Abstract:
The present invention relates to a clinch joiner ( 1 ) for clinch joining ductile layers of material ( 7,8 ), such as metal sheets, and in particular to and a joiner including a die ( 4 ) and a punch ( 2 ). The die comprises: a die anvil ( 44 ) with a body portion ( 48 ) and an anvil surface ( 46 ); at least two die blades ( 56 ) that extend transverse to the anvil surface ( 46 ) to form a die aperture ( 57 ), the separation between the blades ( 56 ) defining a die aperture width and the extension of the die blades above the anvil surface ( 46 ) defining a die aperture depth; at least one pivot recess ( 68 ) in the body portion ( 48 ); a protrusion ( 58 ) on each of the die blades ( 56 ) that is seated in a matching pivot recess ( 68 ) to form a pivot joint by which each die blade ( 56 ) may pivot to constrict and dilate the die aperture ( 57 ); at least one biasing member ( 70 ) by which the die blades ( 56 ) are biased to constrict the die aperture ( 57 ); and a die shield ( 14 ) that limits the extent by which the die blades ( 56 ) may pivot to dilate the die aperture ( 57 ). The pivot joints ( 58,68 ) extend underneath the anvil surface ( 46 ) so that when the die aperture ( 57 ) is dilated the depth of the die aperture is decreased.

Description:
BACKGROUND 
     a. Field of the Invention 
     The present invention relates to a joiner for clinch joining ductile materials, such as metal sheets, and in particular to a joiner including a die and a punch assembly. 
     b. Related Art 
     It is known to join a plurality of sheets of ductile material by causing these to be deformed into an interlocking configuration in a local area. Such joins are made by ductile material joining tools comprising a die with an aperture that is opposite a punch assembly comprising a punch surrounded by a stripper mechanism. Layers of ductile material are sandwiched between the punch assembly and when the punch is pressed towards the aperture, material is drawn into the aperture. The material undergoes plastic deformation in the aperture to flow into a shape in which two or more layers are interlocked, for example by the forming of one layer around another layer. 
     The aperture has a base with an anvil having an anvil surface and at least two side walls formed from movable blades. The blades are generally transverse to the anvil surface and extend in the direction in which the die and punch are pressed together. The blades help define the local area, for example a circular, square or rectangular area, in which the deformation of the layers of sheet material takes place. Once the material has been drawn and flows into the aperture, the blades move away from each other in a radial direction as sheet material flows laterally. 
     The outward movement of the blades is constrained by a die shield, which may be separate from but held in a fixed relationship with the die. In order to provide a compact die, the die shield can be joined to or integral with the die. 
     A circular die and punch can be used to form a clinch joint in which sheet material is symmetrically deformed both axially and radially to form a leak-proof button, for example as disclosed in patent document U.S. Pat. No. 5,150,513. A rectangular die and punch can be used to form a trapezoidal clinch joint (also called a lance joint), in which the sheet material is cut through by the punch along a pair of parallel opposed lines, with the layers of sheet material deformed laterally outwards underneath each of the cuts. 
     One commercially available example of a joining tool for forming a circular clinch joint is the SR 504 Series die and punch tool set from Bollhoff Fastenings Limited of Willenhall, West Midlands, UK. This tool set has a nominal 5 mm aperture with four movable die blades constrained by a separate die shield having an inner diameter of 16 mm. 
     A more compact die and punch would be desirable, for example allowing the sheet material joining tool to get further into corners or other awkward locations when fabricating a structure from the sheet material. However, there is a trade off between the strength of the tool and the maximum thickness of sheet material that may be joined, and the overall size, particularly in the direction transverse to the direction in which pressure is applied. 
     The die shield is arranged so that much of the pressure between the die and punch assembly is born by the die shield and stripper mechanism. However, the die blades experience increasing pressure as material is pressed into the aperture. This pressure can result in restriction of the movement of the die blades, resulting in a bad joint and/or damage to the die blades, particularly if the die blades are made thinner to reduce the lateral extent of the die. 
     A further constraint results from the necessity to include in the die some means of biasing the die blade back towards the anvil surface after the drawing operation by the punch is completed. For this, a coil spring or o-ring can be provided extending around the outside of the die blades. As the die blades move outwards to dilate the aperture, the spring or o-ring becomes stretched or compressed. When the joined sheet material is withdrawn from the aperture, the die blades return to their start position owing to the tension or compression in the spring or o-ring. 
     Because the spring or o-ring extends around the outside of the die blades usually between the die blades and the surrounding die shield, lateral space must be provided for the spring or o-ring. This again limits further reduction in the lateral extent of the die. Furthermore, lateral clearance space can result in a die blade being dislodged from between the anvil and die shield, and being lost from the die. This is very inconvenient in a production environment, as any machine using the sheet metal joiner would then have to be stopped to repair or replace the faulty die. If the faulty die were not spotted immediately, a great deal of rework to joined fabrications might then be required. 
     In some dies, the die blades slide laterally outwards, in which case the aperture depth is unchanged as the die blades move. However, the pressure imparted on the die blades can inhibit such a sliding motion, unless a die shield is used. Such a die shield will increase the lateral extent of the die. 
     Sometimes the die blades pivot outwards about a pivot mechanism below the level of the anvil surface. The pivot mechanism has a pivot axis or pivot point below and laterally outside an edge of the anvil surface. Because of this and the requirement that die blades should have a wide base or pivot with sufficient surface area to withstand the pressures during drawing, the die blades tend to rise when pivoted outwards. This tends to increase the pressure on the die blades, and again limits the amount by which the lateral dimensions of the die can be reduced. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems cited above, and provides a die for a ductile material joiner, and also a ductile material joiner for clinch joining two or more layers of ductile material, which addresses these issues. 
     Accordingly, the invention provides a die for a ductile material clinch joiner, comprising: 
     a) a die anvil, the anvil having a body portion and an anvil surface; 
     b) at least two die blades, the blades extending generally transverse above and below the anvil surface and forming with the anvil surface a die aperture for a die punch, the separation between the blades defining a die aperture width or diameter and the extension of the die blades above the anvil surface defining a die aperture depth; 
     c) at least one pivot recess in the body portion; 
     d) a protrusion on each of the die blades, each protrusion being seated in a matching pivot recess to form with the recess a pivot joint by which each die blade may pivot with respect to the anvil body portion in order to constrict and dilate the aperture width or diameter; 
     e) at least one biasing member by which the die blades are biased to constrict the die aperture; and 
     f) a die shield around the die blades; 
     wherein the pivot joints extend underneath the anvil surface so that when the die aperture is dilated the aperture depth is decreased. 
     Also according to the invention, there is provided a ductile material joiner for clinch joining two or more layers of ductile material, comprising a punch tip and a die according to the invention, the die having an aperture matching the punch tip. 
     In a round clinch joint, the punch tip should match the die aperture with clearance that is sufficient so that ductile material is drawn down between the punch tip and die blades prior to compression of the material against the anvil surface and consequent lateral flow of the ductile material. 
     In a lance joint, the punch tip should match the die aperture with a close clearance so that ductile material is cut along the die blades prior to compression of the material against the anvil surface and consequent lateral flow of the ductile material under the cut. 
     The provision of the pivot joint underneath the anvil surface can be used to reduce the lateral extent of the die blade. This is because the joint then extends laterally further inwards, for example towards a central longitudinal axis of symmetry of the die. 
     At the same time, this arrangement permits the depth of the aperture to be decreased when the aperture is dilated, and this helps to permit a reduction in pressures borne by the die blades as the die blades move outwards during the joining of the sheet material. This facilitates movement of the die blades and hence formation of the joint. For example, the pivot joint will have a pivot point or pivot axis, and provision of at least a part of the pivot joint underneath the anvil surface permitting this axis to be moved inwards. Bringing the pivot axis laterally inwards allows the die blade to pivot so that as the aperture dilates, the depth of the aperture decreases. 
     It is not necessary however, for the aperture depth to decrease as the die blades first start to pivot laterally outwards. For example, the pivot point or pivot axis may be laterally between a pair of longitudinal axes defined by the limits of travel of tips of the dies blades extending above a substantially flat anvil surface, the pair of longitudinal axes being transverse to the anvil surface. Then, the height of the die blade tips above a plane defined by the anvil surface will fall as the die blade become fully dilated. If the pivot point or pivot axis is approximately central between the pair of longitudinal axes, then this height will initially rise as the die blades begin to move laterally outwards, then fall as the die blades move fully outwards. Preferably, the pivot point or axis will be closer to the innermost of the pair of longitudinal axes, so that height of the tip of the die blade above the plane defined by the anvil surface is lower when the aperture is fully dilated than when the aperture is constricted. In any event, depth of the aperture may increase as the aperture is partially dilated, and then decrease as the aperture is fully dilated. 
     The pressure on the die blade is then partially relieved during the time when a maximum pressure would be expected to be exerted on the die blades, i.e. as the sheet material joint forming process completes formation of the joint. 
     The recess underneath the anvil surface may be a part spherical or a part cylindrical pivot socket, in which case the die blade protrusion may be a convex surface matching the pivot socket so that the convex surface rotates in the pivot socket when the die blade is pivoted. The surfaces may be essentially spherical, with clearance provided for manufacturing tolerances. Additional clearance may be required if the pivot joint is not cylindrical. For example, the pivot joint can be part-toroidal with the blade protrusion having a similar part-toroidal shape. Additional clearance must then be provided between the protrusion and the recess to allow for the fact that as the protrusion pivots in the recess, the central part and end parts along the arc of the protrusion will move relatively apart in a longitudinal direction. 
     Preferably, the anvil has a shoulder that extends in a plane below the anvil surface and that supports a die blade endmost portion. The die blade endmost portion may be a portion of the die blade protrusion. In a preferred embodiment of the invention, the shoulder extends tangentially from the pivot recess. 
     It is advantageous if the biasing member is provided between the die blade and the shoulder. The biasing member then does not add to the lateral extent of the laterally biased die blades. The biasing member is preferably a resilient ring, for example a nitrile o-ring, that is compressed between the die blade and the shoulder when the die blade is pivoted to dilate the aperture. 
     There may however, be alternatively or additionally a resilient biasing member, for example a coil spring or an o-ring, stretched around laterally outwards facing surfaces of the die blades below the level of the anvil surface. The effect of a biasing member provided between the die blade and the shoulder may permit a reduction in the size of a second biasing member stretched around laterally outwards facing surfaces of the die blades. This facilitates a reduction in the lateral extent of the die, whilst at the same time maintaining good inward biasing of the die blades. 
     In one embodiment of the invention, the anvil surface is bounded by a generally circular periphery, the die blades extending in an arc around at least part of said periphery. 
     In an alternative embodiment of the invention, the anvil surface is bounded by a generally square or rectangular periphery, the die blades extending along at least two opposed sides of said periphery. 
     It is advantageous if the die shield remains in close proximity with a portion of the die blade adjacent the protrusion at the die blade is pivoted to dilate and constrict the aperture. This helps to keep each die blade retained to the die. 
     In one embodiment, the die shield is integral with the body portion, and the die blades are removably inserted in an opening in a side of the body portion. In another embodiment of the invention, the die shield is removably attached to the body portion and the die shield prevents the die blades from being removed from the die when the die shield is attached to the body portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in partial cross section through a sheet material clinch joiner comprising a punch with a retracted punch tip on one side of a pair of metal sheets, and opposite this a die according to a first embodiment of the invention; 
     FIG. 2 is a view in partial cross section through a sheet material clinch joiner comprising a punch with an extended punch tip on one side of a pair of metal sheets, and opposite this a die according to a second embodiment of the invention, and showing the punch tip deforming the layers of sheet metal into an aperture of the die; 
     FIGS. 3,  4  and  5  are views of the die of FIG. 2 showing, respectively, a view facing into the die aperture and showing a cylindrical symmetry of the die, a cross-sectional view in a plane through a cylindrical axis of the die, and an enlarged view of a portion of the die showing a number of die blades moveable between a central die anvil and a peripheral die shield; 
     FIGS. 6 and 7 are, respectively, views of the die of FIG. 1, facing into the aperture and along a central cylindrical axis of the die; 
     FIGS. 8,  9  and  10  are views showing how the punch tip and second embodiment the die co-operate to form a clinch joint in the two sheets of metal as the punch tip is driven towards the anvil surface; 
     FIG. 11 shows a cross-sectional view through a punch tip and a die according to a third embodiment of the invention, the die having a pair of parallel die blades either side of a central rectangular die anvil and a pair of parallel and integrally formed die shields laterally outwards of the die blades; 
     FIG. 12 is a cross-section through the die of FIG. 11, taken along lines XII—XII; and 
     FIG. 13 is a cross-section through the die of FIG. 11, taken along lines XIII—XIII. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a first embodiment of a sheet metal clinch joiner  1 , comprising a punch assembly  2  and a die  4 . The punch assembly  2  and die  4  are cylindrically symmetric and are aligned along common cylindrical axes  5 , 6 . Between the punch assembly  2  and die  4  are a pair of thin metal sheets  7 , 8  which are aligned transverse to the punch assembly and die axes  5 , 6 . The sheets  7 , 8  are in contact along a common interface  9 . In a sheet material joining operation, the punch assembly  2  is brought towards the pair of sheets  7 , 8  as indicated by arrows  10  until a forward hollow stripper tip  12  of the punch assembly  2  comes into contact with one of the metal sheets  7 , thereby pressing the other metal sheet  8  against a cylindrical die shield  14  of the die  4 . 
     FIG. 2 shows a second embodiment of a sheet metal joiner  101  in which parts common with the first embodiment  1  are indicated by reference numerals incremented by  100 . The operation of the first and second embodiments  1 , 101  is essentially the same. The punch assembly  2 , 102  has a main cylindrical housing  16 , 116  referred to herein as a stripper can. The end of the stripper can  16 , 116  away from the metal sheets  7 , 8 ;  107 , 108  has an open end plugged with a punch holder  18 , 118 . The other end of the stripper can  16 , 116  has a radially inwards directed lip  20 , 120  which terminates in a central circular aperture from which the stripper tip  12 , 112  extends. The stripper tip  12 , 112  has an outwardly directed flange  22 , 122 . An outer cylindrical surface  24 , 124  of the stripper tip  12  is a close sliding fit with a matching cylindrical surface  25 , 125  of the stripper can lip  20 , 120 . In addition, the stripper tip flange  22 , 122  has an outer cylindrical surface  26 , 126  which has a close sliding fit with an inner cylindrical surface  27 , 127  of the stripper can  16 , 116 . The stripper tip  12 , 112  is therefore free to slide axially with respect to the stripper can  16 , 116 . 
     The sliding fit of the stripper tip  12 , 112  within the stripper can  16 , 116  is limited in an outwards direction by contact between the stripper can lip  20 , 120  and the stripper tip flange  22 , 122 . A coil spring  28 , 128  is retained within the stripper can  16 , 116  between the punch holder  18 , 118  and the stripper tip flange  22 , 122 . The coil spring  28 , 128  biases the stripper tip  12 , 112  outwards so that in a rest condition the stripper tip flange  22 , 122  remains in contact with the stripper can lip  20 , 120 . The axial sliding movement of the stripper tip with respect to the stripper can is limited in an axially inwards direction by compression of the spring  28 , 128  against the punch holder  18 , 118 . 
     A cylindrically symmetric punch  30 , 130  is axially centered on the punch axis  5 , 105 , and is set into a cylindrical recess  32 , 132  in the punch holder  18 , 118 . The punch  30 , 130  extends axially along the centre of the stripper can  16 , 116  into the stripper tip  12 , 112 , where the punch  30 , 130  tapers down to a punch tip  34 , 134 . The stripper tip  12 , 112  terminates in a neck  36 , 136  with a cylindrical inner surface  37 , 137  that is a close sliding fit with the cylindrical stripper tip  34 , 134 . 
     When the punch assembly  2 , 102  is moved  10 , 110  up against the metal sheet  7 , 107  the stripper tip  12 , 112  comes first into contact with the metal sheet  7 , 107 . Further movement  10 , 110  then causes the stripper tip  12 , 112  to slide axially with respect to the stripper can  16 , 116 , with the result that the spring  28 , 128  begins to be compressed whilst the punch tip  34 , 134  continues with the motion  10 , 110  towards the metal sheet  7 , 107 . 
     As this is happening, the die  4 , 104  provides a restoring force against the other metal sheet  8 , 108 . Most of the restoring force is provided through the die shield  14 , 114 . 
     The die shield  14 , 114  stands higher than the die blades  56 , 156  with sufficient clearance that the die blades may pivot outwards as the metal layers  7 , 8 ;  107 ;  108  are drawn down by the punch tip  34 , 134 . 
     The first and second embodiments of the die  4 , 104  are shown in more detail in FIGS. 3-7. The die  4 , 104  has a unitary anvil body  40 , 140  which is cylindrically symmetric about die axis  6 , 106 . The anvil body  40 , 140  has at one end a lower stem  42 , 142  that in use is seated in a tool holder (not shown). At the opposite end of the anvil body  40 , 140  is a die anvil  44 , 144  with an anvil surface  46 , 146 . A die main body portion  48 , 148  with a diameter greater than that of the die stem  42 , 142  and die anvil  44 , 144  extends between the die stem and die anvil. 
     The die shield  14 , 114  extends around and is spaced from the die anvil  44 , 144  by a gap  50 , 150 , and is securely attached to the die main body portion  48 , 148  by an interference fit with a rebate  52 , 152  in the die main body portion  48 , 148 . The rebate  52 , 152  has a ledge  54 , 154  that faces in a direction axially towards the die anvil  44 , 144 , so that when the die shield bears the pressure of the punch assembly  2 , 102 , the die shield  14 , 114  is retained by the ledge  54 , 154 . 
     The gap  50 , 150  between the die shield  14 , 114  and the die anvil  44 , 144  is substantially filled by a number of die blades  56 , 156 . The first embodiment of the die  4  has seven die blades  56  equally spaced around the circular periphery of the anvil  44 . The die blades  56  are each separated from adjacent die blades by a short gap  57 . 
     The second embodiment of the die  104  has eight equally spaced die blades  156  around the circular periphery of the die anvil  144 . As can be seen in FIG. 3, there are no gaps between the die blades  156  when the die blades are up against the die anvil  144 . 
     The die anvil surface  146  is flat, except for a peripheral region  161  that is angled  165  down at about 3° in order to relieve pressure on the peripheral region when sheet material  108  is drawn down onto the anvil surface  146 . This helps to avoid abrasion or chipping of the anvil surface peripheral region  161 . 
     The die blades  56 , 156  extend generally transverse above and below the anvil surface  46 , 146  and form with the anvil surface an aperture  57 , 157  for the punch tip  34 , 134 . The separation between the blades  56 , 156  defines the aperture width  59 , 159 , and the extension of the die blades above the anvil surface  46 , 146  defines an aperture depth  60 , 160 . 
     As can be seen best in FIG. 5, each die blade  56 , 156  has a tip  62 , 162  which is recessed a short distance  163  below the level of the die shield  14 , 114 . The die shield  14 , 114  therefore bears most of the pressure imparted by the punch assembly  2 , 102 . 
     Each die blade  56 , 156  has a base  64 , 164  at the opposite end from the die blade tip  62 , 162 . The die blade base  64 , 164  is seated on a shoulder  66 , 166  between the die main body portion  48 , 148  and die anvil  44 , 144 . The shoulder  66 , 166  extends in a plane transverse to the die axis  6 , 106  between the die shield  14 , 114  and a pivot recess  68 , 168  that extends directly underneath the die anvil surface  46 , 146 . Each die blade  56 , 156  has a protrusion  58 , 158  that extends into the recess  68 , 168 , so that the protrusion and recess form a pivot joint by which each die blade  56 , 156  may pivot at its tip  62 , 162  laterally towards and away from the die anvil surface  46 , 146  respectively to constrict and dilate the die aperture  57 , 157 . 
     The recess  68 , 168  is essentially part-toroidal in shape, with the die blade protrusions  58 , 158  having a similar matching part-toroidal shape. There is provided normal manufacturing clearance between the protrusions and the recess, but also an additional clearance along an upper portion of the interface between the protrusions  58 , 158  and the recess  68 , 168 , to allow for the fact that as the die blades pivot, the arcuate ends of each toroidal protrusion will tend to rise and fall slightly whilst the central part of the die blade base  64 , 164  remains in contact with the shoulder  66 , 166 . 
     The first embodiment of the die  4  is provided with one resilient nitrile o-ring  70  that extends around the periphery of the shoulder  66 , 166  in a matching channel formed by a rebate  72  in a lower outer corner of each die blade lower portion  64 . As each die blade  56  pivots outwards, the rebate  72  compresses the o-ring against the shoulder  66 . This biases each die blade  56  back towards the die anvil  44 . 
     The second embodiment of the die  104  has a similar o-ring  170  and rebate  172 , and also a second o-ring  174  which is stretched around and seated in a groove  176  in a laterally outward facing surface  178  of each die blade  156 . The second o-ring  174  is preferably provided proximate the die blade tip  162  in order to provide for a maximum of lateral outwards movement for each die blade  156  in the gap  150  between each die blade  156  and the die shield  114 . 
     FIG. 5 shows in phantom outline how the die blade  156  compresses the second o-ring  174  as each die blade  156  is moved towards the surrounding die shield  114 . This compression and also the stretching of the second o-ring owing to the increased circumference around the die blades  156 , provides an additional biasing force to return each die blade  156  towards the die anvil  146 . 
     It should be noted that the pivot formed by each die blade protrusion  58 , 158  in the recess  68 , 168  is a slightly greater radius from the die axis  6 , 106  than the outer radius of the die anvil surface  46 , 146  and also the inner radius of the die blade tip  62 , 162 . Therefore, as each die blade  56 , 156  pivots laterally outwards, each die tip  62 , 162  rises slightly, but not so far that each die tip  62 , 162  rises above the level of the surrounding die shield  114 . 
     In the first embodiment of the die  4 , the outwards pivoting of each die blade  56  is limited by the contact of each die blade  56  and the surrounding die shield  14 . 
     In the second embodiment of the die  104  the outwards pivoting of each die blade  156  is limited by the contact of the second o-ring  174  and the surrounding die shield  114 . As shown in FIG. 5, at this point the tip  162  of each die blade  156  has dropped by a distance  180  further below the rim of the surrounding die shield  114  and the distance  163  of the die blades when up against the die anvil  144 . This is possible because at the outermost pivot of each die blade  156 , the die blade tip  162  is radially outwards of the effective pivot point or pivot axis of the die blade protrusion  158  in the recess  168 . The maximum outwards deflection for the die blades  56  of the first embodiment of the die  4  is up against the surrounding die shield  14 . Because each die blade  56  in the first embodiment of the die  4  can pivot radially further outwards than each die blade  156  of the second embodiment  104 , the die blade tips  62  of the first embodiment  4  drop further below the level of the surrounding die shield  14  than indicated in FIG.  5 . 
     The first embodiment of the die  4  therefore provides a greater reduction in compressive force on the die blade tips  62  whilst the second embodiment of the die  104  provides an increased radially inwards biasing force to return each die blade  156  to the die anvil  144 . In both embodiments, the radial extent of the die  4 , 104  is minimized by the provision of the recess  68 , 168  under the die anvil  46 , 146 , and also by minimising the gap  50 , 150  between the die shield  14 , 114  and each die blade  56 , 156 . 
     An important benefit of minimising the gap  50 , 150  is that the die shield  14 , 114  can be arranged to be in contact with a heel  82 , 182  of the die blade near the o-ring rebate  72 , 172 . The die  4 , 104  is assembled by first placing each die blade  56 , 156 , and o-ring  70 , 170 , 174  in place around the die anvil  44 , 144 . The die shield  14 , 114  is then inserted over the die blades and die body rebate  52 , 152 . Each die blade  56 , 156  is then securely held by the die shield  14 , 114  which keeps each die blade protrusion  58 , 158  securely engaged in its recess  68 , 168 . 
     Reference is now made to FIGS. 8,  9  and  10  which show for the second embodiment of the invention the operation of the die punch tool  102  and die  104 . In FIG. 8 the punch assembly  102  is being moved  110  towards the metal sheets  107 , 108 . When the stripper tip  112  comes into contact with the upper metal sheet  107 , the punch  130  continues to move towards the upper metal sheet until the punch tip  134  comes into contact with the upper metal sheet  107  and begins to deform plastically both metal sheets  107 , 108  into the die aperture  157 . When the two sheets of metal  107 , 108  have filled the die aperture  157  as shown in FIG. 9, further movement of the punch tip  134  into the die aperture  157  causes lateral plastic deformation of the metal sheets  107 , 108  as shown in FIG.  10 . The lateral movement of the metal sheets  107 , 108  causes each die blade  156  to pivot outwards thereby compressing the o-ring  170  between each die blade  156  and the die shoulder  166 , and also stretching and extending the second o-ring  174  that extends around all of the die blades  156 . 
     In the plastic deformation process, the metal sheets  107 , 108  are deformed into a button shape with the interface  109  doubling back on itself in an s-shape at the edges of the button shape. This locks the two sheets of metal  107 , 108  together at this localized area. 
     When the drawing pressure is relieved, the die tip  134  is withdrawn under the action of the coil spring  128  that was compressed in the drawing process. The punch tip  112  is then removed from the upper metal sheet  107 , and at the same time the die  104  is removed from the lower metal sheet  108 , whereupon each die blade  156  springs back against the die anvil  144  under the biasing action of the o-rings  170 , 174 . 
     FIGS. 11,  12  and  13  show a third embodiment of a sheet metal clinch joiner  201 . For convenience, components of the joiner  201  similar in function to those of a second embodiment  104  are represented by reference numerals incremented by  100 . 
     The joiner  201  has a rectangular symmetry with a pair of straight parallel die blades  256  arranged either side of a rectangular die anvil  244 . The die blades  256  are assembled by slotting each die blade  256  into a gap  250  between the die anvil  244  and a pair of straight parallel die shields  214 . Because the die blades  256  are straight and can be slotted in from one side of the die  204 , the die shields  214  are integral with a main die body portion  248  which is also integral with a die stem  242 . 
     The die  244  could be provided with an o-ring which is compressed between a base portion  264  of each die blade  256  and a suitable internal shoulder. In this example however, just one o-ring  274  is wrapped around the die blades  256  seated in a channel  276  in a laterally outward facing surface  278  of each die blade  256 . The o-ring  274  is also seated in a pair of channels  275  in open ends  277  of the die anvil  244 . The operation of the joiner  201  is similar to that for the first and second embodiments  1 , 101  and so will not be described in detail. The type of joint formed by the die tool  201  is a lance type joint in which sheet material is cut along two parallel lines formed by the scissor-like contact between the die tip  234  and each die blade  256 . The die tip  234  has between the die blades  256  two tapered ends  235  which form a ramp in the sheet materials which are deformed into a die aperture  257 . Compression of sheet materials into the die aperture  257  results in lateral flow of the sheet materials mainly in two opposite lateral directions towards each die blade  256 . This flow causes the sheet materials to flow underneath the cuts initially formed in the materials. 
     Because each die blade  256  has a protrusion  258  extending into a recess  268  underneath an anvil surface  246 , each die blade  256  is able to pivot in a similar manner to the first and second embodiments  1 , 101 , so that each die blade tip  262  drops as the aperture  257  is fully dilated. Because of the cylindrical symmetry of the pivot joint formed by the die blade protrusion  258  in the anvil recess  268 , there is no need as in the first and second embodiments for additional clearance between the die blade protrusion  258  and anvil recess  268  beyond normal manufacturing tolerances. 
     The clinch joining tools described above have a compact lateral dimension relative to the size of the joint made in sheet materials. For example, in the first and second embodiments  1 , 101 , the diameter of the constricted die aperture may be between 2 to 12 mm, in which case the outer diameter of the die shield  14 , 114  will be between, respectively, 7 to 18 mm. The depth of the aperture will depend on the separation between the die blades and thickness of sheet material to be joined, but typically will be between 0.5 to 2 mm. The die blade tips  62 , 262  will when the aperture is constricted be about 0.05 mm below the level of the surrounding die shield  14 , 114 . When the die aperture  57 , 157  is fully dilated, then each die blade tip will be between about 0.10 and 0.15 mm beneath the level of the surrounding die shield  14 , 114 . 
     It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the spirit or ambit of the present invention.