WO2016113254A1

WO2016113254A1 - Method for producing a closed-end blind rivet, and closed-end blind rivet

Info

Publication number: WO2016113254A1
Application number: PCT/EP2016/050471
Authority: WO
Inventors: Stefan Schneider
Original assignee: Newfrey Llc; Tucker Gmbh
Priority date: 2015-01-13
Filing date: 2016-01-12
Publication date: 2016-07-21
Also published as: DE102015100424A1; TW201641182A

Abstract

A method for producing a closed-end blind rivet (1) comprising the following steps: providing a mandrel (2) having a mandrel head (4) and having a breakneck portion (6) located between said head and a stem (5), which breakneck portion has an initial breaking strength, inserting the mandrel head (4) into an opening in a preformed blank (13), and extruding the blank (13) over the mandrel (2) to form a rivet body (3) in such a manner that a first, closed end of the rivet body (3) closely surrounds the mandrel head (4) and the interior of the rivet body (3), wherein the tensile stress on the breakneck portion (6) during extrusion of the rivet body (3) is less than the initial breaking strength of the breakneck portion (6), and forming a flange (9) at a second, open end of the rivet body (3), reducing the tensile strength of the mandrel (2), and thus of the initial breaking strength of the breakneck portion (6), by heat treatment of the blind rivet (1) composed of the rivet body (3) and mandrel (2). The invention also relates to a closed-end blind rivet produced by such a method.

Description

METHOD FOR PRODUCING A CLOSED-END BLIND RIVET,

AND CLOSED-END BLIND RIVET

Method for producing a closed-end blind rivet comprising the following steps: providing a mandrel having a mandrel head, a stem, and a breakneck portion that has an initial breaking strength and is located between the mandrel head and the stem; inserting the mandrel head into an opening in a preformed blank; and extruding the blank over the mandrel to form a rivet body in such a manner that a first, closed end of the rivet body closely surrounds the mandrel head, and the interior of the rivet body is in close contact with the breakneck portion and a portion of the stem, wherein the stress on the breakneck portion during extrusion of the rivet body is less than the established breaking strength of the breakneck portion, and forming a flange at a second, open end of the rivet body. Blind rivets are used to permanently join workpieces that are in contact with one another. For this purpose, the workpieces have holes passing through them that are brought into alignment with one another and into which the blind rivet is placed. Drawing on the mandrel then deforms, and thus fastens, the blind rivet. A fastened blind rivet produces a clearance-free fit in the holes of the workpieces, and clamps the workpieces together.

A closed-end blind rivet and a method of the indicated type for producing the same are known from GB 738,741 A. Closed-end blind rivets are employed when the rivet hole is to be sealed against the passage of media. Soft grades of aluminum are used by preference here as the material for producing the rivet body. During setting of the blind rivet, the rivet body is radially expanded by the mandrel head so that the rivet body seals the holes in the workpieces by a clearance-free fit. During expansion of the rivet body, the rivet body material is swaged and forced radially outward by the mandrel head moving along the rivet body bore, during which process the deformation force acting on the mandrel head increases, and the mandrel head breaks off of the mandrel stem when the breaking strength is reached of a breakneck portion constituting a predetermined breaking point. In this context, the breaking strength of the breakneck portion of the blind rivet must be designed such that the mandrel head can deform the rivet body enough to form an swage-head on the blind side of the workpieces to be joined on the one hand, but on the other hand cannot be pulled into the swage-head or through it and through the rivet hole in the workpieces to be joined, instead breaking off beforehand and remaining in the rivet body on the blind side. Specifically, if the breaking strength of the breakneck portion is too high, then during setting of the blind rivet the mandrel head is pulled into the swage-head formed by the rivet body that joins the workpieces to one another, and the breaking point of the breakneck portion on the mandrel head side projects past the swage-head, a condition known as "spiking." These generally sharp projections present a risk of injury. In the case of colored blind rivets, the visible surface of the blind rivet can be destroyed, and the silvery breaking point of the breakneck portion is visible in the rivet hole of the rivet body, a condition that is unacceptable for visible parts. Furthermore, rivet body material can be pressed into the gap between the nosepiece opening of the riveting tool and the stem when the breaking strength is too high. As a result, the riveting tool jams on the swage-head of the blind rivet and can only be moved away from it through the application of force. A mandrel head that has been drawn into the swage-head also significantly reduces the wall thickness of the rivet body in the region of the rivet hole, considerably reducing the shear strength and tensile strength of the rivet joint. The stack-up of unfavorable dimensional tolerances can also be a problem. In unusual cases, for example when the diameter of the mandrel head is at the lower tolerance limit and the diameter of the rivet hole in the workpieces to be joined is at the upper tolerance limit, and the grip length of the workpieces is on the thin side, it may occur that the breakneck portion does not break and the mandrel with the mandrel head is pulled completely out of the rivet body. Such a pull-through of the mandrel head, which is unfavorable for the strength of the rivet joint, can be prevented by a greater thickness of the material to be joined on the blind side. In practice, however, such as in applications in the automotive industry, comparatively thin blind-side material thicknesses are frequently encountered, with the result that the risk is higher in such applications of rivet mandrel pull-through occurring.

If the breaking strength of the breakneck portion is low, then rivet mandrel pull-through can be reliably avoided. However, the problem then arises of the rivet mandrel being damaged during the process of producing the blind rivet. During production of closed- end blind rivets, the rivet body, which is first present in the form of a blank, and the rivet mandrel are joined to one another in an impact extrusion process in which the rivet body is extruded onto the mandrel. In this process, material of the rivet body flows along the rivet mandrel. While this is taking place, tensile stress arises in the rivet mandrel because of friction. If the basic strength of the mandrel material and the breaking strength established by dimensioning of the breakneck portion are on the low side, then damage or breakage of the mandrel can occur during the impact extrusion process. The problem here is that damage, such as a broken mandrel or even a crack in the breakneck portion, cannot be detected in a rivet exiting the manufacturing tool. During setting of the blind rivet, as well, a breaking strength that is sharply reduced due to pre-existing damage to the rivet mandrel can only be discovered by chance, because inadequate formation of the swage-head is primarily manifested on the blind side of the workpieces, which is not visible.

Based on these problems, the object of the invention is to specify a method for producing a blind rivet, and to create a blind rivet that can be set without projection of the breaking point and without the risk of mandrel head pull-through, even in the case of unfavorable dimensional tolerances and comparatively thin material thicknesses on the blind side. The stated object is achieved by a blind rivet produced in accordance with the method specified in claim 1. Advantageous embodiments of the method are specified in claims 2 through 5.

The method according to the invention for producing a closed-end blind rivet comprises the following steps:

providing a mandrel having a mandrel head, a stem, and a breakneck portion that has an established initial breaking strength and is located between the mandrel head and the stem,

inserting the mandrel head into an opening in a preformed blank, and extrudingthe blank over the mandrel to form a rivet body in such a manner that a closed, first end of the rivet body closely surrounds the mandrel head, and the interior of the rivet body is in close contact with the breakneck portion and a portion of the stem, wherein the tensile stress on the breakneck portion during extrusion of the rivet body is less than the initial breaking strength of the breakneck portion, forming a flange at an open, second end of the rivet body, and reducing the tensile strength of the mandrel and thus the initial breaking strength of the breakneck portion by heat treatment of the blind rivet composed of the rivet body and mandrel.

The method according to the invention makes it possible to choose the tensile strength of the mandrel material and the breaking strength of the breakneck portion to be high enough that damage to the rivet mandrel is reliably prevented during the course of the production process and during shaping of the rivet body. Yet the disadvantages of high mandrel strength and high breaking strength that can occur during setting of the blind rivet are nevertheless avoided in that the blind rivet is subjected to a heat treatment following completion of the mandrel and rivet body and forming of the rivet body, by means of which treatment the tensile strength of the mandrel and hence the breaking strength of the breakneck portion are significantly reduced, so that breakage of the mandrel occurs reliably even when the diameter tolerances of the mandrel head and rivet hole in the workpiece are unfavorable and the material thickness of the workpiece on the blind side is relatively thin. The mandrel head is prevented from being drawn into the swage-head of the set blind rivet and certainly from being pulled through the swage-head, because the reduced tensile strength causes the mandrel breakneck portion to break before reaching the tensile force that would be required for such to occur.

The method according to the invention thus ensures a reliable process for producing the blind rivet by means of high initial breaking strength of the rivet mandrel, and a reliable process for producing the rivet joint by means of the reduction in the rivet mandrel breaking strength of the completed blind rivet that takes place at the end of the production process.

The method according to the invention is primarily suitable for closed-end blind rivets in which a preformed blank that is made of aluminum or an aluminum alloy is formed over the mandrel in order to produce the rivet body. The mandrel of the blind rivet can be made of steel, a steel alloy, or also of an aluminum alloy.

According to a preferred embodiment of the method, the heat treatment of the blind rivet is carried out with regard to temperature and/or duration such that the initial breaking strength of the breakneck portion is reduced by at least 20% owing to the heat treatment. By means of a reduction in the breaking strength in this range, it is possible to reliably achieve the result that the fracture zone of the breakneck portion does not project beyond the outside of the flange when the blind rivet is set.

The heat treatment of the blind rivet according to the invention of necessity also leads to a reduction in strength, and hence in the shear force that the rivet body can absorb. It has proven to be advantageous for the heat treatment of the blind rivet to be carried out such that a reduction in the shear force of the rivet body brought about by the heat treatment is a maximum of 30%. It is possible to compensate for the reduced shear strength of the rivet body through design of the rivet joint, for example by increasing the number of blind rivets used for a rivet joint.

In a preferred application of the method according to the invention, the heat treatment comprises an annealing of the blind rivet at a temperature of 350° C for a duration of 20 min.

The method of the invention is explained in detail below using examples that are shown in the drawings. They show:

Figure 1 a longitudinal section through a blind rivet produced with the method according to the invention,

Figure 2 a schematic representation of the individual steps for producing a closed-end blind rivet according to the invention, and

Figure 3 a cross-section through a blind rivet according to the invention set in a rivet hole of a workpiece.

Figure 1 shows a closed-end blind rivet 1 , which is composed of a mandrel 2 and a rivet body 3. The mandrel 2 has a mandrel head 4 at one end, a stem 5 that forms the other end, and a breakneck portion 6, which is located between the mandrel head 4 and the stem 5. The breakneck portion 6 forms a predetermined breaking point that is intended to break during setting of the blind rivet 1 so that the mandrel head 4 is separated from the stem 5. The mandrel head 4 and the stem 5 have circular cross- sections, with the stem 5 having a substantially uniform diameter, and the diameter of the mandrel head 4 being greater than the diameter of the stem 5 by a factor of approximately 1.5. The breakneck portion 6 can likewise have a circular cross-section, but its cross- section can also deviate from the circular shape. The cross-sectional area of the breakneck portion 6 is smaller than the cross-sectional area of the stem 5, and its size and the strength of the material determine the breaking strength at which the breakneck portion 6 breaks under the stress of tensile force.

The rivet body 3 has a body section 7 with a uniform outside diameter. It is closed at one end by a floor 8, and at the opposite end has a radially outward extending flange 9. The rivet body 3 surrounds the mandrel head 4, the breakneck portion 6, and a portion of the stem 5 adjacent thereto. The hollow inner surface of the rivet body 3 is formed closely onto the outer surface of the mandrel head 4, breakneck portion 6, and stem 5.

The rivet body 3 is preferably made of aluminum or an aluminum alloy. The mandrel 2 is preferably made of steel or another metal with greater strength as compared to the rivet body. Aluminum alloys are also suitable as a material for the mandrel 2.

Figure 2 illustrates the method for producing the blind rivet from Figure 1 and its parts. To produce the rivet body 3, a starting piece 1 1 is cut from a fed wire 10 and is formed into a circular disk-shaped pellet 12. The pellet 12 is then formed by means of pressing tools into a cup-shaped blank 13, which is next subjected to a heat treatment in order to give it the desired properties for further processing.

In parallel thereto, a fed mandrel wire 15 is provided with a breakneck portion 6 by rolling or flanging. Then a mandrel head 4 is formed onto the breakneck portion 6 in a tool, and a stem 5 adjoining the breakneck portion 6 is cut to length, and the now complete mandrel 2 is cleaned.

The mandrel 2 thus produced is then inserted, with the mandrel head 4 foremost, into the opening of the blank 13, after which the blank 13 is extruded in an impact extrusion die to form a rivet body 3 that closely surrounds the mandrel head 4, the breakneck portion 6, and a portion of the stem 5, and that has a flange 9 formed onto the open end of the rivet body 3. During extrusion of the rivet body 3, the material of the rivet body flows along the mandrel 2. Owing to friction, this results in tensile stress on the mandrel 2, which is greatest at the weakest point, the breakneck portion 6. If the rivet mandrel breaking strength established by the formation of the breakneck portion 6 is relatively low, breakage or cracking of the mandrel 2 or of the breakneck portion 6 can occur during the flow process that forms the rivet body 3. The breaking strength of the breakneck portion must therefore be made sufficiently high that damage of the mandrel cannot occur as a result of the extrusion of the rivet body.

Next, the intermediate product of the blind rivet that has been completed to this point is heat treated by annealing at a temperature of, preferably, 350° C during a time period of, preferably, 20 min. In this way, the tensile strength of the mandrel 2 is reduced to such a degree that the tensile force that causes the breakneck portion 6 to break is approximately 20% lower than before the heat treatment.

The strength of the rivet body 3 is likewise decreased in the heat treatment, so the shear strength of the heat-treated blind rivet is lower than the shear strength of an untreated blind rivet. However, the lower shear strength of the rivet body can be taken into account in the design of a rivet joint and compensated for by a larger number of blind rivets where applicable.

After the heat treatment, the blind rivet 1 is cleaned and brought into its final form by postforming of the flange 9.

Figure 3 shows a blind rivet 1 according to the invention fastened in a rivet hole 20 of a workpiece 21. During the fastening operation, the rivet body 3 of the blind rivet 1 inserted into the rivet hole 20 is expanded and axially swaged by movement of the mandrel head 4 toward the flange 9 due to the exertion of a tensile force on the stem while the flange 9 is simultaneously being supported by a backing device, for example the nosepiece of the setting tool. The expansion of the rivet body 3 stops when the mandrel head 4 reaches the region of the rivet body 3 where the rivet body 3 is surrounded by the edge of the rivet hole 20, and is prevented thereby from expanding. The rivet body 3 now opposes the mandrel head 4 with a steeply rising resistance to deformation that causes the tensile force exerted on the stem 5 to rise and brings about a firm contact pressure of the rivet body 3 on the inner edge of the rivet hole 20 on the blind side. Once the described state of deformation is reached, a further increase in the tensile force should no longer advance the mandrel head 4, with the result that the mandrel head is not pulled into, or indeed through, the swage-head 22 formed by the rivet body 3. In consequence of the reduction in breaking strength brought about by the heat treatment according to the invention, a breakage of the breakneck portion 6 now occurs. The mandrel head 4 breaks off of the stem 5. Due to the spring-back that occurs in this process, the mandrel head 4 is projected a short distance into the rivet body 3, while the stem is removed and received by the riveting tool. The final state that is reached is shown in Figure 3.

Figure 3 also shows that even without spring-back of the mandrel 2, the breaking point 23 of the breakneck portion 6 that is left on the mandrel 2 remains inside the opening of the rivet body 3 and would not project past the outside of the flange 9.

Claims

Method for producing a closed-end blind rivet (1 ) comprising the following steps:

providing a mandrel (2) having a mandrel head (4), a stem (5), and a breakneck portion (6) that has an initial breaking strength and is located between the mandrel head (4) and the stem (5),

inserting the mandrel head (4) into an opening in a preformed blank (13), extruding the blank (13) over the mandrel (2) to form a rivet body (3) in such a manner that a first, closed end of the rivet body (3) closely surrounds the mandrel head (4), and the interior of the rivet body (3) is in close contact with the breakneck portion (6) and a portion of the stem (5), wherein the tensile stress on the breakneck portion (6) during extrusion of the rivet body (3) is less than the initial breaking strength of the breakneck portion (6), and

forming a flange (9) at a second, open end of the rivet body (3), characterized by

a heat treatment of the blind rivet (1 ) composed of the rivet body (3) and mandrel (2) that reduces the tensile strength of the mandrel (2) and thus the initial breaking strength of the breakneck portion (6).

2. Method according to claim 1 , characterized in that a preformed blank (13) made of aluminum or an aluminum alloy is employed.

Method according to one of claims 1 or 2, characterized in that a mandrel (2) made of steel or a steel alloy is employed.

Method according to one of the preceding claims, characterized in that the heat treatment of the blind rivet (1 ) is executed in such a way with regard to temperature and/or duration that the initial breaking strength of the breakneck portion (6) is reduced by at least 20% as a result of the heat treatment.

5. Method according to one of the preceding claims, characterized in that the heat treatment comprises an annealing at a temperature of 350° C for duration of 20 min. Closed-end blind rivet (1 ) comprising a mandrel (2) having a mandrel head (4), a breakneck portion (6), and a stem (5), and comprising a rivet body (3) that has a first, closed end and a second, open end from which a flange (9) extends outward, wherein the rivet body (3) is extruded over the mandrel (2) in such a manner that the first, closed end closely surrounds the mandrel head (4), and the interior of the rivet body (3) is in close contact with the breakneck portion (6) and a portion of the stem (5), produced by a method according to one of the preceding claims.