EP0062243B2 - Method of producing bimetallic contact rivets - Google Patents

Description

The starting point of the invention is a method for producing bimetal contact rivets by cold welding with the features specified in the preamble of claim 1. Such a method is e.g. B. is known from GB-A-1 079 080. In the implementation of the known method, two wire sections of different lengths and of different compositions with matching cross sections are separated from a wire supply, arranged one behind the other in a guide bushing, and cold welded to one another at the adjoining cut surfaces by pressure and enlargement of the contact surface. The enlargement of the contact surface is also used to form the rivet head. This can be done in that the abutment, against which the wire sections are pushed out of the guide bush by a compression needle, already has the contour of the rivet head, that is, it also serves as a headmaker, so that when the wire sections between the compression needle and the abutment are compressed, the rivet head is also formed or that cold welding initially creates only a semi-finished blank, the head of which is only given its final shape in a second deformation step by a separate headmaker. Usually the longer wire section is made of copper and the shorter wire section is made of silver.

The copper is used to form the rivet shaft and to form the rear part of the rivet head, while the expensive silver is only used to form the actual contact layer.

Compared to contact rivets made of solid silver, bimetallic rivets with copper shafts and with contact surfaces made of silver have resulted in considerable silver savings. However, the increasing cost of precious metals has led to efforts to reduce the use of precious metals in bimetal contact rivets. So z. B. has already been proposed to provide the precious metal only in the center of the contact surface, but such bimetallic rivets are relatively expensive to manufacture on the one hand, on the other hand, they only seem to save precious metals, because a contact surface requires a certain minimum size for a given application; replacing the precious metal in the edge area with base metal would adversely affect the switching behavior.

One tries to keep the precious metal layers on bimetal rivets as thin as possible. As far as the production of bimetallic rivets is based on plated strand-shaped semi-finished products, it is no problem to keep the precious metal layer as thin as desired. However, the production of bimetallic contact rivets from strand-shaped semifinished products is so complex that such rivets are many times more expensive than rivets which are produced from wire by cold welding. In the latter method, however, the precious metal layer cannot be made as thin as desired. The reason is simply that it is not possible to cleanly cut and handle wire sections of any length. 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.

The invention is based on the object of making available a process which is suitable for mass production and which makes it possible to produce bimetal contact rivets made from wire by cold welding with a thinner noble metal coating than previously.

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 two wire sections by cold welding and that the rivet head is then formed at one end of this blank, where the precious metal is located. For given dimensions of the finished contact rivet, the method according to the invention starts from thinner and correspondingly longer wire sections than the methods belonging to the prior art because of the formation of the blank with an enlarged diameter; in the prior art, the wires from which it is assumed already have the diameter that the shaft of the finished contact rivet should also have. Because the method according to the invention is based on thinner wires, the volume fraction of the noble metal used per contact rivet can be reduced. As mentioned, it is not possible to cut pieces of wire of any length, but if the length of the wire section containing the noble metal can be chosen to be thinner than before, the reduction in cross-section results in the noble metal being saved. The required larger shaft diameter of the bimetallic contact rivet is obtained by compressing the wire sections, which also cold weld them together. The length of the wire sections is shortened to the same extent as the cross section of the wires increases. The length of the noble metal section of the blank formed by upsetting and consequently the thickness of the noble metal layer on the finished molded contact rivet head can therefore be smaller than would be possible if one started with wire sections to produce a bimetallic contact rivet with the same external dimensions that were already in diameter with the Diameter of the shaft of the bimetal contact rivet match.

The increase in diameter that occurs during upsetting should be selected so that perfect cold welding is guaranteed. For the metal pairing copper / silver, it is therefore expedient to choose the speed ratios of abutment to compression needle specified in claim 3. With a value of v _w / v _s <0.25, the contact surface is located in the outer area Wire sections only progressively inadequate welding takes place, while at 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 F_z 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 compression needle, which protrudes from the other end into the guide bush. there are the two wire sections which abut one another with their ends facing one another and abut the abutment or the compression needle with the outer ends. The upsetting needle is then advanced into the guide bush at the speed v _s and the abutment is moved back from the bush at the lower speed v _w synchronously 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 face. The diameter increases continuously along the wire sections while the wire sections are being pushed out of the guide bush. The cross-section increases according to the relationship

where F, the cross-sectional area of the wire sections before upsetting and F _{z 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 occurring during upsetting can take place.

During the upsetting, the upset portion of the wires does not necessarily require lateral guidance. However, a further guide bushing is preferably 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. When two deformation impacts are exerted, the end of the blank in a bushing covered with precious metal is first pre-compressed so far in the free space in front of the bushing that it can no longer buckle during the subsequent second deformation shock. 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.

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

From a copper wire section of 9 mm length and 3 mm diameter and from a silver wire section of 2 mm length and 3 mm diameter, a bimetal contact rivet can be produced by a cold welding process from the prior art, which has the following typical dimensions:

According to the method of the invention, a bimetallic 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 a silver wire section 1.5 mm long and 1.64 mm in diameter. By compression, it becomes a blank of 3 mm in diameter and 9.45 mm in length, of which 0.45 mm is silver. After formation of the head of 6 mm diameter with a remaining shaft length of 3 mm, there is a silver coating on the head with an average thickness of only about 0.11 mm, i.e. H. the amount of silver used is only about 20% compared to the previously described bimetal contact rivet according to the prior art. 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 9) schematically show an example of the sequence of the method according to the invention, showing the most important device elements that 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 bushes 2 and 3 are in alignment with a flat surface 10 of the carrier 1, on which a slide 7 can be displaced. 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 shifted in this way. 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 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 slide 7 is moved in the direction of the arrow 11 (FIG. 1), as a result of which the silver wire section 6a inserted in the guide bushing 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.

The slide 7 is now shifted 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. 3). The second guide bush 13 has a clear cross section, which, for. B. 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 is displaceably guided with a flat end face. This plunger 16 is initially at the end of the guide bush 8, so that the two wire sections 5a 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 by a factor of 3.5, the plunger 16 in the direction of the arrow 17. The upsetting needle 9 thus presses the wire sections 5a 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 and 6a is expanded 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 weld together to form a cylindrical blank 18. As soon as the front end of the compression needle 9 has reached the surface 10, its feed is stopped and the plunger 16 completely withdrawn from the second guide bush 13. The slide 14 is now moved in the direction of the arrow 19 (FIG. 4) 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 bimetallic contact rivet (FIG. 5).

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. 6). 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. 6). By upsetting the head, the end of the blank 18 protruding from the guide bushing 20 does not bend in the subsequent shaping process by which the head is finally shaped.

FIG. 7 also shows the moment of pre-upsetting, namely in a viewing direction rotated by 90 ° (direction of arrow 29 in FIG. 6). 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. 5 and 6), 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. 8). 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 bimetal contact rivet 33 is released. Subsequently, the needle 22 is advanced in the direction of arrow 28 and throws the finished bimetallic rivet 33, which until then was still with its shaft 34 in the guide bush 20, out of this (FIG. 9).

On the device shown, two processing cycles can run in parallel, but at different times, in order to increase the output. This is indicated in Fig. 4, where at the same time with the upsetting of the wire sections 5a and 6a to form a blank 18, the previously made blank 18 with the headmaker 26 is molded onto the head.

Claims (4)

1. A method of producing bimetallic contact rivets by cold welding, comprising the following steps : two segments (5a, 6a), which differ in length and consist of wires which differ in composition but have the same cross-section are introduced one behind the other into a fitting guide bushing (8) ;

the two wire segments (5a, 6a), which abut at their ends, 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 (5a, 6a) ;

the two wire segments are upset so as to effect cold welding in that the distance between the upsetting needle (9) and the abutment (16) is reduced and the upsetting needle is advanced in the guide bushing at the same time ; and

the rivet head (32) is formed at that end of the arrangement consisting of the two wire segments at which the short wire segment (6a) is disposed, characterized in that for the upsetting of the wire segments (5a, 6a) before forming the rivet head (32)

in order to form an approximately cylindrical contact rivet slug having a cross sectional area exceeding the cross sectional area of the wire segments (5a, 6a) before they are upset

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 two 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 two 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 of the upsetting needle (9) to the velocity of the abutment (16).

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