Abstract:
A wiredrawing machine includes dies and capstans for reduces the cross sectional area of the wire. The machine includes a double die situated at the last capstan to reduce the cross sectional wire one final time in such a way that delamination and central bursting is minimized.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention pertains to the art of methods and apparatuses for wet wiredrawing machines, and more specifically to a final double die used with the wet wiredrawing machines. 
   2. Brief History 
   While the invention is subject to a wide range of applications, it is particularly suited for drawing metal wire into high tensile strength, steel wire with increased torsional ductility. In particular, wire is drawn through a plurality of dies in a wire drawing machine whereby the cross section of the wire is reduced by a constant reduction at each die. The total reduction at the final two dies is generally equal to the constant reduction. 
   The hardness of drawn steel wire results from the plastic deformation associated with the drawing process. The wire increases in hardness as it proceeds through the wire drawing machine. If the wire becomes too hard or brittle, breakage occurs during the drawing process or when the wire is subjected to torsion or bending. 
   As the wire is drawn through a die to reduce its cross section, the outer fibers of the wire flow faster or at a higher velocity than those in its center causing a lesser amount of elongation at the center of the wire than at the surface of the wire. A stress differential resulting from this mechanism of elongation induces compressive, longitudinal stresses on the surface of the wire and tensile, longitudinal stresses at its center. Voids, known as central bursts, can occur in the center of the wire when the tensile stresses exceed the breaking strength of the material. The central burst effect can be prevented by controlling the process geometries, such as the die angle and the percent reduction in area. The central bursting zone defines die geometries for which non-uniform deformation through the cross section of the wire is expected. Die geometries defining the central bursting zone do not always result in central bursting. These geometries will, however, always induce the tensile, longitudinal stresses in the wire center and the compressive, longitudinal stresses at the wire surface that can cause voids and fracture during subsequent drawing steps or when the drawn wire is subjected to torsional loading. 
   Strain introduced into the wire by the drawing process increases the tensile strength of the wire. Preferably, this increase is held constant at every die of the draft in a wire drawing machine. Analyses of the formation of central bursts show that bursting is more likely to occur if the increase in tensile strength remains low. Therefore, the wire is drawn through a draft of many dies each having geometry to avoid the central burst zone. Reducing the number of dies in the draft results in a higher reduction of area at each die. This in turn results in an increase in both heat generation and die wear. 
   Ductility of high strength, steel wire is particularly important when the wire is subjected to plastic deformation during manufacture, such as from twisting a plurality of wires into a multi-wire strand. Torsion testing, indicating the minimum number of twists to failure, is a common method of testing wire ductility. Maximum ductility occurs when there is uniform twisting along a gauge length and the final fracture is straight and transverse to the wire axis. Strain localization and delamination (longitudinal splitting) are qualitative indications of a decrease in ductility. 
   SUMMARY OF THE INVENTION 
   It is desirable to provide a method and apparatus to draw high tensile strength, steel wire that has increased torsional ductility. 
   It is a further advantage of the present invention to provide an apparatus and method of drawing steel wire to produce high tensile strength, steel wire with increased torsional ductility. 
   It is a still further advantage of the present invention to produce high tensile strength, steel wire with increased torsional ductility by a relatively inexpensive method and apparatus. 
   In accordance with the invention, there is provided method and apparatus for drawing steel wire through a plurality of dies and drawing capstans alternately arranged in a wire drawing machine. The cross section of the wire is typically reduced by a reduction of about 15% to about 18% at each of the dies. The cross section of the wire at the final double die is reduced by a total amount substantially equal to the reduction at a single standard die. 
   In accordance with the present invention, a method of drawing steel wire to produce high tensile strength comprises the steps of drawing wire through a plurality of dies arranged in a wire drawing device; reducing the cross section of the wire by a constant reduction of about 15% to about 18% at each of the plurality of dies; and reducing the wire at a final double die by a total amount is equal or less than the constant reduction of the previous dies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein: 
       FIG. 1  is a schematic of drawing capstans and dies for drawing metal wire of the present invention. 
       FIG. 1   a  is a schematic of drawing capstans and dies for drawing metal wire of the present invention. 
       FIG. 2  is an enlarged side view of a standard die. 
       FIG. 3  is an enlarged side view of a double die having multiple inserts in a single die casing. 
       FIG. 4A  is a perspective view of a die casing having heat dissipating grooves fashioned in the outer die casing surface. 
       FIG. 4B  is a perspective view of another embodiment of the die casing having heat dissipating grooves fashioned in the outer die casing surface. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the invention only and not for purposes of limiting the same,  FIG. 1  depicts a wire drawing device  10  that produces high tensile strength wire  12 , which may be steel wire. A plurality of substantially identical, standard dies  14  and drawing capstans  16  are alternately arranged in device  10 . The term “standard die” refers to a die having a geometry that reduces the cross section of the wire passing through one conical region of the die. A final die is included at the end of the device that includes multiple wire reduction components, to be discussed in detail in a subsequent paragraph, such that the total reduction of the cross section of the wire at the final die  18  and  19  of the device  10  is equal or less than the reduction at each of the preceding, standard dies. In one embodiment, the device  10  is preferably a wet slip wire drawing machine  10  and the dies may be submerged in a cooling lubricant for use in dissipating heat. 
   With continued reference to  FIG. 1 , the wire  12  may be brass-coated and/or zinc-coated steel wire or filaments. The steel filaments may have a very thin layer of brass, such as alpha brass, with the brass coating itself having a thin zinc layer thereon, or a ternary alloy addition, such as cobalt or nickel. The term “steel” refers to what is commonly known as carbon steel, also called high-carbon steel, ordinary steel, straight carbon steel or plain carbon steel. Such steel owes its properties chiefly to the presence of carbon without substantial amounts of other alloying elements. However, the tensile strength of carbon steel can be increased by small additions of alloying elements, usually less than 1.0%, referred to as “micro-alloyed steels.” High tensile strength steels having a high level of ductility and outstanding fatigue resistance are described in U.S. Pat. No. 4,960,473, which is incorporated herein by reference. Brass is an alloy of copper and zinc which can contain other metals in varying lesser amounts. The ternary alloys employed as coatings in this invention are iron-brass alloys since they contain 0.1 to 10 percent iron. 
   With reference to  FIG. 1 , the wire  12  may pass directly from each standard die  14  to its drawing capstan  16  and then to the next die. The wire  12  may be drawn over capstans  16  with each succeeding capstan running faster than the preceding one to compensate for wire elongation. The reduction in the cross sectional area of the wire between the capstans on this machine with a straight draft, may be a substantially fixed or standard value. This ensures a lower velocity of the wire being drawn than the peripheral velocity of the drawing capstans. The resulting positive slip ensures that all portions of the wire are taut and that there is adequate frictional force exerted on the wire by the capstan to pull the wire through the dies. 
   With reference to  FIGS. 1 and 2 , the die  14  may include a cavity that houses one or more inserts that reduce the cross section of the wire  12 . The wire  12  may enter the die cavity from a first end and move along a first axis of travel through the die  14 . As the wire  12  travels into the cavity, it encounters a wire reducing section  31 . The cross section of the wire  12  may be reduced in the wire reducing section  31 , by a predefined amount as will be discussed in the following paragraph. The wire  12  may then continue through the die  12  to a bearing region  32 . In one embodiment, the final die, which may be a double die, may include two wire reducing sections  31 ,  31 ′, reference  FIG. 3 , and two bearing regions  32 ,  32 ′. In this manner, the wire  12  is reduced twice within the same die  12  as will be discussed in detail in subsequent paragraph. 
   With reference to  FIGS. 1 and 2 , in one embodiment, reference  FIG. 1 , the wire  12  may be reduced by a constant amount of about 15% to about 18% at each standard die  14 . Preferably, the cross section of the wire is reduced at each die  14  by an amount of about 15.5%. An important aspect of the invention is that the total reduction of the cross section of the wire at the final die  18 ′ is substantially equal or less than the reduction at one of the preceding, standard dies. Preferably, the reduction in the next to last die  18  may be about 10% to about 90% of the constant reduction at the preceding, standard dies  14  and the remaining reduction is at the final die  19 . More preferably, the reduction at next to final die  18  is about 30% to about 70% of the constant reduction and the remainder is at the final die  19 . Most preferably, the reduction at the next to final die  18  is about 55% of the constant reduction and the remainder is at the final die  19 . The last reduction in the cross sectional area of the wire within the next to last die  18  is preferably between 2% and 6%. 
   With reference to  FIG. 2 , a standard die  14  is shown having a die angle ‘a’, a bearing surface ‘b’, a back relief angle ‘c’ and an inlet opening diameter ‘d’. Each standard die  14  may have a die angle ‘a’ of about 5 to about 12 degrees. For the purpose of the present invention, each die  14  may have a die angle of about 10 degrees. The final two dies  18  and  19  are substantially identical to the standard dies with the exception of the amount of reduction taken. Each of the final two dies may have a die angle of about 5 to about 12 degrees. Preferably, this die angle is about 10 degrees. The specific die angle ‘a’ in conjunction with the cross sectional areas of inlet opening ‘d’ and bearing surface ‘b’ controls the amount of reduction, as identified above, of the cross area of the wire as it passes through the die. 
   With reference to  FIG. 1   a  and  3 , in the present embodiment, the final two dies  18  and  19  may be incorporated into a single die casing  18 ′, as shown in  FIG. 1   a . The final die may therefore be a single die facilitating two separate reductions of the wire  12 , in one pass through the die  18 ′, termed a double die  18 ′.  FIG. 3  shows a schematic representation of the double die  18 ′. The die  18 ′ may include multiple inserts or alternately may be fashioned from a single insert. The approach angle ‘a’ of the double die  18 ′ may be equivalent to that of a standard die  14 . As the wire  12  passes through the single die  18 ′, the wire  12  may be reduced by the first wire reducing portion  31  and then immediately again by the second wire reducing portion  31 ′. A second approach angle ‘a 1 ’ is shown in  FIG. 3 . As discussed above, the die angle ‘a’ and ‘a 1 ’ may be about 5 to 12 degrees, and preferably 10 degrees. The last reduction in the cross sectional area of the wire within the double die  18 ′ is preferably between 2% and 6%. In an embodiment, a die for drawing associated wire products has at least a first die portion having first and second ends, the at least a first die portion having cavity fashioned at least partially interior to the at least a first die portion for use in drawing an associated wire; wherein the cavity extends from the first end of the at least a first die portion to the second end; wherein the cavity includes at least first and second drawing contours for shaping the associated wire; and, wherein the at least a first drawing contour shapes the associated wire at a substantially similar rate as the at least a second extruding contour, wherein the associated wire travels through the cavity along a first axis; wherein the at least a first drawing contour is substantially conical, wherein the at least a first substantially conical contour forms an angle a with the first axis; and, wherein the angle a is between 5° and 12°, wherein the associated wire has an associated wire diameter D; and, further having a bearing region having a length b, wherein the length b is 0.3 the final diameter of the wire; further having a non-drawing relief contour being substantially conical, wherein the nonextruding relief contour forms an angle c with the first axis; and, wherein the angle c is between 20 and 35°. 
   With continued reference to  FIGS. 1   a  and  3 , the double die  18 ′ may include a first bearing region  24  and a second bearing region  25 . The first bearing region  24  may have a characteristic length ‘b’. Similarly, the second bearing region  25  may have a characteristic length ‘b 1 ’. The lengths ‘b’ and ‘b 1 ’ may be smaller than 0.3 times the wire diameter ‘d’. In one embodiment, the lengths may equal 0.2 times the wire diameter. Alternately, the lengths ‘b’ and ‘b 1 ’ may be greater than 0.6 times the wire diameter and preferably 0.7 times the wire diameter. This greatly reduces the delaminating affect on the wire  12  as it passes through the double die  18 ′. Further, region b may be equal to up to 50 times of the wire diameter. 
   With reference now to  FIGS. 4A and 4B , another aspect of the present invention will now be discussed. The die casing  27  may have grooves  28  fashioned on the front and back faces of the casing  27 . The grooves  28  may be cut into the outer surface of the die casing  27 . However, any manner of fashioning grooves in the die casing may be chosen with sound engineering judgment. In one embodiment, the grooves  28  may be radially fashioned on the front and rear faces or surfaces of the die casing  27 . These faces can be flat and have straight grooves, as shown in  FIG. 4A , or these faces may be curved with curved grooves, as shown in  FIG. 4B . In any manner, the grooves  28  provide the added capacity for the die casing  27  to dissipate heat generated from the wire reduction process. Further, axial grooves extend from the radial grooves. The radial grooves provided on the front and rear surfaces provide for lubrication between a tungsten-carbide insert and the wire  12 . It is noted that the die casing may take any shape, including but not limited to star shaped, as chosen with good judgment to facilitate the dissipation of heat away from the die casing  27 . The width W, length L, and depth D, of the grooves  28  may take any dimension as is necessary to increase the heat dissipating characteristics of the die casing over the traditional generally flat die surface. 
   While the present invention is directed to a wire drawing machine incorporating a straight draft, it is also within the terms of the present invention to substitute a wire drawing machine having a tapered draft. The advantage of a tapered draft is that the cross sectional area of the wire is reduced in a fewer number of dies. With a tapered draft, the amount of reduction in cross section of the wire would be larger at the first dies than with the dies in the constant draft. The amount of reduction at each draft would then become increasingly less until the last few dies. Based upon finite element analysis modeling, testing was performed to arrive at the results described herein. 
   It is apparent that there has been provided in accordance with this invention a method and apparatus of drawing metal wire to produce high tensile strength, steel wire with increased torsional ductility that satisfy the objects, means and advantages set forth hereinbefore. While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.