Patent Application
20130125611
The embodiments of the present invention relate to a self-piercing riveting process and, more particularly, to a die member for use in a self-piercing riveting process.
In the joining of components used in high volume vehicle production, it may be desirable to use mechanical fasteners to help achieve the required strength and durability of joints. One type of mechanical fastener used in vehicle production is a self-piercing rivet (SPR).
The general principles of self-piercing rivet technology are known in the art. To apply a self-piercing rivet to workpieces to be joined, a portion of a first workpiece or panel is placed upon a bearing surface of a die member, so as to overlie a die cavity formed in the die member. Portions of one or more additional panels are then stacked on the portion of the first panel overlying the die cavity. The panels are secured in position with respect to each other and with respect to the die member, to prevent relative movement of the parts during application of the rivet. The die cavity may also contain a die post which assists in forcing a portion of the rivet to spread or deflect radially outwardly when pressure sufficient to pierce the first workpiece is applied to the rivet. The rivet also pierces surfaces of the second panel overlying the first panel. In a known manner, up to four layers of material may be joined using existing SPR processes.
During application of the rivet to the workpieces to be joined, a feature known as an SPR “button” is produced. This SPR button is in the form of a protrusion in a surface of the second panel along a side of the second panel opposite the side pierced by the rivet. One of the challenges encountered during SPR joining is the nucleation and propagation of cracks on the “button” side of the joint, along corners of the button shaped by the floor and walls of the die cavity during the SPR operation. The presence and size of these cracks can affect the quality of the joint and the viability of SPR technology as a fastening option.
Thus, a need exists for a die geometry in which crack formation in the rivet material along the SPR button during formation of the button is reduced or minimized.
In one aspect the embodiments of the present invention, a die member for a self-piercing riveting process includes a die cavity having an axis and a plurality of sides positioned about the axis. Each side of the die cavity extends along a plane which includes a chord connecting two points along a circle centered on the axis.
In another aspect the embodiments of the present invention, a die member for a self-piercing riveting process includes a die cavity formed in the die member. A perimeter of the cavity is formed by a plurality of sides and a plurality of fillet radii. Each end of each side of the die cavity is connected by a fillet radius to an adjacent side of the cavity at an end of the adjacent side.
In another aspect the embodiments of the present invention, a die member for a self-piercing riveting process includes a bearing surface and a die cavity formed in the bearing surface. The die cavity includes a cavity floor and a central axis extending through the cavity floor. A plurality of cavity sides extends between the cavity floor and the bearing surface. At least one of the sides is sloped away from the axis in a direction proceeding from the floor toward the bearing surface.
In another aspect the embodiments of the present invention, a die member for a self-piercing riveting process includes six wall portions, each end of each wall portion being connected to an adjacent wall portion at an end of the adjacent wall portion.
In the drawings illustrating embodiments of the present invention:
The exemplary embodiments described herein provide detail for illustrative purposes and are subject to many variations in structure and design. It is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting.
The terms “a” and “an” herein do not denote a limitation as to quantity, but rather denote the presence of at least one of the referenced items. Also, use herein of the terms “including,” “comprising,” “having” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as allowing for the presence of additional items. Further, the use of terms “first”, “second”, and “third”, and the like herein do not denote any order, quantity, or relative importance of the items to which they refer, but rather are used to distinguish one element from another.
Unless limited otherwise, terms such as “configured,” “disposed,” “placed”, “coupled to” and variations thereof herein are used broadly and encompass direct and indirect attachments, couplings, and engagements. In addition, the terms “attached” and “coupled” and variations thereof are not restricted to physical or mechanical attachments or couplings.
Unless noted otherwise, similar reference numerals appearing in views of different embodiments of the present invention refer to similar elements. For example, reference numeral 59 in
A self-piercing rivet and associated die member of the embodiments of the present invention may be adapted for mass production applications, including automotive applications. Embodiments of the self-piercing rivet and die member disclosed herein are suitable for installation and use in a conventional die press, such as utilized by the automotive industry to join sheet metal parts, including body panels and structural components. In such applications, the press applies one or more self-piercing rivets with each stroke of the press.
An end 24d of wall 24b is formed into a cutting or piercing surface configured to pierce a panel or workpiece in a manner known in the art, when the wall end 24d is forced into contact with the workpiece by application of a pressing force on the rivet 20. If desired, an inner portion of wall 24b adjacent the wall end 24d may be chamfered as shown in
In one particular embodiment, die post outer surface 64 forms an angle J in the range of 9.5 degrees to 30.5 degrees inclusive with respect to axis X. In another particular embodiment, the die post is omitted from the die cavity. In this embodiment, deformation of the rivet wall 24b is produced by pressure of the wall against die surface 56.
In particular embodiments, a multi-sided or polygonal die cavity 52 in accordance with the present invention has a plurality of wall portions or sides 59 extending between die surface 56 and bearing surface 51. Wall portions 59 are straight within the limits of manufacturing tolerances.
In a particular embodiment, the depth of the die cavity as measured from a plane defined by bearing surface 51 to a plane defined by die surface 56 and along a plane extending parallel to axis X is within the range of 1.95 mm to 3.30 mm inclusive.
Referring to
Referring to
In these embodiments, central axis X of the die cavity extends through circle center C. Thus, axis X is spaced an equal distance R from each point P2 at which adjacent chords C2 intersect, as shown in
In the view shown in
In a particular embodiment, the plane along which the side 59 extends perpendicular to a plane defined by the bearing surface 51 and is also perpendicular to a plane defined by cavity floor 56.
Referring to
The procedure set forth above may be used to provide a die cavity having any desired number of cavity sides of equal length (taking into account manufacturing tolerances relating to the lengths of the sides).
In addition, a fillet radius r is formed at each intersection of adjacent wall portions 59 and extends along each of the wall portion intersections between die surface 56 and bearing surface 51. In one embodiment, each radius r has a value within the range 0.25 mm-1.0 mm inclusive. In one particular embodiment, the radii r have a value in the range of 0.75 mm to 3.25 mm inclusive.
In one embodiment, sides 59 have equal lengths with equal angles θ (again, within the limits of manufacturing tolerances) formed between each two adjacent sides and facing into the die cavity.
In one embodiment, as shown in
Referring to
In alternative embodiments, rather than six or eight straight sides, the die cavity 52 may have a greater number of straight sides or a lesser number of straight sides, according to the requirements of a particular process. Thus, while the above examples described hexagonal and octagonal die cavities, a cavity in accordance with an embodiment of present invention may have any desired number of sides of substantially equal length, depending on the properties and thicknesses of the materials to be joined, the number of sheets to be joined, and other pertinent factors. In particular embodiments, cavities having anywhere from three to twenty sides, inclusive, are contemplated.
In addition, a radius r2 is formed at the intersection between die surface 56 and each of wall portions 59. In one embodiment, each radius is has a value within the range 0.25 mm-1.0 mm inclusive. In one particular embodiment, the radii r2 have values in the range of 0.75 mm to 3.25 mm inclusive.
Referring to
In a particular embodiment, all of the sides 59′ of the cavity are sloped outwardly as described above.
In the embodiment shown in
Any of the embodiments of the die member described herein may be formed from steel or any other suitable material or materials.
Referring to
The rivet design, die member design, and process parameters are specified so that rivet wall portion 24b does not pierce completely through the thickness of second panel 102 during formation of the die button. The portion of the second panel deflected into die cavity 52 expands radially until it abuts cavity wall portions 59.
It is believed that crack nucleation in the rivet is related to the lack of ductility which often exists in high strength alloys (including aluminum based materials) from which the rivet may be formed. It is believed that the cracks observed in SPR buttons nucleate and grow after a critical stress or cumulative strain is achieved in a given material. During self-piercing rivet processes, material is displaced and is subjected to significant multi-axial stresses and strains during SPR button formation within the die cavity. Often, if cracks are initiated in the SPR button, the cracks are observed along the button edge and surface. It is believed that the largest cumulative strains in the rivet material occur along surfaces of the button located the greatest distance from the central axis of the die cavity, due to significant material displacements required and due to the need for the die cavity to accommodate the volume of the deformed rivet.
It has been found that the geometry of the die cavity can play a significant role in controlling displacement of the rivet material during formation of the SPR button. It is believed that an SPR button formed in a multi-sided die cavity 52 defined as described above using a circle C′ with a radius R will experience less crack formation than an SPR button formed in a circular die cavity having the radius C′. The material of second panel 102 is prevented from deforming uniformly radially outwardly by the straight wall portions 59. Thus, rather than deforming to a circular configuration having the uniform radius R of circle C′, the outer boundary of the SPR button acquires the shape of the multi-sided die cavity 52. Thus, it is believed that use of straight wall portions 59 in restricting or confining deformation of the SPR button material aids in mitigating crack formation and crack propagation along the outer surfaces of the SPR button 150.
It is also seen that, as the number of straight wall portions forming the sides of die cavity 52 increases, the area of the floor 56 of cavity 52 increases, more closely approaching the floor area that would be provided with a circular cavity having the radius R. This increase in floor area allows a relatively greater radial expansion of the material forming the die button. Thus, in a self-piercing rivet application in which the area or space that may be occupied by the riveted joint is restricted, the die cavity floor area available for expansion of the die button can be maximized within a permissible circular joint area or die button area πR2 of circle C′ while eliminating or mitigating crack formation that would otherwise occur during uniform radial expansion of the die button material.
The number of die cavity sides may also be specified so as to take into account the cavity volume needed to accommodate a given rivet size while still minimizing cumulative strain during deformation of a rivet material having a given ductility. This design flexibility with regard to die cavity dimensions also aids in eliminating or mitigating crack formation.
The optimum configuration of wall portions 59 can be determined iteratively and/or analytically to meet the requirements of a particular application, based on factors such as rivet design, panel materials and thicknesses, permissible SPR button area, and other pertinent factors.
It will be understood that the foregoing description of the present invention is for illustrative purposes only, and that the various structural and operational features herein disclosed are susceptible to a number of modifications, none of which departs from the spirit and scope of the present invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.
