Description

Technical Field

[0001] The invention relates to the field of drawing dies used for drawing metal wires. The drawing die according claim 1 is particularly suited for drawing metal wire in liquid lubricants also called‘wet wire drawing’. The nib of the drawing die is held in a nib holder according a second aspect of the invention. Also a method to insert the nib into the nib holder is described representing a third aspect of the invention.

Background Art

[0002] In the production of metal wires like steel, copper alloy, aluminium alloy wires for example, the wire is drawn through subsequently smaller orifices in drawing dies resulting in a stepwise reduced diameter of the wire. The drawing dies are held in a drawing bench that is provided with multiple capstans in between the drawing dies that pull the wire through the orifices. A lubricant is applied to the wire to reduce the friction between the drawing die and the wire passing by.

[0003] As the plastic deformation of the wire and the friction between the wire and the drawing die generates a lot of heat the drawing die must be cooled to prevent premature failure of the drawing die. In dry wire drawing, i.e. when using dry powder as a lubricant, the drawing dies are individually provided with a coolant supply. In wet wire drawing the drawing dies as well as the wire are immersed in a liquid emulsion that acts as lubricant as well as a coolant. In the remainder of the application consistent use will be made of the word‘coolant’ although it is understood by the skilled artisan that this ‘coolant’ also has other functions such as lubrication.

[0004] This application focuses on drawing dies for use in wet wire drawing

processes and machines,‘wet wire drawing dies’ or simply referred to as ‘drawing dies’ in what follows. In a typical wet wire drawing bench between ten and fifty drawing dies can fit. A typical wire mill uses between hundreds and millions of drawing dies each year.

[0005] Known wet wire drawing dies consist of a cylindrical nib that is held in a cylindrical metal casing. The nib is made of a very hard material in which an axial, funnel shaped orifice is machined and polished. The casing is made of machine steel. One problem with known wet wire drawing dies is that the thermal transport of heat is not always optimal as the heat has to exit the nib and must be taken up by the casing that on its turn is in contact with the coolant. In order to overcome this cooling problem different suggestions have been made in the prior art:

[0006] GB87225 (D1 ) describes a nib holder (called a‘die holder’ in D1 ) wherein the nib 17 (called‘pellet’ in D1 ) is held in the holder that is provided with a ‘chamber’ with‘radial ports’ leading from the perimeter of the chamber in order to supply cooling medium immediately around the nib. The holder is intricate to make and the coolant must be injected into the radial ports in order to cool the nib.

[0007] DE596506 (D2) describes a nib holder‘F’ for a dry drawing die wherein cooling channels are provided in the nib holder. The holder is difficult to make and the cooling liquid must be supplied separately.

[0008] GB298821 (D3) describes a housing for a drawing die wherein the

housing consists of a hollow member enclosing the die, the hollow member being provided with inlet openings in front of the housing and outlet openings at the rear of the housing, the inlet and outlet opening communicating with an annular space between nib and housing for the passage of a cooling liquid in the die. The holder is intricate to make and a forced passage of cooling liquid through the drawing die is needed to have good cooling.

[0009] CN201324454U (D4) describes a nib holder provided with outer

circumferential cooling grooves. It is the experience of the inventors that the cooling grooves easily accumulate dirt thereby reducing the cooling capacity of the dies.

[0010] In order to overcome the drawbacks of the prior art, the inventors have come up with a completely new design of a drawing die that will now be disclosed in detail. Disclosure of Invention

[0011 ] It is a prime object of the inventors to provide a drawing die of which the nib can be cooled well. It is a further object of the inventors to provide a drawing die and associated nib holder that is easy to manufacture and has a reduced cost both in terms of material and manufacturing cost. Another object of the invention is to provide a method to mount the nib into the nib holder. A still further object of the invention is to provide a method to produce the nib holder.

[0012] According a first aspect of the invention a wire drawing die with the

features of claim 1 is presented. The wire drawing die comprises a nib and a nib holder, the nib having a through hole. The through hole will generally have a funnel or trumpet shape. The wire enters at the wider entry of the funnel, called‘entry cone’ and exits by the narrower spout. After the spout the through hole opens again in an‘exit cone’. The through hole defines a nib axis.

[0013] The nib has a frustoconical and/or cylindrical mantle that is coaxial to the nib axis. The mantle of the nib can thus be completely a cylinder over the complete axial length of the nib. Or the mantle of the nib can be that of a single frustocone over the complete axial length of the nib. A frustocone is the frustum of a cone. The mantle can also be a combination of a cylinder surface over one part of the axial length and a frustocone over the remaining part of the axial length. Alternative and equivalent wording is that the nib has a‘substantially cylindrical mantle’.

[0014] According the invention the nib holder leaves at least 20% of the surface of the mantle of the nib not covered and this in a limited number of not covered areas. With‘not covered area’ is meant that the nib holder does not contact the nib at that area. Possibly the areas are disjoint meaning that a line can be drawn between the areas on the mantle of the nib, not entering the areas. The total surface of the not covered areas is at least 20% of the surface of the mantle. For clarity: with the surface of the mantle is meant the surface of the frustoconical and/or cylindrical circumferential mantle of the nib thereby excluding the axial extremal - top and bottom - areas of the nib. The number of - possibly disjoint - not covered areas is two or three or four or five or six or seven or eight, up to and including twelve.

[0015] When the number of not covered areas is less than two the nib holder will not be able to hold the nib sufficiently. When more than twelve uncovered areas are present the nib holder becomes difficult to produce.

[0016] In an alternative and equivalent way of describing the invention at least 20% of the surface of the mantle is visible i.e. is in the line of sight when viewing the nib held in the nib holder in all directions perpendicular to the nib axis.

[0017] As the coolant fully surrounds the drawing die when immersed in coolant during wire drawing it results that the coolant can reach the nib mantle unobstructed and immediately. As the coolant has free access to the mantle of the nib, the nib is cooled very well. There is no need for cooling feed channels as the nib is directly exposed to the coolant due to the open structure of the nib holder. Due to the open structure of the nib holder there is no risk that cooling channels become obstructed - for example due to dirt in the coolant - that would inhibit the cooling of the die.

[0018] In a reduced form of the invention the drawing die solely consists of the nib and the nib holder and no other parts.

[0019] In further preferred embodiments at least 30%, 40%, 50%, 60%, 70% or even 80% of the surface of mantle remains not covered by the nib holder. Although the cooling will further improve with increasing disclosure of mantle surface of the nib it becomes increasingly more difficult to hold the nib in the die holder. The purpose of the inventive drawing die is to expose as much mantle of the nib as possible while the nib is held in the nib holder to improve the cooling of the nib.

[0020] In a further alternative and equivalent way of describing the invention the nib holder covers at most 80% of the surface of the mantle of the nib in a number of areas covered by the nib holder, said number of covered areas being one, two, three, four, five, six, seven or eight up to and including twelve. Mutatis mutandis the cooling capacity of the nib will increase when the coverage of the nib mantle by the nib holder reduces to at most 70, 60; 50, 40, 30 or even 20 % of the surface of the mantle of the nib. [0021] The nib is made of a hard material such a cermet, hardmetal, cemented carbide, polycrystalline diamond (PCD) compacts or monocrystalline diamonds from natural or synthetic origin. Mainly used are cemented carbide such as tungsten carbides with e.g. a cobalt containing binder.

Also preferred for use in the invention is PCD compacts.

[0022] In an alternative definition of the invention, the nib holder comprises a base plate from which clamps protrude. The clamps serve to hold the nib firmly. The clamps are unitary to the base plate meaning that the base plate and the clamps are made of one unitary, single piece of material: there are no welds nor mechanical connections between the base plate and the clamps. The base plate has at least one through hole that is coaxial to the nib axis where through, during use, the wire is drawn. The wire is pulled in the direction from the clamps towards the base plate.

[0023] In line with the invention, the clamps cover at most 80% of the surface of the nib mantle. Less coverage such as less than 70, 60, 50, 40, 30 or even 20% will further increase the cooling of the nib, but at the expense of being held less well in the die holder. It is preferred that at least 20% of the surface of the mantle is in line of sight when viewing the nib held in the nib holder in all directions perpendicular to the nib axis. The amount of surface visible can also be more than 30, 40, 50, 60, 70 or even 80% of the surface of the mantle.

[0024] The base plate can have any shape but more preferred the base plate has a polygonal shape, for example a regular polygonal shape, or a Reuleaux polygon shape. The number of vertices is preferably equal to the number of clamps.

[0025] In another highly preferred embodiment the base plate has the shape of a ring or annulus.

[0026] In a further preferred embodiment the clamps that protrude from the

baseplate are angularly evenly distributed. If, for example, the number of clamps is three, the angle between clamps is 120° as viewed from the nib axis. In case the number of clamps is‘N’ the angle between clamps as viewed from the nib axis is 360 N.

[0027] In a further preferred embodiment a recess is provided around the through hole in the base plate of the nib holder. The recess serves to receive the base of the nib. The‘base of the nib’ is one of the surfaces axially delimiting the nib at the side of the nib exit cone.

[0028] The number of clamps is at least two - as one needs at least two clamps to hold something - or any higher number such as two, three, four, five, six, seven or eigth up to and including twelve. Increasing the number of clamps complicates manufacturing. Preferably the number of clamps is three, four, five or six. Depending on the manufacturing method, an even or odd number of clamps may be preferred. When making the nib holder by means of machining an even number of clamps is more preferred while in contrast therewith an odd number of clamps may be more preferred when casting the die holder.

[0029] In a highly preferred embodiment, the transition from the base plate to the clamps protruding from that base plate is rounded. More specific the transition from the base plate to the clamps is rounded between the clamps. With rounded is meant that the transition does not have sharp angles wherein dirt could accumulate. A radius of curvature of at least half a millimetre is a minimum. One, two or more millimetre is better.

[0030] In a further preferred embodiment the clamps protruding for the base plate extend at least two thirds of the axial length of said nib out of the base plate. Less extending of the clamps results in too low holding force for the nib. More preferred is that the clamps protrude about the axial length of the nib out of the base plate. More is not needed as this leads to superfluous material.

[0031] In another preferred embodiment of the invention the clamps are provided with a protrusion, a projection, bump at the radial inner side of the clamp at the end opposite of the base plate. This protrusion prevents that the nib is pulled out of the nib holder when during wire drawing the machine is stopped and the wire recoils.

[0032] According a second aspect of the invention a nib holder is presented. The nib holder has a base plate with clamps protruding out the base plate. The clamps and the base plate are one whole i.e. unitary. The clamps and the base plate are made of the same uninterrupted piece of material. The base plate has a through hole at the centre of the base plate for letting the wire pass. The nib holder is specifically designed to be used in the drawing die as described in the first aspect of the invention.

[0033] According a preferred embodiment the nib holder has a symmetry axis going through said through hole that is perpendicular to the base plate. In a preferred embodiment the protruding clamps are angularly evenly distributed around the symmetry axis.

[0034] In a further preferred embodiment the base plate of the nib holder is a ring.

The number of protruding clamps is two, three, four, five, six, seven or eight or up to and including twelve.

[0035] Preferably at the transition where the clamps extend, rise out of the base plate, the transition is rounded. By preference the radius of curvature at the transition is higher than half a millimetre, for example one or two or more millimetre. This is to prevent dirt accumulation during use of the drawing die in the wet wire drawing machine.

[0036] Further preferred embodiments of the nib holder may show the following additional features:

- The circumferential (relative to the nib axis) thickness of the protruding clamps diminishes radially towards the centre of the nib holder. In this way sufficient surface of the nib mantle remains uncovered while the clamps remain strong enough to hold the nib.

- The radial inner top of the clamps is provided with a small protrusion, projection, bump for holding the top of the nib. This ensures that the nib will remain in the nib holder when the wire being pulled through recoils at a sudden stop of the machine.

- The clamps are mechanically held by a retainer ring connecting all clamps. The retainer ring fits in notches made circumferentially in the top of the clamps. The retainer ring serves to give additional strength to the clamps.

[0037] According a third aspect of the invention a method to clamp a nib in a nib holder is provided thereby forming a drawing die according the invention. The method starts by providing a nib holder according the second aspect of the invention. Next to that a nib is provided. The nib has either a cylindrical or a frustoconical circumferential mantle or a concatenation of both. In a further step a force is applied at the centre of the base plate while holding the outer rim of the base plate. The force is exerted from the side opposite to the side of the base plate where the clamps protrude. This force opens the clamps very slightly, but enough to allow a temporary grip of the nib. This allows to insert the nib between the protruding clamps. In a final step the nib is completely pressed into the nib holder. Possibly the nib base is received in the recess of the base plate.

[0038] In an alternative implementation of the method the method can be

combined with the heating of the nib holder for example when using purely cylindrical nibs. However, care should be exercised that the combination of the force applied at the centre of the base plate with the heat applied does not lead to a plastic deformation of the die holder.

[0039] According a fourth aspect of the invention a method to produce the nib holder according the above by metal injection moulding is described. The method comprises the steps of:

- Providing a mixture of a binder comprising a metal powder;

- Injection moulding the mixture into a mould of which the internal cavity has the shape of the nib holder, thereby forming a fresh nib holder;

- Removing the fresh nib holder from the mould;

- Debinding the binder from the fresh nib holder thereby forming a baked nib holder;

- Sintering the baked nib holder to maximal densification;

- Cooling the sintered nib holder resulting in a finished nib holder. Brief Description of Figures in the Drawings

[0040] Figure 1 shows a prior art wire drawing die;

[0041] Figure 2 shows a first wire drawing die according the invention;

[0042] Figure 3 shows a second wire drawing die according the invention;

[0043] Figure 4 shows a third wire drawing die according the invention;

[0044] Figure 5 shows a fourth wire drawing die according the invention;

[0045] Figure 6 illustrates the method to insert the nib into the nib holder;

[0046] Like items in different figures carry the same unit and tens number, the hundred number indicating the number of the figure. Mode(s) for Carrying Out the Invention

[0047] Figure 1 shows a prior art drawing die 100 comprising a nib holder 102, also called‘casing’, and a nib 104. The nib has a through hole 110 through which the wire is guided. The wire enters at the wide entrance cone 106 and exits through the smaller exit cone 108. The through hole thus has the shape of a funnel.

[0048] The nib is made of cemented tungsten carbide with cobalt used as a

binder. The size and finish of the nibs is standardised in international standards such ISO 1684 and ISO 2804 The size of the nib is indicated with D*H wherein‘D’ is the outer diameter of the nib (usually in mm) and ΊT is the axial height of the nib (also usually expressed in mm). A typical nib size is e.g. 12x10 or 16x13. The outer mantle shape of the nib is generally cylindrical (when the nib is inserted into the casing by means of ‘hot casing’) or is a combination of a cylindrical surface with a frustoconical chamfer at the side of the exit cone (when the nib is inserted into the casing by means of‘cold casing’). Nibs are commercially available from suppliers such as Ceratizit, Hyperion and many others.

[0049] In prior art drawing dies the nib holder is a steel cylinder with a central bore hole wherein the nib is tightly fitted (by means of‘hot’ or‘cold’ casing). As the mantle of the nib is in close contact with the steel casing it is a general belief that the heat generated in the nib will easily pass the nib-to-steel barrier. However due to dirt or remaining lubricants on the nib or casing, a micro gap between nib and casing can result leading to a thermal barrier thereby reducing the heat extraction and an in increased nib temperature during drawing.

[0050] Figure 2 shows a first embodiment of the drawing die 200. It consists of a nib 204 that is a conventional nib that is held in nib holder 202. The nib holder is of an open design such that about 50% of the outer mantel of the nib 204 is not covered in three disjoint areas. The sum of the not covered areas amounts to about 50% of the mantle surface. Alternatively worded: the nib holder only covers about 50% of the surface nib, leaving the remaining part directly visible. [0051] The ratio of the surface that is not covered to the total surface of the mantle can be verified by a number of means for example:

- By spray painting - a‘line-of-sight’ covering technique - the whole drawing die, removing the nib, measuring the painted surface and comparing it to the surface of the mantle of the nib;

- By computer assisted visual techniques based on pictures of the drawing die. The surface area of the uncovered mantle of the nib is calculated taking into account the cylindrical shape of the nib.

[0052] Constructing a drawing die in this manner leaves, during use of the

drawing die, the nib directly exposed to the coolant in which the drawing die is submerged. In this way a strong cooling effect of the nib is obtained, better than having the mantle of the nib being completely enclosed in the prior art cylindrical casing. It is important in this that the coolant can have unimpeded, unhindered, direct contact with the nib without the need of additional coolant feed - through for example channels - to the nib.

[0053] In the shape of the nib holder 202 one can discern a base plate 212 from which three clamps 214, 214’, 214” protrude out of the plane of the base plate 212. The clamps 214, 214’, 214” are distributed along the nib axis under an angle of 120° i.e. are angularly evenly distributed around said axis. As the nib holder 202 is machined from one single piece of material, the clamps and base plate are unitary. The base plate 212 is in this case a ring provided with a central through hole for letting the wire pass.

[0054] A second practical embodiment of the drawing die 300 is depicted in

Figure 3. The drawing die 300 is similarly made out of a single piece of material wherein clamps 314 protrude out of a base plate 312 form the nib holder 302. The difference with the first drawing die is that the transition 316 from the base plate to the protruding clamps is rounded. The rounding prevents that dirt can accumulate in the corner between the base plate and the clamp. Also in the base plate 312, a circular recess is provided to receive the base of the nib (not visible in the Figure 3).

[0055] Note that in both first and second practical embodiment the clamps taper, get gradually thinner radially towards the nib axis i.e. the circumferential thickness of the clamps diminishes towards the nib axis. In this way the surface of nib mantle remains uncovered while the strength of the clamps is not compromised.

[0056] In the third practical embodiment of a drawing die 400 as depicted in

Figure 4, the nib 404 is held in the die holder 402 by means of six angularly evenly distributed clamps 414. The transition from the base plate 412 to the clamp is rounded with a radius of curvature of about 1.5 mm. Note that in this case the width of the clamps 414 remain constant.

Additional, protrusions 422 are provided at the top of the clamps, at the radially inner side. The protrusions 422 are to prevent that the nib 404 would be pulled out of the nib holder 402 by the wire when the drawing machine suddenly stops and the wire recoils, bounces back.

[0057] In a fourth practical embodiment of a drawing die 500 as depicted in

Figure 5, the six clamps 514 are connected with a retainer ring 520. The retainer ring fits in circumferential notches 524 made in the top of the clamps. The retainer ring adds additional strength to the clamps 514.

[0058] The material of which the nib holder is made of is tool steel or machine steel. Examples are EN 11 SMnPb30 and EN C45E or similar.

[0059] The following methods can be used to produce the nib holders:

First there is the method of machining the nib holder. In this method material is removed from a round steel rod corresponding to the outer dimensions of the die holder. For example the gaps between the protruding clamps are milled away by chisels or drills at the rod end. The procedure ends with the mill turning of the rod end. With current numerically controlled machines the procedure can be easily automated.

[0060] Second there is the method of casting. A mould can be made by a lost wax technique representing the nib holder. Pouring molten metal in the mold and opening the mold after solidification results in a cast. The cast must further be finished to obtain the desired tolerances which make this method somewhat less preferred.

[0061] Thirdly the technique of metal injection moulding is used. The technique comprises the steps of:

- Thoroughly mixing a binder with a metal powder to obtain a homogeneous feedstock mixture or

obtaining a readymade, commercially available feedstock; - Injection moulding the mixture into a mould thereby forming a‘green’ nib holder. The mould cavity has the shape of the nib holder;

- Removing the green nib holder from the mould;

- Debinding the binder from the green nib holder thereby forming a ‘brown’ nib holder;

- Sintering the brown nib holder to maximal densification. During

sintering the original dimensions of the brown nib holder will shrink. This shrinkage has to be accounted for in the design of the nib holder mould.

The colours‘green’ and‘brown’ are customarily used in the field of metal injection moulding but do not bear any relation to the true colour of the part. They are only indicative of the state of the sleeve:‘green’ can be substituted by‘fresh’ and‘brown’ by‘baked’. The technique allows to make die holders in large quantities and at low cost. Nib holders made by this technique show some tiny bubbles of entrapped gasses inside the metal. The tiny bubbles are an indication that this metal injection moulding technique has been used.

[0062] Figures 6,‘a’ to‘d’ illustrate the method of how to insert the nib into the nib holder according the invention.

[0063] In Figure 6‘a’ a nib holder 602 according the invention and a nib 602 for insertion into the nib holder 602 are provided.

[0064] The nib holder 602 is placed in clamps 630, 630’ that grip the base plate 612 of the nib holder 602 in between the protruding clamps 614 of the nib holder 602. A stamp 632 is pressed against the centre of the base plate. Figure 6‘b’.

[0065] By exerting force 634 via the stamp 632 the protruding clamps 614 of the nib holder open. By driving the nib 604 by a force 636 the nib is completely inserted into the nib holder. See Figure 6‘c’.

[0066] After release of all forces, the nib is firmly held in between the clamps 614 as illustrated in Figure 6‘d’.