EP0062248B2 - Method of producing trimetallic contact rivets - Google Patents

Description

The starting point of the invention is a method for producing tri-metal contact rivets by cold welding with the features specified in the preamble of claim 1. Such a method has become known from US-A 4073425.

In the implementation of the known method, three differently composed wire sections with matching cross sections are separated from a wire supply and are aligned in succession in a transversely displaceable guide bush between a compression needle displaceable in the guide bush and an abutment arranged outside the guide bush. The middle wire section is usually made of copper, whereas the two outer, mostly shorter wire sections usually consist of silver.

In a first deformation step, only two of the wire sections are initially cold-welded to one another in the known method, by pushing the compression needle against the abutment held in place, an increase in diameter leading to cold welding occurring in a free space in front of the guide bush in the region of the abutting surfaces of two wire sections, while the two other abutment surfaces are still inside the guide bush.

In the next process step, the annular bead created by the cold welding is pressed flat and then sheared off by pushing the three wire sections out of the one guide bushing through the compression needle and pushing them into an opposite guide bushing, in which a further compression needle is arranged to be advanced. Between this compression needle and another abutment opposite it, the wire sections are compressed again in a subsequent deformation cut, whereby those two abutting surfaces of the wire sections, which were in the guide bush during the first deformation process, lie in free space and experience a cross-sectional enlargement leading to cold welding. while the two abutting surfaces welded together in the first deformation cut now lie in the guide bush in question and are not deformed again.

With the second deformation process leading to cold welding, the rivet head of the tri-metal rivet is also preformed; in a further step, it is finally shaped into its final shape using a special headmaking tool.

A major disadvantage of the known method is that in addition to the steps of cutting and positioning the three wire sections, a total of four steps for plastic deformation are required to produce the tri-metal rivet, with new deformation tools sometimes having to be positioned between these four deformation steps.

The invention is based on the object of making available a process which is particularly suitable for mass production and which manages with fewer deformation steps than the known process.

This object is achieved by a method with the features specified in claim 1. It is essential here that an essentially cylindrical blank with an enlarged diameter is first produced from the three wire sections by cold welding and that the rivet head is then formed at one end of this blank, where the noble metal is located. During the upsetting process, the three wire sections are completely pushed out of the guide bush for the purpose of forming the essentially cylindrical blank.

The increase in diameter that occurs during upsetting should be selected so that perfect cold welding is guaranteed. For the metal pairing of silver / copper / silver, one therefore expediently selects the speed ratios from abutment to upsetting needle specified in claim 3. With a value v _{W /} v _s <0.25, there is only a progressively inadequate welding in the outer area of the abutting surfaces of the wire sections, while with a value of v _{w /} v _s above 0.5 the cross-sectional increase becomes too small for a perfect cold welding .

worin F, die Querschnittsfläche der Drahtabschnitte vor dem Stauchen und F₂ jene nach dem Stauchen bedeutet.At the start of the upsetting process, the abutment lies against the end of the guide bush. Between the abutment and the upsetting needle, which protrudes from the other end into the guide bushing, there are the three wire sections, which face each other in pairs with their mutually facing ends and which abut the abutment needle or the upsetting needle with the two outer ends. The compression needle is then advanced into the guide bushing at the speed _Vs and the abutment is moved back from the bushing at the lower speed _{Vw in} synchronism with it. An upsetting cannot take place inside the guide bushing, since the wall of the guide bush prevents the cross-section of the wire sections from increasing. Rather, the compression takes place in the space between the end of the guide bush and the abutment, which faces this end. The diameter increases continuously along the wire sections while the wire sections are being pushed out of the guide bush. The increase in cross-section takes place according to the relationship

where F, the cross-sectional area of the wire cuts before upsetting and F _{2 means} those after upsetting.

It is irrelevant whether the upsetting needle is moved against the abutment or the abutment against the upsetting needle during upsetting. It is important that during the upsetting there is a space outside the guide bush, into which the cross-sectional enlargement that occurs during upsetting can take place.

During the upsetting, the upset portion of the wires does not necessarily require lateral guidance. Preferably, however, another guide bushing is used for this purpose, the clear cross section of which is just F ₂ or slightly larger. The abutment is then displaceably mounted in this second guide bush. The second guide bushing can also advantageously be used to hold the blank while it is being transferred to a headmaking tool, and possibly also during the head molding process itself.

The rivet head can be formed on the blank in a known manner by one or two deformation shocks. Tall occupied end of the end plugs in a sleeve blank in the free space as far as first pre-dipped in front of the bush is that it can no longer during the subsequent second deformation impact umknicken _- by applying two deformation blows with the precio is. The second deformation blow is carried out with a press ram (headmaker) which has a recess whose contour matches the contour of the contact rivet head. If only one deformation blow is carried out, it is carried out with the headmaker and pre-upsetting is not necessary.

In contrast to the known method explained at the outset, the method according to the invention requires only one deformation step instead of four deformation steps before shaping the rivet head. This means that machines that work according to the method according to the invention can produce much more cost-effectively than those that produce according to the known method. Another advantage of the invention is that the essentially cylindrical blank is produced by a continuous flow process of the material, as a result of which the metallurgical structure is very much cheaper and more homogeneous than in the case of a tri-metal contact rivet produced according to the known method.

Finally, it is also advantageous that, according to the invention, tri-metal contact rivets with a particularly thin noble metal layer can be produced. Given the dimensions of the finished contact rivet, the invention starts from thinner and correspondingly longer wire sections than the shaft of the finished tri-metal contact rivet because of the formation of the cylindrical blank with an enlarged diameter due to upsetting. If one assumes thinner wire sections than the shaft diameter of the finished tri-metal contact rivet, the volume fraction of the precious metal used per contact rivet can be reduced. It is not possible to cut pieces of wire of any length; therefore, if the length of the noble metal-containing wire section can be chosen to be thinner than before, then the saving of noble metals results from the reduction in cross-section. Experience has shown that silver wire with a diameter D requires a minimum length of the wire sections of approximately 0.5 D to 0.8 D, the lower value 0.5 D for very thick and the upper value 0.8 D for very thin wires. Shorter wire sections can hardly be handled anymore and no longer have a sufficiently smooth cutting surface suitable for cold welding.

The required larger shank diameter of the tri-metal contact rivet is obtained by compressing the wire sections, which also cold weld them together. The length of the wire sections is shortened by the compression to the same extent as the cross section of the wires increases. The length of the noble metal sections of the blank formed by compression and consequently the thickness of the noble metal layer on the fully formed contact rivet head can therefore be smaller than would be possible if the same outer dimensions of the wire sections were used to produce a tri-metal contact rivet, which already had a diameter with the Match the diameter of the shaft of the tri-metal contact rivet.

• Aus einem Kupferdrahtabschnitt von 9 mm Länge und 3 mm Durchmesser und aus zwei Silberdrahtabschnitten von 2 mm Länge und 3 mm Durchmesser lässt sich nach einem Kaltschweissverfahren aus dem Stand der Technik (z. B. DE-A 2 555 697) ein Trimetallkontaktniet herstellen, welches folgende typische Abmessungen aufweist:
  

A number example is given to illustrate the possible savings in precious metals:

• A tri-metal contact rivet can be produced from a copper wire section of 9 mm length and 3 mm diameter and from two silver wire sections of 2 mm length and 3 mm diameter using a cold welding process from the prior art (e.g. DE-A 2 555 697) has the following typical dimensions:
  

According to the method of the invention, a tri-metal contact rivet with essentially identical external dimensions can be produced from a copper wire section 30 mm long and 1.64 mm in diameter and from two silver wire sections each 1.5 mm long and 1.64 mm in diameter. By compression, it becomes a blank of 3 mm in diameter and 9.90 mm in length, of which 2x0.45 mm are silver. After formation of the head of 6 mm in diameter with a remaining shaft length of 3.45 mm, there is silver on the head layer with an average thickness of only about 0.11 mm; the same savings result at the shaft end of the contact rivet, where another rivet head with silver coating is created when riveting; the amount of silver used is only about 20% compared to the previously described tri-metal contact rivet. Thanks to the silver savings, the height of the rivet head has been reduced by 0.39 mm while the copper insert has remained the same. If necessary, this can be compensated for by using more copper.

The accompanying drawings (FIGS. 1 to 10) schematically show an example of the sequence of the method according to the invention, showing the most important device elements which are required to carry out the method.

In a carrier 1 there are two cutting bushes 2 and 3 parallel to each other with the same internal width, to which a copper wire 5 and a silver wire 6 are fed from a wire supply in the direction of arrow 4 by a loading device (not shown). The two wires have matching diameters (Fig. 1). The free ends of the two cutting bushings 2 and 3 lie in alignment with a flat surface 10 of the carrier 1, on which a slide 7 can be moved. The slide 7 has, parallel to the cutting bushes 2 and 3, a continuous guide bushing 8 with the same inner diameter as that of the cutting bushings 2 and 3. A compression needle 9 is arranged displaceably in the guide bushing 8.

The manufacturing process begins with the slide 7 being displaced such that the guide bush 8 is aligned with the cutting bush 3 (FIG. 1); the upsetting needle 9 is positioned so that its front end 9a is at a distance from the surface 10 which corresponds to the length of the first silver wire section 6a to be cut off. The silver wire 6 is advanced until it abuts the end 9a of the upsetting needle, and then the slider 7 is moved in the direction of the arrow 11 (FIG. 1), whereby the silver wire section 6a inserted in the guide bush 8 is sheared off.

The slide 7 is now moved until the guide bush 8 is aligned with the cutting bush 2; at the same time the upsetting needle 9 is withdrawn by a distance which corresponds to the length of the copper wire section 5a to be cut off (FIG. 2). Now the copper wire 5 is advanced in the direction of arrow 4 until it abuts the silver wire section 6a. The slide 7 is then moved in the direction of the arrow 12 (FIG. 2), as a result of which the copper wire section 5a is sheared off.

By moving the slider 7 in the direction of the arrow 12, it is also brought into alignment with the cutting bush 3 in which the silver wire 6 is inserted. The upsetting needle 9 is again pulled back a short distance, the silver wire 6 is advanced by the same distance and the silver wire section 6b protruding from the cutting bush 3 is sheared off (FIG. 3). A first silver wire section 6a, a second silver wire section 6b and, in between, a longer copper wire section 5a are now located in the guide bush 9 one behind the other and with their end faces abutting one another.

The slide 7 is now moved further in the direction of the arrow 12 until the guide bush 8 is aligned with a second guide bush 13, which is arranged continuously in a second slide 14, which is parallel to the first slide 7 between the first slide 7 and the carrier 1 in a step-shaped recess 15 of the carrier 1 is displaceable (FIG. 4). The second guide bush 13 has a clear cross section, which e.g. is larger by a factor of 3.5 than the clear cross section of the first guide bushing 8. In the guide bushing 13, a plunger 16 mounted in the carrier 1 and having a flat end surface is displaceably guided. This plunger 16 is initially at the end of the guide bush 8, so that the three wire sections 5a, 5b and 6a between the compression needle 9 and the plunger 16 are kept largely free of play.

Now the upsetting needle 9 is pushed into the guide bushing 8 in the direction of the arrow 17 and pulled back in synchronism with it, but at a reduced speed of the thrust egg 16 in the direction of the arrow 17 by a factor of 3.5 (see above). The upsetting needle 9 thus presses the wire sections 5a, 5b and 6a against the slower plunger 16, which serves as an abutment. The consequence of this is that the cross section of the wire sections 5a, 5b and 6a widens by a factor of 3.5; the compression occurs when the material enters from the first guide bush 8 into the second guide bush 13. The two wire sections 5a and 6a and 5a and 6b weld together and form a cylindrical blank 18. As soon as the front end of the compression needle 9 has reached the surface 10 , their feed is stopped and the plunger 16 is completely withdrawn from the second guide bush 13. The slide 14 is now moved in the direction of the arrow 19 (FIG. 5) until the guide bush 13 is aligned with an equally wide guide bush 20 in the carrier 1. The blank 18 is positioned between two displaceable needles 21 and 22 guided in these two guide bushings 13 and 20 in such a way that it projects into the guide bushing 20 to a length which corresponds to the shaft length of the finished tri-metal contact rivet (FIG. 6).

The guide bush 20 and the needle 22 are then moved back by a certain preselectable distance L in the direction of the arrow 23. Synchronously with this, the needle 21 is moved in the same direction 23 (FIG. 7). In this way, a free space 24 is created between the slider 14 and the guide bushing 20, in which the rivet head is pre-compressed. This is done by advancing the needle 21 in the direction of arrow 23 against the stationary needle 22 as an abutment (FIG. 7). By upsetting the head it is achieved that in the following order Molding process by which the head is finished, which does not bend the end of the blank 18 protruding from the guide bushing 20.

FIG. 8 also shows the moment of pre-upsetting, namely in a viewing direction rotated by 90 ° (direction of arrow 29 in FIG. 7). After pre-upsetting the rivet head, the pre-upsetting needle 21 is withdrawn and the slide 14 is moved in the direction of arrow 29. At the same time, a tool slide 25 is moved in the direction of arrow 31, which is arranged parallel to the slide 14. The pre-compression needle 21 and a plunger 26 serving as a headmaker are mounted parallel to one another in the tool slide 25. By moving the headmaker 26 and an opening 30 located between the headmaker 26 and the carrier 1 in the slide 14 in front of the guide bush 20 with the blank 18 therein. The headmaker 26 has in its end face, which is normally at the level of the end face of the guide bush 20 in its starting position (FIGS. 6 and 7), a recess 27 which has the contour of the contact rivet head to be formed.

The guide bush 20 is now pushed together with the needle 22 inserted therein in the direction of the arrow 28 and strikes the pre-compressed blank 18 against the resting headmaker 26, as a result of which the head 32 obtains its final shape (FIG. 9). The tool slide 25 is then moved in the direction of the arrow 28; it moves away from the carrier 1 and takes the headmaker 26 and the pre-compression needle 21 with it, so that the finished tri-metal contact rivet 33 is released. The needle 22 is then advanced in the direction of the arrow 28 and throws the finished tri-metal contact rivet 33, which until then had been with its shaft 34 in the guide bush 20, out of the latter (FIG. 10).

On the device shown, two processing cycles can run in parallel but at different times to increase the output. This is indicated in FIG. 5, where at the same time with the upsetting of the wire sections 5a, 6a and 6b to form a blank 18, the previously manufactured blank 18 with the headmaker 26 is molded onto the head.

Claims (4)

1. A method of producing trimetallic contact rivets by cold welding, comprising the following steps:

- three segments (5a, 6a, 6b) of different lengths, which consist of wires that differ in composition but have the same cross-section, are aligned one behind the other in a guide bushing (8) having an inside cross-section which agrees with the cross-section of the wire lengths, the wire segments are arranged between an upsetting needle (9), which is longitudinally displaceable in the guide bushing (8), and an abutment (16), which is disposed outside the guide bushing (8) and coaxial to the upsetting needle (9) and has an effective cross-sectional area which exceeds the cross-sectional area of the wire segments,

- the wire segments are upset with simultaneous increase of the cross-section and cold welding of the wire segments,

- the rivet head is formed at one end of the assembly consisting of the three wire segments, characterized in that wire segments (5a, 6a, 6b) are upset to form an approximately cylindrical slug of the contact rivet in a shaping step in a continuously proceeding operating in which the upsetting needle (9) is moved in the guide bushing (8) toward the abutment (16) as far as to the end of the guide bushing so as to eject the three wire segments and the abutment (16) is moved away from the upsetting needle (9) at the same time at a velocity which is lower than that of the upsetting needle (9), the two velocities having a constant ratio.

2. A method according to claim 1, characterized in that the ratio of the velocities of the upsetting needle (9) and of the abutment (16) is adjustable.

3. A method according to claim 1 or 2, characterized in that where wire sections of copper and wire sections of silver are used, the ratio of the velocity (v_s) of the upsetting needle (9) and the velocity (v_w) of the abutment (16) is selected between

and preferably between

4. A method according to claim 1 or 2, characterized in that the upset portion of the three wire sections is guided during the upsetting operation in a second guide bushing (13), in which the abutment (16) is displaceable and which has an inside cross-section that is related to the inside cross-section of the first guide bushing (8) as the velocity for the upsetting needle (9) to the velocity of the abutment (16).

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