Lathe, a machine tool that performs turning operations that remove unwanted material from a workpiece (either wood or metal). which is rotated against a cutting tool. Several types of lathes are used: the speed lathe has a cutting tool supported on a rest and is hand manipulated; an engine lathe's cutting tool is clamped onto a power-driven slide; screw-cutting lathes have a lead screw that drives the carriage on which the cutting tool is mounted; a turret lathe has a pivoted holder for cutting tools; a CNC lathe is a computer controlled lathe that is programmed numerically (computer numerically controlled).
The Coming of the Lathe
The first mechanically operated tool, or machine tool, was the lathe, used for cutting and shaping. It was in wide use among the ancient Greeks.
The earliest type was the treadle-operated pole lathe, which used the energy of a bent bow or pole to spin a piece of wood against a cutting head. Another major breakthrough in the design of tools, as of machinery in general, was the discovery of the principle of gears, which were in use in China by the 3rd millennium BC. Gears can be used to match the speeds of two machines by using meshing wheels of different diameters; the twisting effort needed can also be changed by gearing.
But the amount of power that can be passed on through gears, pulleys or any other means of transmitting and multiplying energy is governed in part by the efficiency of these devices. Over the centuries, people found ways of increasing efficiency by using more durable materials (metal, for example, wears more slowly than wood), and by cutting gears and other parts more accurately, to reduce the loss of energy through friction or free play between surfaces.
Accuracy in the manufacture of tools, machinery and instruments depended until the 17th century on the skills of craftsmen working with hand tools and foot-operated pole lathes. But as greater standards of accuracy were demanded by navigators, astronomers, artillerymen, surveyors and scientists, new and improved machine tools were continually being devised.
One machine which depended more than any other on precise gear-cutting was the mechanical clock. By about 1700, the clockmakers' lathe was in use.
The rigidity given to it by the incorporation of metal in its construction allowed metal parts to be cut with greater uniformity and to within finer limits than ever before. This was the first precision machine tool, and was adopted by makers of other articles that required great accuracy in their manufacture.
The cutting head of the clockmakers' lathe of the later 18th century could be moved a few millimeters at a time by turning a finely threaded screw.
Tool development was self-propagating: the next step forward was the lathe on which could be made screws with even finer threads. A practical screw-cutting lathe was devised by the English instrument-maker, Jesse Ramsden, in 1770. It was no accident that an instrument maker was responsible for this step forward, for instruments such as microscopes and telescopes required screws for fine adjustment.
Another instrument requiring fine adjustment was the dividing-engine- a device first developed in France in the 18th century, which improved on hand tools by enabling more finely divided scales to be inscribed on instruments such as sextants and micrometers. It also enabled screws with finer threads and more precisely cut gearwheels to be designed for many purposes. One such purpose was the improvement of the screws and gear-wheels contained in the dividing-engine itself. This led to more accurate dividing-engines, and so refinement bred refinement.
Without precision tool-making, there could have been no Industrial Revolution.
The steam engine as improved by James Watt, which so greatly accelerated industrial production, could not have been made without John Wilkinson's cylinder boring lathe. In an engine worked by steam under pressure, the piston and cylinder must fit together tightly enough to contain the steam, yet still with enough freedom to allow the piston to move. Wilkinson's lathe was the first machine capable of boring cylinders with sufficient accuracy.
The steam engine also drove manufacturing machines faster than ever before, so they were under greater stress. The accurate fitting of their parts thus became more critical, and the precision machine tool assumed still greater importance.
Tools and Mass Production
Because precision machine tools could produce parts to a high degree of accuracy, they facilitated mass production - the high-speed, large-scale manufacture by machines of standardized, interchangeable components or finished products.
In 1798 the American inventor Eli Whitney began work on a contract to deliver 10,000 muskets to the US army within two years - an unprecedented task for one gun manufacturer to undertake in the USA at that time, and the first instance of mass production.
Whitney would not have attempted it without precision lathes fitted with guides, shaping parts made to fixed patterns.
In 1802 the British Navy decided to mechanize the manufacture of pulley blocks for ships' rigging, for the sake of speed and economy. To designs by the engineer Marc Isambard Brunel, the tool-maker Henry Maudslay made an integrated series of 43 machine tools to carry out the whole manufacturing process, from sawing the original elm logs to shaping the channels in the pulley blocks. By this means, blocks of three different sizes were produced at the rate of 100,000 a year, the skilled workforce was reduced from 110 men to 10, and one-third of the capital invested was recovered in a single year.
Some of Eli Whitney's gun-making machinery had been driven by water power. But the idea of making natural power do the work of muscle power in operating machines was already at least 2,000 years old. The power of running water, turning the blades of a water wheel, which then turned grindstones through gears, was tapped in the Middle East about 200 BC. Until the general introduction of the steam engine, water was the major source of industrial energy. Among the tools it powered were grindstones for milling com, saws, forge hammers and ore crushers. Some of these tools were also operated by windmills, using another natural source of power.
In the 19th century, steam supplanted water as the major energy source for powered tools. Henry Maudslay's pulley block-making tools were steam powered.
James Nasmyth's steam hammer was another of this new generation of machine tools. The steam engine had the great advantage that it did not need to be situated near running water; nor was it dependent on such unpredictable factors as water flow and wind force.
Other sources of power came into limited use alongside steam. Hydraulic power - the power exerted by the resistance of water or other liquids to compression - was used by Joseph Bramah in 1795 to drive a press. It is still used to power lifting gear, such as jacks and fork-lift trucks. The spread of piped gas supplies made it possible to use gas engines, in which the combustion of coal gas in a cylinder drove a piston, on an industrial scale.
But not until the coming of electricity was steam superseded as the main power source for driving power tools and other machinery.
The principal advantage of electricity was its manageability. With little or no loss of power, a smooth flow of energy could be relayed to wherever it was needed. In a laboratory experiment of 1821, the British scientist Michael Faraday demonstrated the principle of the electric motor: a metal wire situated in the magnetic field of a magnet rotated when electric current was passed through it. In another experiment ten years later, Faraday demonstrated the principle of the dynamo, or electricity generator, which is basically the motor working in reverse - when an outside power source rotated a metal disc in a magnetic field, electric current was produced in the disc.
Electric power on an industrial scale became available with the invention of a practical and successful dynamo by the Belgian Zenobe Theophile Gramme in 1870. But this machine produced only direct current. This current could be transmitted and used only at the voltage at which it was produced; it could not be distributed to users over a wide area needing different voltages. These disadvantages were overcome by the general adoption of alternating current, which can be 'stepped-up', or increased in voltage, by transformers to the very high voltages needed to avoid power loss during transmission, and then 'stepped-down' by other transformers to suit local needs.
With the development of the practical alternating-current dynamo and electric motor by the Croatian-born engineer Nikola Tesla in the 1880s, electricity began to take over from steam as the main provider of power for machine tools and industrial machines. When electric motors became more compact, they could be used also to drive small tools such as hand drills. The competition from other forms of power is mainly limited to hydraulic power, and to the petrol motor. The latter is used where a power tool must be self-contained, as in the case of the chain saw.
Today, the use of electricity in tools is no longer limited to turning a shaft. For example, very hard materials such as quartz and tungsten carbide can be drilled by using ultrasonics. A drilling rod is electrically vibrated so that it produces ultrasonic sound waves - waves of such high frequency that they are inaudible to the human ear. These waves drive liquid-suspended abrasive particles which cut through the workpiece; the tool does not touch the work.
In the process of electro-chemical machining , used for cutting hard metals, the current itself is the tool. The metal to be shaped is positively charged, so that current flows from it to a negatively charged mould, removing metal from the workpiece until it takes on the shape of the mould.
Building the quality of precision into a tool helped to remove both human error and the variations found in finished work done by hand. Logically, the next step was to remove the unreliable, expensive human operator altogether. A beginning was made in France in 1801, when the French weaver Joseph Marie Jacquard designed automatic control for pattern-weaving looms by means of punched cards. His machines responded to the instructions in the form of patterns of holes in the cards, as do some modern computers.
In the USA in the 1860s, a lathe was devised that responded automatically to instructions in the form of patterns of projections on revolving cylinders.
These mechanical machine guides were early examples of automation at work.
Nowadays, machine tools can be electronically guided by computers (CNC Machining Centers) to make identical or varying parts automatically, according to instructions uploaded from notebook computers.