Mechanics: The Science of Machinery. A. Russell Bond
was intermittently revolved against a chisel which was rested on a block and guided by the workman. Small work could be turned out on such a lathe with considerable precision, but when it came to large parts, particularly parts of steel, the workman was easily tired by the effort of operating the pedal and was apt to be irregular in the guiding of the tool.
FIG. 24.—PRIMITIVE POLE LATHE
Up to the middle of the eighteenth century practically no advance had been made over the ancient lathe of the Egyptians, and when, 150 years ago, the steam engine was invented the task of building the engine seemed almost insuperable.
James Watt was a maker of mathematical instruments, a man of great skill and precision as a craftsman, but he dealt with parts of small dimensions. When he conceived of his steam engine, he mentally pictured the various parts as turned out with all the accuracy and finish that was possible in the diminutive members of a scientific instrument. To him it seemed perfectly feasible to turn a cylinder which would be practically perfect in contour, and to fit it with a piston around which no steam could leak. With the lathe then in existence such a fit was easily possible on small work. But when he undertook to have the cylinder of his engine bored, he discovered that there was no machine that could begin to do the work properly. In fact, when Smeaton, who was a prominent engineer of that time, investigated Watt’s steam engine, he declared that it was such a complicated piece of work that neither tools nor workmen existed that could build it. In Watt’s first engine, the cylinder was only six inches in diameter and two feet long, and a special type of boring machine was devised to bore the forged cylinders. But the boring was so irregular that when the piston was inserted and the steam was turned on, nothing would stop the flow of steam that leaked around the piston. In vain did James Watt use cork, oiled rags, tow, paper, and even old hats to stop the leakage. However, the boring machine was improved and later a cylinder, eighteen inches in diameter, was bored with such accuracy that the large diameter exceeded the small diameter in the worst place by only ⅜ of an inch. This Watt considered a very good bit of turning. To-day cylinders of that size that vary from true by half the thickness of the paper that this is printed on would be thrown out as defective.
It was in 1769 that Watt invented the steam engine, but that great event did not mark the dawn of the present era of machinery. For a quarter of a century thereafter there was little progress in the development of machine tools. A boring machine was built that did fair work. There were a few sawmills in which wind power was employed to drive the saw. But lathes were still driven by foot power and the cutting tool was still held and guided by hand.
MAUDSLEY’S “GO-CART”
The real father of the present era was a very clever British mechanical engineer, Henry Maudsley, who undertook to eliminate the uncertainties of the human hand by clamping the cutting tool of the lathe in a rest and arranging the rest to slide along the length of the lathe or transversely toward or away from the center. These two motions made it possible to accomplish all that the workman could accomplish by hand and at the same time the tool was held so firmly that accuracy and precision of turning was assured. Furthermore, he provided this slide rest with a nut that engaged a screw driven through suitable gearing by the lathe spindle. Then, as the work revolved, the slide rest was compelled to move along the bed of the lathe at a uniform rate. By varying the gearing, the speed of the slide rest and the tool it carried could be varied at will, thus making it possible to cut screw threads of any pitch desired with a degree of accuracy unattainable by hand.
Remarkable as was this improvement, it met with the usual opposition that every real advance in machinery received in those days. People referred to the slide rest as Maudsley’s “go-cart,” but it proved such an important element of the lathe and so very valuable that before long it was universally adopted. From that time on the skill of the workman began to lose its importance. The man began to give way to the machine. Precision was possible in large as well as small work. The human element was also dispensed with in the driving of the lathe. The foot pedal was superseded by the steam engine, and the machine came to be known as the engine lathe.
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.
Following the milling machine, came the grinder, in which a revolving wheel of an abrasive material served to wear away the surface of a piece of work, and with this form of machine steels of great hardness could be finished with accuracy and a high polish.
THE INTERCHANGEABLE SYSTEM
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