In using the term “interchangeable” it must be remembered that the meaning attached to this word grew during these years. The interchangeability of 1813 would not have been considered satisfactory in 1855, much less so today. When Hall completed his first hundred rifles at Harper’s Ferry in 1824, it is said that “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 system of gauging will have a familiar sound to the older mechanics who grew up before the days of the micrometer. When Colonel North was given his first contract for the rifles and furnished two models to work from, these models were so unlike that he asked to have one set aside and that he be allowed to gauge his work from the other. Of the various tools associated with interchangeable manufacture, drilling jigs were in use very early, probably from the start. The filing jig is said to have been invented by Selah North, the son of Colonel North, but it was used by Whitney almost as early. Both Whitney and Colonel North were using plain milling by 1820. A light sort of milling machine is shown in the French Encyclopedia of 1772, already referred to, but the first successful one was built by Mr. Whitney some time prior to 1818. This machine, still in existence and now in the possession of the Sheffield Scientific School of Yale University, is shown again in Fig. 28.[169] In 1817 to 1822 we find the introduction of forging in hand dies, barrel turning by special machinery, and the Blanchard lathe for gun-stocks. Receiving gauges are said to have been used at Middletown in 1829, and were regularly in use at Springfield by 1840. Automatic machinery for the woodworking was first invented by Blanchard in 1818 for the Springfield armory. The accuracy of these machines, shown in Fig. 29, outran that of the metal work of the time. To accommodate the variations still present in the metal parts Blanchard devised a machine which used the actual lock plates as formers and cut the stocks to match, and such machines were used at Springfield until 1840. By that time the work on the metal parts could be made as accurately as the stocks, and this method was no longer necessary. A modern degree of accuracy in shaping of the metal portions was not possible until the miller came into general use for irregular shapes, which was some time in the forties. By 1842, for the new musket to be manufactured at Springfield, there was a complete set of model jigs, taps and gauges. The profiling machine was developed by F. W. Howe, R. S. Lawrence, and E. K. Root, from 1848 to 1852. A drop hammer with dies was used by Hall of Harper’s Ferry in 1827, the head of which was raised by a moving chain and freed by a trip at the desired height. Later Peck invented his lifter, using a strap. The Root drop hammers we have already mentioned. The board drop is largely the work of Spencer.