However, there were other factors entering into the development of our machinery industry. There were other requirements beside these machine-tools, and which these supplied inade quately. Screw-bolts and nuts were needed for putting the machines together. Originally iron screw-bolts had been made by means of a split die provided with spiral threads, by rotating either the bolt or the die backwards and for wards until the thread was partly cut, while a was screwed into the nut first from one side, then the other, until the bolt was found to fit into the nut. These bolts were not inter changeable. This primitive system of bolt manufacture continued until 1847, when the solid die with sectional threads patented by Phi lents W. Gates was generally introduced. This die cut the thread at one pass, then the rotation was reversed to unscrew it from the bolt, which marked the thread and was liable to mutilate the die. No compensation for wear was possible. Nothing approaching perfection was attained until 1857, when William Sellers devised a bolt machine in which dies to cut the thread at one pass, and adjustable to size, could be opened and closed while running continuously in one direc tion. Since then ordinary screw-bolts have been made interchangeable. In a few years this machine of Sellers' was introduced into Eng land and also continental Europe. The first turret lathe was built by Stone in 1854. It was provided with automatic mechanism for turning the turret in 1855, and perfected to its present condition in 1858. The gear-cutter was another of the early machine-shop tools. This was simply a revolving milling-cutter, mounted upon a spindle above the dividing-plate. The wheel to be cut was forced against this. The machine was adjusted by hand. Such work was slow and very expensive; and up to 1867 the teeth of nearly all wheels, even for fine machines, were cast. In this year a machine was devised by William Sellers which limited the work of the operative to adjusting the wheel to be cut to the cutter. The machine was otherwise auto matic; and it was now possible for one work man to attend to several machines, thereby greatly diminishing the cost of such work. From that time cast wheels have been no longer allowable in first-class machines. Another typical machine-tool that deserves mention is the milling-machine. It has received its great est development in this country. The manufac ture of metal-working machinery has become highly specialized, and only the older establish ments produce a number of different types of machines. New establishments usually make only one type of machine, or at most one class embracing tools of similar type. Some estab lishments make only engine-lathes, others only planers, others nothing but milling-machines. The effect of specialization has been to make machine-tools more efficient, and, it may be added, more varied. If, for instance, a new construction is designed, as in the automobile, or airplane, and strangely fashioned parts are required, a machine that will make these parts is always forthcoming.
The development of more accurate machin ery, as indicated above, led to the system of interchangeability. In themanufacture of machinery this is an economic principle of the greatest importance. Under more primitive methods individual parts had to be fitted to gether with great difficulty and at much expense. Now all these parts are made by machines with such exactness that they are completely inter changeable; and there is no.longer any ques tion as to their fitting. This method of manu facture has increased the output of the indi vidual workman and reduced the cost of pro duction tremendously. Though first applied in Europe, it remained for the United States to demonstrate its feasibility and actitally' put it into successful operation, and it is generally referred to as the American system. For the
economical manufacture of any kind of ma chinery in which many parts are to be inter changeable certain definite conditions must be met; and success economically requires that every part shall be finished without the inter vention of a skilled workman. The machine must be so designed that it will not only work automatically but work with a high degree of accuracy. Reference standards must be pro vided with which to compare the several parts in order to determine the amount of variation permissible between the standard and the prod uce; every part must come from the machine in the final finished form. Such are some of the details that make necessary a careful study of every part in order to design a machine that will perform each operation with the most efficiency.
The principle of interchangeability was first suggested and put into operation in a small way by Le Blanc, in France. Its importance and possibilities were at once recognized by Amer ican inventors, and its first application in the United States was in the manufacture of fire arms in our government arsenals under the di rection of Eli Whitney, the inventor of the cotton-gin. Simeon North, a maker of pistols, and a neighbor of Whitney, also adopted the system. By 1818 both of these men were using drilling and filing jigs. The growth of the sys tem was slow, being confined for a time to the principal parts; but even in this undeveloped condition it proved successful economically. In 1822 Calhoun, who was then Secretary of War, remarked to Whitney that his improvements were saving the government $25,000 a year at the two public armories. The drop-hammer and dies were first used by Hall, at Harper's Ferry in 1827. Whitney adopted this machinery and thus was able to cut from red-hot metal all the smaller parts of a gun in a form closely ap proximating the finished article. These forged parts were then subjected to the more accurate milling-machine, which turned out the parts in uniform condition, no matter how varied their Shape may have been when they came from the forging-press. It only remained for the drill to fashion the bearings for the working parts and bore the holes to secure the parts together. This was a comparatively simple matter when once the order of procedure had been deter mined and the guiding templets or jigs pro vided. The wooden stocks of the gun were also made by machinery and with sufficient ac curacy to make them interchangeable. This was accomplished by means of a turning-lathe designed by Thomas Blanchard and patented by him in 1820. After the stock came from the lathe the groove for the barrel and the cavity for the lock were hollowed out by special machinery. Measured by the standards of to day all this work was crude; but the gun of that day was itself crude, and these roughly inter changeable parts served their purpose. Machine tools were then both inaccurate and limited in variety, so that they could not he expected to turn out the various parts with mathematical accuracy. Further, at that time there were no such delicate measuring instruments as we have to-day. The most refined measuring instrument known then was the vernier caliper; and the smallest deviation from the standard that could be detected with this contrivance was, at best, perhaps the thousandth part of an inch. In 1848 the interchangeable system was applied to watch-making at the Waltham factory, and in 1850-51 it was adopted for sewing-machines. The profiling machine was developed between 1848 and 1852 by F. W. Howe and E. K. Root. Since then have come great developments in the quality of machine-tools and in their wonderful adaptability to changing needs. Measuring in struments have now been so highly developed that a variation of the twenty thousandth part of an inch can be detected immediately and with perfect accuracy.