Method for drilling a hole

The invention relates to a self-piercing fastening element, a method for manufacturing a fastening element, and a method for drilling a hole into a fiber-reinforced composite material.

In the field of devices for fastening a part to at least a further part it might be desirable to establish a hole in at least one of the two parts. The hole may be established using a drill and bit, or the hole might even be established by rotation of the fastening element itself during insertion, if the fastening element has a suitable piercing element at its end. In either case, the drilling process can affect the integrity of the region surrounding hole. For example, the drilling process may cause brittleness, or stress concentrations, at the edge of the hole, which alters the functional integrity of the part when a fastening element, inserted through the hole, applies a load to the compromised region. This issue is of particular concern for non- metallic materials, such as composite materials. Therefore, the piercing element for establishing the hole must be suitable for the respective material. Prior art, especially involving composite materials, shows the tendency that the hole cannot be established without amending the integrity of the material at the circumference of the hole which is to be established, for example, due to burning, melting or delamination of the material. Further, prior art shows the tendency that a non-uniform hole diameter with depth is obtained. Furthermore, prior art shows the tendency that the drilling force for drilling a hole and/or the drilling moment which is exerted are too high and/or the saw tooth shows extensive tooth wear and/or the time for drilling the hole is too high. With this regard, it has to be kept in mind, that the fastening element is designed to pierce or drill a hole through an assembly of at least two components and after the piercing or drilling through the layers, the fastening element will be kept in place to fasten the components together. Thus, a self-piercing fastening element is designed to pierce or drill a hole only once. The piercing part is designed for single use. The material used for the element or part of the self-piercing fastening element which serves for piercing or drilling the hole is assumed to be inexpensive because even the part of the fastening element which is used to pierce or drill the hole will be kept in place and is not a part which is recovered after drilling the hole. Thus, the material used for piercing the hole usually has a low hardness.

It is an object of the present invention to provide a fastening element which is improved with regard to at least one of the afore-mentioned disadvantages and/or is improved with regard to its ability to self-pierce at least one of the parts to be fastened, especially a part which is a fiber-reinforced composite material. Further, it is an object of the present invention to improve a method for drilling a hole into a fiber-reinforced composite material.

The object is solved according to the independent claims. Advantageous embodiments are the subject matter of the dependent patent claims and can be gathered from the following description.

The gist of the invention is to use a piercing end having teeth which are arranged around a rotation axis, wherein the teeth comprise an optimized geometry with regard to the piercing. The invention has recognized that the geometry of teeth cutting the material is very important. The invention has recognized that with regard to the above-mentioned disadvantages and the establishing of a smooth hole, especially in a fiber-reinforced composite material, the teeth geometry can be designed with regard to two features: (a) the front face of the teeth cutting the material comprises a different design in at least two sections which each comprise at least one section which is substantially straight and/or (b) the teeth cutting the material are misaligned.

Concerning the two different designs of the front face of the teeth, an upper and a lower front face section are considered, and the upper front face section has a relative angle with regard to the longitudinal axis of the rotation axis which is different to the relative angle between the lower front face and the longitudinal axis of the rotation axis. The upper front face section is used when beginning the drilling of the hole. The teeth are rotated and due to the wear the upper front face section can be worn down and the lower front face section can be the section which can be the one which is used after approximately 50% of the cutting thickness and especially when leaving the material at the end of drilling. Especially, when a fiber reinforced plastic is pierced, a negative or 0° angle with regard to the longitudinal axis of the (upper) front face section is preferred because a positive cutting angle could tear fibers out of the material which would result in increasing delamination. Especially, when a fiber reinforced plastic is pierced, a positive angle with regard to the longitudinal axis of the (lower) front face section is preferred because it is advantageous to pull the fibers towards the saw.

With regard to the misalignment of the teeth, the teeth can comprise a misalignment with regard to the distance to the center around which the teeth are arranged. This misalignment is preferably a misalignment with regard to the top part of the teeth. The misalignment can be used to prevent friction between the teeth acting as a saw and the material in which the whole is to be pierced or drilled. The misalignment can be adapted to obtain at least substantially a constant diameter of the hole throughout the piercing or drilling.

The invention provides a self-piercing fastening element having a hollow body. The hollow body comprises an expandable portion and a piercing end, wherein the piercing end comprises teeth for drilling a hole, which are substantially circularly arranged around a center of the piercing end. The teeth (a) have an upper front face section and a lower front face section extending at least partially straight, the upper front face section having a relative angle with regard to the longitudinal axis of the fastening element which is different to the relative angle between the lower front face and the longitudinal axis of the fastening element and/or (b) the teeth are misaligned so that (i) the teeth comprise a misalignment with regard to the distance to the center around which the teeth are arranged and/or (ii) the teeth extend in different directions.

Providing the afore-mentioned tooth design affects delamination when the fastening element enters and exits the material in which the hole is to be established and/or a uniform hole diameter with depth. Additionally, the tooth wear and drilling force as well as the drilling moment are in a suitable range. Preferably, the drilling force can be below 200N, preferably below 170N, more preferably below 160N, most preferably below 150N and/or the drilling moment can be below 10Nm.

The term "fastening element" within the description as well as according to the invention encompasses any element which can be used to fasten one part to another part. The two parts can contact each other by a flat area section. One of the two parts can be a panel. The other part can be a further panel or a layer structure. Preferably, the material(s) to be pierced can be selected from the group comprising glass and carbon fiber thermoset and thermoplastic materials, car body steels (XCG10/10, P260, SPFC590, SABC1480) and aluminum. The two parts which can be fastened by the fastening element can have the shape of a sheet or at least have a sheet-like section. The fastening element can be inserted into a hole, the hole can be established in at least one of the two parts. The fastening element can comprise arms or at least a deformation section to engage one of the parts, especially the part into which the hole is established, wherein the arm or deformation section cooperates or interacts with a counterface of the fastening element so as to keep the two parts in contact to each other by exerting a force by the counterface as well as the arm or deformation section on the two parts. The two parts are held together or fastened to each other by arranging them between the counterface and the arm/deformation section. The fastening element can be especially a rivet or a clip. The diameter of the hole can be between approximately 4mm and approximately 20mm. The term "hollow body" within the description as well as according to the invention encompasses at least a section of the fastening element which is hollow so as to ensure that a further part or further section of the fastening element might be in the hollow body or so as to ensure the expansion of the fastening element. The hollow body can comprise a deformable portion to expand the fastening element with regard to its original circumference. The deformable portion can cooperate with the counterface.

The term "expandable portion" within the description as well as according to the invention encompasses a deformable element or deformable portion on the hollow body which can be especially deformed in a direction transverse the longitudinal direction of the fastening element or the hollow body.

The term "piercing end" within the description as well as according to the invention encompasses an end at the fastening element or the hollow body that is formed, especially having teeth, so as to establish the hole by rotation of the piercing end with regard to the part into which a hole is to be established.

The term "teeth" within the description as well as according to the invention encompasses at least two tooth-shaped elements. The term "tooth" within the description as well as according to the invention encompasses a protrusion or projection which extends substantially in the direction of the longitudinal direction of the fastening element or the hollow body such that the included angle between the tooth and the longitudinal axis is not greater than 45°. Preferably, the included angle between the tooth and the longitudinal axis is not greater than 30°. More preferably, the included angle between the tooth and the longitudinal axis is not greater than 20°. Most preferably, the included angle between the tooth and the longitudinal axis is not greater than 10°. A tooth can preferably have a height which is smaller than about 5mm, preferably the height of a tooth is in range of about 1.5mm to 4mm. Preferably, the height of a tooth is in the range of about 1.5mm to 3mm. More preferably, the height of a tooth is in the range of about 1.0mm to 2.5mm. Especially the height of a tooth can be approximately 1.9mm.

The term "substantially circularly arranged" within the description as well as according to the invention encompasses an arrangement of elements, especially teeth, so that deviations from the circle are allowed, especially the elements (teeth) might not all have the same distance to a center but a group of teeth can have the same distance to the center and a further group of elements can have a (slightly) different distance to the center than the first group. An overall impression of a circular shape is sufficient as long as the teeth in rotation establish the hole.

The term "front face" within the description as well as according to the invention encompasses the face of a tooth that is in front with regard to the rotation direction.

The term "upper" within the description as well as according to the invention encompasses a section that is more spaced apart from the hollow body than the lower section. The term "upper" within the description as well as according to the invention encompasses that the respective section is arranged at the longitudinal end of the tooth, especially at the longitudinal end of the fastening element. The extension of the upper front face section in the direction of the longitudinal axis of the fastening element can be in the range of approximately 0.1mm to approximately 0.8mm, preferably in the range of approximately 0.2mm to approximately 0.7mm, more preferably in the range of approximately 0.3mm to approximately 0.7mm. The extension of the upper front face section in the direction of the longitudinal axis of the fastening element can be between approximately 0.5mm to approximately 0.7mm, especially approximately 0.6mm. The length of the upper front face section can be adapted with regard to the wear of the material during drilling or piercing such that due to the wear the upper front face section can be worn down and the lower front face section can be the section which is used after approximately 50% of the cutting thickness and especially when leaving the material at the end of drilling.

The term "lower" within the description as well as according to the invention encompasses a section that is less spaced apart from the hollow body than the upper section. Especially, the lower section can be adjacent, directly or indirectly, to the upper section, more spaced apart from the longitudinal end of the fastening element.

The term "at least partially straight" within the description as well as according to the invention encompasses that a macroscopic distance is regarded to be straight, wherein small deviations of a straight line are allowed and will be encompassed by the meaning of "straight"

The term "longitudinal axis of the fastening element" within the description as well as according to the invention encompasses that the fastening element has an axis of rotation which is parallel or along the longitudinal (spatial) extension of the fastening element. The term does not require that the fastening element has to be symmetric around the rotation axis or longitudinal axis, however, it is not excluded. The term "longitudinal axis of the fastening element" can also refer to the longitudinal axis of the hollow element which might coincide with the longitudinal axis of the fastening element, or the longitudinal axis of the fastening element and the longitudinal axis of the hollow body can be parallel to each other. The longitudinal axis of the fastening element and/or the longitudinal axis of the hollow body can intersect the center around which the teeth are arranged. The term "misalignment" within the description as well as according to the invention encompasses a misalignment of the teeth in such a way that the teeth do not have the same distance to the center, especially the distances between the top of the teeth are different. At least two groups of teeth can be provided, each of the teeth of one group has substantially the same distance to the center, and the two distances of the two groups differ. The different distance with regard to the top of the teeth can be obtained such that the basis or feet of the teeth has a different distance to the center and/or such that the teeth extend in different directions having a different angle to the longitudinal axis of the fastening element or the rotation axis. If each group of teeth have a different angle with respect to the longitudinal axis, this design can be best seen in a direction in which the teeth are arranged around the center. This misalignment can be formed from a sheet material such that the teeth are bent in two different directions. The distance of the top of the teeth to the center can be determined as the shortest line to the longitudinal axis of the fastening element (intersecting the center) or the rotation axis (intersecting the center). This distance is the distance measured directly to the longitudinal axis of the fastening element or the rotation axis. Thus, the distance considered can be the projection of the distance of the top of the tooth to the longitudinal axis of the fastening element or the rotation axis traverse to the respective axis. The term "misalignment" can encompass the meaning that at least two different groups of teeth are provided which have a different distance between the upper part of the tooth and the longitudinal axis of the fastening element or the rotation axis. The misalignment can be specified in such a way that an average distance is determined considering the respective groups and the misalignment is given by the different distance to this average distance. The misalignment can be provided in such a way that a sheet metal is bent in two different ways with regard to the teeth, one bending that is substantially traverse to the longitudinal direction of the fastening element or hollow body and one bending that results in a teeth that again substantially extends in the direction of the longitudinal direction of the fastening element or the hollow body. One group of teeth can be bent closer to the center (first bend in the direction of the center) and the other group of teeth can be bent further away from the center (first bend away from the direction of the center). The misalignment can be easily obtained by this kind of bending. In case the fastening element comprises a sheet material, the sheet plane can be the reference to the center.

According to a preferred embodiment, the relative angle between the upper front face section and the longitudinal axis of the fastening element is smaller than the relative angle between the lower front face section and the longitudinal axis of the fastening element. This can lead to optimized results with regard to melting, burning or delamination and/or tooth wear.

According to a preferred embodiment, the relative angle between the upper front face section and the longitudinal axis of the fastening element is about 0° to about 5° (especially, the relative angle can be a negative angle with regard to the longitudinal axis, which means that the upper front face section can be inclined towards the rotation direction (a negative angle corresponds to upper front face section being rotated slightly clockwise with respect to the portion of the longitudinal axis above the teeth), and the relative angle between the lower front face section and the longitudinal axis of the fastening element is about 5° to about 15° (especially, the relative angle can be a positive angle with regard to the longitudinal axis, which means that the lower front face section be inclined away from the rotation direction). The upper front face section can include an angle of preferably about 90° with the longitudinal axis of the fastening element. The upper front face section can include an angle of about 89° or about 91 ° with the longitudinal axis of the fastening element. The upper front face section can include an angle of about 88° or about 92° with the longitudinal axis of the fastening element. The upper front face section can include an angle of about 87° or about 93° with the longitudinal axis of the fastening element. The upper front face section can include an angle of about 86° or about 94° with the longitudinal axis of the fastening element. The upper front face section can include an angle of about 85° or about 95° with the longitudinal axis of the fastening element. The lower front face section can include an angle of preferably about 80° with the longitudinal axis of the fastening element. The lower front face section can include an angle of about 79° or about 81 ° with the longitudinal axis of the fastening element. The lower front face section can include an angle of about 78° or about 82° with the longitudinal axis of the fastening element. The lower front face section can include an angle of about 77° or about 83° with the longitudinal axis of the fastening element. The lower front face section can include an angle of about 76° or about 84° with the longitudinal axis of the fastening element. The lower front face section can include an angle of about 75° or about 85° with the longitudinal axis of the fastening element. The term "about" in the context of (deviations of) values of angles with regard to this description and according to the invention encompasses a deviation of about 2°, especially 1°, preferably less than 1°, more preferably less than 0.8°, most preferably less than 0.6°.

Considering the establishing of the hole, the upper front face, which has an angle with regard to the longitudinal axis of the fastening element or the rotation axis, can be related to the insertion angle, the angle with which the fastening element penetrates the material first. This insertion angle is preferably about 90° or a negative angle because a positive cutting angle could tear fibers out of the material when sawing fiber reinforced plastic. The lower front face, which has a different angle with regard to the longitudinal axis of the fastening element or the rotation axis, can be related to the finishing angle, the angle with which the fastening element finishes the hole. The finishing angle is preferably about 80°, this value is assumed to significantly reduce drilling force and can result in smaller single fiber strands. A positive cutting angle seems advantageous for the hole finishing, especially with regard to sawing a fiber reinforced plastic because it is advantageous to pull the fibers towards the tooth.

According to a preferred embodiment, the misalignment with regard to the center is about 0.10mm to about 0.20mm. The misalignment of the teeth with regard to the center means that two teeth or two groups of teeth are arranged to the center such that the distance of one of the two teeth differs to the distance of the other of the two teeth by about 0.10mm to about 0.20mm to the average distance. The distance is measured with regard to the top of the teeth. The top of the teeth is the respective reference point. The distance of a tooth to the center is measured as the projection with regard to the direction traverse to the longitudinal direction of the fastening element of the distance from the top of the tooth to the average distance to the center. Accordingly, the average distance of the teeth from the center is measured as the average of all the projections of the distances between the center and the top of each tooth with regard to the direction traverse to the longitudinal direction of the fastening element or the rotation axis. The distance to the average distance to the center of at least one of the teeth can differ from the distance to the average distance to the center of a further tooth by about 0.15mm. The distance to the average distance to the center of at least one of the teeth can differ by about 0.155mm or about 0.145mm. The distance to the average distance to the center of at least one of the teeth can differ by about 0.14mm or about 0.16mm. The distance to the average distance to the center of at least one of the teeth can differ by about 0.13mm or about 0.17mm. The distance to the average distance to the center of at least one of the teeth can differ by about 0.12mm or about 0.18mm. The distance to the average distance to the center of at least one of the teeth can differ by about 0.11mm or about 0.18mm. The distance to the average distance to the center of at least one of the teeth can differ by about 0.10mm or about 0.20mm. The term "about" in the context of (deviations of) values for spatial distances with regard to this description and according to the invention encompasses a deviation of about 0.009mm, preferably less than 0.008mm, more preferably less than 0.007mm, most preferably less than 0.006mm.

In a preferred embodiment, the teeth create a U-shape profile with regard to a direction around the center. Especially two succeeding teeth in the circular direction form a leg of the U. The teeth can substantially extend in the same direction which is substantially a direction parallel to the longitudinal axis of the fastening element or the hollow body. Thus, the teeth can have a shape or form obtained by bending protrusions at the piercing end twice so that the protrusions are misaligned with regard to the distance to the center around which the protrusions are arranged, wherein at least part of the protrusions form the teeth. One bending can be substantially traverse to the longitudinal direction of the fastening element or hollow body and one bending can result in a protrusion/teeth that again substantially extends in the direction of the longitudinal direction of the fastening element or the hollow body. One group of teeth can be bent closer to the center (first bend in the direction of the center) and the other group of teeth can be bent further away from the center (first bend away from the direction of the center). A substantially constant hole diameter can be obtained throughout the thickness of the material to be pierced or drilled.

According to a preferred embodiment, the misalignment of the teeth alternates. Preferably, the misalignment of the teeth alternates between two adjacent teeth. The misalignment can alternate though as an alternation is provided with a pattern of two substantially similarly or equally aligned teeth which are followed by at least one tooth which is misaligned with regard to the two predecessors. The misalignment can have a 2-1-2-1-... , 3-1-3-1-... , 3-2-3-2-... pattern. Other patterns, especially "irregular patterns" 3-2-1-3-2-1-... or 3-2-2-3-2-2-... are not excluded.

According to a preferred embodiment, the teeth differ in height. According to this, two groups of teeth can be formed, one group being larger in height than the other group. The groups can alternate though as one tooth of one group is immediately followed by a tooth of the other group. With this embodiment, one can have a sharp cutting edge (the teeth having the lower height) once the first group of teeth (the teeth having the greater height at the beginning before establishing the hole) is worn down partially.

According to a preferred embodiment, the number of teeth is smaller or equal to 18. Preferably, the number of teeth is less 18, 17, 16, 15, 14, 13 or 12. According to the teeth design of the invention, this number leads to good results with regard to degradation, the force to be used to establish the hole and the degree of exit delamination.

Preferably, the teeth can be arranged symmetrically around the center in such a way that the teeth are arranged point-symmetrically to the center. The teeth can be arranged at constant circumferential intervals; for example, the center of each tooth can be positioned every 20 degrees, for a total of 18 teeth around the circumference of the piercing element.

According to a preferred embodiment, the fastening element comprises a rod which extends axially inside the hollow body, wherein the rod and the hollow body are adapted to cooperate with each other to drive it in rotation about a longitudinal axis and, when axial traction is applied, to deform said expandable portion radially. According to this embodiment, a very efficient fastening element can be provided which can establish the hole by itself.

According to a preferred embodiment, the fastening element is a rivet.

According to a preferred embodiment, the rivet can comprise a bearing collar or flange, wherein the hollow body, which can be especially tubular, is arranged between the bearing collar and the piercing end. The hollow body extends axially and is radially expandable. The hollow body can comprise a cylindrical first portion that is adjacent to the bearing collar and a cylindrical second portion that is adjacent to the piercing end. The hollow body can further comprise an expandable third portion in the form of a shank or drum, especially having strips which can be folded on each other when expanded, between the cylindrical first portion and the cylindrical second portion. The rod, which can extend axially inside the hollow body, can co-operate with said hollow body to drive it in rotation about its axis and, when axial traction is applied to deform the expandable third portion radially. The rod can include a breakable zone that is positioned to be flush with the bearing collar when the third portion of the hollow body is fully expanded radially.

The piercing end can be part of a component which further comprises the hollow body. The hollow body can be formed as a single piece together with the piercing end. As an alternative the piercing end can be connected to the hollow body directly or indirectly. The connection can be detachable, especially after or during expansion.

According to a preferred embodiment, the rod can comprise at one end a first rod portion that extends inside the cylindrical first portion of the hollow body. Further, the first rod portion can project beyond the bearing collar outside the hollow body.

According to a preferred embodiment, the rod can comprise at the end, which is spaced apart from the one end of the rod, a second rod portion is at least partially surrounding by the second portion of the hollow body in such a manner as to be blocked axially in the hollow body.

According to a preferred embodiment, the rod can comprise a third rod portion between the first and second rod portions, the third rod can include the breakable zone and the third rod portion can be arranged in such a manner that a fraction of the third rod portion becomes inserted in the first portion of the hollow body when the third portion of the hollow body is fully expanded radially.

According to a preferred embodiment, the first portion of the hollow body has an inside surface that is provided with claws so as to prevent the rod from moving in a translational direction.

According to a preferred embodiment, the rod is fastened to the second portion of the hollow body by a tab-and-notch system.

According to a preferred embodiment, the rod is fastened to the second portion of the hollow body by welding.

According to a preferred embodiment, the expandable third portion of the hollow body has slots with strips that are interposed between the slots. Further, the expandable third portion can have an annular zone of weakness in a transverse midplane.

According to a preferred embodiment, the strips have outside surfaces that are abrasive. According to a preferred embodiment, the piercing end of the hollow body is in the form of a hole saw. According to a preferred embodiment, the fastening element is made of formed sheet metal. The metal can be a soft steel, low carbon steel or mild steel, especially in a bainite state, e.g. DC01, C45E or the like. The sheet metal can be rolled and/or welded. The invention further provides a method for manufacturing a fastening element having a hollow body comprising an expandable portion and a piercing end, the piercing end comprising teeth for drilling a hole, which are substantially circularly arranged around a center of the piercing end. The teeth are provided by bending protrusions at the piercing end twice so that the protrusions are misaligned with regard to the distance to the center around which the protrusions are arranged, wherein at least part of the protrusions form the teeth. As explained above, the piercing end can have teeth comprising a misalignment such that the top of the teeth have a different distance to the center at least for two teeth of the piercing end. One bending can be substantially traverse to the longitudinal direction of the fastening element or hollow body and one bending can result in a protrusion/teeth that again substantially extends in the direction of the longitudinal direction of the fastening element or the hollow body. One group of teeth can be bent closer to the center (first bend in the direction of the center) and the other group of teeth can be bent further away from the center (first bend away from the direction of the center). According to a preferred embodiment, a sheet material which is rolled and welded to obtain the hollow body is provided. Manufacturing the fastening element by using a sheet material provides the possibility of diverse designs. A sheet material can be processed more easily than a solid body. Further, the misalignment can be easily obtained by referring to the sheet plane which can be used as a reference to the center of the fastening element.

The invention further provides a method for drilling a hole into a fiber-reinforced composite material, wherein a piercing end of a rotation body comprising teeth for drilling a hole is used, wherein the teeth are substantially circularly arranged around a center of the piercing end, characterized in that a piercing end is used which comprises teeth having an upper front face section and a lower front face section extending at least partially straight, the upper front face section having a relative angle with regard to the longitudinal axis of the rotation body which is different to the relative angle between the lower front face and the longitudinal axis of the rotation body and/or (i) teeth comprising a misalignment with regard to the distance to the center and/or (ii) teeth extending in different directions.

The term "fiber-reinforced composite material" within the description as well as according to the invention encompasses a material such as SMC (thermoset with glass fiber) or thermoplastics with long glass or carbon fiber. The term "rotation body" within the description as well as according to the invention encompasses a longitudinal extending element, preferably an at least partly rod-like or hollow body-like element, to which a force, especially a rotation force, can be applied so that the rotation body can be brought into rotation around the longitudinal axis of the rotation body. The rotation body can be formed in the form of the rod or hollow body as explained above.

According to a preferred embodiment of the method, the relative angle between the upper front face section and the longitudinal axis of rotation body is smaller than the relative angle between the lower front face section and the longitudinal axis of the rotation body. According to a preferred embodiment of the method, the relative angle between the upper front face section and the longitudinal axis of the rotation body is about 0° to about 5° and the relative angle between the lower front face section and the longitudinal axis of the rotation body is about 5° to about 15°. According to a preferred embodiment of the method, the misalignment with regard to the center is about 0.10mm to about 0.20mm.

The invention further provides a piercing element, especially a saw or drill, which can be part of a self-piercing fastening element, which can be the one explained in the present description. The piercing element has a piercing end, the piercing end comprising teeth for drilling a hole, which are substantially circularly arranged around a center of the piercing end, and the wear of the teeth is such that at least 20% of the height of the teeth is worn down in a single drilling process, especially in a material selected from the group comprising glass and carbon fiber thermoset and thermoplastic materials, car body steels (XCG 0/10, P260, SPFC590, SABC1480) and aluminum. Preferably, wear of the teeth is higher than 25% of the height of the teeth. More preferably, wear of the teeth is higher than 30% of the height of the teeth. Most preferably, wear of the teeth is higher than 40% of the height of the teeth. Especially preferable, wear of the teeth is higher than 50% of the height of the teeth. Wear of the teeth can be in the range of approximately 0.6mm to approximately 1.5mm. More preferably, wear of the teeth is in the range of approximately 0.7mm to approximately 1.4mm. Most preferably, wear of the teeth is in the range of approximately 0.8mm to approximately 1.3mm.

In a preferred embodiment, wear of the teeth is considered such that the geometry of the teeth is adapted to the hole to be drilled and/or the material to be drilled such that (i) the drilling diameter is substantially the same during drilling and/or (ii) the cutting angle of the teeth changes during drilling.

The invention further provides a method for drilling a hole into at least one part. Especially, the part can be a part which can be fastened to another, further part. Especially, a fastening element as described above can be used in the method. A piercing end of a rotation body comprising teeth is used for drilling the hole. The piercing end comprises teeth for drilling the hole, which are substantially circularly arranged around a center of the piercing end. The piercing end is rotated and after drilling the hole, wear of the teeth is such that at least 20% of the height of the teeth is worn down.

The explanations with regard to the fastening element, especially explanations with regard to similar terms used, hold for the method or the piercing element as well.

Exemplary embodiments of the invention will now be described in greater detail with reference to the drawings.

It is shown in the drawings:

Fig. 1a, b a piercing end comprising teeth substantially circularly arranged around a center according to the invention;

Fig. 2 a flattened visualization of the teeth according to Fig. 1 ;

Fig. 3a a misalignment of teeth in a flattened, isometric view according to the invention;

Fig. 3b a misalignment of teeth in a flattened, isometric view according to the invention;

Fig. 3c a misalignment of teeth in a schematic drawing; Fig. 4 a perspective view of a self-piercing rivet having a piercing end;

Fig. 5 a perspective view of a self-piercing rivet having a piercing end according to a further embodiment of the rivet; and Fig. 6 the self-piercing rivet of Fig. 5 together with a rod.

Fig. 1a and 1b show by way of simplified illustrations an end of a self-piercing fastening element 1. The fastening element 1 comprises a hollow body 2 which has a piercing end 3. The piercing end 3 comprises teeth 4 for drilling a hole. The hollow body 2 can be driven in a rotational manner around the rotation axis A. The piercing end 3 comprises twelve teeth 4. The teeth 4 are substantially circularly arranged around a center of the piercing end 3. The center is intersected by the rotation axis A which falls together with the longitudinal axis B of the fastening element 1. The teeth 4 are based on a virtual plane visualized in Fig. 1b as a dashed line P. ln Fig. 2 a schematic drawing shows the teeth 4 in more detail. The teeth 4 are flattened for the purpose of better illustration. The teeth 4 have an upper front face section 5 and a lower front face section 6. The upper front face section 5 as well as the lower front face section 6 are formed by a straight line. The upper front face section 5 has a relative angle with regard to the longitudinal axis B of the fastening element 1 which is different to the relative angle between the lower front face 6 and the longitudinal axis B of the fastening element 1.

As can be seen from Fig. 2, the relative angle between the upper front face section 5 and the longitudinal axis B of the fastening element 1 is smaller than the relative angle between the lower front face section 6 and the longitudinal axis B of the fastening element 1. The relative angle between the upper front face section 5 and the longitudinal axis B of the fastening element is about 0° and the relative angle between the lower front face section 6 and the longitudinal axis B of the fastening element 1 is about 10°; the lower front face section has a positive angle with regard to the longitudinal axis B.

In Fig. 3a to Fig. 3c misalignment of the teeth 4 is shown. In Fig. 3a a misalignment is shown schematically such that the teeth 4 extend in a slightly different direction. Two groups of teeth 4 are provided. One group of teeth 4 extends in the same direction. The other group of teeth 4 extends in another direction. The two groups have a different angle with regard to the longitudinal direction B of the fastening element 1. The basis of the two groups of teeth 4 has the same distance to the center. The misalignment can be obtained from a sheet material that forms the hollow body 2 and by bending the teeth 4 in different directions. The top of the teeth 4 has a misalignment with regard to the sheet material (the basis) and the top of the teeth 4 has a different distance to the center. The difference to the center is specified by the sheet material wound around the center and the bending of the teeth 4. The bending is shown schematically in more detail in Fig. 3c, the left hand diagram.

In Fig. 3b a misalignment is shown schematically such that the teeth 4 are offset to each other with regard to the basis of the teeth 4. From the point of manufacture, the teeth 4 shown in Fig. 3b can be obtained by a two-bending process of a sheet material. Two groups of teeth 4 are provided. One group of teeth 4 is bent first in a direction towards the center and the other group is bent in a direction away from the center. Each of the teeth 4 is bent a second time. The second bending is such that the teeth 4 extend substantially in the direction of the longitudinal axis B of the fastening element 1. The two-stage bending leads to a misalignment that the top of the teeth 4 has a misalignment with regard to the sheet material (the basis) and the top of the teeth 4 has a different distance to the center. The difference to the center is specified by the sheet material wound around the center and the bending of the teeth 4. The bending is shown schematically in more detail in Fig. 3c, the right hand diagram. Fig. 3c shows that the misalignment with regard to the center is about 0.15mm in the embodiment shown. Further, the misalignment of the teeth 4 alternates between two adjacent teeth 4. Fig. 4 and 5 show a self-piercing rivet as an example for a self-piercing fastening element 1 according to the invention. The self-piercing rivet comprises a supporting flange 10 and a hollow body 2. The hollow body 2 is tubular shaped and expandable. The body 2 comprises at one end a cylindrical portion 11 adjacent to the flange 10. At the other end, the body 2 comprises a piercing end 3.

In the embodiments shown in Fig. 5 and 6 the piercing end 3 is attached to the hollow body 2.

Further, the embodiments shown in Fig. 4 to 6 show an expandable portion 7. The expandable portion 7 is between the cylindrical portion 11 and the piercing end 3 (Fig. 4) or the end of the hollow body 2 which receives or connects to the piercing end 3 (Fig. 5 and 6).

Fig. 6 shows as an example for the embodiment of Fig. 5 (which holds as well for the embodiment of Fig. 4) a rod 8 which extends axially inside the hollow body 2. The rod 8 can be in engagement with a drive to rotate the hollow body 2 together with the piercing end 3.

Fig. 5 and 6 show an embodiment according to which the piercing end 3 is attached by connecting means 9 which are arranged so as to separate the piercing end 3 from the hollow body 2 during expansion of the expandable portion 7.