Abstract:
The invention relates to a self-drilling fastening element ( 10 ), comprising a rotationally symmetrical drilling tip ( 16 ) and a shaft ( 14 ), wherein the cross-section of the drilling tip ( 16 ) is bounded by a first radius (R 2 ), the origin ( 22 ) of which lies outside the rotational axis ( 18 ) of the drilling tip ( 16 ) on the side opposite the rotational axis ( 18 ), near the drilling tip ( 16 ), and a second radius (RI) near the shaft ( 14 ), wherein the origin ( 26 ) of the second radius (RI) is farther from the rotational axis ( 18 ) than the origin ( 22 ) of the first radius (R 2 ) and likewise lies on the opposite side of the rotational axis ( 18 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a self-drilling fastening element, for example a drilling screw or a self-drilling blind rivet, comprising a rotationally symmetrical drilling tip and a shank. 
         [0002]    Self-drilling fastening elements of this type are increasingly in demand, since the expensive pilot drilling and thread-cutting can be dispensed with. 
       PRIOR ART 
       [0003]    Originally, cutting drilling tips were frequently used in this context. However, a drawback of this is the resulting shavings which have to be removed. By contrast, in the meantime non-cutting methods have become established, in which the material in which the fastening element is to be placed is heated—by a suitably formed tip, which is pressed at a high rotational speed against the workpiece in which the fastening element is to be placed—and thus becomes plastic. The fastening element is subsequently screwed into the workpiece which has been softened in this manner. Advantageously, not only does this not produce any shavings, but in addition a tube, i.e. a material bank, is produced and can be used to increase the length of the supporting thread after the fastening element has been placed. 
         [0004]    Corresponding fastening elements according to the prior art had conical (for example DE 10 2006 034 585), flattened and hollow (DE 10 2006 034 583), or spherical (for example 10 2006 034 584) tips. Recently, spherical tips in particular have been preferred. The purpose of the spherical shape was to provide the best possible friction radius and thus achieve intense heating of the workpiece. 
         [0005]    Prior art self-drilling fastening elements conventionally have a relatively small radius at the tip, in such a way that the tip is spherical in form. This small, spherical contact surface heats the material. An extending, conical hole-forming part having a relatively sharp angle is attached to this tip. 
         [0006]    It has been found that prior art tips have to be pressed with a particular force so as to generate sufficient friction, at a sufficiently high rotational speed, for the material to start flowing. Further, prior art tips do not provide optimum tube formation. 
       DESCRIPTION OF THE INVENTION 
       [0007]    Starting from this prior art, the object of the present invention is therefore to provide a drilling tip for a self-drilling fastening element which minimises the shaping time and simultaneously improves the performance of the connection, in particular by way of optimum tube formation. According to the invention, the self-drilling fastening elements should be manufactured as economically as possible, even in the case of high sheet metal thicknesses or when they are to be placed in two unholed metal sheets which are positioned one on top of the other (in particular in the case of self-drilling blind rivets). 
         [0008]    According to the invention, this object is achieved in that the cross-section of the drilling tip is bounded by a first radius close to the tip and a second radius close to the shank, the origin of the second radius being further than the origin of the first radius from the axis of rotation. 
         [0009]    It is particularly preferred for the two radii to transition tangentially into one another. 
         [0010]    Further, it is particularly preferred for the second radius to transition into the shank via a conically extending portion. This conically extending portion makes optimum tube formation possible. 
         [0011]    In this case, the conical portion preferably has an angle α of 28° to 40°, preferably 32°, between the flanks thereof. 
         [0012]    Further, in this case it is particularly preferred for the second radius to transition into the shank via a concavely curved portion having a radius of which the center point is positioned on the same side of the axis of rotation. 
         [0013]    This improves the tube formation even more. 
         [0014]    Further, it has been found to be highly advantageous for at least the portion between the second radius and the shank to be provided with grooves which circulate radially perpendicular to the axis of rotation. 
         [0015]    In this case the grooves preferably have a semi-circular cross-section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention is explained in greater detail in the following by way of the appended drawings, in which: 
           [0017]      FIG. 1  is a side view of a self-drilling screw having a drilling tip according to the invention; 
           [0018]      FIG. 2  is the section A-A from  FIG. 1 ; 
           [0019]      FIG. 3  is the detail of the tip from  FIG. 2 ; 
           [0020]      FIG. 4  is a side view of a self-drilling screw having a further drilling tip according to the invention having encircling grooves; 
           [0021]      FIG. 5  is a side view of a self-drilling screw having yet another drilling tip according to the invention having a concavely curved portion; and 
           [0022]      FIG. 6  is a side view of a self-drilling screw having a further drilling tip according to the invention having a concavely curved portion and radially encircling grooves. 
       
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       [0023]      FIG. 1  shows a self-drilling screw according to the invention, which is adapted for flowing hole drilling. The screw  10  consists of a head  12  and a shank  14 , which is provided with a self-cutting external thread and ends in a tip  16 . 
         [0024]    The screw  10  is formed with rotational symmetry about an axis of rotation  18 . 
         [0025]    The tip  16  comprises a first region  20 , in which the section along the axis A-A through the tip  16  has an outer contour which follows a radius R 2 , the center point of which is positioned laterally outside the axis of rotation  18  on the opposite side of the axis of rotation  18 . 
         [0026]    This region  20 , in which the cross-section of the tip  16  is bounded by the radius R 2 , is adjoined towards the shank  14  by a further region  24 , in which the cross-section of the tip  16  is externally bounded by a further, larger radius R 1 , having an origin  26  which is also positioned on the opposite side of the axis of rotation  18  but is much further away from the axis of rotation. 
         [0027]    This second region of the tip  16  is adjoined towards the shank  14  by a conical portion  28 , which subsequently transitions with a slight radius into the shank  14 . 
         [0028]    In this case, the conical portion  28  of the tip  16  has an angle of 28-40°, preferably 32′, between the lines externally bounding the cross-section thereof. 
         [0029]    In the embodiment shown, the screw head  12  has a hexalobular external driving feature  30 . Naturally, any other conventional driving features may similarly be used. 
         [0030]    A tip configured according to the invention can equally be used for self-drilling blind rivets which use flowing hole drilling and for any other self-drilling fastening elements. 
         [0031]      FIG. 2  is a sectional view along the section line A-A (the axis of rotation  18 ). 
         [0032]    In this case an M 5  screw is selected as an example of the dimensioning. 
         [0033]    The tip  16  thereof is shown in detail in  FIG. 3 . 
         [0034]      FIG. 3  clearly shows that the outer contour of the tip  16 , shown in section in this figure, actually has a proper point rather than a spherical shape at the end remote from the shank. This point results from the meeting of the two radii R 2 , the origin of which is positioned outside the axis of rotation  18  on the respectively opposite side. 
         [0035]    The following is an example of the dimensioning for an M5 screw: 
         [0036]    The first radius R 2  is 1.5 mm and the origin  22  thereof is 1.3 mm away from the tip towards the shank  14  and 0.7 mm away from the axis of rotation  18 . The adjoining radius R 1  is 10.8 mm and the origin thereof is 5.7 mm away from the tip and 8.9 mm away from the axis of rotation. This radius subsequently transitions into a conical portion  28 , the flanks of which form an angle of 32°. 
         [0037]      FIG. 4  shows a further embodiment according to the invention of a self-drilling screw. 
         [0038]    The head and shank correspond to the screw according to  FIG. 1 , and the drilling tip is also provided with the two radii R 1  and R 2  and the conical portion  28 . 
         [0039]    However, the entire conical portion  28  and part of the portion curved with radius R 1  are provided with radially extending grooves  32  arranged horizontally with respect to the axis of rotation  18 . These grooves have a semi-circular cross-section. 
         [0040]    One the one hand, the grooves  32  further improve the tube formation and reduce the material bank on the penetration side of the self-drilling screw, and further, the increased friction means that even if the screw has a low rotational speed during drilling, sufficient heat energy is available to plasticise the workpiece material. 
         [0041]      FIG. 5  shows a further embodiment according to the invention of the self-drilling screw. In this case too, the screw head  12  and the screw shank  14  are formed as in  FIG. 1 . 
         [0042]    However, the drilling tip is modified in such a way that instead of the conical portion  28 , a concavely curved portion  128  is arranged between the shank  14  and the second radius R 1 . This concave configuration means that the tube formation is further optimised and, most importantly, prevents plasticised material from being thrown up counter to the penetration direction of the self-drilling screw. Because according to the invention barely any material is thrown up on the penetration side of the screw, more material is available for the tube formation, and the screw can also be placed in correspondingly thinner material. 
         [0043]    Finally,  FIG. 6  shows a combination according to the invention of the features of  FIGS. 4 and 5 , a concavely curved portion  128 , having a radius R 3  of which the center point is positioned on the same side of the axis of rotation, being provided between the second radius R 1  and the shank  14 , and this portion  128  and part of the region of the second radius R 1  being provided with radially extending grooves  32 , which are arranged horizontally with respect to the axis of rotation  18  and have a semi-circular cross-section. 
         [0044]    This combination of features according to the invention provides a minimum material bank on the penetration side as well as optimum tube formation and high plasticisation even at relatively low rotational speeds, by way of the increased friction due to the grooves  32 . 
         [0045]    The embodiment according to the invention has the major advantage that this new shape of the tip or cap minimizes the shaping time and improves the force transfer through the connection. In this way, a fastening element according to the invention can be placed even in thick metal sheets or in two unholed metal sheets which are positioned one on top of the other much more economically. This is of particular relevance for use with self-drilling blind rivets. 
         [0046]    The configuration according to the invention of the tip  16  provides rapid shaping into thick metal sheets and rapid heating on the point where the fastening element is placed. Offsetting the origins of the shaping radii R 1 , R 2  from the axis of rotation  18  of the fastening element  10  provides an absolute point. The adjoining frictional cone, formed by the radii R 2  and R 1  and by the conical portion  28 , optimizes the tube formation. This is particularly relevant for self-drilling screws. 
         [0047]    By comparison with the prior art, the present invention also has the advantage that particularly rapid formation is provided even at high material thicknesses of the workpiece in which the fastening element  10  is placed. The frictional cone  28 , in connection with the two portions  20  and  24 , thus provides good heating and particularly pronounced tube formation on the underside of the metal sheet. 
         [0048]    This frictional cone which is specially configured according to the invention promotes the tube formation. This results in a larger number of supporting thread turns on the counter support when the present invention is applied to a screw. 
         [0049]    According to the prior art, a relatively small radius was provided at the tip. This small, spherical contact surface was used to heat the material of the workpiece in which the fastening element was to be placed. 
         [0050]    According to the invention, the material of the workpiece is heated not by the tip formation, but instead by the following radii R 2 , R 1  which are offset from the centre, transition tangentially into one another, and have an adjoining conical shaped part  28 . In the specific combination thereof according to the invention, this shaping provides a large contact surface, correspondingly rapid heating of the material, and good tube formation. 
         [0051]    Thus, according to the invention the shaping time is reduced and at the same time the number of supporting thread turns is increased. The present invention can therefore be used even with very high sheet metal thicknesses and two or more unholed metal sheets.