Читать книгу Mechanics: The Science of Machinery - A. Russell Bond - Страница 18

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The most notable advance in machine work came early in the nineteenth century, when what was known as the “American System” of manufacture, or the interchangeable system, was introduced. As long as mechanics were obliged to perform their operations largely by hand, it was impossible to attain great accuracy. Each workman put his own individuality into the work. As a consequence, no two pieces were of exactly the same size or shape. This was true even with the early power-driven machine tools. The parts might be very close to the same size, but careful measurements showed that they varied by a minute fraction of an inch. Hence, when a machine was assembled the unyielding metal parts had to be filed and trimmed and hammered to fit them together. If any accident occurred to a machine, the damaged part could not be replaced by another taken from stock. The entire machine had to go back to the shop where an experienced mechanic would make a new part to replace the damaged one. In those days a machine was not manufactured but was built as an individual mechanism, just as a house or a boat is built to-day.

With the advent of accurate machine tools came the idea of standardizing the parts so that hundreds and thousands of pieces could be made of exactly the same dimensions, and in assembling a machine the parts could be picked at random from the stock and put together without the use of special tools and without requiring any special fitting. This was of special importance in the tools of warfare, because armies need quantity production, i. e., rifles, cannon, etc. More machines of the same kind were required for an army than for any other organization or line of work. At the close of the Napoleonic War the British Government had 200,000 parts of muskets either partly finished or waiting repairs. Their muskets were made after the old system. Each one was built separately with its parts individually fitted together, so that whenever any part was injured, the musket had to be laid aside and sent back to the workshop for repairs.

Long before that time, the idea of making standard guns had been hit upon in France. Thomas Jefferson, while Minister to France, in 1785, wrote of the French system which was then being developed by a mechanic named Le Blanc. He was building a musket in which the parts were of standard pattern, and which could be assembled by taking pieces haphazard as they came to hand and putting them together without special fitting. Thomas Jefferson called the attention of the American Government to this system and showed that it was possible to produce muskets cheaper by that method of manufacture. However, our Government at that time failed to avail itself of the opportunity of utilizing this system of manufacture.

Later on, the idea was taken up in this country by Eli Whitney and by Simeon North. When Whitney attempted to introduce the system, he was laughed at by French and English ordnance officials, and even our own Government officials were skeptical, particularly when they found that it required so much preparation in the way of machinery and designing of parts before a single musket was completed. It seemed like a waste of money to invest in so much preparation. But Whitney was soon able to silence all his critics by taking to Washington ten pieces of each part of a musket and then selecting at hazard from each pile of pieces the requisite parts and putting together ten muskets. It was not long before the foreign governments saw the importance of this method of manufacture. Great Britain later adopted it in the making of her own rifles and called the process the “American System.”

But it was not only in the field of rifles that interchangeable manufacture made itself felt. The New England clock industry provides an interesting illustration. At first the clocks were made of wood, but early in the nineteenth century, a clock maker, Chauncey Jerome by name, designed a brass clock in which the parts were made on the interchangeable system. Instead of building each clock as a separate piece of work, clocks were turned out by the thousands, and at an extremely low price. Soon he had flooded this country with his clocks and began to look around for other markets. Machinery had been used by other clock makers in producing wooden clocks, and movements which had cost $50 each in 1840 had been reduced to $5. But Chauncey Jerome’s clock was made of brass and by means of the interchangeable system of manufacture he could produce it for less than 50 cents. The clock was such a success in this country that Jerome decided to try it abroad. Consequently he made arrangements with an agent in England and shipped over a large consignment. The British Government was astonished at the low price of the clock and was convinced that it had been undervalued. At that time, they had a simple and ingenious method of punishing a consigner who undervalued the goods he wished to introduce into the country. This consisted in promptly appropriating the property at the price given in the invoice. In due course of time, much to Jerome’s astonishment, he received a letter from the British Government stating that his clocks had been confiscated, and with the letter came a check paying for them at the invoice price. Jerome was not in the least dejected by the rebuke; on the contrary, he was rather elated, for, as far as he could figure it out, he had a spot-cash buyer for his goods and no selling expenses. He did not mind at all letting the British Government have the clocks at the invoice price. So he decided to try again with a larger shipment. To his great delight this shipment met the same fate as the first, and in due course another good British check arrived. Thus encouraged, Jerome sent over a third and still larger shipment, but by that time Johnny Bull began to suspect that the Yankee clock maker was getting the best of the bargain and, finally convinced that clocks really could be produced with profit at the low invoice price, he permitted them to enter his country. With this striking example, the fame of the Yankee system of manufacture spread over the world.

In order to have two parts alike, they must be placed under a machine in exactly the same way. In other words, they must be set in “jigs” or frames which are fitted into the machine in such a way that the tools will approach the work from exactly the same angle or penetrate the work to exactly the same depth in ten, or a hundred, or a thousand, or a million pieces, as the case may be. Making jigs and dies consumes a great deal of time in preparation work, but once the preparation stage has been passed, articles are produced with wonderful rapidity and very little waste of time. Formerly it was necessary to determine the location of each hole in a casting separately and spend precious time in adjusting the work to the proper position under the tool. If the hole was to be threaded, it had to pass through several separate operations.