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Cold Working Mechanically working metals at temperatures below the critical range (cold working) results in strain-hardening the metal. In fact, the metal becomes so hard that it is difficult to continue the shaping process without annealing the metal. Because the errors due to shrinkage are eliminated in cold working, a more compact and better metal is obtained than in hot-worked metal. The strength, hardness, and elastic limit are increased, but the metal's ductility is decreased. Because this makes the metal brittle, it must be heated from time to time during the working to remove the undesirable effects. A number of cold-working processes are used in industry; however, the ones of interest to airframe repairers are cold rolling, cold drawing, stamping, and pressing. Each is discussed in the paragraphs that follow. Cold Rolling. Cold rolling is the process of reducing the cross-sectional area of pieces of metal by passing them between revolving cylinders or rolls at room temperature. In a steel mill, a rolling mill has, in addition to cranes and other handling equipment: - Soaking pits where ingots are brought to the appropriate temperature.
- Reheating furnaces.
- Rolling stands.
- Straightening and cooling tables.
- Cut-off shears.
- Coilers and decoilers for strips.
- Roll-grinding machines.
A rolling stand consists of two or more rollers positioned one above the other and in groups of two or more stands side by side. The operation consists of passing a piece of metal between two or more rollers, subjecting it to compression. The compression compacts and lengthens or compacts and widens the particular piece of metal. The action depends on the rollers' dimensions and the compression applied. In producing cold-rolled sheet and strip and cold-drawn rods and bars of various dimensions, the oxide is removed from the hot-rolled material by pickling; and the material is further reduced on cold-rolling mills. Bars, sheets, and strips are cold-rolled to obtain a desired surface finish, improve dimension tolerances, impart improved physical properties, and make lighter gauges than can be made on hot-strip mills. Cold Drawing. Cold drawing is used in making seamless tubing, wire, streamlined tie rods, and other forms of stock. Wire is made from hot-rolled rods of various diameters. These rods are picked in acid to remove scale, dipped in lime water, and then dried in a steam room where they remain until ready for drawing. The lime coating adhering to the metal lubricates the rod during the drawing operation. The rod size used for drawing depends upon the diameter desired in the finished wire. To reduce the rod to the desired size wire, the rod is drawn cold through a die. One end of the rod is filed or hammered to a point and slipped through the die opening. It is then gripped by the jaws of the draw and pulled through the die. This series of operations is done on a mechanism called a draw bench. To reduce the rod gradually to the desired size, the wire is drawn through successively smaller dies. Because each of these drawings reduces the wire's ductility, it must be annealed from time to time during the drawing operation. Although cold working reduces the wire's ductility, it increases the wire's tensile strength. In making seamless steel aircraft tubing, the rod is cold drawn through a ring-shaped die with a mandrel, metal bar, inside the tubing to support the tube during the drawing operation. This forces the metal to flow between the die and the mandrel and controls the wall thickness and the inside and outside tube diameters. Stamping and Pressing. Stamping and pressing sheet metal into molds or dies are methods of shaping or forming various-sized parts or complete pieces. In a broad sense, the two terms have little difference in meaning. However, stamping is generally applied to forming small objects that can be shaped by one rapid blow of a machine. Pressing, on the other hand, describes the process that uses a slow, steady stroke or movement to form a large piece or section. Making parts by stamping and pressing brings the aviation industry one step nearer to mass production. However, constant changes in aircraft design make it necessary to alter or replace dies frequently.
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