RIVET DIES, RIVET SETTING TOOL AND ASSOCIATED METHOD

The present invention relates to rivet dies, a rivet setting tool incorporating such dies and associated methods. The rivet dies, rivet setting tool and method have particular, but not exclusive, application to forming a joint with a self-piercing rivet whereby the rivet is inserted into a workpiece including sheet material.

Self-piercing rivets take three main forms. There are semi tubular, fully tubular and solid self-piercing rivets. Solid self-piercing rivets usually comprise a head and a solid (non-hollow) shank. When a solid self-piercing rivet is used to form a joint in a workpiece, a slug of material of the workpiece is punched out (i.e. removed from the workpiece) by the rivet to form a hole all the way through the workpiece and the hole is filled by the rivet. The workpiece and/or shank of the rivet are then deformed so as to create a joint due to interlock between the workpiece and the rivet. Such joints may be more susceptible to stress and/or corrosion due to the fact that a slug of material of the workpiece has been removed to create the hole which is filled by the rivet.

A semi tubular self-piercing rivet generally has a head and a partially hollow shank that defines a central rivet cavity, the central rivet cavity only extending part way along the length of the rivet as it does not extend all the way through the rivet head. A fully tubular self-piercing rivet has a hollow head and a hollow shank that define a central rivet cavity, the central rivet cavity extending all the way through the length of the rivet. The central rivet cavity is defined by a bore that extends all the way through the rivet, from top to bottom (i.e. all the way through both the head and the shank of the rivet). Both semi tubular and fully tubular rivets are driven by a punch into a workpiece of sheet material such that it pierces the top sheet (or sheets, if the workpiece includes more than two sheets) and forms a mechanical interlock with the bottom sheet. It is often desired for the head of the rivet to be substantially flush with the upper surface of the top sheet. Since the bottom sheet is not pierced through, there is a reduced risk of corrosion occurring in the completed joint, as compared to, say, a conventional rivet or a solid self-piercing rivet. The invention described herein is only intended for use with semi-tubular and fully tubular self-piercing rivets, which do not remove a slug of material from the workpiece during joining.

Using self-piercing rivets in a joining process reduces the number of production steps, as compared to conventional riveting in which a hole first has to be drilled into the sheet material, the rivet inserted, and then its projecting end upset. Self-piercing riveting usually uses a die to support the workpiece while the rivet is inserted. The die may have a flat surface, or may have a cavity to accommodate and/or direct the plastic flow of the workpiece material to assist with deformation of the rivet shank to produce the required interlock.

Self-piercing riveting, often in combination with adhesive, has been used to great commercial success in the automobile industry, where lightweight materials such as aluminium have been adopted for vehicle body panels and other components in the interests of weight reduction and therefore reduced energy consumption. It is often difficult or not feasible to spot weld aluminium such as high strength aluminium, particularly to steel, owing to aluminium's high thermal conductivity, low melting range and propensity to form an oxide surface film.

More recently in the automotive industry, there has been a move towards using high strength sheet metals. It has been established that conventional rivets and/or dies are not always suitable for use with some workpieces, for example, those made of thick stack, high strength light metal alloys such as 6000 series and 7000 series aluminium alloy. As is well known in the art, 6000 series and 7000 series aluminium alloys may also be referred to as AA6xxx and AA7xxx alloys respectively.

In addition to AA6xxx and AA7xxx alloys, the invention described herein is suitable for use with aluminium die cast materials.

The higher strength and low ductility of such material generally means that the rivet experiences higher stress during the joining operation, and this is compounded when a flat die or die with a shallow die cavity of conventional geometry is used. Conventional self-piercing rivets utilised in combination with conventional dies are not capable of withstanding these high stresses in such a manner that the deformation of the rivet shank remains controllable to ensure that the final joint is of satisfactory quality. Simply manufacturing the rivet from a higher strength material does not generally achieve the desired results, because the corresponding reduced ductility can cause the rivet shank to crack as it attempts to deform during insertion. In order to form a suitable joint with satisfactory strength and corrosion resistance, the shank of the rivet needs to have sufficient column strength to pierce the top sheet of material without buckling, but flare outwardly during insertion in a repeatable and predictable manner without tearing or cracking.

It is desirable to produce self-piercing riveted joints in the high strength materials without having to increase significantly the setting forces required. It is also desirable to reduce the setting force required for a rivet setting operation. Doing so will mean that lower cost rivet setting apparatus can be used to create a satisfactory joint and/or, by reducing the forces exerted on the rivet setting apparatus during rivet setting operations, the lifetime of the rivet setting apparatus and reliability will be increased.

Finally, one way that has been contemplated of creating satisfactory rivet joints in high strength workpieces is to heat the workpiece prior to or during the riveting operation, to thereby soften the workpiece. Such a requirement not only increases the complexity of the rivet setting operation, but also increases the cost and energy requirement for carrying out a riveting operation. It is therefore desirable to provide a way of carrying out a rivet setting operation where it is not necessary to heat the workpiece; that is to say, the rivet setting operation can be carried out substantially at room temperature.

It is one object of the present invention to provide rivet dies, a rivet setting tool and associated methods which obviate or mitigate disadvantages with existing apparatus and methods, whether mentioned above or otherwise, and/or to provide an improved or alternative rivet die, rivet setting tool or method.

According to a first aspect of the invention there is provided a rivet die for a rivet setting tool configured to set a fully tubular or semi tubular self-piercing rivet into a workpiece, said rivet having an outer shank diameter D, the rivet die comprising a main body and a mounting stem depending from the main body, wherein the main body is generally cylindrical about a central axis; wherein the main body has an upper surface, wherein a die cavity is formed in the upper surface; wherein the die cavity includes a central bore which extends along the central axis all the way through the mounting stem; and wherein the diameter of the central bore at the upper surface is less than the outer shank diameter.

When a die according to this aspect of the present invention is used to carry out a rivet setting operation on a workpiece, the central bore of the rivet provides a large volume into which material from the lower sheet of the workpiece can flow. By accommodating material of the lower sheet of the workpiece in the bore during the rivet setting process, this reduces the amount of material of the lower sheet of the workpiece that flows radially outwards. Reducing the amount of material of the lower sheet of the workpiece that flows radially outwards during the rivet setting process minimises the likelihood of cracking resulting from an impression in the lower sheet of the workpiece developing into a compressed fold. Other advantages of this die geometry are discussed further below in the description.

The upper surface of the main body may include an outer annular wall which surrounds the die cavity.

The die cavity may include a tapered region which adjoins the bore and which is radially outboard of the bore, the tapered region being defined by a surface which faces the axis.

The tapered region acts to ease displacement of material of the lower sheet of the workpiece into the bore, thereby maximising the discussed benefits of the bore.

According to a second aspect of the invention there is provided a rivet setting tool configured to insert a fully tubular or semi tubular self-piercing rivet, having an outer shank diameter D, into a workpiece, the rivet setting tool including a rivet die according to the preceding aspect located beneath a punch reciprocally movable by an actuator.

According to a third aspect of the invention there is provided a method of manufacturing a product or a component of a product, the method comprising fastening together two or more layers of a workpiece using a rivet setting tool according to the previous aspect of the invention, wherein the actuator advances the punch to set a fully tubular or semi tubular self-piercing rivet, having an outer shank diameter D, into the workpiece.

According to a fourth aspect of the invention there is provided a rivet die comprising a main body, wherein the main body is generally cylindrical about a central axis; wherein the main body has an upper surface, wherein a die cavity is formed in the upper surface and located on the central axis, the die cavity being defined in part by a substantially radial base portion of the upper surface; wherein the die cavity is bounded by a raised annular rim located on the upper surface, the rim being defined at least in part by a rim portion of the upper surface; wherein the upper surface has a transition portion which adjoins the base portion and the rim portion, the transition portion including a first and second sections, the first section being located radially inboard of the second section, and wherein the first section has a radius of curvature in a plane on which the axis lies which is less than a radius of curvature in said plane of the second section.

When a die according to this aspect of the present invention is used to carry out a rivet setting operation on a workpiece, the transition portion helps guide material of a lower sheet of the workpiece radially outwards as it moves away from the die cavity. This results in a higher quality of joint in the workpiece by minimising the potential for formation of cracks. Other advantages of this die geometry are discussed further below in the description.

The centre of curvature of the first section may be located above the first section of the transition portion of the upper surface and the centre of curvature of the second section is located below the second section of the transition portion of the upper surface.

According to a fifth aspect of the invention there is provided a rivet setting tool for inserting a rivet into a workpiece including a rivet die according to the fourth aspect of the invention located beneath a punch reciprocally movable by an actuator.

According to a sixth aspect of the invention there is provided a method of manufacturing a product or a component of a product, the method comprising fastening together two or more layers of a workpiece using a rivet setting tool according to the fifth aspect of the invention.

According to a seventh aspect of the invention there is provided a rivet die rivet die for a rivet setting tool, the rivet die comprising a main body, wherein the main body is generally cylindrical about a central axis; wherein the main body has an upper surface, wherein a die cavity is formed in the upper surface and located on the central axis; wherein the die cavity is bounded by a raised annular rim located on the upper surface; wherein an outer land is located on the upper surface bordering the annular rim and being located radially 35 outboard of the annular rim, wherein a maximum height of the rim parallel to the central axis is greater than a maximum height of the die cavity and a maximum height of the outer land.

When a die according to this aspect of the present invention is used to carry out a rivet setting operation on a workpiece, the annular rim and outer land co-operate to help guide material of a lower sheet of the workpiece radially outwards and down towards the die as it moves away from the die cavity. This results in a higher quality of joint in the workpiece by minimising the potential for formation of cracks and minimising dishing of the workpiece. Other advantages of this die geometry are discussed further below in the description.

The outer land may be generally flat such that it lies generally perpendicular to the central axis.

The annular rim may comprise a first annular surface and a second annular surface which is radially outboard of the first annular surface, and wherein the first annular surface has a first gradient which is greater than a second gradient of the second annular surface, each gradient being defined as the magnitude of the rate of change of the position of the relevant surface along the axis as a function of radial position along the relevant surface.

The rivet die for a rivet setting tool may be configured to set a rivet having an outer shank diameter D. A minimum diameter, perpendicular to the axis, of a portion of the annular rim having said maximum height may be less than the outer shank diameter D.

According to an eighth aspect of the invention there is provided a rivet setting tool for inserting a rivet into a workpiece including a rivet die according to the previous aspect of the invention located beneath a punch reciprocally movable by an actuator.

According to a ninth aspect of the invention there is provided a method of manufacturing a product or a component of a product, the method comprising fastening together two or more layers of a workpiece using a rivet setting tool according the previous aspect of the invention.

According to a tenth aspect of the invention there is provided a rivet setting tool configured to insert a rivet, having an outer shank diameter D, into a workpiece, including a rivet die according the sixth aspect of the invention located beneath a punch reciprocally movable by an actuator.

According to an eleventh aspect of the invention there is provided a method of manufacturing a product or a component of a product, the method comprising fastening together two or more layers of a workpiece using a rivet setting tool according to the previous aspect of the invention, wherein the actuator advances the punch to set a rivet, having an outer shank diameter D, into the workpiece.

In accordance with any of the methods discussed above, the workpiece may comprises one or more layers of i) AA7xxx aluminium alloy, ii) AA6xxx aluminium alloy or iii) die cast aluminium.

The method may be carried out at room temperature, without heating of the workpiece.

The product may be a motor vehicle.

According to a twelfth aspect of the invention there is provided a product or component of a product manufactured using any of the above-discussed methods.

In each aspect of the invention above the die cavity may be configured, in use, to receive a portion of a workpiece undergoing a rivet setting operation.

The advantages set out above in relation to the features of one of the aspects of the invention apply equally to other aspects of the invention having the same or equivalent features. The optional features set out above in relation to any one aspect of the invention may be combined, where appropriate, with any of the other aspects of the invention.

Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:

FIG. 1 is a perspective view of a rivet setting tool;

FIG. 2 is a sectioned view through part of the apparatus of FIG. 1;

FIG. 3 shows a computer simulated sectioned view through a portion of a known rivet setting tool, rivet and workpiece, at different stages as a rivet is set in the workpiece;

FIG. 4 shows an enlarged computer simulated sectioned view through the known rivet setting tool, rivet and workpiece of FIG. 3 at a particular stage of the rivet being set in the workpiece;

FIG. 5 shows a computer simulated sectioned view through a portion of a rivet setting tool according to an embodiment of the present invention, rivet and workpiece, at different stages as a rivet is set in the workpiece;

FIG. 6 shows a computer simulated sectioned view through a portion of a rivet setting tool according to another embodiment of the present invention, rivet and workpiece, at different stages as a rivet is set in the workpiece;

FIG. 7 shows a schematic enlarged annotated sectioned view through a portion of the die which is part of the rivet setting tool shown in FIG. 6;

FIG. 8 shows a sectioned view through a die of a rivet setting tool according to a further embodiment of the present invention;

FIG. 9 shows a computer simulated sectioned view through a portion of a rivet setting tool according to an embodiment of the present invention (which includes the die shown in FIG. 8), rivet and workpiece, at different stages as a rivet is set in the workpiece;

FIG. 10 shows a sectioned view through a die of a rivet setting tool according to a further embodiment of the present invention;

FIG. 11 shows a computer simulated sectioned view through a portion of a rivet setting tool according to an embodiment of the present invention (which includes the die shown in FIG. 10), rivet and workpiece, at different stages as a rivet is set in the workpiece; and

FIG. 12 is a sectioned view through a portion of a rivet setting apparatus according to the present invention which includes the die shown in FIG. 10.

FIG. 1 shows a rivet insertion apparatus 2 and an associated carrier. The carrier comprises a C-frame 4, which has an upper jaw 6 and a lower jaw 8. A die assembly 10 is provided on the lower jaw 8 of the C-frame. The rivet insertion apparatus 2 inserts rivets into a workpiece (not depicted) which is located over the die assembly 10, as described further below.

The rivet insertion apparatus 2 comprises an electric drive 12 that operates to drive an inertially driven reciprocal punch (not visible in FIG. 1) which moves axially in a cylindrical housing 14 and a nose assembly 16. Although an inertial electric drive is depicted, other forms of drive may be used. For example, the electric drive may comprise an electric motor, which directly drives the punch (and which may be controlled based upon real-time feedback regarding the position of the punch). In another example, a hydraulic drive may be used. However, an electric drive may be preferred because it does not require delivery of hydraulic fluid (delivering hydraulic fluid may be difficult, and the hydraulic fluid may pose a health and safety risk if it leaks). The reciprocal punch is used to insert rivets from the nose assembly 16 through a workpiece (not depicted). The rivet insertion apparatus 2 may further comprise an additional drive (not visible) which may be used to clamp the nose assembly 16 onto the workpiece during insertion of a rivet and during upsetting of the rivet (as described further below). The electric drive 12 and the additional drive may be independently controllable (e.g. using a control apparatus). The additional drive may for example be an electric drive or a hydraulic drive. An example of a drive which may be used to clamp the nose assembly 16 onto the workpiece is described in U.S. Pat. No. 5,752,305.

Rivets are supplied under air or gas pressure via a delivery tube (not shown) to the nose assembly 16. The rivets are then inserted through the workpiece. In an alternative arrangement, the rivets may be supplied by transportation to the nose assembly 16 in carrier tape.

A control system 23 is configured to control delivery of rivets to the nose assembly 16, and is configured to control operation of the reciprocal punch. The control system 23 may also control other parts of the rivet insertion apparatus 2, such as the drive which moves the punch and the drive which moves the nose. The control system 23 may comprise a processor and a memory, the memory storing instructions regarding operation of the rivet insertion apparatus 2. The processor may process the instructions and provide outputs which control operation of the rivet insertion apparatus 2.

The die assembly 10 is shown in more detail in FIG. 2. The die assembly 10 includes a die 13 supported on the lower jaw 8 of the C-frame 4 by a die holder adapter 18 that is received in a bore 19 through the jaw 8. The die 13 is generally cylindrical about a central axis, with a head (or main body) 20 defining an open die cavity 21 for facing the setting tool 10, and a depending stem 22 that extends along the central axis and is of reduced diameter compared to the head 20 such that an annular surface 23 extending radially relative to the central axis is defined on the underside of the head 20. The adapter 18 has a generally cylindrical body with a first end 25 that is received in a snug fit in the bore 19 in the jaw 8 of the C-frame 4 and a second hollow end 26 that receives the die stem 22 such that the annular surface 23 of the die is seated on an upper surface 27 of the second end 26. A sealing member such as, for example, a O-ring or the like may be provided between the adapter 18 and an upper surface 28 of the surface of the arm 14 in which the bore 19 is defined. The adapter body has a radially outward extending flange 29 defined part way along its outer surface with one of the radially extending faces being seated on the upper surface 28 of the arm 14 immediately around the bore 19. The second hollow end 26 is tapered inwardly and terminates in the annular upper surface 27 on which underside surface 23 of the head 20 is supported. A cylindrical bore 30 extends within the adapter body from the second end 26 to a position substantially half way along its length and receives the die stem 22 by a slip fit or friction fit. The body is also penetrated by two small diameter passages: a first 31, which extends along a central longitudinal axis of the body from the first end 25 to the cylindrical bore 30, and a second 32 that extends radially from the first passage 31 to the periphery of the flange 29. In each case, the passages have respective enlarged first and second entry ports 33, 34 to allow connection to a hose for the supply of pressurised air. The passages 31, 32 and entry ports 33, 34 are not of importance for understanding the present invention and, as such, further discussion regarding them is omitted.

It is to be appreciated that, in other embodiments, the bore 19 in the lower jaw 8 of the C-frame can have a reduced diameter such that the die holder adapter 18 can be eliminated, in which case the lower jaw 8 of the C-frame in the region around the bore 19 serves to hold the die via its stem directly.

FIG. 3 shows a computer simulated sectioned view through a portion of a known rivet setting tool, rivet and workpiece, at different stages as a rivet is set in the workpiece. In particular, the view shows the calculated stress at any given point within the system. In this figure, as with other figures within this document showing computer simulated views, only the stress level within the rivet and the workpiece are shown. That is, the stress level in the die, nose and punch are not shown.

FIG. 3a shows a first stage in the rivet setting process. A workpiece 40 is shown received between the opposed nose assembly 16 and die 13. The workpiece comprises an upper sheet 40a of aluminium 7075 T61A having a thickness of 2.0 mm, and a lower sheet 40b of aluminium 7075 T61A having a thickness of 3.0 mm.

The die 13 is a DG11-100 (available from Atlas Copco IAS UK limited), which has a die cavity 21 with a depth A (parallel to the axis of the die) of 1.0 mm.

FIG. 3b shows a second stage in the rivet setting process. A rivet 42 is driven by a punch 44 into the upper sheet 40a of the workpiece 40.

In the depicted rivet setting process, the rivet 42 is driven by the punch 44 towards the die 13 with a velocity of 380 mm/s and with a maximum driving force of about 70 kN.

In the present case, the rivet 42 is a BG0G46E self-piercing rivet (available from Atlas Copco IAS UK limited). The rivet 42 has a head 42a and a partially hollow shank 42b which defines a central rivet cavity 42c.

In FIG. 3b, the tip of the shank 42b of the rivet 42 is driven into the upper sheet 40a of the workpiece 40 and, consequently, some of the material of the upper sheet 40a is received by the central rivet cavity 42c. The driving of the rivet into the upper sheet of the workpiece causes the lower sheet of the workpiece to be deformed such that a portion of the lower sheet 40b of the workpiece 40 is received within the die cavity 21 of the die 13.

The tip of the rivet shank has penetrated 60-70% (in terms of thickness of the sheet) through the upper sheet of material. Approximately 30-40% of the volume of the central rivet cavity (which may also be referred to as the rivet bore volume) is filled with material of the upper sheet of the workpiece. High levels of stress (approximately 2000 MPa) are observed at the tip of the shank of the rivet. The lower sheet of the workpiece is in contact with the surface defining the bottom of the die cavity. Stress within the portion of the workpiece contacting the bottom of the die cavity is in the region of 650 MPa. Material roll over (discussed in more detail below in relation to FIG. 4) is present at the die radius leading edge 21a. The die radius leading edge 21a is the portion of the upper surface of the die, which is in contact with the workpiece, radially spaced outboard of, but radially closest to, the portion of the upper surface defining the bottom of the die cavity which is contact with workpiece during this stage. The die radius leading edge has created an impression into the lower sheet of the workpiece.

Within this figure and subsequent figures, the arrows within the workpiece indicate the relative magnitude and direction of force being exerted on the relevant portion of the workpiece as a result of the rivet being driven into the workpiece.

FIG. 3c shows a third stage in the rivet setting process. In this stage, the rivet has been driven further into the workpiece such that the shank of the rivet has been driven all the way through the upper sheet 40a of the workpiece 40 and the tip of the shank 42b of the rivet 42 contacts the lower sheet 40b of the workpiece. Material of the upper sheet of the workpiece has almost completely filled the central rivet cavity. As compared to the second stage, in the third stage the lower sheet of the workpiece has been further deformed such that a larger portion of the lower sheet 40b of the workpiece 40 is received within the die cavity 21 of the die 13. This may also be referred to as there being greater die impression of the workpiece (and, in particular, of the lower sheet) in the third stage as compared to the second stage. As compared to the second stage, the shank 42b of the rivet in the third stage has flared slightly outwards relative to a central longitudinal axis of the rivet.

In this stage, the rivet (in particular the tip of the shank of the rivet) has pierced completely through the upper sheet material. Stress of approximately 2000 MPa has propagated from the tip of the rivet shank through the length of the rivet shank. More than about 90% of the volume of the central rivet cavity is filled with material of the upper sheet of the workpiece. Initiation of flaring of the shank of the rivet can be seen as the tip of the rivet shank begins to pierce the lower sheet of the workpiece. The material of the lower sheet of the workpiece is beginning to flow radially from the centre of the die, within the die cavity, towards a sidewall 21b of the die cavity.

FIG. 3d shows a fourth stage in the rivet setting process. In this stage, the rivet has been driven further into the workpiece such that the tip of the shank 42b of the rivet 42 has penetrated slightly into the lower sheet 40b of the workpiece. As compared to the third stage, in the fourth stage the lower sheet of the workpiece has been further deformed such that a larger portion of the lower sheet 40b of the workpiece 40 is received within the die cavity 21 of the die 13. As compared to the third stage, the shank 42b of the rivet in the fourth stage has flared slightly outwards relative to the longitudinal axis of the rivet.

In this stage, greater than about 50% of the length of the rivet (parallel to the axis) has been inserted into the workpiece, with the tip of the shank penetrating the lower sheet of the workpiece. The central rivet cavity volume is completely filled with material of the workpiece. The material of the lower sheet of the workpiece continues to flow radially outwards within the die cavity. Deflection is present between the lower sheet of the workpiece and a generally radial, generally flat portion 21c of the upper surface of the die which defines a rim 21d of the die which is radially outboard of the die cavity 21. This deflection is caused by high levels of compressive force on the workpiece (exerted by the rivet and the die) during rivet insertion, and radial flow of the material of the lower sheet of the workpiece in the die due high tension forces within the lower sheet of the workpiece.

FIG. 3e shows a fifth stage in the rivet setting process. In this stage, the rivet has been driven further into the workpiece such that more of the shank 42b of the rivet 42 has penetrated into the lower sheet 40b of the workpiece. As compared to the fourth stage, in the fifth stage, the lower sheet of the workpiece has been further deformed such that a larger portion of the lower sheet 40b of the workpiece 40 is received within the die cavity 21 of the die 13. As compared to the fourth stage, the shank 42b of the rivet in the fifth stage has flared slightly outwards relative to the longitudinal axis of the rivet.

In this stage, greater than approximately 75% of the length of the rivet (parallel to the axis) has been inserted into the workpiece. Displacement of the rivet and the upper sheet of the workpiece causes the lower sheet of the workpiece to further flow in a radially outward direction. Material of the workpiece flowing out of the die cavity (which may also be referred to as a die pocket) combined with the compressive forces exerted by the workpiece on the surface of the die defining the bottom of the die cavity create increased sheet deflection giving a ‘dished’ appearance to the workpiece 40.

The initial roll over impression formed in the second stage at the die radius leading edge 21a has now moved radially outwards as the lower sheet moves radially outwards over the die radius leading edge 21a, such that the impression is now situated over the generally flat portion 21c of the upper surface of the die which defines the rim 21d of the die.

FIG. 3f shows a final, sixth stage in the rivet setting process. In this stage, the rivet has been driven further into the workpiece such that even more of the shank 42b of the rivet 42 has penetrated into the lower sheet 40b of the workpiece. In addition, the head 42a of the rivet has been driven by the punch such that it is substantially flush with portions of an upper surface of the upper sheet of the workpiece. As compared to the fifth stage, in the sixth stage, the lower sheet of the workpiece has been further deformed such that a larger portion of the lower sheet 40b of the workpiece 40 is received within the die cavity 21 of the die 13. As compared to the fifth stage, the shank 42b of the rivet in the sixth stage has flared slightly outwards relative to the longitudinal axis of the rivet. The combination of the shank of the rivet being received by both upper and lower sheets of the workpiece, and the shank of the rivet flaring outwards results in an interlock being formed between the rivet and the upper and lower sheets of the workpiece, and hence a joint being formed between the upper and lower sheets of the workpiece.

In the sixth stage, an underside 42d of the head 42a of the rivet is seated so that the underside 42d is near flush with the top sheet 40a of the workpiece. Maximum interlock between the rivet and the sheets of the workpiece (and hence between the sheets of the workpiece themselves) has now been achieved. Gaps between the upper and lower sheets of the workpiece, which have formed during the rest of the rivet setting process, are now reduced due to the action of the head 42a of the rivet squeezing the upper and lower sheets together. The bottom sheet of the workpiece has achieved maximum radial displacement. It can be seen at this stage in the process that there are portions of the upper surface of the die that never contact the workpiece at any point during the rivet setting process. Given that a large portion of the lower sheet of the workpiece does not contact the die (and, in particular, the portion of the surface of the die that forms the die cavity) during the rivet setting process, this results in the said portion of the lower sheet of the workpiece being unsupported during the rivet setting process. This may allow excessive movement of the workpiece relative to the die and/or crack initiation, resulting in an unsatisfactory rivet joint between the sheets of the workpiece.

The impression in the lower sheet of the workpiece, formed in the second stage at the die radius leading edge 21a, is still over the generally flat portion 21c of the upper surface of the die which defines the rim 21d of the die. It is possible that the final seating of the rivet head positon will put additional stress in this area.

FIG. 4 shows an enlarged computer simulated sectioned view through the known rivet setting tool, rivet and workpiece of FIG. 3 at a particular stage of the rivet being set in the workpiece. In particular, the view is shown at the fourth stage of the rivet setting process as discussed above.

As already discussed, the die radius leading edge 21a creates an impression 40c in the lower sheet 40b of the workpiece 40. As the rivet setting process continues, for example, by the fourth stage as shown in FIG. 4, the impression 40c reforms or develops during radially outward displacement of the material of the lower sheet 40b to form a small lap or fold 40d.

As the rivet setting process advances, the fold is compressed at the die radius leading edge due to the radially outward movement of the lower sheet of the workpiece. The fold may be nearly fully compressed to form a collinear face with the rest of the outer surface of the lower sheet within the die cavity. However, a small gap between the compressed fold and the rest of the outer surface of the lower sheet remains. It is thought that this small gap may cause circumferential and/or radial cracking in a rivet joint formed using the above-described rivet setting process. Clearly, the possibility of crack formation in a rivet joint means that such a rivet joint between the sheets of the workpiece is unsatisfactory.

In light of the above issues with the rivet setting process for a known geometry of die, the applicant has contemplated a number of alternative solutions to the problem of unsatisfactory joint formation.

First, in order to reduce the likelihood of cracking resulting from an impression in the lower sheet of the workpiece developing to a compressed fold, the applicant has contemplated increasing the volume of the die cavity to accommodate more material and potentially reduce the amount of material displacement in the die.

Secondly, in order to reduce excessive movement of the workpiece relative to the die and/or crack initiation due to a large portion of the lower sheet of the workpiece not contacting the die (and therefore being unsupported) during the rivet setting process, the applicant has contemplated a die profile that has greater contact with the surface of the lower sheet of the workpiece during deformation of the workpiece during the rivet setting process. It is thought that such a die profile that has greater contact with the lower sheet of the workpiece will reduce pressure and stress concentration within the joint, and hence reduce the likelihood of cracking. The die profile chosen considers the natural flow and behaviour of the material and limits any undesirable material flow.

The only difference between the rivet setting tool shown in FIG. 5 and the known rivet setting tool shown in FIGS. 3 and 4 is that the die 13′ is a die in accordance with an embodiment of the invention instead of a known die.

Again, the rivet 42 used is a BG0G46E and the workpiece 40 comprises an upper sheet 40a of 2.0 mm thickness 7075 T61A (7000 series) aluminium and a lower sheet 40b of 3.0 mm thickness 7075 T61A (7000 series) aluminium. The velocity of the punch 44 towards the die 13′ during the rivet setting operation is 380 mm/s and the maximum force the punch can exert on the rivet during the rivet setting operation is about 70 kN.

The die 13′ is generally cylindrical about a central axis, and comprises a head (or main body) 20′ defining an open die cavity 21′ and a depending stem (not shown) that extends along the central axis and is of reduced diameter compared to the main body 20′.

The main body 20′ is generally cylindrical about a central axis B. The main body 20′ has an upper surface 20a′. A die cavity 21′ is formed in the upper surface 20a′ and located on the central axis B. The die cavity 21′ is bounded by a raised annular rim 21d′ located on the upper surface 20a′. An outer land 20b′ is located on the upper surface 20a′ bordering the annular rim 21d′. The outer land 20b′ is located radially outboard of the annular rim 21d′. In the present embodiment the outer land 20b′ is a generally flat, generally radial surface—i.e. it lies generally perpendicular to the central axis B. In other embodiments this need not be the case.

A maximum height C of the rim 21d′ parallel to the central axis B (and in a direction opposite to that in which the stem of the die depends from the main body of the die) is greater than a maximum height of the die cavity D and a maximum height of the outer land E. In the present embodiment the maximum height of the die cavity D is less than the maximum height of the outer land E. In other embodiments the maximum height of the die cavity and the maximum height of the outer land may be the same, or the maximum height of the die cavity may be greater than the maximum height of the outer land.

The annular rim 21d′ comprises a first annular surface 21e′ and a second annular surface 21f which is radially outboard of the first annular surface 21e′. The first annular surface is generally equivalent to the sidewall 21b of the die cavity 21 discussed above in relation to the known die geometry, and so the first annular surface may be referred to as the sidewall of the die cavity.

The first annular surface 21e′ has a first gradient which is greater than a second gradient of the second annular surface 21f′. Each of the gradients are defined as the magnitude of the rate of change of the position of the relevant surface along the axis as a function of radial position along the relevant surface. In other words, the gradient is the magnitude (i.e. is not affected by whether the surface faces inward towards the axis or outwards away from the axis) of the rate of change of the position of the relevant surface in the direction parallel to the axis, as a function of the radial position on the relevant surface.

The gradient of a particular surface can be determined with reference to two separate points on the surface in the conventional manner, as follows. If the first point on the surface has a radial position (i.e. distance from the axis) R1 and an axial position (distance from a given point along the axis) A1, and the second point on the surface has a radial position R2 and an axial position A2 (distance from the same given point along the axis), then the gradient G of the surface is given by:

$G = ❘ (A 1 - A 2) / (R 1 - R 2) ❘$

A surface which has a relatively large gradient may be said to be relatively steep, whereas a surface which has a relatively small gradient may be said to be relatively shallow.

The first annular surface 21e′ is inward facing, i.e. faces towards the axis; whereas the second annular surface 21f′ is outward facing, i.e. faces away from the axis.

In the present embodiment the annular rim 21d′ comprises an intermediate surface 21g′ which is located between the first annular surface 21e′ and the second annular surface 21f′. The intermediate surface is generally radial—i.e. it extends in a direction which is generally perpendicular to the axis. The intermediate surface may be a generally flat surface or may be a generally radial portion of a curved surface.

The second annular surface 21f′ has a greater radial extent than first annular surface 21e′. That is to say, the maximum radial separation between two points on the second annular surface 21f′ is greater than the maximum radial separation between two points on the first annular surface 21e′.

The stages of the rivet setting process shown in FIG. 5 are equivalent to those already discussed in relation to FIG. 3. However, some points of difference between the rivet setting process shown in FIG. 5 and that shown in FIG. 3 are discussed below.

In the second stage shown in FIG. 5b, which may also be referred to as the upper sheet material piercing stage, the tip of the shank of the rivet 42 has penetrated approximately 40-50% through the thickness of the upper sheet 40a of the workpiece 40. Approximately 40% of the volume of the central rivet cavity has been filled with material of the upper sheet of the workpiece. High levels of stress, in the order of 2000 MPa are present at the tip of the shank of the rivet. The lower sheet 40b of the workpiece is in contact with the rim 21d′ and experiences stresses in region of 650 MPa. The lower sheet 40b has filled about 50% of the volume of the die cavity 21′. Lower stresses are experienced by the portion of the lower sheet of the workpiece which is received in the die cavity (and also in the portion of the lower sheet of the workpiece which is above the portion received in the die cavity).

In the third stage shown in FIG. 5c, which may also be referred to as the upper sheet pierced stage, the rivet 42 (and, in particular, the tip of the shank of the rivet) has pierced completely through the upper sheet 40a of the workpiece. Stress in the region of 2000 MPa has propagated from the tip of the rivet shank through the entire length of the rivet shank. The central rivet cavity is completely filled by material of the upper sheet of the workpiece. Flare initiation (i.e. the beginning of the shank of the rivet deforming outwards, away from the longitudinal axis of the rivet) can be seen as the tip of the rivet shank begins to pierce the lower sheet of the workpiece. The die cavity is now completely filled with material of the lower sheet of the workpiece, the portion of the lower sheet of the workpiece received in the die cavity showing stress in the region of 250 MPa. The material of the lower sheet of the workpiece continues to flow radially outwards over the upper surface of the die, and, in particular, over the rim 21d′ of the die 20′.

In the fourth stage shown in FIG. 5d, which may also be referred to as the flare initiation stage, more than about 60% of the length of the rivet (parallel to the axis) has been inserted into the workpiece with the tip penetrating the lower sheet of the workpiece. The central rivet cavity is still completely filled. The rivet shank is beginning to flare into the bottom sheet of the workpiece. The die cavity is completely filled by material of the bottom sheet of the workpiece. The lower sheet of the workpiece continues to spread radially outwards, along the surface of the rim of the die. In particular, the lower sheet of the workpiece flows along and maintains contact with second annular surface 21f. Deflection in a generally upward direction is present between the outer land 20b′ and the lower sheet of the workpiece.

In the fifth stage shown in FIG. 5e, which may also be referred to as the rivet flaring & material ride out stage, more than about 75% of the length of the rivet (parallel to the axis) has been inserted into the workpiece. The rivet has interlock in the lower sheet of the workpiece, thereby interlocking the upper and lower sheets of the workpiece. Displacement of the rivet and material of the upper sheet of the workpiece causes the material of the lower sheet of the workpiece to further flow across the surface of the die. Material of the lower sheet of the workpiece flowing out of the die cavity, combined with the compressive forces exerted on the bottom surface of the die cavity by the workpiece create increased deflection of the sheets of the workpiece giving a ‘dished’ appearance to the workpiece.

In the sixth stage shown in FIG. 5f, which may also be referred to as the rivet head flush stage or end of cycle stage, the underside of the head of the rivet is seated so that the underside is near flush with the top (or upper) sheet of the workpiece. Maximum interlock is now achieved between the rivet and the lower sheet of the workpiece, and hence between the upper and lower sheets of the workpiece. The material of the lower sheet of the workpiece has achieved maximum radial displacement. This highlights areas where the upper surface of the die never contacts the lower sheet of the workpiece.

If the sixth stage of the rivet setting process using a known die, as shown in FIG. 3f, is compared with the sixth stage of the rivet setting process using a die in accordance with the present invention, it can be seen that the joint formed using the die in accordance with the present invention has a number of advantages.

First, a greater surface area of the lower surface of the lower sheet of the workpiece contacts the die in the case of the die according to the present invention. This means that the workpiece is more supported by the die during the rivet setting process. Greater support of the workpiece during the rivet setting process results in less stress and unwanted deformation in the formed joint, and hence a better quality of joint.

Secondly, there are no gaps between the bottom/sidewall of the die cavity and the lower sheet of the workpiece with the die according to the present invention. Conversely, the gaps present in the case of the known rivet geometry mean that, during the rivet setting process, extra stress is caused at the point where the die radius leading edge contacts the lower sheet of the workpiece. As already discussed, this results in an impression in the lower sheet of the workpiece, which can finally result in a compressed fold in the material of the lower sheet and hence cracking of the joint. The lack of gaps resulting from use of the die according to the present invention results in a better joint in which the risk of the joint cracking is reduced.

Thirdly, the joint formed using the die according to the present invention does not include any gaps between the upper and lower sheets of the workpiece, adjacent the rivet, unlike the joint formed using the known geometry of die. The lack of gaps between the sheets of the workpiece means that the joint between the two sheets is stronger and more stable.

Finally, the ‘dishing’ of the workpiece which has undergone the rivet setting process using the die according to the present invention is considerably reduced as compared to that of the workpiece that has undergone the rivet setting process using the die of known geometry. Whilst the ‘dishing’ itself does not have an adverse effect on the quality of the joint formed by the rivet, the ‘dishing’ means that the shape of the resulting riveted workpiece is deformed. This may be aesthetically undesirable and/or may prevent the workpiece from being able to function correctly.

All of the benefits of the joint formed using the die according to the present invention result from the presence of the outer land outboard of the raised annular rim, and, in particular, that the outer land has a height which is less than that of the annular rim. In this way, during the rivet setting process, the annular rim and outer land co-operate to help guide material of the lower sheet of the workpiece radially outwards and down towards the die as it moves away from the die cavity.

The raised annular rim provides a further benefit. As can be seen in FIG. 3, with known die geometries, during a rivet setting operation, the first portion of the die to contact the underside of the workpiece is the outer rim 21c which is a significant radial distance from the location at which the rivet pierces the workpiece. This means that the portion of the workpiece underneath the rivet is initially unsupported until the workpiece is deformed such that it contacts the portion of the upper surface of the die which forms the base of the die cavity. In contrast, as can be seen clearly in FIG. 5, with a die according to the present invention, due to the presence of the raised rim at a location which is generally underneath the workpiece at the location at which the rivet pierces the workpiece, the portion of the workpiece underneath the rivet during the riveting operation is supported by the die for the entirety of the riveting operation. This means that the workpiece is more supported by the die during the rivet setting process. Greater support of the workpiece during the rivet setting process results in less stress and unwanted deformation in the formed joint, and hence a better quality of joint.

In some embodiments a minimum diameter F, perpendicular to the axis B, of a portion 21g′ of the annular rim 21d′ having said maximum height is less than an outer shank diameter S of a rivet with which the die is configured to be used. In particular, the portion of the annular rim which has said maximum height is the portion of the annular rim which, in use, extends along the axis B, closest to the punch of the rivet setting apparatus. In the present embodiment it is adjacent to and radially outboard of the die cavity. The outer shank diameter S of the rivet is a diameter perpendicular to the axis B (which corresponds to the longitudinal axis of the rivet). For the avoidance of doubt, the outer shank diameter is measured when the rivet is in its unset state (i.e. before a riveting operation is carried out by the rivet setting apparatus using the rivet). The outer shank diameter is the diameter of the outer surface (i.e. surface that faces radially outwards) of the shank of the rivet. Usually the outer surface of the shank of the rivet is a surface of the shank of the rivet which runs parallel to the axis. However, in embodiments of rivet in which the outer surface of the shank of the rivet does not run parallel to the axis, the outer shank diameter may be considered to be the greatest diameter of the tip of the shank of the rivet.

By having a minimum diameter of the maximum height portion of the annular rim which is less than the outer shank diameter of the rivet with which the die is configured to be used, this helps to guide the shank of the rivet over the raised annular rim as it flares outwards (as shown in FIGS. 5c-5f). This helps the rivet to deform in consistent manner, which may help to reduce stress in the rivet and enhance the interlock between the rivet and the workpiece, thus resulting in a higher quality joint.

In some embodiments the minimum diameter F, perpendicular to the axis B, of a portion 21g′ of the annular rim 21d′ having said maximum height is greater than an inner shank diameter I of a rivet with which the die is configured to be used. The inner shank diameter I of the rivet is a diameter perpendicular to the axis B (which corresponds to the longitudinal axis of the rivet). For the avoidance of doubt, the inner shank diameter is measured when the rivet is in its unset state (i.e. before a riveting operation is carried out by the rivet setting apparatus using the rivet). The inner shank diameter is the diameter of a portion of the inner surface (i.e. surface that faces radially inwards) of the shank of the rivet, said portion running/lying parallel to the axis.

In some embodiments, the die cavity may include a central bore of the type discussed in more detail below. Some of these embodiments may include a tapered region of the type discussed in more detail below.

The only difference between the rivet setting tool shown in FIG. 6 and the known rivet setting tool shown in FIGS. 3 and 4 is that the die 13″ is a die in accordance with an embodiment of the invention instead of a known die.

Again, the rivet 42 used is a BG0G46E and the workpiece 40 comprises an upper sheet 40a of 2.0 mm thickness 7075 T61A (7000 series) aluminium and a lower sheet 40b of 3.0 mm thickness 7075 T61A (7000 series) aluminium. The velocity of the punch 44 towards the die 13″ during the rivet setting operation is 380 mm/s and the maximum force the punch can exert on the rivet during the rivet setting operation is about 65 kN.

The die 13″ is generally cylindrical about a central axis, and comprises a head (or main body) 20″ defining an open die cavity 21″ and a depending stem (not shown) that extends along the central axis and is of reduced diameter compared to the main body 20′.

The main body 20″ is generally cylindrical about a central axis B. The main body 20″ has an upper surface 20a″. A die cavity 21″ is formed in the upper surface 20a′″ and located on the central axis B. The die cavity 21″ is defined in part by a substantially radial base portion 21h″ of the upper surface 20a″. That is to say the base portion 21h″ is substantially flat and extends in a substantially radial direction. In other words the base portion 21h″ lies in a plane which substantially perpendicular to the axis B. The die cavity 21″ is bounded by a raised annular rim 21d″ located on the upper surface. The rim 21d″ is defined at least in part by a rim portion 21i″ of the upper surface 20a″. In the present example, the rim portion extends in a direction that is generally perpendicular to the axis.

However, in other embodiments, this need not be the case. The upper surface 20a″ has a transition portion 21j″ which adjoins the base portion 21h″ and the rim portion 21i″.

As best seen in FIG. 7, the transition portion 21j″ includes a first section 21k″ and second section 21l″. The first section 21k″ is located radially inboard of the second section 21l″. The figure has been annotated to show several radii of curvature. The first section 21k″ has a radius of curvature R1 in a plane on which the axis B lies which is less than a radius of curvature R2 in said plane of the second section 21l″.

It can be seen that the centre of curvature C1 (in a plane on which the axis B lies) of the first section 21k″ is located above the first section of the transition portion of the upper surface, and the centre of curvature C2 of the second section is located below the second section of the transition portion of the upper surface. Within the context of this portion of the description, when the terms above and below are used, above means in a direction towards where the workpiece contacts the die in use, and below means in a direction away from where the workpiece contacts the die in use.

The stages of the rivet setting process shown in FIG. 6 are equivalent to those already discussed in relation to FIG. 3. However, some points of difference between the rivet setting process shown in FIG. 6 and that shown in FIG. 3 are discussed below.

In the second stage shown in FIG. 6b, also known as the upper sheet material piercing stage, the tip of the shank of the rivet has penetrated 50-60% of the thickness of the upper sheet of the workpiece. Approximately 30% of the volume of the central rivet cavity 42c is filled with material of the upper sheet 40a of the workpiece 40. High levels of stress, of approximately 1900 MPa, can be seen at the tip of the shank of the rivet. The lower sheet of the workpiece is contact with the bottom 21h″ of the die cavity. Stress within the workpiece above the die cavity is in the region of 650 MPa.

In the third stage shown in FIG. 6c, also known as the upper sheet pierced stage, the rivet (and, in particular, the tip of the shank of the rivet) has pierced completely through the upper sheet of the workpiece. Stress of approximately 2000 MPa has propagated from the tip of the shank through the length of the shank. The entirety of the volume of the central rivet cavity 42c is filled by the material of the upper sheet of the workpiece.

Flare initiation can be seen as the tip of the shank of the rivet begins to pierce the lower sheet of the workpiece. A good level of die impression (i.e. when material deforms to conform to the profile of the upper surface of the die) is seen in the lower sheet of the workpiece. The material of the lower sheet of the workpiece is beginning to flow radially outwards.

In the fourth stage shown in FIG. 6d, which may also be known as the flare initiation stage, greater than about 50% of the length of the rivet (parallel to the axis) has been inserted into the workpiece, with the tip of the shank of the rivet penetrating the lower sheet of the workpiece. The volume of the central rivet cavity is completely filled with material of the workpiece. The material of the lower sheet of the workpiece continues to fill out the die cavity and move radially outwards along the transition portion of the upper surface of the die. Deflection is present between the flat outer rim portion of the die surface and the lower sheet of the workpiece. The deflection is such that the lower sheet of the workpiece is spaced above the surface of the die at the rim portion. The deflection is caused by high levels of compression during the rivet insertion and high levels of tension radial flow of the material of the lower sheet of the workpiece in the die cavity.

In the fifth stage shown in FIG. 6e, which may also be known as the rivet flaring and material ride out stage, greater than about 80% of the length of the rivet (parallel to the axis) has been inserted into the workpiece and the rivet has created interlock in the lower sheet of the workpiece. Displacement of the rivet and upper sheet of the workpiece causes the lower sheet of the workpiece to further flow outwards in a radial direction. Material of the lower sheet of the workpiece flowing out of the die cavity, combined with the compressive forces exerted by the workpiece on the surface at the bottom of the die cavity create increased sheet deflection giving a ‘dished’ appearance to the workpiece. The material dishing continues causing the material of the workpiece to ride out upwards away from the transition portion and rim portion of the upper surface of the die.

In the sixth stage shown in FIG. 6f, also known as the rivet head flush stage or as the end of cycle phase, the underside of the head of the rivet is seated so the underside of the head of the rivet is near flush with the upper sheet of the workpiece. Maximum interlock is now achieved between the rivet and the lower sheet of the workpiece, and hence between the upper and lower sheets of the workpiece. Gaps between the upper and lower sheets of the workpiece are reduced due to the head of the rivet squeezing the sheets together.

The material of the lower sheet of the workpiece has achieved maximum radial displacement. This highlights areas where the upper surface of the die never contacts the lower sheet of the workpiece.

The dished appearance of the workpiece due to the deflection between the lower sheet and die remains in the completed joint. However, the completed joint is an improvement over the joints created with a die of known geometry. This is discussed in more detail below.

If the sixth stage of the rivet setting process using a known die, as shown in FIG. 3f, is compared with the sixth stage of the rivet setting process using a die in accordance with the present invention shown in FIG. 6f, it can be seen that the joint formed using the die in accordance with the present invention has a number of advantages.

First, a greater surface area of the lower surface of the lower sheet of the workpiece contacts the die in the case of the die according to the present invention. This means that the workpiece is more supported by the die during the rivet setting process. Greater support of the workpiece during the rivet setting process results in less stress an unwanted deformation in the formed joint, and hence a better quality of joint.

Secondly, there are no gaps between the bottom/sidewall of the die cavity and the lower sheet of the workpiece with the die according to the present invention. The gaps present in the case of the known rivet geometry mean that, during the rivet setting process, extra stress is caused at the point where the die radius leading edge contacts the lower sheet of the workpiece. As already discussed, this results in an impression in the lower sheet of the workpiece, which can finally result in a compressed fold in the material of the lower sheet and hence cracking of the joint. The lack of gaps resulting from use of the die according to the present invention results in a better joint in which the risk of the joint cracking is reduced.

All of the benefits of the joint formed using the die according to the present invention result from the upper surface of the die having a transition portion having a first section located radially inboard of the second section, the first section having a radius of curvature which is less that of the second section. In this way, during the rivet setting process, the transition portion helps guide material of the lower sheet of the workpiece radially outwards as it moves away from the die cavity.

FIG. 8 shows a sectioned view through a die of a rivet setting tool according to a further embodiment of the present invention.

The die 13″ is generally cylindrical about a central axis B, with a head (or main body) 20″ defining an open die cavity 21″ for facing the setting tool, and a depending stem 22′″ that extends along the central axis (in an opposite direction to that in which the upper surface of the die-discussed below-faces) and is of reduced diameter compared to the head 20″′.

The main body has an upper surface 20a′″, the die cavity 21″ being formed in the upper surface 20a′″. The die cavity 21′″ includes a central bore 21m″ which extends along the central axis B into the mounting stem 22′″.

FIG. 9 shows a computer simulated sectioned view through a portion of a rivet setting tool according to an embodiment of the present invention (which includes the die 13″ shown in FIG. 8), rivet and workpiece, at different stages as a rivet is set in the workpiece. Each of the stages shown in FIGS. 9a to 9f are generally equivalent to the stages shown in relation to the known rivet setting apparatus in FIGS. 3a to 3f.

The central bore of the rivet provides a large volume into which material from the lower sheet of a workpiece can flow when the die takes part in a rivet setting process. The remainder of upper surface of the die outside of the bore imparts enough reaction force on the workpiece during the rivet setting process to produce a satisfactory joint. By accommodating material of the lower sheet of the workpiece in the bore during the rivet setting process, this reduces the amount of material of the lower sheet of the workpiece that flows radially outwards. Reducing the amount of material of the lower sheet of the workpiece that flows radially outwards during the rivet setting process minimises the likelihood of cracking resulting from an impression in the lower sheet of the workpiece developing into a compressed fold.

The presence of a central bore has a number of other benefits.

Given that the central bore has, in effect, a ‘bottomless’ structure, the die cavity has an effective limitless volume. This has several benefits from the standpoint of conducting a rivet setting operation.

First, the bore allows the punch to ‘over-drive’ a rivet into the workpiece, such that the head of the rivet ends up beneath an upper surface of the workpiece. This is because the material of the workpiece which is displaced by the presence of more volume than usual of the rivet embedded in the workpiece is received in the central bore.

Secondly, the tolerances involved in driving the punch for a rivet setting operation are increased. If the punch of a rivet setting apparatus inadvertently ‘over-drives’ the rivet into the workpiece then this will not matter—the bore can accommodate any excess material from the workpiece caused by the overdriving of the rivet into the workpiece. As such the rivet setting apparatus including a die according to the present invention may be said to have an improved process window as compared to that including dies of known geometry.

Thirdly, with a conventional die geometry, if a rivet is driven into the workpiece such that the volume of the workpiece displaced into the die cavity exceeds the volume of the die cavity, then the pressure/stress within the workpiece and rivet (particularly the shank of the rivet) will increase as the workpiece and rivet are compressed. This results in additional stress in the workpiece and rivet which may affect the quality of the created joint. Furthermore, increased stress in the workpiece and rivet will cause increased force to be exerted on the die and/or punch or the rivet setting apparatus. The increased force exerted on the die and/or punch will result in a reduced operating lifetime of the die and/or punch. Furthermore, the increased force exerted on the punch may mean that a more expensive actuator to drive the punch, which can withstand the increased forces, may be required. A die geometry according the present invention, incorporating a bore, overcomes these potential issues—the die cavity is never completely filled by the workpiece as the workpiece can be received by the central bore. In addition, reduced stress within the rivet during a rivet setting operation utilising a die according to the present invention may mean that a softer rivet can be used as compared to that for a die of conventional geometry.

Finally, given that the central bore can accommodate an effectively limitless volume of material, this gives greater flexibility regarding the workpieces (and rivet geometries) that a die according to the present invention can be utilised with. In particular, whereas an existing die may only be optimal for use with a particular workpiece (having a defined total thickness of sheets) and/or rivet geometry, given the defined volume of the die cavity; a die according to the present invention may be suitable for use with a relatively wide range of total workpiece thicknesses and/or rivet geometries. This is advantageous as the same die can be used for a wide range of different rivet setting operations, without the need for using a different die. It follows that a single die of the type of the present invention can be used in place of several different dies of a known type, thereby reducing the number and cost of dies required to carry out a number of different rivet setting operations.

In the present embodiment the central bore 21m″ extends all the way through the mounting stem 22″. This need not be the case in other embodiments. For example, the central bore may extend all the way through the head of the rivet die, but only partially through the mounting stem.

However, a central bore which extends all the way through the mounting stem has a number of potential benefits.

First, the fact that the bore extends all the way through the die means that it may be possible to inspect and/or access the workpiece through the die. This can be seen clearly in FIG. 12, which shows a sectioned view through a portion of a rivet setting apparatus according to the present invention which includes the die 13′″ shown in FIGS. 10 and 11. The central bore 21m′″ of the die 13″ forms part of a conduit 46 which extends all the way from the underside of the lower jaw 8, through the jaw, adapter 18 and die 13″, to above the die. This provides access from beneath the die, through the die, to above the die. Said access may be referred to as being a ‘line of sight’.

Several potential uses of this feature include: i) analysing properties of the punch between rivet setting operations (whilst a workpiece is not located in the rivet setting apparatus); ii) analysing properties of the workpiece when located in the rivet setting apparatus—e.g. presence or otherwise of workpiece, thickness of workpiece, material of workpiece, quality of ‘button’ formed on underside of workpiece after rivet setting operation; and iii) use of compressed air or tool via the through-bore to aid with ejecting the workpiece from the die after a rivet setting operation.

In addition, the central bore may be of a relatively small diameter. In light of this the central bore may, if it did not have a ‘bottomless’ (or through-bore) structure, be susceptible to becoming blocked or misshaped due to the accumulation of contamination in the form of swarf, debris, dust and/or plating etc. from the workpiece. The presence of a through-bore enables any contamination to fall through the central bore as opposed to accumulating. What is more, it may be possible to actively clean the central bore of contamination-compressed air or another cleaning agent may be directed into the bore via the opening of the bore at the mounting stem end of the die.

In some embodiments the diameter T (or minimum diameter), perpendicular to the axis B, of the central bore 21m′″ is less than an outer shank diameter S of a rivet with which the die is configured to be used. In particular, the diameter may be the diameter of the central bore as it passes through the mounting stem. In the case where the die includes a tapered region (see below), the diameter T may not be a diameter of the tapered region. What is meant by the outer shank diameter S of the rivet has already been discussed above and applies equally here.

By having a diameter of the central bore which is less than the outer shank diameter of the rivet with which the die is configured to be used, this ensures that the upper surface of the die can exert a sufficient reaction force on the workpiece (particularly on the portion of the workpiece which is directly below the rivet) to ensure that the rivet does not pierce all the way through the workpiece—as already discussed, this would result in a joint of reduced integrity and/or which is undesirably susceptible to corrosion.

In some embodiments the diameter T (or minimum diameter), perpendicular to the axis B, of the central bore 21m′″ is greater than an inner shank diameter I of a rivet with which the die is configured to be used. What is meant by the inner shank diameter I of the rivet has already been discussed above and applies equally here. In alternative embodiments the diameter T (or minimum diameter), perpendicular to the axis B, of the central bore 21m″ is less than the inner shank diameter I of a rivet with which the die is configured to be used. The upper surface 20a′″ of the main body 20′″ includes an outer annular wall which surrounds the die cavity 21″. The annular wall or rim 21d′″ is defined at least in part by a rim portion 21i″ of the upper surface 20a″. In the present example, the rim portion extends in a direction that is generally perpendicular to the axis. However, in other embodiments, this need not be the case.

The upper surface of the die of the present embodiment defines a die cavity which has a generally w-shaped cross-sectional profile, as seen in the figure. However, it will be appreciated that the present invention, relating to the presence of a central bore, may equally be applied to any other geometry of die, for example, one in which the die cavity is defined in part by a generally radial portion of the upper surface of the die, which defines a flat bottom of the die cavity. Such a geometry of die is shown in FIGS. 10 and 11.

FIG. 10 shows a sectioned view through a die of a rivet setting tool according to a further embodiment of the present invention. The die cavity 21″ is defined in part by a substantially radial (flat) base portion 21h″ of the upper surface 20a′″. FIG. 11 shows a computer simulated sectioned view through a portion of a rivet setting tool according to an embodiment of the present invention (which includes the die 13′″ shown in FIG. 10), rivet and workpiece, at different stages as a rivet is set in the workpiece. Each of the stages shown in FIGS. 11a to 11f are generally equivalent to the stages shown in relation to the known rivet setting apparatus in FIGS. 3a to 3f.

In addition, a central bore according to the present aspect of the invention may be applied to any of the die geometries according to other aspects of the invention discussed in relation to FIGS. 5 to 7.

In the embodiments of the present invention shown in FIGS. 8 to 11 the die cavity 21″ includes a tapered region 21n′″ which adjoins the bore 21m′″ and is radially outboard of the bore 21m′″. The tapered region 21n″ is located at an upper end of the bore, i.e. the end of the bore which is located furthest along the axis B away from the stem 22′″. The surface (or portion of the upper surface of the die) that defines the tapered region faces toward the axis B. The tapered region may also be referred to as a counter-sunk region/portion.

The tapered region acts to ease displacement of material of the lower sheet of the workpiece into the bore, thereby maximising the benefits of the bore discussed above.

In the present description, use of a die according to the invention as part of a rivet setting apparatus to carry out a rivet setting operation on a workpiece is described in relation to a workpiece comprising two sheets of 7000 series aluminium. It will be appreciated that the invention may be used with any appropriate type of workpiece. Such a workpiece may include sheets of any appropriate material, any number of sheets equal to two or more, and any appropriate thickness of the sheets of the workpiece.

Furthermore, dies according to the present invention may be used as part of a rivet setting process which utilises any appropriate size and geometry of rivet, formed from any appropriate material. However, the invention is of most relevance to self-piercing rivets.

The workpiece which undergoes the rivet setting process may be any appropriate workpiece. For example, it may be used for manufacturing a product or a component of a product. The product may be any appropriate product. In some cases, it may be a vehicle.

RIVET DIES, RIVET SETTING TOOL AND ASSOCIATED METHOD

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