Lesson 11

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Mechanical working processes

The shaping of metal by mechanical means in either the hot or cold state is called mechanical working. In mechanical working processes the metal is shaped by pressure. This excludes machining or grinding, and the casting of molten metal into a mould. Mechanical working processes include rolling, drawing, extrusion, smith forging, die forging and press working.

The initial materials for the mechanical working of metals are ingots and billets of various cross sections and weights.

Mechanical working involves the plastic deformation of either cold or heated metal by the external action of special tools such as mill rolls, forging dies, flat hammer dies or extruding dies. Mechanical working changes the shape and dimensions of the billet but the volume of a metal shaped remains substantially constant throughout the process.

The plasticity of a cast metal having a coarse-grained dendritic structure proves to be less than that of a metal which had undergone deformation and has, therefore, a more fine-grained structure. As a rule, the ductility of a metal increases with temperature, and its resistance to deformation is reduced.

The rate of deformation in mechanical working affects the plasticity of the metal in various ways. An increase in the deformation rate within a definite limit is accompa­nied by a decrease in plasticity. A further increase in this rate, after a certain point, improves the plasticity.

The degree of deformation, both in hot and, especially, in cold working, must be maintained within known limits. Rates exceeding the upper limit lead to the formation of cracks, tears and other discontinuities in the metal.

The type of stressed state and the type of deformation also affect the ductility of the metal being shaped in the course of mechanical working operation. In the various mechanical working processes there are usually two dif­ferent stressed state systems three-dimensional compression of the metal when compressive stresses act along all the three axes, as in smith forging, closed-die forging, rolling and extrusion; and three-dimensional stressed state of the metal when compressive stresses act along two axes and a tensile stress along the third, as in drawing. Metal has a higher ductility in the first system of stressed state than in the second.

The three systems of deformation to which metal is most frequently subjected in mechanical working are compressive strain along one axis and tensile strain along the two others; compressive strain along the first axis, tensile along the second and none along the third; and compressive strain along two axes and tensile strain along the third.

Lesson 12

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Machine -tools

Before metal is formed into the required shape, it has to undergo a number of processes, such as casting, rolling, forging, welding, piercing, trimming, spinning, bending, drawing, etc. The machines which work on the surface of the metal piece, or provide it with grooves or holes or with a thread, are called machine-tools.

The most common machine-tool found in almost any workshop is the lathe, in which the job spins round its axis while a cutting tool is working on it. Its main parts are: the headstock, which usually contains the gearing mechanism; the chuck, into which the job is clamped; the tailstock, which can be moved along the bed to adjust the distance between the chuck and the tailstock centre; and the carriage consisting of the saddle and the cross slide on which the toolholder with the tool is mounted. The speed at which the lathe turns can be adjusted either by regulating the speed of the motor driving it, or by the gearing. The automatic turret lathe is a perfection of the ordinary lathe. Its tools are changed automatically. A worker skilled in the use of a lathe is cabled a turner.

Other machine-tools work on plane surfaces, for example milling machines, and also planing and shaping machines. A broaching machine employs broaches to make non-circular holes. Circular holes are drilled by a drilling machine or bored by a boring machine or a boring mill. Gear cutting machines include gear milling machines, bobbing machines, gear shapers and gear grinding ma­chines. Thread milling machines are used in the production of threads on shafts and on other machine elements; whilst die-sinking machines turn out dies, which are used in the process of stamping.

All these machines use cutting tools, usually made of high-speed steel; grinding machines, on the other hand, use abrasives such as diamonds, diamond dust, corundum, silicon carbide, etc. Honing and lapping machines also use abrasives; they enable us to obtain a smooth finish of less than 0.001 mm.