Dies have been used to deform, shape or form metal wire, fiber, rod, cylinder or bar stock, and similar materials. Dies are typically fabricated in the art by attaching a hard, wear- and chip-resistant die core to a softer and tough housing or container, wherein the container material may be shaped more easily than the hard die core to allow rigid reversible attachment to the drawing or extruding machine. The attachment of the hard die core to the housing may be made with a braze, a combination of braze and solder adhesion, a variety of wedges such as interference, a press or thermal-shrink fit, or a sinter process.
Dies may be used to reduce the diameter of a wire, to create a surface roughness on a stock material, or to create a useful shape or profile from the stock material through processes such as wire drawing and extrusion. In a wire drawing process, wire may be pulled through a hole or draw passage in a die core, typically under high tension and at high speed. A wire may be reduced in diameter, wherein the draw passage diameter is less than that of the wire being pulled through the passage. In an extrusion process, a metal bar stock may be pushed through a shaped die to impart a specific profile which may be subsequently cut and bent into usefully articles. In extrusion processes, a hole or profile may be machined, cut or drilled into the die to impart a particular shape, dimension, and/or surface texture for the article.
Because a workpiece is being forced through the die core to impart a shape or reduce dimensions, the workpiece is deformed, creating internal pressure on the die. Drawing or extruding a non-linear shape into a workpiece creates significant internal and non-uniform stresses on the die. Various approaches may be used to manage material non-linear elastic and plastic deformations of the wire or bar stock in order to achieve the desired final article dimension. Such deformation of the workpiece may be known as die swell. Such techniques may include polishing the inner diameter of the die core (the opening of the draw passage). Polishing may control die wear and impart a finished surface on the shaped workpiece. To guide a workpiece through the draw passage, a cone or similar shape may be machined in the soft container material.
Typically, die cores experience wears, chips, and/or micro cracks of the inner diameter of the core, causing the wire or extruded article diameter, shape and/or surface finish to deviate over time. At a threshold level of deviation in article shape, diameter or surface, the die core may be removed. A larger diameter draw passage or profile may be formed in the die core, removing the chip, crack or damage. The die core may then be reused in shaping larger workpieces, such as a metal wire or bar stock. The die is then removed and a larger diameter hole or profile is drilled and/or polished. This process may be repeated until the draw passage or profile reaches about 50% of the diameter of the die core, or until a large crack develops in the die. At this point and with a high wear rate, the die may be retired from use.
In drawing lubricated wire or bar stock, the shear strength of the wire or bar and the deformation rate determine the internal pressure subjected to the die. Harder, less deformable wire, drawn at faster speeds, with larger diameter changes in a small die bearing area increases the pressure on the die. A die lifetime is related to the ratio of applied internal pressure, the die tensile strength, die material selection, and the geometry of the die. The reduction in strength as the die wall thins may be predicted by the uniform, isotropic, low-strain, elastic, single-body Lame equation for maximum bearable internal pressure, P, for a die of tensile (hoop) strength T, wall thickness, t, and inner diameter, Di, shown below (Hall, Rev. Sci. Instr., 37(5), 568-571, 1966). In the equation shown below, as the wall gets thinner, P approaches zero. In other words, the maximum bearable pressure a given die of strength T can support vanishes as the wall wears down. The maximum bearable pressure for a thick die is limited to material strength, T and reaches 60% of that strength for w=2. The maximum incremental improvement in strength with die wall thickness occurs for w approaching 1 or for a thin ring of support.
      P    =          T      ⁡              [                                            w              2                        -            1                                              w              2                        +            1                          ]                  w    =          1      +              2        ⁢                                  ⁢                  t          /                                    D              i                        .                              
Limits to die lifetime typically manifest as chipping, cracking and progressive diameter increase and/or loss of shape precision. Longer lasting dies making more precise shapes at high production rates with better surfaces require higher strength dies.
The apparent strength, T, of the die is the superposition of intrinsic material strength (derived from its manufacture), geometry (w) and any external applied stress that counteracts the internal pressure in use. Uniform external compression is frequently used to counter uniform internal pressure and strengthen die materials.
Compression on the die can be achieved by shrinking a material around the die. One approach in the art is co-sintering a hard diamond die inside a carbide ring. An example of such an approach is described in U.S. Pat. No. 4,016,736 to Carrison et al., which is incorporated herein by reference in its entirety. This method creates high compression via chemical bonding, thus ideally imposes no tensile stresses on the materials. The compression developed depends on the extent of sintering, strength of the particle bonds and defects. In practice however, non-uniform shrinkage and defects creates local tensile stresses and shape distortion in the sintered bodies, which limit compression.
Another approach is disclosed in International Patent Application Publication No. WO 79/00208, filed by Bieberich, incorporated herein by reference in its entirety, wherein compression and attachment of a die is achieved via powder metal sintering and melting. The metal powder shrinks and contracts around the diamond die creating compression, ideally without tension. Compression developed this way is limited by the thermal stability of the die, restricting this method to low melting, soft metals or incomplete sintering.
U.S. Pat. No. 4,392,397 to Engelfriet et al., incorporated herein by reference in its entirety, discloses a different approach wherein the co-sintered carbide ring is replaced with a steel ring press fit around the die material, wherein the ring may be hardened by thermal treatment to increase compression on the die. This method creates high tensile stresses in the steel ring directly proportional to the compression on the die. These tensile stresses can crack the steel ring placing a limit on the compression achieved by this approach.
U.S. Pat. No. 5,957,005 to Einset et al., incorporated herein by reference in its entirety, discloses a method of improving die compression by shaping the sinter-bonded carbide sleeve to redistribute the compression on the PCD die derived from the sintered carbide. Compression improved this way is of course restricted to the PCD die to which it is permanently sinter-bonded.
Another method of providing compression is by wrapping a thin steel ribbon under tension around a die as reported in Groenbaek, “Optimization of tool life & performance through advanced material and prestress design”, ICFG/NACFG International Cold Forging Conference, Columbus, Ohio, Sep. 2-3, 2003. This design may also be viewed at http://www.strecon.com/Products. The completed wrap may be welded or crimped to hold the compression. After welding, discrete steel rings may be placed around the steel ribbon to provide reinforcement. However, this technique requires special steel tensioning and welding equipment, which results in expensive processing and expensive die products. In addition, the resulting container system is not disposable.
There is still a need for an improved diamond wire drawing die with extended service time and capability to draw larger diameter wire and forms for a given die diameter. There is also a need for an improved diamond wire drawing die that resists sliding wear, bulk bending and friction heat (thermal expand/contract or thermal-chemical wear). This need extends to low cost, flexibility, and reliable compression systems for many different sizes and types of dies.
The present invention is directed to solving one or more of the problems described above.