Drawing operations involve the forcing of metal through a die by means of a tensile force applied to the exit side of the die. Most of the plastic flow is caused by the compressive force which arises from the reaction of the metal with the die. Usually the metal has a circular symmetry., but this is not an absolute requirement.
Rods, tubes, and extrusions are often given a cold finishing operation to reduce the size, increase the strength, improve the finish, and provide better accuracy. In general, the preparatory step in cold drawing of bars, tubes that have been hot-rolled is that of removing all traces of scale. This is best done by immersing them in a vat of dilute sulphuric acid from 15 to 30 minutes., removing, and washing in fresh water. After the scale is removed, the material is washed in lime to remove the acid. The lime, plus soap or oil, acts as a good drawing lubricant.
Wire drawing - All the wire that is available is produced by cold-drawing through dies. As in the case of cold rolling, the raw material for drawing is rolled bar from hot rolling mill. The size of rod varies from about 6 to 19 mm in diameter, depending upon the size of the finished wire. When required for drawing into wires this may be in coils several hundred meters in length.
Dies are made of chilled cast iron, hardened alloy steel, cemented tungsten carbide, and diamonds. The selection of the die material depends upon the composition of the wire as well as upon the size of the wire and the finish required. In passing through the dies, the steel is severely cold worked and must be annealed. To soften the wire, it is passed through a furnace where it is heated to a temperature in the vicinity of, or slightly lower than, the critical range.
Both single-drafts or continuous - drawing processes may be used. In the first method, the drawing operation is started by pointing the end of the rod and pushing it through the tapered hole in the die. The end is gripped by tongs and sufficient wire is pulled through the die so that the wire can be attached to a power-operated reel. The reel is then rotated to draw the wire through the die at the desired rate. In single-draft method, the wire is drawn several times through a single die until the necessary reduction has been obtained. In view of the enormous pressure between the die and rod being drawn, it is necessary to lubricate the area of contact. In continuous - drawing the wire is fed through the several dies and draw-blocks are arranged in series to obtain the necessary reduction.
Tube Drawing - This method produces tubes having smaller diameters or thinner walls than can be obtained by hot rolling. This is also used to get dimensional accuracy, smooth surface, and improved physical properties of tubes already made by hot rolling process.
Cold-drawing seamless tubing is similar to the corresponding process in solid bar stock. Hot rolled tubing must first be treated by pickling and washing to remove all scale and then covered with a suitable lubricant. One end of the tube is reduced in diameter to permit it to enter the die, and a mandrel that has an outside diameter smaller than the workpiece inside diameter is passed through the tube stock. In drawing through with the tongs, in inside and outside , diameters of the tube plastically conform to the diameters of the mandrel and die, respectively. Light finishing requires only a single pass, while deep reduction necessitates two or more passes of the tube through the die.
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