, tunnels had been built with temporary timber shoring, but Brunel invented an extendable, waterproof, cast-iron shield that moved ahead of the construction as the excavation progressed. Maudslay built the device. His workshop also produced a variety of specialized machines—for printing, pressing, and minting.
He invented a machine for punching holes in boilerplate (an operation previously carried out by hand) that greatly speeded up riveting. He was best known for his pioneering marine steam engines, with which he equipped at least forty vessels during his lifetime. When Isambard Kingdom Brunel, Marc’s son and likewise an engineer, built the
Great Western,
the first steamship to cross the Atlantic, it was Maudslay’s firm, now run by
his
son, that built the
750
-horsepower engine, the largest in the world.