A device for cutting metal, wood or other material into a circular shape. The object to be 'turned' or shaped is revolved by the machine; and a cutting tool, firmly supported, is brought up against it.
Lathes are of many kinds, and range in size from the miniature tool used by a jeweller to the monster lathe capable of turning castings many feet in diameter.
History of the Tool-making Tool
The Lathe has the importance in technological history of having been one of the few machines which were engaged in commodity production before the Industrial Revolution. After this revolution it became the greatest tool of all: the machine which makes machines.
The earliest form was the pole-lathe, invented probably in Greece. Like the bow-drill, from which it may have developed, the spindle of the pole-lathe is rotated by means of a cord. The top end of the cord is tied to a pliant bough or 'pole'; it is then looped around the spindle, and the bottom end is tied to a treadle under the turner's feet. Press down the treadle, the bough yields, and the spindle turns, in one direction; release the tension and the spindle turns in the other direction. The spindle could also be turned by a rope pulled to and fro by the turner's mate, as with a form of lathe used by the Japanese before their industrialization.
In Great Britain turners were busy with their pole-lathes till recent years in the Chilterns, in Pembrokeshire, Cardiganshire and Armagh, producing table-legs, bowls and platters. In turning such things as a table-leg, the length of wood itself could provide the spindle, and was therefore fitted between the dead points of the lathe. In turning bowls, wheel-hubs, etc., the wood had to be fixed on the end of the spindle, and so necessarily lay outside the spindle bearings. This turning on a 'chuck' needed a stronger frame, and also a motive-power greater than cord, treadle and pole could develop. The Japanese lathe gives a hint of the way in which these requirements were met in ancient times.
One might expect the lathe to have been linked at any rate to the potter's wheel, which was invented in Mesopotamia and used in Crete about 2000 B.C., in Egypt possibly a thousand years earlier. However, for close on two thousand years it did not occur to anyone to turn a harder material than unbaked clay, although the bronze chisels already used by carpenters could take a good edge and would have served admirably for shaping wood on a lathe. Egyptian tomb frescoes depict the painstaking manufacture, by hand, of alabaster vases of exact circular section inside and out. Before the fifth century B.C. the mouldings on the legs of Egyptian furniture were also carved, although they closely resemble the kind of ornament which later was to be turned on the lathe. Stool legs depicted on ivories from Megiddo in Palestine dated between 1250 and 1150 B.C. appear to have been carved as well.
When the lathe appears in Egypt, some time in the sixth or fifth century B.C., it had been introduced, we may suspect, from the Greek world around the Aegean, since in this area its history can be traced a little farther back. Climatic conditions around the northern Mediterranean coast are less favourable for the preservation of wooden objects buried in the earth than either the humid climate of more northerly latitudes or the dry sands of the Egyptian desert. Thus the existence of the earliest turnery can only be inferred indirectly from the shapes of pottery vessels, which must have been ceramic imitations of turnery. In the Geometric period of Greek antiquity (1000-700 B.C.) some shallow earthenware dishes, with curved profiles and close-fitting lids, have just the shape of copies of wooden prototypes developed on the lathe. The pictures painted on the pottery of sixth- and fifth-century Greece leave no doubt of the skill and popularity of the turner's craft, at least as far as furniture is concerned.
Cups with elaborate profiles were also produced on the lathe. None has survived in Greece itself, but a turned wooden kylix of Greek form was recovered from an Iron Age chieftain's grave in Upper Bavaria, belonging to the sixth or fifth century B.C. Its graceful lines and slender ribbing and a loose, solid and irremovable wooden ring contrived round the foot - a turner's tour de force - proclaim an advanced stage of technique. We may imagine that this knowledge of turning joined the cultural streams which radiated from the Greek homeland, as much by colonization as by trade, reaching such distant places as the southern parts of France, Germany and Russia, as well as the nearer shores of Italy. In the sixth century B.C. the Greek turner's products, and no doubt knowledge of his machine too, would accompany the very considerable export of Greek pottery to Etruria, thence to be conveyed by the Etruscans to the Celtic inhabitants of northern Italy.
It was Celtic immigrants of the Early Iron Age who introduced the pole-lathe into Britain. In the latter part of the third century B.C. they entered the country from the south-west. At one of their lake villages at Glastonbury in Somerset excavators found a turned wooden tub, twelve inches in diameter with walls five and a half inches high. This would have been a creditable piece of turnery at any period. The Belgic invaders who entered Britain from 75 B.C. onwards must have brought with them knowledge of a lathe capable of very precise work. They turned large pear-shaped urns (some nearly two feet tall) in shale, dividing them into a number of sections made to fit exactly together by means of rabbeted edges. The hardness of shale is sufficient to blunt the ordinary modern chisel very rapidly, and for a time the problem of the tool used by the Belgae in their turning remained unsolved. The excavation of a factory of shale armlets at Kimmeridge in Dorset (where the best jet-black shale is obtained) suggested a solution: for here flint flakes were used, and experiment showed that these were effective in cutting shale on the lathe, although their brittleness allowed them only a short life. So primitive flint was used with one of the earliest of machines.
The Belgae were also masters of a technique allied to wood-turning, the spinning or planishing of thin bronze. In this process which was already known to the bronzeware factories of Capua in Italy, a metal sheet is turned against a shaped mould and with a hard point gradually pressed into the desired shape. Bowls and shield-bosses were manufactured by this means. Spinning of this kind is still carried on in at least one workshop in Great Britain, where noses for fighter planes were made during the First World War, and after the war, samovars.
The Modern Manual Lathe
A versatile machine tool that supports and rotates objects, called workpieces, so that various cutting operations can be performed on them. Most lathes incorporate toolholders for bringing the tools against the work. Lathes are usually used on cylindrical metal pieces, but irregularly shaped objects and wood and plastic pieces can also be worked on them. The most useful lathes for a variety of metalworking operations are engine lathes and the related bench and toolroom lathes. They are capable of precision work and require skilled operators. The parts of an engine lathe are shown in the accompanying diagram. The motor drives a spindle in the headstock, and the speed of the spindle is controlled by pulleys or gears or by a variable-speed motor. The motor also drives the feed rod, which can be engaged by a gear system in the apron of the carriage assembly to move the carriage along the lathe and to move the cutting tool into the workpiece. The handwheels on the' apron are used for manual control of these movements.
The most common lathe operation is the reduction of the diameter of a cylindrical piece by a process called turning. The piece to be turned is supported between steel points, called centers, which are lodged in center holes drilled in the ends of the piece. The tailstock is moved along the lathe to accommodate work of different lengths. The piece is rotated by a clamp, called a dog, which extends around the piece and engages a faceplate attached to the spindle, and by the headstock center, which is also attached to the spindle. The tailstock center remains fixed, with the work rotating on it. For this reason, it is called the dead center.
The cutting is carried out by a small tool of hard steel that is set in a toolholder. The toolholder fits into a tool post fixed to the compound rest at the top of the carriage assembly. The compound rest can be fixed at any horizontal angle to the work and can be moved toward it or away from it. It is moved toward the work until the tool is cutting at the proper depth. Then it is fixed in position, and the entire carriage is moved down the length of the work at a speed suitable for an even cut.
In addition to turning and other operations, such as tapering, that are done on the side of the work, lathes are capable of operations on the end, or face, of a piece. A chuck or a faceplate is used to fasten the work to the headstock. For drilling or for threading a hole the proper tool is fixed on the tailstock and brought against the face. Other operations, such as making the face perpendicular to the sides, are carried out from the compound rest.
In the turret lathe a hexagonal turret replaces the tailstock. The turret can hold six different tools, and these are brought against the face in sequence. Side operations are carried out by tools held in a four-sided post on the carriage. Turret lathes can repeat a series of operations with no adjustment and are thus used for quantity production.
Speed lathes are simple lathes on which the tools are held against the work by hand. They are used in some preliminary metalworking operations, in the polishing of metal parts, and in woodworking.
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