Device (3; 201; 202) with rotating blades for processing a web material (N; N1; N2), comprising: a support structure (17; 217); a rotating blade-holder (15; 15B; 203; 205), which rotates around a rotation axis and on which a group of rotating blades (19; 19B; 203A; 205A) is arranged; an anvil blade (21; 21B; 21C; 204; 206) carried on said support structure (17; 217) adapted to co-act with said group of rotating blades (19; 19B; 203A; 205A); wherein each rotating blade (19; 19B; 203A; 205A) and the anvil blade (21; 21B; 21C; 204; 206) are so configured that, during rotation of the rotating blade-holder (15; 15B; 203; 205), each rotating blade (19; 19B; 203A; 205A) and the anvil blade (21; 21B; 21C; 204; 206) start reciprocal contact at a first end of the rotating blade (19; 19B; 203A; 205A), and terminate the reciprocal contact at a second end of the rotating blade (19; 19B; 203A; 205A), the point of reciprocal contact between each rotating blade (19; 19B; 203A; 205A) and the anvil blade (21; 21B; 21C; 204; 206) gradually moving from the first end to the second end of the rotating blade (19; 19B; 203A; 205A); a sensor (31; 104; 105; 107; 231; 232) adapted to detect a parameter indicative of a mutual interaction between the group of rotating blades (19; 19B; 203A 205A) and the anvil blade (21; 21B; 21C; 204; 206) generating a signal of mutual interaction between blades and anvil blade, said signal lasting for the time frame of interaction between the rotating blade (19; 19B; 203; 205) and the anvil blade (21; 21B; 21C; 204; 206) during the movement of the point of reciprocal contact from said first end to said second end; characterized in that the sensor (104; 231; 232) is adapted to detect a voltage applied between the anvil blade (21; 21B; 21C; 204; 206) and the group of rotating blades (19; 19B; 203A; 205A), said voltage signal containing information concerning the mutual interaction between the fixed anvil blade and the rotating blades. a support structure (17; 217); a rotating blade-holder (15; 15B; 203; 205), which rotates around a rotation axis and on which a group of rotating blades (19; 19B; 203A; 205A) is arranged; an anvil blade (21; 21B; 21C; 204; 206) carried on said support structure (17; 217) adapted to co-act with said group of rotating blades (19; 19B; 203A; 205A); wherein each rotating blade (19; 19B; 203A; 205A) and the anvil blade (21; 21B; 21C; 204; 206) are so configured that, during rotation of the rotating blade-holder (15; 15B; 203; 205), each rotating blade (19; 19B; 203A; 205A) and the anvil blade (21; 21B; 21C; 204; 206) start reciprocal contact at a first end of the rotating blade (19; 19B; 203A; 205A), and terminate the reciprocal contact at a second end of the rotating blade (19; 19B; 203A; 205A), the point of reciprocal contact between each rotating blade (19; 19B; 203A; 205A) and the anvil blade (21; 21B; 21C; 204; 206) gradually moving from the first end to the second end of the rotating blade (19; 19B; 203A; 205A); a sensor (31; 104; 105; 107; 231; 232) adapted to detect a parameter indicative of a mutual interaction between the group of rotating blades (19; 19B; 203A 205A) and the anvil blade (21; 21B; 21C; 204; 206) generating a signal of mutual interaction between blades and anvil blade, said signal lasting for the time frame of interaction between the rotating blade (19; 19B; 203; 205) and the anvil blade (21; 21B; 21C; 204; 206) during the movement of the point of reciprocal contact from said first end to said second end;