A cutting unit for cutting at least one continuous rod transversely into a succession of segments, the cutting unit comprising: a rotary cutting drum with at least one radial blade; a cutting block device comprising a number of blade-guide bushes, each of which has a U-shaped cross section, is advanced a given distance together with and enclosing the continuous rod, and has a transverse through slit engaged by the blade of the cutting drum during the cutting operation; a wheel which rotates about a respective central axis of rotation, and supports the blade-guide bushes with the interposition of an epicyclic gear train to keep the blade-guide bushes parallel at all times to the travelling direction of the continuous rod; a first electric motor for rotating the cutting drum about a central axis of rotation; and an actuating system for actuating and imparting to the blade-guide bush a given cyclic law of motion synchronized with rotation of the cutting drum and comprising: a second electric motor mechanically independent of the first electric motor; and a control device for controlling the second electric motor to synchronize the second electric motor with the first electric motor; the cutting unit being characterized in that: a) it is provided a number of interchangeable wheels having respective blade-guide bushes; b) each wheel is designed to cut segments of a length within a given range; c) each wheel comprises a transponder, in which an identification code of the wheel is memorized; and d) the cutting block device comprises a transponder reader for reading the transponder and the identification code of the wheel to automatically determine whether or not the cutting block device is fitted with the right wheel. a rotary cutting drum with at least one radial blade; a cutting block device comprising a number of blade-guide bushes, each of which has a U-shaped cross section, is advanced a given distance together with and enclosing the continuous rod, and has a transverse through slit engaged by the blade of the cutting drum during the cutting operation; a wheel which rotates about a respective central axis of rotation, and supports the blade-guide bushes with the interposition of an epicyclic gear train to keep the blade-guide bushes parallel at all times to the travelling direction of the continuous rod; a first electric motor for rotating the cutting drum about a central axis of rotation; and an actuating system for actuating and imparting to the blade-guide bush a given cyclic law of motion synchronized with rotation of the cutting drum and comprising: a second electric motor mechanically independent of the first electric motor; and a control device for controlling the second electric motor to synchronize the second electric motor with the first electric motor; the cutting unit being characterized in that: a) it is provided a number of interchangeable wheels having respective blade-guide bushes; b) each wheel is designed to cut segments of a length within a given range; c) each wheel comprises a transponder, in which an identification code of the wheel is memorized; and d) the cutting block device comprises a transponder reader for reading the transponder and the identification code of the wheel to automatically determine whether or not the cutting block device is fitted with the right wheel. a) it is provided a number of interchangeable wheels having respective blade-guide bushes; b) each wheel is designed to cut segments of a length within a given range; c) each wheel comprises a transponder, in which an identification code of the wheel is memorized; and d) the cutting block device comprises a transponder reader for reading the transponder and the identification code of the wheel to automatically determine whether or not the cutting block device is fitted with the right wheel.