Эксперт по сдаче вступительных испытаний в ВУЗах
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TEXT 1
FOUR INDUSTRIAL REVOLUTIONS
The history of mechanical engineering goes back to the time when the man first tried to make machines. We can call the earlier rollers, levers and pulleys, for example, the work of mechanical engineering.
Mechanical engineering, as we understand it today, starts from the first Industrial Revolution.
People have labelled as «revolutions» three episodes in the industrial history of the world and now we are entering the fourth.
The first industrial revolution took place in England between 1760 and 1840. Metal became the main material of the engineer instead of wood, and steam gave man great reserves of power. This power could drive not only railway engines and ships but also the machines which built them.
In the second revolution, from 1880 to 1920, electricity was the technical driving force. It provided power for factories that was easier and cheaper to control than steam. It was marked also by the growing importance of science-based industries such as chemicals and electrical goods, and the use of scientifically-designed production methods such as semi-automatic assembly lines.
The third industrial revolution coincided with the advent of automation – in its inflexible form. In this revolution, the main features were advances in the control of manufacturing processes so that things could be made more cheaply, with greater precision and (often) with fewer people. And this change, which occurred around the middle of the 20th century, also featured a new machine that was to greatly influence the world, the electronic computer.
What is the fourth industrial revolution? The fourth industrial revolution will be characterized by automated machines that are versatile and programmable and can make different things according to different sets of computer instructions. It will be characterized by flexible, automated machinery, the most interesting examples of which are robots.
TEXT 2
STEEL
The most important metal in industry is iron and its alloy – steel. Steel is an alloy of iron and carbon. It is strong and stiff, but corrodes easily through rusting, although stainless and other special steels resist corrosion. The amount of carbon in steel influences its properties considerably. Steels of low carbon content (mild steels) are quite ductile and are used in the manufacture of sheet iron, wire, and pipes. Medium-carbon steels containing from 0.2 to 0.4 per cent of carbon are tougher and stronger and are used as structural steels. Both mild and medium-carbon steels are suitable for forging and welding. High-carbon steels contain from 0.4 to 1.5 per cent of carbon, are hard and brittle and are used in cutting tools, surgical instruments, razor blades and springs. Tool steel, also called silver steel, contains about 1 per cent of carbon and is strengthened and toughened by quenching and tempering.
The inclusion of other elements affects the properties of the steel. Manganese gives extra strength and toughness. Steel containing 4 per cent of silicon is used for transformer cores or electromagnets because it has large grains acting like small magnets. The addition of chromium gives extra strength and corrosion resistance, so we can get rust-proof steels. Heating in the presence of carbon or nitrogen-rich materials is used to form a hard surface on steel (case-hardening). High-speed steels, which are extremely important in machine-tools, contain chromium and tungsten plus smaller amounts of vanadium, molybdenum and other metals.
Quenching is a heat treatment when metal at a high temperature is rapidly cooled by immersion in water or oil. Quenching makes steel harder and more brittle, with small grains structure.
Tempering is a heat treatment applied to steel and certain alloys. Hardened steel after quenching from a high temperature is too hard and brittle for many appli¬cations and is also brittle. Tempering, that is re-heating to an intermediate temperature and cooling slowly, re¬duces this hardness and brittleness. Tempering tempera¬tures depend on the composition of the steel but are fre¬quently between 100 and 650 0С. Higher temperatures usually give a softer, tougher product. The colour of the oxide film produced on the surface of the heated metal often serves as the indicator of its temperature.
Annealing is a heat treatment in which a material at high temperature is cooled slowly. After cooling the metal again becomes malleable and ductile (capable of being bent many times without cracking).
All these methods of steel heat treatment are used to obtain steels with certain mechanical properties for cer¬tain needs.
TEXT 3
MACHINING
Machining – shaping of parts by metal removal – can produce symmetrical shapes with smooth surfaces and dimensional accuracies not generally attainable by most lubrication methods. Machining of large-volume parts is best accomplished by screw machines. Lathes, drilling machines, milling machines and grinders are used to perform machining operations.
Screw-machined parts are made from bar stock or tubing fed intermittently and automatically through rapidly rotating hollow spindles. The cutting tools are held on turrets and tool slides. Operations are controlled by cams or linkages that position the work, feed the tools, hold them in position for the proper time, and then retract the tools. Finished pieces are automatically separated from the raw stock and dropped into a container.
Screw machines can be turning, threading, boring, cutting off, with special attachments or not. All machining is done on surfaces of rotation concentric with the axis of the part.
Bushings, bearings, pins, nuts, bolts, shafts and many other simple and complex forms are among the thousands of products produced on cut-off, done without stopping the forming operations. Piercing, slotting, trimming are performed on special equipment. Other operations include spot welding, seam welding and curving.
Materials. Any of the ductile metals and alloys that can be formed by other processes can be roll-formed. Most common of these are steel and aluminum roll-formed for decorative and architectural applications.
Brass is commonly roll-formed for applications where its strength, corrosion resistance or ability to be plated is needed. The process is also used to form copper electrical conductors of different types.
Other metals less commonly used for roll-formed shapes, include zinc and magnesium alloys, titanium and some others.
Cut stock less than 20 in. in length does not feed through a roll-forming machine easily: parts adopted to continuous forming from coil stock with automatic cut-off are most economical.
Material can be as thin as 0,005 in. and as thick as 1 in. Most roll-formed production is in the range between 0,020 and 0,125 in. thick. Width of stock can be from less than 1 in. to 100 in.
