|This Week in History
February 14 20, 1827
Francis PrattSetting the Standards for American Machine-Tool Development
There were many skilled mechanics and inventors in 18th- and 19th-Century New England, but it was Francis Pratt, born in Woodstock, Vermont on February 15, 1827, who set out to establish a uniform set of measurements and gages in both America and Europe which would firmly establish the machine-tool industry's ability to produce absolutely interchangeable parts. That goal had been laid out in the 1790s by Eli Whitney, backed by President Thomas Jefferson, and partially implemented by several generations of mechanics before the advent of the Civil War.
However, much remained to be accomplished in 1848 when Francis Pratt completed his apprenticeship with a Lowell, Massachusetts machinist. After working for four years at the Gloucester Machine Works in New Jersey, Pratt went to work at the Colt Armory in Hartford, Connecticut, which had been established by Samuel Colt to produce his "six-shooter" pistols with interchangeable parts. Transferring to the Lincoln Iron Works as its foreman two years later, Pratt worked on designing and producing the "Lincoln" miller for the Colt Armory. This all-purpose miller became the leading American machine tool, used for producing a multiplicity of products. More than 150,000 of these machines were eventually produced and sold throughout America and the world.
But Pratt, like many other mechanics, was not satisfied with the rather broad range of measurement tolerances for the machines and their products. He arranged for a fellow worker at the Colt Armory to join him at the iron works, and in 1860, they became partners. This partner was Amos Whitney, born in Maine, and a member of a branch of Eli Whitney's family. Whitney had previously worked for the Essex Machine Company, which built cotton-spinning machinery, locomotives, and machine tools. The two mechanics rented a small room and began working on their own account, while still keeping their jobs at the iron works. With the advent of the Civil War, they moved into gun manufacturing, and had all the work they could handle.
By 1865, they had left their positions at the iron works and were erecting a new building in Hartford for their machine-tool company. This progression, from apprentice, to journeyman, to foreman, to partner in a firm, was a typical experience for many talented mechanics and inventors. What distinguished this company from many others with equally talented employees was its partners' dedication to exploring the basic principles of machine design and measurement.
From the very early days of their partnership, Pratt and Whitney had become teachers of other mechanics, and were suggesting new methods of work and new means of accomplishing rapid and high-quality production. Like the work of Alexander Dallas Bache (the great-grandson of Benjamin Franklin) with the U.S. Bureau of Weights and Measures, Pratt and Whitney were struggling to establish standards of length and durability of metals. For example, when John Hall completed his first hundred rifles at Harper's Ferry in 1824, "the joint of the breech block was so fitted that a sheet of paper would slide loosely in the joint, but two sheets would stick." This crude method of measuring tolerances would change by the time of the Civil War to a mechanic's ability to measure in terms of 1/32 of an inch.
But, the actual size of that inch was still not established, and varied widely, as did the foot and the yard. Francis Pratt decided to establish the actual size of those measurements and to invest in research on hardening steel so that a product could not only be worked to an accurate size, but its material would also be able to maintain that exact dimension. Accuracy from product to product, and from machine tool to machine tool, would ensure the production of truly interchangeable parts in large quantities and at a much lower cost. This was no easy task.
To tackle the problem, Pratt and Whitney established a Gage Division in the company which gave itself the task of setting a practical standard inch of exact dimensions. The work was completed over a number of years, and the standard was accurate to millionths of an inch. This project required cooperation with scientists, and in 1879 came an opportunity to work with an eminent astronomer. William A. Rogers was a mathematician, physicist, and astronomer who then worked at Harvard University's observatory. He was put in charge of the newly erected eight-inch meridian circle, and his chief task was the observation of the catalogued stars between 50-55 degrees north declination. This was part of an international project by the German Astronomische Gesellschaft to establish the most accurate positions of all of the sky's brightest stars.
Rogers' work on the star catalogue required him to develop greater refinements in his equipment, and he became proficient in making more accurate screws for his apparatus, and in calculating their errors. In 1879, Rogers was sent to Europe by the American Academy of Arts and Sciences to obtain copies of the British imperial yard and the French meter. Pratt and Whitney now entered into a cooperative agreement with Rogers and his assistant George Bond, a graduate of Stevens Institute of Technology, to develop a machine which could make absolutely correct measurements within a limit of 1/50,000 of an inch.
Rogers travelled to London and Paris and obtained reliable transfers of the British yard and the French meter. These, and the United States standard yard, designated as Bronze No. 11, were painstakingly compared with the standard bars made by Rogers for Pratt and Whitney. The U.S. standard yard was furnished with the cooperation of the Coastal Survey, which contained the Bureau of Weights and Measures. To do the work of comparison, the two scientists developed a large machine called the Rogers-Bond Comparator.
By 1880, Pratt and Whitney had a set of master bars, accurate within millionths of an inch, and these became the standard for its machines and products. By 1885, the company had developed the Pratt & Whitney Standard Measuring Machine, which could construct and duplicate recognized standards of length. Professor Rogers went on to apply optical methods to the measurement of very tiny changes in length, and he undertook meteorological research to learn more about the coefficient of expansion in metal objects and thermometers.
The achievement of accurate, standard measurements for American machine tools ensured their use around the world. Pratt & Whitney's machines went to Great Britain, Prussia, Russia, Turkey, Egypt, Spain, Sweden, and Denmark. But there was another feature to American machine tools that made them even more useful. There were talented machine-tool designers in Britain and France, but the industries there, except for arsenals, did not adopt the machines in the first half of the 19th Century. The tradition of the guildshand finished productswas still strong. And, most of the mechanics were specialists in some particular field, again because of the remnants of the feudal system.
American mechanics, in contrast, tended to be generalists. They could work on locomotives, guns, sewing machines, looms, bicycles or what-have-you. Many of the machine tools they built were also very flexible. A machine to bore the barrel of a rifle could also drill a hole in the crankshaft of an automobile so that oil could reach the bearings. And, thus, companies which had produced arms during the Civil War had no trouble in converting to making typewriters, sewing machines, or bicycles. And when the first plants were built for making automobiles, the machine tools again made an easy transition to the new invention.
The republican culture of the United States also produced mechanics who could understand both the details and the larger context of the machine tools, and were equipped to suggest minor improvements or major ones of tool design or work methods. And although the machines enabled more work to be done by fewer people, and could be run by semi-skilled workers once they were set up by a master mechanic, they also required a highly-skilled corps of those mechanics to keep them accurate and improve their performance. It was this group that provided the flexibility to convert the machines to any improvement in technology, or to design new machines for a mode of technology never before imagined.