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Wire

From Academic Kids

This article refers to metallic wire. For other usages of the word, see wire (disambiguation).

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Wires.jpg
Wires

A wire is a single, usually cylindrical, elongated strand of drawn metal. Wires are used to bear mechanical loads and to carry electrical energy and/or communications signals.

Wire has many uses. It forms the raw material of many important manufactures, such as the wire-net industry, wire-cloth making and wire-rope spinning, in which it occupies a place analogous to a textile fibre. Wire-cloth of all degrees of strength and fineness of mesh is used for sifting and screening machinery, for draining paper pulp, for window screens, and for many other purposes. Vast quantities of aluminium, copper, nickel and steel wire are employed for telephone and data wires and cables, and as conductors in electric power transmission, and heating. It is in no less demand for fencing, and much is consumed in the construction of suspension bridges, and cages, etc. In the manufacture of stringed musical instruments and scientific instruments wire is again largely used. Among its other sources of consumption it is sufficient to mention pin and hair-pin making, the needle and fish-hook industries, nail, peg and rivet making, and carding machinery; indeed there are few industries into which it does not enter.

Not all metals and metallic alloys possess the physical properties necessary to make useful wire. The metals must in the first place be ductile and strong in tension, the quality on which the utility of wire principally depends. The metals suitable for wire, possessing almost equal ductility, are platinum, silver, iron, copper, aluminium and gold; and it is only from these and certain of their alloys with other metals, principally brass and bronze, that wire is prepared. By careful treatment extremely thin wire can be produced. Special purpose wire is however made from other metals (e.g. Tungsten wire for light bulb and vacuum tube filaments, because of its high melting temperature).

Contents

History of wire production

In antiquity, jewellery often contains, in the form of chains and applied decoration, large amounts of wire that is accurately made and which must have been produced by some efficient, if not technically advanced, means. In some cases, strips cut from metal sheet were made by pulling them through perforations in stone beads. This causes the strips to fold round on themselves to form thin tubes. This strip drawing technique was in use in Egypt by the 2nd Dynasty. From the middle of the 2nd millennium B.C most of the gold wires in jewellery are characterised by seam lines tha follow a spiral path along the wire. Such twisted strips can be converted into solid round wires by rolling them between flat surfaces or the strip wire drawing method. Strip and block twist wire manufacturing methods were still in use in Europe in the 7th century A.D, but by this time there seems to be some evidence of wires produced by true drawing. Square and hexagonal wires were possibly made using a swaging technique. In this method a metal rod was struck between grooved metal blocks, or between a grooved punch and a grooved metal anvil. Swagging is of great antiquity, possibly dating to the beginning of the 2nd millennium B.C in Egypt and in the Bronze and Iron Ages in Europe for torcs and fibulae. Twisted square section wires are a very common filigree decoration in early Etruscan jewllery. In about the middle of the 2nd millennium B.C a new category of decorative wires was introduced which imitated a line of granules. Perhaps the earliest such wire is the notched wire which first occurs from the late 3rd, early 2nd millennium B.C in Anatolia and occasionally later.

Wire drawing was not introduced into England before the second half of the 17th century. Wire is usually drawn of cylindrical form; but it may be made of any desired section by varying the outline of the holes in the draw-plate through which it is passed in the process of manufacture. The draw-plate or die is a piece of hard cast-iron or hard steel, or for fine work it may be a diamond or ruby. The object of utilizing precious stones is to enable the dies to be used for a considerable period without losing their size, and so producing wire of incorrect diameter. Diamond dies must be rebored when they have lost their original diameter of hole, but the metal dies are brought down to size again by hammering up the hole and then drifting it out to correct diameter with a punch.

Drawing wire

Wire is often reduced to the desired diameter and properties by repeated drawing through progressively smaller dies. The wire can be reheated for further drawing; the annealing is done in cast-iron pots, holding coils of wire which are raised to a red heat and then allowed to cool. Although the wire is kept air-tight as much as possible, some amount of scaling occurs, and pickling must be done to remove this scale before redrawing.

An important point in wire-drawing is that of lubrication to facilitate the operation and to lessen the wear on the dies. Various lubricants, such as oil, are employed. Another method is to immerse the wire in a copper (II) sulfate solution, so that a film of copper is deposited which forms a kind of lubricant, easing the drawing considerably; in some classes of wire the copper is left after the final drawing to serve as a preventive of rust.

The wire-drawing machines include means for holding the dies accurately in position and for drawing the wire steadily through the holes. The usual design consists of a cast-iron bench or table having a bracket standing up to hold the die, and a vertical drum which rotates and by coiling the wire around its surface pulls it through the die, the coil of wire being stored upon another drum or "swift" which lies behind the die and reels off the wire as fast as required. The wire drum or "block" is provided with means for rapidly coupling or uncoupling it to its vertical shaft, so that the motion of the wire may be stopped or started instantly. The block is also tapered, so that the coil of wire may be easily slipped off upwards when finished. Before the wire can be attached to the block, a sufficient length of it must be pulled through the die; this is effected by a pair of gripping pincers on the end of a chain which is wound around a revolving drum, so drawing the pincers along, and with them the wire, until enough is through the die to be coiled two or three times on the block, where the end is secured by a small screw clamp or vice ready for the drawing operation. Wire has to be pointed or made smaller in diameter at the end before it can be passed through the die; the pointing is done by hammering, filing, rolling or swaging in dies, which effect a reduction in diameter. When the wire is on the block the latter is set in motion and the wire is drawn steadily through the die; it is very important that the block shall rotate evenly and that it shall run true and pull the wire in an even manner, otherwise the "snatching" which occurs will break the wire, or at least weaken it in spots.

Continuous wire-drawing machines differ from the single-block machines in having a series of dies through which the wire passes in a continuous manner. The difficulty of feeding between each die is solved by introducing a block between each, so that as the wire issues it coils around the block and is so helped on to the next die. The speeds of the blocks are increased successively, so that the elongation due to drawing is taken up and slip compensated for. The operation of threading the wire first through all the dies and around the blocks is termed "stringing-up." The arrangements for lubrication include a pump which floods the dies, and in many cases also the bottom portions of the blocks run in lubricant. The speeds at which the wire travels vary greatly, according to the material and the amount of reduction effected.

Finishing, covering, and insulating

Wires and cables for electrical purposes are covered with various insulating materials, such as cotton, rubber, or plastic, wrapped in concentric fashion and further protected with, substances such as paraffin, some kind of preservative compound, bitumen or lead sheathing or steel taping. The stranding or covering machines employed in this work are designed to carry supplies of material and wind it on to the wire which is passing through at a rapid rate. Some of the smallest machines for cotton covering have a large drum, which grips the wire and moves it through toothed gears at a definite speed; the wire passes through the centre of disks mounted above a long bed, and the disks carry each a number of bobbins varying from six to twelve or more in different machines. A supply of covering material is wound on each bobbin, and the end is led on to the wire, which occupies a central position relatively to the bobbins; the latter being revolved at a suitable speed bodily with their disks, the cotton is consequently served on to the wire, winding in spiral fashion so as to overlap. If a large number of strands are required the disks are duplicated, so that as many as sixty spools may be carried, the second set of strands being laid over the first.

For the heavier cables, used for electric light and power, and submarine cables, the machines are somewhat different in construction. The wire is still carried through a hollow shaft, but the bobbins or spools of covering material are set with their spindles at right angles to the axis of the wire, and they lie in a circular cage which rotates on rollers below. The various strands coming from the spools at various parts of the circumference of the cage all lead to a disk at the end of the hollow shaft. This disk has perforations through which each of the strands pass, thence being immediately wrapped on the cable, which slides through a bearing at this point. Toothed gears having certain definite ratios are used to cause the winding drum for the cable and the cage for the spools to rotate at suitable relative speeds which do not vary. The cages are multiplied for stranding with a large number of tapes or strands, so that a machine may have six bobbins on one cage and twelve on the other.

Rubber covering of wires and cables is done by passing them through grooved rollers simultaneously with rubber strips above and below, so that the rubber is crushed on to the wires, the latter emerging as a wide band. The separate wires are parted forcibly, each retaining its rubber sheathing. Vulcanizing is afterwards done in steam-heated drums. However, the conductor needed to be tinned to provide some relief to stripping away the natural rubber. Since the mid1960s, the insulation has been plastic or polymers exhibiting properties simular to rubber.

Many auxiliary machines are necessary in connection with wire and cable-covering, as plant for preparing the rubber and paper, etc., cutting it into strips, winding it, measuring lengths, etc.

Wire gauges

In commerce, the sizes of wire are estimated by gauges which consist of plates of circular or oblong form having notches of different widths round their edges to receive wire and sheet metals of different thicknesses. Each notch is stamped with a number, and the wire or sheet, which just fits a given notch, is stated to be of, say, No. 10, 11, 12, etc., of the wire gauge.

Gauges may be broadly divided into two groups, the empirical and the geometrical. The first include all the old ones, notably the Birmingham (B.W.G.) and the Lancashire or Stubs. The origin of the B.W.G. is lost in obscurity. The numbers of wire were in common use earlier than 1735. It is believed that they originally were based on the series of drawn wires, No. 1 being the original rod, and succeeding numbers corresponding with each draw, so that No. 10, for example, would have passed ten times through the draw plate. But the Birmingham and the Lancashire gauge, the latter being based on an averaging of the dimensions collated from a large number of the former in the possession of Peter Stubs of Warrington, have long held the leading position, nod are still retained and used probably to a greater extent than the more recent geometrical gauges. There is no need, therefore, to give an account of the other and less known gauges which have been used by manufacturers. In no case is there any regular increment of dimensions from which a regular curve could be drawn.

The first attempt to adopt a geometrical system was made by Messrs Brown & Sharpe in 1855. They established a regular progression of thirty-nine steps between the English sizes, No. 0000 (460 mils or about 12 mm) and No. 36 (5 mils or about 0.13 mm). Each diameter was multiplied by 0.890526 to give the next lower size. This is now the American wire gauge, and is used to a considerable extent in the U.S.A.

The Imperial Standard Wire Gauge, which has been sanctioned by the British Board of Trade, is one that was formulated by J. Latimer Clark. Incidentally, one of its recommendations is that it differs from pre-existing gauges scarcely more than they differ among themselves, and it is based on a rational system, the basis being the mil. No. 7/0, the largest size, is 0.50 in. (500 mils or 12.7 mm) in diameter, and the smallest, No. 50, is 0.001 in. (1 mil or about 25 m) in diameter. Between these the diameter, or thickness, diminishes by 10.557%, and the weight diminishes by 20%.

The circular forms of gauge are the most popular, and are generally 3 3/4 in. (95 mm) in diameter, with thirty-six notches; many have the decimal equivalents of the sizes stamped on the back. Oblong plates are similarly notched. Rolling mill gauges are also oblong in form. Many gauges are made with a wedge-like slot into which the wire is thrust; one edge being graduated, the point at which the movement of the wire is arrested gives its size. The graduations are those of standard wire, or in thousandths of an inch. In some cases both edges are graduated differently to serve for comparison between two systems of measurement. A few gauges are made with holes into which the wire has to be thrust. All gauges are hardened and ground to dimensions.

References

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