MANUFACTURING A FASTENER WITH PROFILE DRAWING

Description

Technical Field

[0001] The invention relates to a method for manufacturing a fastener with reduced cross-section. An example of a fastener is a nail.

Background Art

[0002] Fasteners with reduced cross-section are well known in the art.

[0003] US-A-4,755,091 and US-A-4,973,211 disclose nails in the form of a star, i.e. a nail with a cylindrical core and three or more ribs protruding from said core. These star-like nails have a cross-section that is reduced to a great extent.

[0004] A lot of advantages are attributed to these star-like nails.

[0005] A first advantage is that these nails use much less steel. It is claimed that the star-like nails use 50% less steel material than the traditional nails with a round cross-section.

[0006] A second advantage is that these star-like nails have an increased surface area compared to a round nail. This increased surface area and the star- like form lead to an increased holding power.

[0007] A third advantage is that these nails lead to less splitting of the wood when hammering the nail in the wood since some of the fibres in the wood are broken by the ribs.

[0008] Despite all these advantage there are hardly any star-like nails available on the market. The reason has been explained in US-B1 -7,097,403: difficulties in manufacturing. Indeed the complex shape of the star-like nails is difficult to manufacture in a continuous and economic way by means of rollers or Turks heads. Batch manufacturing star-like nails from conventional nails with round cross-section, as disclosed in US-A- 5,836,826, has also shown not to be competitive.

[0009] US-B1 -7,097,403 discloses now a fastener with a cross-section deviating from the known cross-sections in the prior art. The cross-section of the nail disclosed in US-B1 -7,097,403 has an imaginary bisecting parting line. At least two ribs form an angle of about 90° with this parting line and protrude from this parting line. So the ribs are no longer starting from a center core and radially protruding. This type of fastener is made by means of two opposing half-dies.

[0010] The use of two opposing half-dies, however, requires a high maintenance since these half-dies must continue to perfectly fit to one another. In addition, the splitting of the dies limits the speed of drawing.

Disclosure of Invention

[0011] It is an object of the present invention to avoid the drawbacks of the prior art. [0012] It is an aspect of the present invention to provide for an economic way of manufacturing a fastener. [0013] It is another aspect of the present invention to provide for an improved way of manufacturing a star-like nail. [0014] According to the invention, there is provided a method of manufacturing a fastener.

The method comprises the following steps: a. providing a steel wire with a cross-section having an initial convex form; b. drawing the steel wire in two or more drawing steps by means of two or more single piece profile drawing dies thereby further reducing the cross-section and thereby changing the cross-section into a cross- section having indentations and concave parts. The indentations have amplitudes which become greater with each drawing step.

[0015] The terms "single piece drawing die" refer to a drawing die that is not made out of two halves.

[0016] In an embodiment, the initial convex form is either round or polygonal.

[0017] In a preferred embodiment of the invention, the final cross-section of the fastener has at least three ribs radiating from a central core.

[0018] In another preferred embodiment of the invention, the cross-section has a cylindrical core and has at least three ribs protruding from the core. The length of this protrusion is smaller than the diameter of the cylindrical core. This has the advantage of providing a more robust structure where the ribs are less likely to break off.

[0019] Most preferably, the number of ribs is equal to four.

[0020] The above-mentioned method of drawing by means of consecutive single piece profile drawing dies has resulted in speeds of drawing at the final drawing step of 300 m/min and more.

Brief Description of Figures in the Drawings

[0021] The invention will now be disclosed in more detail with reference to the accompanying drawings in which:

- FIGURE 1 , FIGURE 2 and FIGURE 3 show consecutive cross-sections of the single piece drawing dies.

Mode(s) for Carrying Out the Invention

[0022] FIGURE 1 , FIGURE 2 and FIGURE 3 illustrate the progressive die design.

[0023] FIGURE 1 shows a cross-section of a first so-called first rougher die 10. The hard metal around the die is of one single monolithic piece. The cross-section where the steel wire is drawn through, has a square-like cross-section with four straight sides 12 and four rounded edges 14. The die 10 (and also the other dies) may have an inner coating of wolfram carbide, titanium nitride or diamond like carbon.

[0024] FIGURE 2 shows a cross-section of an intermediate die 20. The sides 22 of the cross-section are no longer straight but exhibit an indentation 26. The edges 24 are rounded. The cross-section area may be between 70 % to 80 % of the cross-section area of the first rougher die.

[0025] FIGURE 3 shows a cross-section of a so-called finish die 30. The four sides 32 exhibit an indentation 36 which is larger than the indentation 26 of the previous die. The four edges 34 remain rounded. The cross- section area may be between 45 % and 60 % of the cross-section area of the first rougher die.

[0026] The die angles of the dies 10, 20 and 30 preferably range between 4° and 8°. The number of single piece profile drawing dies may vary depending upon the dimensions of the wire fed into the first rougher die 20.

[0027] The steel wire is a low carbon steel, e.g. of the type 1006. This is a plain carbon steel with a carbon content of maximum 0.08 weight %, a manganese content of ranging between 0.25 and 0.40 weight %, a maximum sulphur content of 0.05 weight % and a maximum phosphorus content of 0.04 weight %.

[0028] In order to allow the high speed of drawing and steel wire reduction and to maintain a high die life, a coating such as a zinc phosphate coating is preferred on the steel wire. Suitable drawing lubricants are used throughout the drawing process.