Blind rivet
This application is a continuation of U.S. Ser. No. 570 411, filed Jan. 12, 1984, which is a continuation of U.S. Ser. No. 465 420, filed Feb. 10, 1983, which is a continuation of U.S. Ser. No. 154 687, filed May 30, 1980, all now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a blind rivet as described in the and, more particularly, to a blind rivet for use with grips of different thicknesses which includes a body with a head, a shank, a bore therethrough, and a section of reduced wall thickness at a location spaced from the free end of the shank, and includes a drawing mandrel having a shank receivable in the body bore and having a head at one end of the shank which has a maximum diameter substantially corresponding to the outside diameter of the body shank. 
     BACKGROUND OF THE INVENTION 
     A blind rivet of this kind can be used not only for the grips which normally correspond to its shank length but also for smaller gripping thicknesses. The head of the drawing mandrel upsets the shank during the riveting operation, during which process the shank zone which projects from the parts to be connected and has a reduced wall thickness is deflected and forms a collar-shaped bead between the material surface and the closing head. Due to their wide range of use, blind rivets of this kind make unnecessary the production, acquisition and maintenance of stocks of a large selection of blind rivet sizes for all the gripping thicknesses which are required. Also, the risk of any confusion of the sizes occurring is considerably reduced. 
     A blind rivet of this kind which is known from obvious prior use consists of aluminium. Its shank has, approximately in the centre of its length, a zone of a reduced outside diameter and thus a reduced wall thickness. During its production, the shank of the blind rivet is pressed in its central zone between gripping dies. During this process, the aluminium rivet material is deflected on both sides of the die into the ungripped shank zones. This known blind rivet thus necessitates an elaborate production. Above all, the described deformation can only be brought about in aluminium rivets. It is not possible in steel rivets. 
     There is furthermore known a blind rivet (DE AS 1 099 276) which has sections of varying wall thickness. In the shank, which is cylindrical on the outside, there extends a cylindrical bore from the set head to approximately two thirds of the shank length. The remainder of the shank bore tapers conically towards the free shank end, as a result of which the wall thickness is also conically increased in this section. The drawing mandrel for this blind rivet has a half-round head whose diameter on its underside is between the shank diameter and the minimum hold diameter of the rivet. The edge of the head forms a cupped gripping point which is directed towards the shank end of the rivet. The drawing mandrel portion that directly adjoins the head is formed as a cone which increases away from the head. For the production of the riveted connection, this known rivet is deformed in two stages: At the start of the riveting operation, the conical zone of the drawing mandrel shank is outside the rivet shank bore. As soon as the thick end of this section is pulled into the rivet bore, it expands the section having the conical rivet shank bore in that it presses the shank wall to the outside in this section. When the head of the drawing mandrel comes into abutting contact with the rivet shank, the rivet shank has a bore which continues cylindrically at the same diameter and a section which conically widens on the outside towards the free rivet shank end. The second deformation is substantially effected in that the head of the drawing mandrel divides, with its cupped gripping point, the free shank end of the rivet shank into an external zone and an internal zone; the external zone, which is not covered by the drawing mandrel head, being pressed outwardly during this further pull-through for the closing head formation. The internal zone is pressed by the drawing mandrel head into the hollow space which is provided between the conical drawing mandrel section and the shank bore of the rivet. As the drawing mandrel is pulled through, the external zone of the rivet shank end flows around the head of the latter and largely envelops it. Following the breaking-away of the drawing mandrel at a predetermined breaking point, which is close to the thick end of the conical shank section that is close to the head, there comes about a largely tight riveted connection with a fixed drawing mandrel head and head shank. This known blind rivet has nothing in common with the species of the subject matter of the application since it is not intended for grips of varying thickness and the design of the drawing mandrel is different. The ratio of the drawing mandrel head diameter to the diameter of the rivet shank is in the case of the known blind rivet essential for the solution to the problem formulated in connection with this rivet, which solution is achieved by the described deformation of the blind rivet shank. An upsetting effect which with smaller gripping thicknesses might lead to the formation of the collar-shaped bead required therefor cannot be brought about in the case of the known blind rivet during the riveting operation. 
     Furthermore known is a blind rivet with sections of varying wall thickness (CB PS 860 123), wherein the shank bore has at the free shank end a section of reduced diameter and the shank has in this zone an outside diameter which conically increases towards the free end. This blind rivet is set by means of a pull-through mandrel which does not stay in the riveted joint. The pull-through mandrel has a shank with a conically widened end. As it passes through the rivet shank bore, the conical end of the pull-through mandrel presses the entire projecting zone of the blind rivet shank to the outside and, due to the external conical shape of this zone, brings it into gripping contact with the material to be connected. After the pull-through of the entire mandrel, the riveted connection has a bore of a continuous identical inside diameter. The pull-through mandrel of this configuration cannot exercise any upsetting effect at all. A hollow rivet connection only comes about if the shank length and the shank shape are exactly adapted to the intended gripping thickness. Should a closed riveted connection be desired, then a cylindrical pin has to be driven into the hollow rivet by hand. For the mentioned reasons, this rivet is not suitable for varying grips. 
     It is the object of the invention to provide a blind rivet of the above-mentioned type which can be easily produced from any material, especially steel, and which can nevertheless be used for a wide gripping range. 
     SUMMARY OF THE INVENTION 
     According to the invention, this problem is solved by providing a rivet of the above-mentioned type wherein the section of reduced wall thickness is formed by a step increase in the diameter of the bore in the body shank. 
     During the setting of the blind rivet according to the invention, the projecting shank zone is deformed by upsetting. The head of the drawing mandrel, which covers the entire free end surface of the blind rivet shank, exerts this effect, which leads to the formation of the closing head when used in the grip that corresponds to the shank length and results in a bead formation in a smaller grip. Compared to the aluminium rivet described at the beginning, there is provided, in addition to the possibility of using any desired rivet material and a simpler production, the further advantage that the outside shank diameter can remain the same throughout. In the case of rivets used in the maximum gripping range, the shank thus bears everywhere uniformly against the wall of the rivet hole. In the smaller gripping range, the radial movement of the material already starts on the outside diameter of the rivet during the upsetting of the rivet, whereas in the case of the known aluminium rivet the first portion of the radial path serves for first reaching the normal shank outside diameter and thus the diameter of the rivet bore. The stepped transition of the diameter increase results in the material which advances faster in the zone of the step urging the zone having the larger bore diameter to the outside and thus determining the direction of bulging. As the upsetting process progresses, the head of the drawing mandrel then gradually penetrates between the bulged-out shank walls. The afore-described blind rivets with differences in diameter in the shank bore are deformed by the expanding of the bore and not by upsetting during the riveting operation and are not usable for varying gripping thicknesses and thus cannot contribute to the solution of the problems with which the invention is concerned. 
     Advantageously, the bore section having a reduced wall thickness has a bore diameter which somewhat exceeds the diameter of the drawing mandrel. In this zone, it is possible for rivet material to penetrate during the penetration of the drawing mandrel head into the upset shank zone. 
     In a preferred constructional form, the underside of the drawing mandrel head is flat and thus bears with its entire surface against the end of the blind rivet shank. 
     In another constructional form, the underside of the drawing mandrel head may be bevelled so as to rise from its external edge zone towards the drawing mandrel shank. In this form, too, there comes about the axial upsetting effect which is material to the invention. 
     The actions of the longitudinal bore on both sides of the step may extend cylindically and thus be produceable by simple drilling. 
     In another constructional form, the longitudinal bore widens conically from the step to the opening in the set head. The wall thicknesses are thus the thinner the smaller the gripping thickness is, so that the minimum wall thickness of the shank portion that projects from the grip is directly above the contact face of the collar-shaped bead that is being formed. Good bulging and thus reliable overlapping and contact-making are ensured in this way. 
     An advantageous further development of the blind rivet according to the invention consists in that the drawing mandrel has an approximately spherical-cap-shaped head with at least three plane-surface bevels which extend so as to be inwardly inclined to the top, Due to the bevels, the head of the drawing mandrel is pulled particularly easily into the deforming rivet shank and is satisfactorily overlapped and held by the rivet material. A firm grip of the drawing mandrel head can thus be brought about even when hollow steel rivets are used. 
     In a preferred constructional form, the bevels may extend as far as the lower head surface in such a way that the circumference thereof runs in a straight line in the zone of each bevel. The head has a contour which, depending on the number of bevels, is reminiscent of a triangle or polygon. This results in a particularly effective overlapping of the rivet shank material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some exemplified embodiments of the invention will be described with reference to the accompanying drawings, in which: 
     FIG. 1 shows a blind rivet, including the drawing mandrel, in a section, 
     FIG. 2 shows a different constructional form of a blind rivet without a drawing mandrel in a section, 
     FIG. 3 shows a riveted connection in a longitudinal section, 
     FIG. 4 shows another riveted connection in a longitudinal section, 
     FIG. 5 shows an elevation of a drawing mandrel, and 
     FIG. 6 shows a top view of the drawing mandrel. 
    
    
     DETAILED DESCRIPTION 
     The blind rivet shown in FIG. 1 has a spherical-cap-shaped set head 1 and a shank 2. A longitudinal bore, which is designated 3 as a whole, extends through the entire blind rivet. It is composed of a cylindrical section 3a having a diameter d 1  and a cylindrical section 3b having a diameter d 2  ; the diameter d 2  being a few tenths of a millimeter larger than the diameter d 1 . The section 3a extends from the end of the shank 2 that is remote from the head to approximately mid-way of the shank length 1, and the section 3b extends subsequently to the surface of the set head 1. The transition from the section 3a to the section 3b occurs abruptly along with the formation of a step 4. More specifically, as shown in FIG. 1, the transition 4 is a steplike increase in the bore diameter which defines a generally axially facing shoulder 4a, the radially inner and outer edges of the shoulder 4a being defined by sharp, distinct corners 4b and 4c. 
     The rivet shown preferably consists of steel. The dimensions given hereunder are meant to give an approximate picture of the measurement conditions: 
     
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Shaft length    1        12        mm                                     
Shaft outside diameter                                                    
                D        4.75      mm                                     
Length of section 3a                                                      
                1.sub.1  6         mm                                     
Bore diameter of                                                          
                d.sub.1  2.8       mm                                     
section 3a                                                                
Bore diameter of                                                          
                d.sub.2  3.5       mm                                     
section 3b                                                                
Possible gripping                                                         
                s        from 1 to 8                                      
                                   mm                                     
thicknesses                                                               
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     A drawing mandrel, which is designated 5 as a whole, is inserted in the blind rivet. it has an approximately spherical-cap-shaped head 5a with a flat underside or bottom surface which directly contacts the end surface of the shank 2. The diameter of the bottom surface thereof is approximately equal to the diameter D of the shank outside diameter. The drawing mandrel shank has a diameter d 3  which differs from the daimeter d 1  of the smaller shank bore only to such an extent that the drawing mandrel is introduceable into the blind rivet shank without difficulty. The drawing mandrel shank 5b has a predetermined breaking point 6. 
     FIG. 2 shows a longitudinal section through another constructional form of a blind rivet with a set head 1&#39;, a shank 2&#39; and longitudinal bore 3&#39;. The longitudinal bore 3&#39; comprises a cylindrical section 3a&#39; which extends with a diameter d 1  from the shank end that is remote from the head to approximately the longitudinal centre thereof. There follows, along with the formation of a step 4&#39;, a section 3b&#39; with a diameter d 2  which widens conically from the step 4&#39; to the surface of the set head 1&#39; up to an opening diameter d 3 . The rivet shown in FIG. 2 also consists of steel. 
     FIGS. 3 and 4 show in longitudinal sections a riveted joint produced with a blind rivet shown in FIG. 1 with grips of different thickness. 
     FIG. 3 is the example for a minimal gripping thickness s 1  ; FIG. 4 is the sample for a larger gripping thickness s 2 . The formation of the closing head 7 is effected by using the drawing mandrel 5, including the drawing mandrel head 5a, the shank 5b breaking at the predetermined breaking point 6. 
     In the case of a riveted joint with the minimal gripping thickness s 1 , the shank of the multi-range rivet shown in FIG. 1 projects farther from the rivet bore than is necessary for the formation of a closing head. As soon as the upsetting operation commences through the pull on the drawing mandrel 5, the shank zone with the bore section 3b, in other words with the smaller wall thickness, is deflected to the outside. The subsequently forming closing head 7 and the head 5a of the drawing mandrel 5 which penetrates deeper into the bore presses the bulging shank zone as a collar-shaped bead 8 on the surface of the material to be riveted and thus brings about the firm fit of the rivet. FIG. 3 shows the completed connection after the breaking-away of the drawing mandrel shank 5b. The &#34;folding&#34; of the shank material in the zone of the bead 8 is indicated at 8a. 
     FIG. 4 shows a riveted connection wherein the grip has the maximum thickness s 2  that corresponds to the shank length 1. The rivet shank zone of reduced wall thickness, in other words the entire bore section 3b of increased diameter, is located within the rivet bore. During the riveting operation, there has formed a normal closing head 7&#39;. Within the scope of the play between the shank diameter and the rivet bore, the zone with the reduced wall thickness has come into particularly good abutting contact with the wall of the rivet hole, due to the slight concavity. 
     The drawing mandrels 5 shown in FIGS. 5 and 6 each have three bevels 10 at their heads 5a. In the case of the drawing mandrel shown in FIG. 5, the bevels extend over the underside of the head 5a that is directed towards the shank so as to be inclined inwardly towards the surface of the head. In the case of the head shown in FIG. 6 in a top view, the bevels 10 extend as far as the lower head surface in such a way that the surface equals a circular surface having cut-off segments, that is to say the circumference extends in a straight line in the zone of each bevel. As the drawing mandrel head penetrates into the rivet bore, the bevels make it possible for the rivet material to overlap the head in the zone of the bevels in a particularly effective manner and thus reliably to hold the head. 
     As shown in broken lines in FIG. 1, the underside of the drawing mandrel head may be beveled slightly. 
     The invention is not confined to the exemplified embodiments shown. For example, the longitudinal bore of the shank may have three sections having different diameters which increase towards the set head. Furthermore, the transition may occur in the form of a short bevel instead of a radial step. Depending on the hollow rivet material used, the length of the bore sections may vary. Of course, the set head, too, may differ in shape, as required. The same applies to the head of the drawing mandrel. Its base surface need not be flat but may extend inwardly in a roof-like manner. Its external edge need not form a circular surface, even without bevels, but may, for example, be a polygon. What is essential is that the largest diameters of the lower edge should correspond to the outside diameter of the rivet shank so as to ensure that the drawing mandrel head brings about an axial upsetting effect during the riveting operation. If necessary, the outside diameter of the drawing mandrel head may be somewhat larger than the outside diamter of the rivet shank. A very minimal undersize is also conceivable.