driven by a motor 19 which is secured to carriage 10 another motor 20 is secured to support 16 for driving a crank disc 21 a tie rod 22 is hinged or mounted to crank disc 21 and connected to a knife holder 24 which is guided in side guards 23 a cutting knife 25 with a circular cross section is secured to knife holder 24 a horizontally extending plate 26 having beveled front and rear edges is placed on supporting table 8 and secured to support 16 a counterknife 27 cooperates with knife 25 is provided in plate 26 another plate 28 is provided above plate 26 in a position slightly spaced therefrom on its upper side plate 28 is provided with an angular adjustment mark 29 fig1 motor 19 forms a part of a known light sensing guide system not shown having a light barrier 30 as its measuring element light barrier 30 comprises a light emitter 31 supported on the lower side guard 23 a mirror 32 which is adjustably supported on plate 28 and two phototransistors 33 as light receivers mirror 32 has a roof like reflecting surface so that the vertically incident light beams are reflected upwardly in two oppositely oblique directions due to this design phototransistors 33 can be located at spaced apart positions which eliminates erroneous sensing the lateral distance between cutting knife 25 and central line m of.