and more preferable to be 0 5 u00b0 3 0 u00b0 on the other hand the angle formed between the cam surfaces 61 63 and the vertical plane is preferable to be in the range of 5 u00b0 50 u00b0 and more preferable to be 10 u00b0 45 u00b0 such a difference in the angle formed by the cam surfaces makes it possible to lessen the urging force of the die block 48 than that of the die block 50 and consequently to carry out the groove formation process while holding a portion of the cylindrical material 61 where little deformation is expected under a suitable restraining force in the middle portion of the upper base 66 an attaching block 68 is secured and on the lower side thereof a preliminary flange forming die 70 is secured the die 70 is provided with an opening 72 for receiving the head of the punch 36 leaving a slight clearance between the two and it is at the same time tapered to be of funnel shape convergent from the fixed end to the free end thereof the earlier mentioned preliminary forming process performed with such an apparatus will be described a suitably prepared cylindrical material 51 is fitted on the punch assembly 43 before the upper base 66 accompanied by the complementary cams 64 65 is descended the movable cams 56 57 are inwardly moved under pressure due to the cam action appearing there resisting the biasing force of the above mentioned resilient members along with this movement the die blocks 48 50 respectively secured.