toward the inside of the duct 2 under the effect of that pressure the end segment 11 is pierced and forms the hole 23 the method includes a step for pre positioning the flared end 16 on the outer neck 7 this step is shown in fig5 the outer neck 7 is positioned against the wall 9 at the end segment 11 across from the hole 23 the method also includes a step for forming the flared end illustrated in fig6 this step is also carried out with a die 27 cooperating with a punch 28 the die 27 has an opening 29 the die 27 also comprises rounded edges 30 designed to give the outer neck 7 its final shape the punch 28 has an irregular diameter the head of the punch 28 has a diameter corresponding to the diameter of the duct 2 the shape of the remaining portion of the punch is conjugated with the shape of the and segment 11 and the flared end 16 the outer neck 7 and the end segment 11 deform under the effect of the pressure applied by the punch the outer neck 7 adopts a rounded shape the edge of the die being inclined at approximately 90 u00b0 relative to the horizontal plane the end segment 11 adopts its final shape revealing the bend of the flared end 16 fig7 illustrates the steps for positioning the inlet of the duct 2 across from the flange 3 the positioning of the duct 2 across from the flange 3 is done such that.