Abstract: A mechanical guidance system for automatically guiding one or more electrodes of an arc-welding unit along a groove delimited by the facing chamfered end surfaces of two metal parts to be welded together, the guidance system being of the type comprising a main carriage which is moved along a fixed guide extending parallel to the groove, a secondary carriage hinged to the main carriage and fitted with an electrode support block, and a guidance device comprising running members in contact with the surfaces of the metal parts on either side of the groove and feeler members which penetrate into the groove, wherein the guidance device includes two feeler members which are in contact with respective ones of the two side walls of the groove and which are respectively situated towards the front and towards the rear of the secondary carriage relative to the displacement direction of the main carriage.