An Improved Process for Metal Wire Drawing and a Tool for Actuating the Process.

Technical Field The invention relates to a drawing process where the metal wire is coated with a thin surface layer of material to improve drawing, the wire being passed through several different drawing holes.

Background Art A process of the above-described type is known, in which the wire is coated with a lubricant which serves both to reduce friction during the forced passage through the draw-plate and to obtain a wire with a smooth surface and a constant section.

Prior-art drawplates exhibit a reduction tract, where the metal wire is subjected to plastic deformation. This reduction tract has an angle of convergence which is normally comprised between 12 and 14°, but in any case never exceeds 20-25°.

This is due to the fact that drawplates with holes having an angle of convergence above this produce irregular drawing results thanks to excessive traction forces on the wire, which can be exacerbated to the point of breakage in further drawing operations. In any case, such levels of traction lead to rapid wear on the wire.

Prior-art drawing processes of the above type generally comprise a further phase, after the last of the drawing operations, in which the wire is treated to remove the surface layer of lubricant. This leads to several drawbacks. Firstly, the process is made complicated by the need to predispose suitable equipment for removing the lubricant from the wire after the process; secondly, working times are

considerably affected, with consequent increases in manufacturing costs.

The main aim of the present invention is to eliminate the above-described drawbacks by providing an improved drawing process in which a surface layer of the wire can be removed simply and economically.

An advantage of the invention is that after the final drawing stage a subsequent phase is does not necessarily required in order to remove a coating from the wire.

A further advantage of the present invention is that a high-quality metal wire is obtained, having a smooth external surface and straight constant transversal section.

A further aim of the present invention is to provide a tool which simply and economically removes a surface layer of coating from the metal wire being drawn.

These aims and advantages and more besides are all attained by the invention as it is characterised in the appended claims.

Disclosure of Invention Further characteristics and advantages of the present invention will better emerge from the detailed description that follows of a preferred but non-exclusive embodiment of the invention, illustrated purely by way of a non-limiting example in the accompanying figure of the drawing, in which: figure 1 shows a schematic and partial longitudinal section of a draw-plate according to the invention during use.

With reference to figure 1,1 denotes in its entirety a tool for drawing metal wires, which comprises a draw-plate with a hole 2 having a longitudinal axis x-x, an inlet 3 and an outlet 4 for a metal wire 5. Arrows 6 indicate the wire advancement direction as it passes through the tool 1. The tool 1 is in fact the last draw-plate of a series thereof through which the wire passes in order to reach its final section size. Using Roman numerals I to V, five planes orthogonal to the axis x-x are

indicated, arranged one after another in the advancement direction 6 of the wire 5 and distanced one from another so that they identify 4 distinct but coaxial zones of the drawplate tool 1.

The first zone, comprised between planes I-I and II-II, is cone-shaped in order for the wire to be introduced; for this reason the angle of convergence can be quite large. This first cone I-I performs no plastic deformation on the wire 5, merely introducing it into the body of the tool 1.

The second zone, comprised between II-II and Ici-111, comprises a converging reduction cone, with an angle a of convergence which in the embodiment illustrated is about 32-33°. The wire 5 is subjected to plastic deformation in this zone. This is in fact the most important part of the draw-plate tool 1, as it is here that the wire section is reduced.

The angle of convergence of the first zone (introduction cone) can advantageously be equal to the angle of convergence of the second zone (reduction cone). The two cones could be made into one continuous cone, with no change in the angle of convergence, which would make the tool 1 easier to manufacture.

The third zone, comprised between planes III-III and IV-IV, is a constant-section cylinder, having a same diameter as the wire diameter D2 on completion of drawing, i. e. as it exits the output 4. The fourth zone, comprised between plates IV-IV and V-V, is an outlet cone, with a relatively large angle of divergence which does not interact contactingly with the drawn metal wire 5.

In other embodiments, not illustrated, of the invention, the angle oc of convergence of the second reduction zone can be different from the one shown in figure 1; in the further embodiments, the angle a of convergence is preferably comprised between 30 and 85 degrees and is in any case more than about 30°.

Also possible would be angles a of convergence considerably greater than 85°,

indeed, it would be possible to use angles of up to 120° and above.

The tool 1 can be used to actuate a metal wire drawing process as in the present invention. The process involves coating the wire to be drawn with a thin layer 7 of material which aids the subsequent drawing. The above-mentioned material might be, for example, a lubricant reducing friction during drawing, giving a drawn wire with a smooth surface and constant section.

The process reduces the section of the wire 5 by a series of gradual drawing operations. In the first operations, but excluding the final one, the drawing holes have a relatively small angle of convergence, for example between 12° and 14°, and in any case less than 25°. In the final operation, the tool 1 as described herein is used, with an angle of convergence as shown between planes 11-11 and Ici-111, exhibiting a relatively high angle a of convergence, greater than 30° and preferably between 30° and 85°. Angles a above 85° can be used.

The metal wire 5 at the inlet 3 to the final draw-plate, i. e. the tool 1, has a diameter Dl, while at the outlet the diameter is D2, smaller than Dl. Furthermore the metal wire 5 at the inlet 3 has a layer of surface coating 7, relatively thin, made, for example of a lubricating material. Experiments have shown that when the metal wire 5 passes through the hole 2 of the draw-plate 1, the layer of surface coating 7 does not pass through, being stopped at the start of the reduction passage from 11-11 to 111-111 and being removed to form a waste scurf which rolls back towards the inlet 3 of the hole 2, whence it can be easily removed.

At the outlet 4 the metal wire 5 exhibits a diameter D2 which is smaller than Dl at the inlet 3, and further exhibits a smooth external surface cleaned of the layer 7 of material it presented at the inlet 3.

The tool 1 therefore has a double function ; firstly it reduces the section of the wire, through a drawing action; then it removes the layer of lubricant coating the

wire, through a sort of scouring of the lubricant.

The choice of the shape of the tool 1 (especially the choice of the angle a of convergence at the inlet 3 and the outlet 4 of the reduction cone) depends on various factors. In particular it is necessary to find a proper balance among the following three parameters: the percentage of reduction of the wire section ; the degree of wire cleanliness required (which increases proportionately as the angle a increases) and the working life of the tool (which increases as angle a is reduced).