Abstract:
There is provided a blind rivet including an axially extending hollow tubular body having a tail end face at one end and a pre-formed radially enlarged flange at the other end, together with a mandrel having a stem which extends co-axially though the body and having enlarged head adjacent the tail end of the body, the stem having a weakened region immediately adjacent the head. There is also provided a method of fastening at least two workpieces using such a blind rivet, including providing the blind side of the workpieces with a countersunk region to accommodate the conically deformed tail end of the body after setting, substantially flush therewith.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. Ser. No. 11/652,962, filed on Jan. 12, 2007, issued as U.S. Pat. No. 7,650,681, which is a divisional of U.S. Ser. No. 10/430,742, filed May 6, 2003, which claims priority to Great Britain Application No. 0210463.6, filed May 8, 2002, all of which are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed towards a blind rivet and more particularly to a blind rivet of the head break type whereby a mandrel head is ejected from the rivet body after setting. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional blind rivets comprise an outer tubular shell or body having an enlarged flange at one end, together with a mandrel associated therewith, such mandrel comprising a cylindrical stem extending through the tubular rivet body so as to be coaxial therewith, the stem having a radially enlarged head at one end for engagement with an end face (tail end) of the rivet body remote from the enlarged flange. The blind rivet is then passed through a preformed hole in a workpiece until the flange engages with the edge of the hole and is held in engagement therewith during a setting operation. During setting, the remote end of the rivet, which is disposed inwardly of the work pieces (the blind side), is then compressed towards the flange by drawing the mandrel stem, and hence the mandrel head, back towards the flange, whereby the deformed portion of the rivet body compresses the work piece therebetween with the flange itself. 
         [0004]    Of this type of blind rivet there are two main types. A break stem type blind rivet relies on the mandrel head entering the tail end of the rivet body and being pulled through the rivet body so as to be encapsulated thereby, until the mandrel head meets resistance as the expanded rivet body engages the blind side of the work pieces, whereby the subsequent increase in load on the mandrel stem causes this stem to break at a predefined weakened region so that the mandrel head is retained within the rivet body to form an expanded portion or bulge of the rivet body on the blind side of the work piece thereby compressing the work pieces between this enlarged region and the flange. 
         [0005]    Alternatively, a break head type blind rivet relies upon the mandrel head being ejected following setting. In this type of blind rivet, the mandrel head has an abrupt change of section between the mandrel stem and the mandrel head to present a shoulder portion which engages the tail end of the blind rivet to exert a substantially axial force thereon thereby compressing and deforming the end of the rivet against the blind side of the work piece to form a flattened bulge between which the work pieces are again compressed against the flange. Here again a weakened region of the stem will again break, but since the mandrel head has not being drawn into the rivet body it is simply ejected away therefrom when the stem breaks due to the high reaction force between the deformed rivet body and the head. 
         [0006]    However, in both these conventional types of blind rivet, a defined or enlarged section of the blind rivet is maintained on the blind side of the work piece presenting a projection which may limit the available workspace on the blind side of the work pieces. This is particularly relevant in the use of blind rivets to manufacture small cabinets, such as those used in the computer industry, whereby the internal projection of a set blind rivet may interfere with the placement of components within such cabinets. 
         [0007]    It is therefore an object of the current invention to provide an improved blind rivet which alleviates the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the present invention there is now provided a blind rivet comprising an axially extending hollow tubular body having a tail end face at one end, and a pre-formed radially enlarged flange at the other end, the rivet further comprising a mandrel having a stem which extends co-axially through the tubular body and having an enlarged head adjacent the tail end of the body, this head having an outer diameter greater than an inner diameter of the body, wherein the mandrel has a uniform conical shoulder extending between the outer diameter of the head and the stem, the stem being further provided with a weakened region immediately adjacent this conical shoulder. Preferably the angle of this conical surface will be inclined at an angle between 91 and 110° to the axis will usually lie between 98 and 102°. Ideally, this angle will be at 100°. The use of such a conical surface to form a conical shoulder at these angles has been found to transmit a force, as the mandrel head is drawn into the rivet body, which has an axial component serving to compress the rivet body and also has a axially inclined component which serves to deform the tail end face of the rivet body conically outwards of its axis. 
         [0009]    It is also preferred that the head diameter of the mandrel head is between 4% and 9% greater than an outer diameter of the tubular rivet body. For this reason, the rivet is intended to be inserted into a preformed hole extending through a workpiece/s which must have a diameter greater than this maximum head diameter and which is usually provided to have a diameter 10% greater than the body diameter. However, the hole diameter may lie in a range of 9% and 15% greater than the body diameter, dependent on the head diameter. This increased diameter of the head alleviates the possibility of the head being drawn into the rivet body during setting. 
         [0010]    Furthermore, it is preferred that the mandrel head has an outer cylindrical diameter, usually co-axial with the rivet body, having an axial length of at least 1 mm, again to alleviate the possibility of the conically deformed body encompassing the mandrel head during the setting operation and, further, to prevent the head from becoming distorted (or “dished”) under the large loads applied thereto during a setting operation. 
         [0011]    It is also preferred that the axial length of the rivet body will be between 25% and 55% greater than the grip thickness of such rivet, the grip thickness being defined as the combined thickness of the work pieces being joined together by this rivet and hence the thickness to be gripped thereby. This predefined rivet body length ensures that there is sufficient rivet body material to form an appropriate rivet joint and to flow into the preformed hole in the workpieces without resulting in excess rivet body material projecting excessively from the blind side of the connected work pieces once set. 
         [0012]    Further according to the present invention there is also provided a method of fastening together at least two work pieces using a blind rivet, which method comprises the steps of selecting an axially extending blind rivet having a body with a length between 25% and 55% greater than the combined thickness of the at least two work pieces, the body also having a known outer diameter, then, pre-forming a hole through the workpieces, which hole having a diameter between 9% and 15% greater than the outer diameter of the rivet body, and subsequently inserting the blind rivet through this hole; this method then provides the step of applying an axial setting force to the rivet body by use of a mandrel head having a conical shoulder, whereby this setting force has a first axial component which is exerted in the axial direction to compress the rivet body into the hole and a second annular component to deform the rivet body conically outwards of its axis. Furthermore, it is preferred that the hole forming step comprises providing the blind side of the hole with a counter-sunk region so that the conical deformation of the rivet body is deformed into the counter-sunk region of the hole, so as not to project substantially proud of the blind side of the workpiece. 
         [0013]    Preferably, this method will comprise the selecting and inserting of a blind rivet according to the present invention and as described above. 
         [0014]    The preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying, illustrative drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a partial cross-section of a blind rivet according to the present invention inserted through a workpiece; and 
           [0016]      FIG. 2  is a cross-sectional view of the set rivet body of the rivet of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring now to  FIG. 1 , a blind rivet assembly  10  is shown inserted, but not set, in a preformed hole  12  extending between two work pieces  14  and  16 . The rivet assembly  10  comprises a hollow tubular rivet body  18  having an enlarged counter-sunk flange  20 , this counter-sunk head tapering inwards along an axis A of the rivet body for complimentary receipt within a pre-formed counter-sunk region  44  of the hole  12 . 
         [0018]    The remote tail end  22  of the rivet body  18 , axially opposed to the flange  20 , has a substantially flat end face extending perpendicular to the rivet axis A. The rivet body length L is predetermined to be between 25% and 55% greater than the combined depth D of the two work pieces  14  and  16 . This depth D is commonly referred to as the grip thickness of a blind rivet—defined as the thickness of material to be gripped by the set rivet. In the present embodiment, depth L is approximately 30% greater than depth D. As is conventional for blind rivets, the assembly  10  is further provided with a mandrel  30  comprising a cylindrical mandrel stem having an enlarged head  34  at its blind end thereof which lies adjacent to the tail end  22  of the rivet body  18 . The mandrel stem  32  is of substantially uniform cross-section area along its axial length and which diameter is substantially equal to the internal diameter of the tubular rivet body  18  so as to form a complimentary frictional fit therewith. 
         [0019]    The mandrel head  34  has a maximum outer diameter defined by a radial wall  36  extending coaxial about the axis A and having an axial length of at least 1 mm. This minimum axial length of the mandrel head is required to provide appropriate strength to the head to alleviate the possibility of the mandrel head distorting, or “dishing” when it is subject to large stresses during the rivet setting operation and particularly when the circumference of the head is engaged with the rivet body to effect deformation thereof (as described below). 
         [0020]    The diameter of this wall  36  is greater than the maximum outer diameter of the rivet body so as to be at least 4% greater than the body diameter, but no greater than 9% thereof. 
         [0021]    The shoulder  38  of the mandrel head, extending between the outer wall  36  and the mandrel stem, is axially inclined to form a substantially uniform conical surface which has a preferred angle of inclination, relative to the axis A, of 100°, although this angle of conical angular inclination can lie between the ranges of 91 and 110° to remain within acceptable operational parameters. This angular range has been determined to be appropriate for effecting the required deformation of the rivet during setting. 
         [0022]    The remote end of the mandrel head is also provide with a conical surface about axis A to act as an alignment mechanism for the assembly  10 , whereby the inclined surfaces thereof help align the assembly with the hole  12  as it is being inserted therein. 
         [0023]    The mandrel head diameter is required to be at least 4% greater than the outer diameter of the rivet body to alleviate this head being pulled into the body during the setting operation, which could prevent the head being ejected after setting. However, this diameter must not exceed 9% greater than the rivet body otherwise too large a hole will be required through the workpieces to allow the rivet to pass therethrough, making the filling of this hole difficult during setting (possibly leaving the set rivet loosely set) or the closer the hole diameter is to the diameter of the flange the risk of failure of the set rivet increases. 
         [0024]    Furthermore, the mandrel stem  32  is provided with a preformed weakened region  42  immediately adjacent this conical surface  38 . As is conventional for blind rivets, this weakened region  42  is formed as a narrowed portion of the stem resulting from squaring off the stem diameter to provide four flats about the diameter so as to present a substantially square cross-sectional area immediately adjacent the head being thinner, and hence weaker, than the main mandrel stem. Alternatively, the weakened portion could result from two or more diametrically opposed indentations or even a circumferential groove. What is important here is that such weakened portion be immediately adjacent the mandrel head. 
         [0025]    Since the mandrel head  34  has a diameter greater than the rivet body  18 , it is essential that the preformed hole  12  be of a diameter at least equal to that of the maximum mandrel head diameter and is usually formed to be 10% greater than the maximum diameter of the body (since the maximum diameter of the head is limited to 9% greater than the maximum diameter of the body). However, the hole  12  can be formed to have a diameter of between 9% and 15% greater diameter than that of the maximum body diameter, dependent on the exact diameter ratio of the mandrel head to rivet body. Usually the blind rivet system utilising a rivet according to the present invention will be compatible with standard hole sizes made with existing standard size drill bits (i.e. the rivet body diameter being such that the holes are 10% greater in diameter) although different size rivets could also be produced but requiring specific hole forming tools (drill bit) which will be of appropriate diameter to use with such rivets. 
         [0026]    Furthermore, the preformed hole  12 , whilst having a counter-sunk region  44  to accommodate the counter-sunk head  20  of the rivet body, will also have formed on the blind side region  21  of the workpieces (i.e. the inner side or that side opposite to the setting tool) a conical counter-sunk region  46  extending about the periphery of the hole. 
         [0027]    Once the rivet assembly  10  has been inserted through this preformed hole  12  it may be set using a conventional blind rivet setting tool (not shown) which comprises a nose portion for restraining the rivet body flange (or head)  20  in solid abutment with the work piece  16 , whilst having a displaceable jaw mechanism for clamping the mandrel stem  32  and exerting a setting force F in the direction shown substantially by the arrow F in  FIG. 1 . 
         [0028]    The application of the setting force F draws the mandrel head into engagement with the flat surface of tail end portion  22  of the rivet body which, due to its conical shoulder  38 , exerts a setting force having a first component extending in an axial direction and which serves to compress the rivet body  18  towards the rivet flange  20 , resulting in radial expansion of the compressed (reduced length) rivet body so as to fill the enlarged hole  12  (this radial expansion being outwardly, due to the mandrel stem being maintained through the body  18 ). The conical shoulder  38  also provides a secondary force component perpendicular to the conical surface of the such  38  which serves to deform the tail end  22  of the rivet body radially and conically outwards so that the mandrel head  34  does not substantially enter this shortened rivet body which, due to the axial length of the walls  36  (and the restricted length of the rivet body) is also unable to encompass and surround the mandrel head  34 . Rather, some of the material of tail end  22  of the rivet  18  is partially displaced axially towards the rivet head  20  with the remaining material of the rivet body in this tail end region forming a considerably thinned conical outer wall ( FIG. 2 ) which is displaced into the counter-sunk region  46  of the hole  12  so as not to project externally of the secured workpieces. Thus the combination of the conical shoulder  38  of the mandrel and the countersunk region  46  serve to provide such conical spreading of the tail end of the body. 
         [0029]    Continued axial displacement of the mandrel in direction F results in an increased resistance force encountered by the mandrel head  34  as it encounters the workpiece  14 , and which subsequently prevents further displacement of the mandrel head. Continued application of the setting force F the increases the stress on the mandrel stem in a conventional manner until resulting failure of the mandrel stem  32  at the weakened region  42 . The resultant re-action force between the rivet head  34  and the rivet body  18  causing ejection of the rivet head  34  therefrom. 
         [0030]    The resultant set blind rivet is shown in  FIG. 2  having a compressed axial length L 2 , substantially equal to the grip thickness D, whereby the axially displaced rivet body material has been squeezed into the enlarged hole  12  to provide a rivet body having an increased body thickness  18 ′ in this region and which has a substantially conical, cup-shaped tail end  22 ′ formed into the counter-sunk region  46  of the preformed hole  12 . In this manner, a substantially rigid rivet joint is formed which does not extend substantially proud of the blind side of the work piece  14 . 
         [0031]    Furthermore, the angular setting range of the conical shoulder has been determined so that the appropriate deformation forces can be applied to compress and conically deform the rivet body in the manner described above. If the angle of this conical surface was greater than 110 degrees then there would be a risk that insufficient axial force would be applied during setting and that the head could be pulled into the rivet body. If this angle was less than 90 degrees then there would be no angular force component to effect conical deformation of the tail end of the rivet and the mandrel head is likely to be ejected without the rivet being correctly set. 
         [0032]    In this preferred embodiment, the rivet body head  20  is shown to be counter-sunk so that this head  20  is also maintained flush with the outer work piece  16  once set, although this feature is optional and it will be appreciated that the current invention may utilize a conventional blind rivet which does not use a counter-sunk head. 
         [0033]    The importance of providing the length L within the predefined ranges discussed above is to provide sufficient material of the rivet body to allow appropriate deformation of the rivet body to fill the enlarged hole  12  after setting and to form a sufficiently robust set-end of the rivet that will resist any applied tensile loading on that joint, but not to result in excess rivet body material which cannot be accommodated by the counter-sunk region  46  of the hole  12  and may project beyond the blind side of the workpiece. 
         [0034]    While the above detailed description describes the preferred embodiment of the present invention, it should be understood that the present invention is susceptible to modification, variation and alteration without deviating from the scope and fair meaning of the subjoined claims.