fig1 shows a torque converter 1 including a front cover 3 formed by a first embodiment of the present invention . in the figure , o -- o represents a rotation axis of the torque converter 1 , an engine ( not shown ) is arranged at the left in the figure , and a transmission ( not shown ) is arranged at the right in the figure . the torque converter 1 is a mechanism for transmitting a torque from a crank shaft of the engine to a main drive shaft of the transmission . the torque converter 1 is basically formed with a front cover 3 that is configured for coupling to the crank shaft of the engine through a flexible plate , an impeller 4 , a turbine 5 and a stator 6 . the front cover 3 and an impeller shell 4a of the impeller 4 are welded together at their outer peripheral portions to form a working fluid chamber between them . the front cover 3 is formed of a circular plate - like main portion 32 , a boss 31 and an outer peripheral connection portion 33 , and is provided with a radially outer corner portion 34 between the main portion 32 and the outer peripheral connection portion 33 . the boss 31 is formed radially inside the main portion 32 for positioning and centering the front cover 3 with respect to parts at the engine side . the outer peripheral connection portion 33 having a cylindrical form is formed radially outside the main portion 32 , and is fixed to the outer peripheral portion of an impeller shell 4a at its end near the transmission . a method of forming the radially outer corner portion 34 of the front cover 3 is described below . the impeller 4 is formed of the impeller shell 4a , a plurality of impeller blades 4b fixed to the inner surface of the impeller shell 4a , and an impeller hub 4c fixed to the inner periphery of the impeller shell 4a . the turbine 5 is opposed to the main portion 41 of the impeller shell 4a with the working fluid chamber therebetween , and is basically formed of a turbine shell 5a and a plurality of turbine blades 5b fixed to the turbine shell 5a . a turbine hub 8 having a spline aperture 8a for coupling to the main drive shaft , which extends from the transmission , is fixed to the inner periphery of the turbine shell 5a . the stator 6 is arranged between the impeller 4 and the turbine 5 , and is fixed to a stator shaft , which is fixed to a housing of the transmission , through a one - way clutch 7 . torque transmitted from the crank shaft of the engine is supplied to the front cover 3 through a flexible plate ( not shown ). the torque is transmitted to the impeller shell 4a via the front cover 3 . thereby , the impeller 4 rotates , and the working fluid flows from the impeller shell 4a toward the turbine 5 . the flowing working fluid rotates the turbine 5 so that the torque of the turbine 5 is output to the main drive shaft through the turbine hub 8 . description is now given on the method of forming the radially outer corner portion 34 of the front cover 3 . first , a circular steel plate is prepared by cutting a blank flat plate into a predetermined configuration . the steel plate is then subjected to a preliminarily pressing process to form the front cover 3 having a general form shown in fig2 . it should be understood that the form depicted in fig2 is that of an annular member shown only in cross section . the preliminary pressing process may be any of a number of pressing step , well know in the art , for attaining the shape depicted in fig2 . typically at least one pressing step is required in order to attain the shape depicted in fig2 although , two or more steps may be necessary . thereafter , a finish pressing process is performed with dies 11 and 12 shown in fig2 and 3 . more specifically , the die 11 is pressed downward against the unfinished impeller shell 4a , with the unfinished impeller shell 4a resting on the die 12 as shown in fig2 . in the finish pressing process , a surface 11a of the die 11 pushes downward a free end 3a of the outer peripheral connection portion 33 of the front cover 3 remote from the radially outer corner portion 34 . at the same time , the unfinished impeller shell 4a is confined between the surfaces 11b , 11c , 12a and 12b of the dies 11 and 12 . thus , internal compressive stresses act on the outer peripheral connection portion 33 and the radially outer corner portion 34 in a direction parallel to the direction of the thickness as indicated by an arrows s 1 and s 2 in fig3 . this suppresses reduction in thickness , which is often caused by extension of the radially outer corner portion 34 . in some cases , the internal compressive stresses increase the thickness of the corner portion 34 owing to compression . therefore , the radially outer corner portion 34 , which is finished by the press working as shown in fig3 has a lager thickness than the prior art . accordingly , the required thickness of the blank can be reduced compared to prior art configuration while maintaining a strength , and the production cost can be reduced . in the prior art , the finish press working is performed without applying a compressive force to the radially outer corner portion 34 . in this case , the finish press working reduces the thickness of the blank from 5 . 00 mm to a value from 4 . 40 to 4 . 55 mm , and thus the thickness decreases about 9 %. in contrast to this , according to the method of this embodiment , the press working changes the thickness of the blank from 5 . 00 mm into a value of 4 . 95 - 5 . 10 mm , and thus the thickness does not substantially decrease . fig4 shows the torque converter 1 including an impeller shell 4a formed by a second embodiment of the invention . in fig4 o -- o represents the rotation axis of the torque converter 1 , the engine ( not shown ) is arranged at the left in the figure , and the transmission ( not shown ) is arranged at the right in the figure . the torque converter 1 is a mechanism for transmitting a torque from the crank shaft of the engine to the main drive shaft of the transmission , and is basically formed of the front cover 3 coupled to the crank shaft of the engine through the flexible plate , the impeller 4 including the impeller shell 4a , the turbine 5 and the stator 6 . the front cover 3 and the impeller shell 4a of the impeller 4 are welded together at their outer peripheral portions to form the working fluid chamber between them . the impeller 4 is formed of the impeller shell 4a , the plurality of impeller blades 4b fixed to the inner surface of the impeller shell 4a , and the impeller hub 4c fixed to the inner periphery of the impeller shell 4a . the impeller shell 4a is formed of the main portion 41 , an outer peripheral connection portion 42 and a radially inner corner portion 43 . the main portion 41 is opposed to the turbine 5 . the radially outer corner portion 42 is formed radially outside the main portion 41 , and is fixed to the front cover 3 . the radially inner corner portion 43 is formed radially inside the main portion 41 , and is fixed to the impeller hub 4c . a method of forming the radially inner corner portion 43 of the impeller shell 4a is described below . the turbine 5 is opposed to the main portion 41 of the impeller shell 4a with the working fluid chamber therebetween , and is basically formed of the turbine shell 5a and the plurality of turbine blades 5b fixed to the turbine shell 5a . the turbine hub 8 having the spline aperture 8a for coupling to the main drive shaft , which extends from the transmission , is fixed to the inner periphery of the turbine shell 5a . the stator 6 is arranged between the impeller 4 and the turbine 5 , and is fixed to the stator shaft , which is fixed to the housing of the transmission , through the one - way clutch 7 . an operation of the torque converter 1 will be described below . the torque sent from the crank shaft of the engine is supplied to the front cover 3 through the unillustrated flexible plate . the torque is transmitted to the impeller shell 4a . thereby , the impeller 4 rotates , and the working fluid flows from the impeller shell 4a toward the turbine 5 . the flowing working fluid rotates the turbine 5 so that the torque of the turbine 5 is output to the main drive shaft through the turbine hub 8 . description is now given on the method of forming the radially inner corner portion 43 of the impeller shell 4a . first , a circular steel plate , which was prepared by cutting a blank plate , is preliminarily pressed to form the impeller shell 4a having a cross section form shown in fig5 . next , finish press working is performed with dies 61 and 62 shown in fig5 and 6 . more specifically , the die 61 is pressed downward again the impeller shell 4a laid on the die 62 as shown in fig5 . in this finishing work , the radially inner corner portion 43 and a portion 43a are compressed in a direction parallel to the direction of thickness of the impeller shell 4a by the force indicated by arrows s 3 and s 4 in fig5 while the impeller shell 4a is being pressed . thereby , the finish press working shown in fig6 does not significantly reduce the thickness of the radially inner corner portion 43 . therefore , the required thickness of blank of the impeller shell 4a can be small so that the weight of the torque converter 1 can be reduced while maintaining a strength and a rigidity of the impeller shell 4a . in the prior art , the finish press working is performed without applying a compressive force to the radially inner corner portion 43 . in this case , the bending press working for forming a corner of 5 mm in radius reduces the thickness of the blank from 3 . 850 mm to 3 . 11 mm , and therefore the thickness decreases about 19 %. in contrast to this , according to the method of the present invention , the press working for forming a corner of 5 mm in radius reduces the thickness of the blank from 3 . 85 mm to 3 . 57 mm , and therefore the thickness decreases only about 7 %. also , according to the method of this embodiment , the bending press working for forming a corner of 5 . 5 mm in radius reduces the thickness of the blank from 3 . 85 mm to 3 . 76 mm , and therefore the thickness decreases only about 2 %. the above numbers and percentages relating to the present invention were derived from tests conducted using the methods and die configurations of the present invention . according to the invention , the compressive force in the direction crossing the direction of thickness of the blank is applied to the corner portion during the finish press working so that the reduction in thickness of the corner portion can be suppressed . accordingly , the thickness of the finish - pressed corner portion can be larger than that in the prior art so that the required thickness of the blank can be reduced while maintaining the strength , and therefore the production cost can be reduced . also , according another aspect of the invention , the first step is performed prior to the finish press working in the second step . in this first step , the blank is shaped such that the compressive force in the direction crossing the direction of thickness of the blank will be applied to the corner portion during the finish press working . this suppresses reduction in thickness of the corner portion which may be caused by the finish press working in the second step . accordingly , the required thickness of the blank can be reduced as compared with the prior art , while maintaining the strength and rigidity of the corner portion , and therefore the production cost and the weight can be reduced . it should be appreciated that in fig2 , 5 and 6 the depicted members are shown in cross section and as such , only half of the cross section is depicted . the line 0 -- 0 is a centerline about which there is symmetry of the front cover and impeller shell . various details of the present invention may be changed without departing from its spirit or its scope . furthermore , the foregoing description of the embodiments according to the present invention are provided for illustration only , and not for the purpose of limiting the invention as defined by the appended claims and their equivalents .