Wire is one of the generic types of metallurgical products, together with plates, sheets, bars, tubes. Encyclopaediae generally define metallic wire as a “ single strand or rod of metal, usually cylindrical “. The history of wire making goes way back in Antiquity .
The first known writing relating to wire appears in the Bible (Ex.39:3): “ And they did beat the gold into thin plates, and cut it into wires...) However, archeological discoveries date the art of wire making to much earlier times, probably as far back as 4000 BC : a necklace containing gold wire was found in the tomb of an Egyptian Pharaoh who reigned about 2750 BC and there are wire-containing jewelry and ornaments made by Assyrians in the 1700's BC.
The manufacturing of wire was for a long time limited to jewelry and similar decorative items using “soft” materials such as gold or bronze. Utilitarian uses started to appear in the latter years of BC, as shown by 3 bronze wires twisted into a cable found in Pompei. For many centuries, wire was manufactured by hammering the ductile metals gold & bronze into thin sheets. Then hammers and files were used to transform the thin strips into short round pieces , which could eventually be brazed into longer wires . There is however evidence that even in the antique Egypt some wire were actually drawn through tapered holes , the crude predecessors of “dies”.
Modern wire manufacturing done by drawing through dies can be traced to the 300 AD to 700 AD period. Wire manufacturing by drawing through dies became common in the 12th to 14th centuries, in France, England and Germany: in those times, wire was drawn by hand. German wire manufacturers started to use waterpower to replace hand operation in the Middle Age. Also, German manufacturers of the Düsseldorf area discovered about 1650 the advantage of using lubricants (such as stale beer!) to draw hard steel.
Thus, the basic method of wire manufacturing, i.e. drawing a soft metal through a hard, incompressible die has remained unchanged for centuries. Obviously, modern industrial wire manufacturing has developed for productivity and quality a number of sophisticated technologies pertaining to:
- Wire rod (feed material coming from the hot rolling mill) quality and special coatings
- Drawing lubricants (Na or Ca based soap–type compounds)
- Diamond (oil drawing) or carbide dies (dry drawing) of increasing high technology
- Very productive highly automated multi-hole drawing machines
- Continuous processes combining cleaning, annealing, coating and skin pass sizing
The above narrative obviously only pertains to the “metallic wire” and not to the increasingly important glass wire involved in the “fiber optics” industry.
Wire drawing is a metalworking process used to reduce the cross-section of a wire by pulling the wire through one or more dies. There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes for wheels, and stringed musical instruments. Although similar in process, drawing is different from extrusion, because in drawing the wire is pulled, rather than pushed, through the die. Drawing is usually performed at room temperature, thus classified as a cold working process, but it may be performed at elevated temperatures for large wires to reduce forces.[1]
Of the elemental metals, copper, silver, gold, and platinum are the most ductile and immune from many of the problems associated with cold working.
Process
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Wire drawing conceptThe wire drawing process is quite simple in concept. The wire is prepared by shrinking the beginning of it, by hammering, filing, rolling or swaging, so that it will fit through the die; the wire is then pulled through the die. As the wire is pulled through the die, its volume remains the same, so as the diameter decreases, the length increases. Usually the wire will require more than one draw, through successively smaller dies, to reach the desired size. The American wire gauge scale is based on this. This can be done on a small scale with a draw plate, or on a large commercial scale using automated machinery.[1][2] The process of wire drawing changes material properties due to cold working.
The area reduction in small wires is generally 15–25% and in larger wires is 20–45%.[1] The exact die sequence for a particular job is a function of area reduction, input wire size and output wire size. As the area reduction changes, so does the die sequence.[3]
Very fine wires are usually drawn in bundles. In a bundle, the wires are separated by a metal with similar properties, but with lower chemical resistance so that it can be removed after drawing.[citation needed] If the reduction in area is greater than 50%, the process may require an intermediate step of annealing before it can be redrawn.
Commercial wire drawing usually starts with a coil of hot rolled 9 mm (0.35 in) diameter wire. The surface is first treated to remove scales. It is then fed into a wire drawing machine which may have one or more blocks in series.
Single block 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 wire until enough can be coiled two or three times on the block, where the end is secured by a small screw clamp or vice. When the wire is on the block, it is set in motion and the wire is drawn steadily through the die; it is very important that the block rotates evenly and that it runs true and pulls the wire at a constant velocity, otherwise "snatching" occurs which will weaken or even break the wire. The speeds at which wire is drawn vary greatly, according to the material and the amount of reduction.
Machines with continuous blocks differ from single block machines by having a series of dies through which the wire is drawn in a continuous fashion. Due to the elongation and slips, the speed of the wire changes after each successive redraw. This increased speed is accommodated by having a different rotation speed for each block. One of these machines may contain 3 to 12 dies.[2] The operation of threading the wire 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.[4]
Often intermediate anneals are required to counter the effects of cold working, and to allow further drawing. A final anneal may also be used on the finished product to maximize ductility and electrical conductivity.[5]
An example of product produced in a continuous wire drawing machine is telephone wire. It is drawn 20 to 30 times from hot rolled rod stock.[2]
While round cross-sections dominate most drawing processes, non-circular cross-sections are drawn. They are usually drawn when the cross-section is small and quantities are too low to justify rolling. In these processes, a block or Turk's-head machine are used.[6]
Lubrication
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Lubrication in the drawing process is essential for maintaining good surface finish and long die life. The following are different methods of lubrication:[1]
Various lubricants, such as oil, are employed. Another lubrication method is to immerse the wire in a copper(II) sulfate solution, such that a film of copper is deposited which forms a kind of lubricant. In some classes of wire the copper is left after the final drawing to serve as a preventive of rust or to allow easy soldering.[citation needed]The best example of copper coated wire is in MIG wire used in welding.[10]
Mechanical properties
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The strength-enhancing effect of wire drawing can be substantial. The highest strengths available on any steel have been recorded on small-diameter cold-drawn austenitic stainless wire.[citation needed]
Drawing dies
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Diagram of a carbide wire drawing dieDrawing dies are typically made of tool steel, tungsten carbide, or diamond, with tungsten carbide and manufactured diamond being the most common.[2] For drawing very fine wire a single crystal diamond die is used.[2] For hot drawing, cast-steel dies are used. For steel wire drawing, a tungsten carbide die is used. The dies are placed in a steel casing, which backs the die and allow for easy die changes.[2] Die angles usually range from 6–15°, and each die has at least 2 different angles: the entering angle and approach angle.[2]
See also
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Notes
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References
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