one example of the implementation of this invention is shown in fig1 - 3 of the drawings , wherein numeral 10 generally indicates a hemming station for panel hemming . in station 10 , a robotic roller apparatus 11 is positioned adjacent a panel assembly 12 , which is supported by rigid tooling or an anvil 14 . the tooling or anvil would also be provided with a suitable retainer , such as a clamping device not shown , for holding the panel assembly 12 in position during the hemming operations . the panel assembly includes an outer panel 16 , with an end flange 17 , and an inner panel 18 being hemmed together by the roller apparatus 11 . the apparatus 11 includes a robotic mount section 21 carried by a robot , not shown , and contains a prehem ( 45 degree ) roller 22 and a final roller 24 . these rollers bend the flange 17 from a 90 degree angle to a 180 degree angle in two steps during a single pass of the roller apparatus . the prehem ( 45 degree ) roller 22 is flexible and programmable for rotation about the y - axis to achieve appropriate prehem bending . rotation to position the angle of this roller 22 may be controlled by any suitable mechanism , one of which is represented by a gear 26 and an electric stepping motor 28 incorporated into the roller apparatus 11 . this degree of freedom is necessary to ensure proper bending through all contours of the panel . to adjust the height of the prehem roller 22 , the robot can rotate the roller apparatus 11 about the axis of the final hem roller 24 for a third degree of freedom . the invention is further illustrated in fig2 and 3 with axial views along the bend axis of the hemline ( y - axis ) and along the rotational axis of the final roller 24 ( x - axis ). fig2 shows the relative positions of the two rollers . the rotational flexibility of the prehem roller 22 , provided by a gear 26 and a stepping motor 28 , would allow this embodiment to be operable in reverse . for example , the roller apparatus 11 could be rotated about the y - axis so that the roller 24 would act as the pre hem roller and the roller 22 could be used as the final roller . this embodiment may also enable the forming of the very sharp , pinch shape described in u . s . pat . no . 6 , 672 , 121 , and could be used with all sheet metal alloys , including , but not limited to , heat treatable aa6111 - t4 , non - heat treatable aa5182 , aa5754 & amp ; aa5083 , and any steel sheet application . in another application , this embodiment could be used to bend the initial flange from 180 degrees to 90 degrees followed by the flat hemming operation . roller flanging would allow the elimination of the flange press operation leading to further cost reductions . further embodiments of the invention are provided which incorporate retrogression heat treatment for softening aluminum alloys for hemming . one example for implementing this concept is shown in fig4 - 10 . as seen in fig4 , a roller hemming station 30 includes a robotic roller apparatus 31 positioned adjacent to a panel assembly 32 , which is supported by rigid tooling or an anvil 34 . the panel assembly 32 includes an outer panel 36 , with a hem flange 37 , and an inner panel 38 being hemmed together by the roller apparatus 31 folding the flange 37 against the inner panel 38 . the robotic roller apparatus 31 is carried by a robot ( not shown ) through a mount section 40 , and contains an induction coil 42 , an air - quenching knife 44 , and roller bearings 46 , which support a hem roller 48 . vertical ( z - direction ) and transverse ( x - direction ) positions of the induction coil 42 , with respect to a bend axis of the hemline , are controlled by guide rollers , including a side roller 50 in the x - direction and a top roller 52 the z - direction . a first compression spring 54 acts between an x pivot arm 56 and a z pivot arm 58 , supporting the x pivot arm . a second compression spring 60 , supported by the end of arm tool mount 40 , acts against the z pivot arm 58 to maintain appropriate proximity between the induction coil 42 and the hemline material to obtain the proper retrogression heat treatment . this flexibility , with guide rollers , compression springs and pivot arms , is necessary to allow the coil to follow the product contour while the robot motion is programmed for optimal positioning of the rollers to flat - hem the flange . the induction coil 42 is powered and water - cooled with flexible connections ( not shown ) running through the components of the roller apparatus 31 and tool mount section 40 , along the robot arm ( not shown ) to a power supply ( not shown ). the invention is further illustrated in fig5 through 10 . fig5 is a front axial view along the bend axis of the hemline and shows the relative positions of the guide rollers 50 , 52 and the proximity of the induction coil 42 to the hem flange 37 . fig6 is a top view also showing the guide roller positions along with the hem roller 48 . fig7 is the corresponding front view of all components of the roller apparatus 31 . the flexibility of the guide rollers 50 , 52 to maintain proper proximity for the induction coil to the hemline material is illustrated in the front view of fig8 and the bottom view of fig9 . fig1 shows the roller apparatus 31 in both assembled and exploded conditions . the rht process involves rapid heating to dissolve unstable particles and rapid quenching to maintain the resulting supersaturated solution condition that allows sufficient cold deformation for flat hemming . after heating , the solutionized material at the hemline will be quenched by the air knife and cold rolled to the flat hem condition . in another use , this invention could be applied to the hemming of non - heat treatable aluminum alloys such as 5182 , 5754 and 5083 . the induction heating process would anneal the material in the hemline to remove the prior cold work , followed by the air quench and cold roller bending . this would enable severe bending during the roller hem process to achieve the sharp , pinch hem shape , and would be an application of a preforming annealing process for stamping aluminum sheet rather than the retrogression application , which applies to age - hardenable alloys . in yet another embodiment of this invention , the induction coil could be used to heat the hemline material , which could then be “ hot - hemmed ” to create an extremely sharp outer bend radius that may provide a desirable appearance . hot hemming would not likely apply to age - hardenable aluminum alloys , but rather would be applicable for 5xxx aluminum alloys , magnesium sheet alloys and steel sheet alloys . with either hot or cold bending embodiments , this process may enable the forming of the very sharp , pinch hem shape described in u . s . pat . no . 6 , 672 , 121 . the pinch hem geometry , while possible with conventional hemmers , would most likely only be feasible with the roller hemming method . still another embodiment of the invention involves a method called friction roller hemming . referring to fig1 , numeral 61 indicates an apparatus and process for carrying out friction hemming of a sheet metal outer panel 62 with an inner panel , not shown . the outer panel 62 has a flange 64 angled initially in the 90 - degree open position . a roller apparatus 65 including a solid roller 66 contacts the sheet metal flange 64 and is rotated to create heat from friction as it bends the flange in a closing direction . the roller 66 has a generally cylindrical end 68 and an inwardly adjacent flared shoulder portion 70 adapted to engage a bend portion 72 of the flange 64 to concentrate heating of the flange in the bend portion . the roller 66 is carried on a shaft / axis 74 connected with an electric motor 76 mounted in a robotic mount section ( not shown ). the motor spins the roller 66 in the rotational direction of arrow 78 while the roller is moved in the lateral direction of arrow 80 . the lateral motion of the roller applies , against the sheet metal flange 64 , a force vector having components p x , p y and p z , causing the flange to bend in a closing direction . numeral 82 represents a line / area of contact between the roller and the flange upon which the force vector acts and in which friction heating occurs . the bend portion 72 of the hemline is deformed during the hemming process to produce a sharp , crisp bend radius 84 needed for the desired appearance . heat generated by friction between the roller 66 and the flange 64 locally softens the sheet metal in the bend portion 72 , enhancing bending plasticity while resisting shear band localization and failure by cracking at the sharp , outer hem radius 84 . the curved shape of the roller is designed to localize the frictional heat in the bend portion 72 during an initial pass ( pre - hem step ). the robot ( not shown ) repositions the roller 66 to flatten the hem on a final pass . this method can be used to produce the sharp , flat “ pinch ” hem geometry described in u . s . pat . no . 6 , 672 , 121 . while the invention has been described by reference to certain preferred embodiments , it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described . accordingly , it is intended that the invention not be limited to the disclosed embodiments , but that it have the full scope permitted by the language of the following claims .