There are many ways of working metals now in common use. Metals may be cast in a molten state, or they may be pressed and molded into shape in a cold state, or they may be hammered either cold or hot, but in nearly all cases in which metal is removed in order to form a piece of work, the chisel is used as a cutting instrument. This is perfectly apparent in lathes and planers, but not quite so apparent in sawing, drilling, filing, and grinding. A drill is merely a spiral chisel which revolves upon its own center. A saw is a gang of tiny chisels, and a file consists of still smaller chisels which are broader than those of the saw. In grinding we have rough surfaces in which particles of emery or carborundum act as tiny chisels. The shears, the punch, and the cutting torch are practically the only exceptions to the rule that metals are always cut by chisels, and even the shears may be conceived as consisting of a pair of broad coacting chisels, while it takes little imagination to see a form of chisel in the punch. The cutting torch is, of course, in no sense a chisel. In the cutting of metals the work may move against a fixed tool or the tool may move against a fixed piece of work. In a lathe, it is the work that revolves or rotates against the tool. In the drill and the milling machine the tool revolves against the work. In the planer, the tool is fixed and the work slides against it. The shaper reverses the operation; the work is fixed and the tool moves in a rectilinear direction. Up to the nineteenth century practically the only machines for cutting metals were the lathe and a crude form of boring machine. The machinists of that day had not reached the stage where they were able to produce anything but round work on a machine. The planer had not been born. It was for this reason that Watt had a great deal of difficulty in getting rectilinear motion for the piston of his engine. He had to invent a complicated system of links and levers in order to obtain a practically parallel motion to guide his piston in and out of the cylinder. When the planer was invented and it was possible to produce straight surfaces with a considerable degree of accuracy, all of Watt’s ingenious parallel motions, went into the discard and the cross-head and guides took their place. It was not until long after the planer had been invented that Eli Whitney, the American genius of cotton-gin fame, conceived the milling machine. He reversed the operation of the lathe by placing the cutting tool on the revolving spindle and sliding the work against it. Milling cutters consist of wheels formed with a number of cutting edges or chisels which are arranged either on the periphery of the wheel or on the face of the wheel.