and in which a piston 50 slides and the pipe 42 39 cut lengthwise into the same thickness of the support and which comes out in the lower part of a cylindrical housing 57 in which the piston 56 moves provided with a gudgeon 58 an elastic medium or a spring 60 placed between the piston 56 and the upper surface surface of the housing 57 pushes the gudgeon 58 downwards as fig3 illustrates in detail pressurized fluid fed to each tube 42 it spreads into the cylindrical housing 42 34 through a regulation valve of a known type 46 and activates the piston 50 which locks the support 48 in 52 against an element 54 united to the beam 10 and therefore also locking the translation of the blade 34 the same pressurized fluid compressed air upstream of the adjustment valve 46 flows into tube 42 39 to activate the piston 56 which in contrast to the elastic medium or spring 60 raises the gudgeon 58 releasing its extremity 62 from the corresponding hold 68 of a pierced plate 64 joined to the support 66 of the relative counter blade 36 in this position the blade 34 can be made to descend using known mechanical and pneumatic means until it corresponds to said counter blade 36 and then cuts the bands of material when the supply of pressurized fluid is suspended the spring 60 pushes the gudgeon 58 downwards and inserts the extremity 62 into a corresponding hole 68 in the plate 64 in this way the blade is connected to the counter blade simultaneously the piston 50 is turned off so that support 48 is unlocked from the element 54 as illustrated in fig4 under these.