jig units 53 having the mandrels mounted thereon whereby the replacing of the mandrels 18 which takes the longest period of time in the arrangements for the press can be performed with high efficiency by the use of the auxiliary driving means 30 at the portions of the rotary index 48 other than the tube enlarging portion thereof clamping means 45 and 45a each comprising a plurality of first clamping units are provided on the right and left sides of the tube enlarging portion of the rotary index 48 respectively to suppot the ends of the fins in the direction of their lengths 5 more particularly the means 45 and 45a are respectively constituted by clamping units 44 44a and clamping units 44b 44c which are adapted to be driven independently of each other and each of which has a stroke length sufficient for permitting it to support the fins of maximum and minimum lengths 5 desired to undergo the tube enlargement each first clamping unit is provided with a through hole 50 for permitting the passage therethrough of the mandrel or mandrels 18 which are out of the fin area when the same are lowered for the purpose of enlarging the tubes 3 second clamping units 47a and 47b which are arranged in the direction of the width 10 of the fin are provided in a heat exchanger insertion jig 46 in the same manner as is in the.