Automatic mechanical guide system for one or more torches of a welding unit in a groove demarcated by the bevelled end surfaces of two metal parts to be welded together, of the type comprising a main carriage that moves along a fixed circular guide during use more or less parallel to the groove, a secondary carriage that is connected to the main carriage by a jointed connecting assembly and is equipped with a torch holder and a guide device comprising roller components (10a, 10b) in contact during use with the surfaces of the metal parts bordering said groove and tracer components (11a, 11b) that enter into the groove during use, the guide device comprising - a first tracer and roller assembly (10a, 11 a) that is located at the front of the secondary carriage, in the direction of travel of the main carriage during use and comprises a front tracer component in contact during use with one of the two side walls of the groove and a front roller component rolling during use on the surface of one of the two metal parts on one side of the groove - and a second tracer and roller assembly (10b, 11b) that is located at the rear of the secondary carriage and comprises a rear tracer component in contact during use with the other side wall of the groove and a rear roller component rolling on the surface of the other metal part on the other side of the groove, the torch(es) (T1, T2) then being mounted on the secondary carriage so that they are between the first and second tracer and roller assemblies (10a, 11a; 10b, 11b), the jointed connecting assembly comprising - a first pivot axis is that is perpendicular to the direction of travel of the main carriage and is located in a plane parallel to the plane of the circular guide - and a second pivot axis that is perpendicular to the direction of travel of the main carriage and perpendicular to the plane of the circular guide, the first and second pivot axes (A1, A2) being located between the first and second tracer and roller assemblies (10a, 11a; 10b, 11b), - lateral pushing means (20, 21) being provided that act on the secondary carriage to urge it to pivot about the first axis so that each tracer component (10a, 10b) stays in contact during use with the associated wall of the groove and pushing means that constantly, during use, push the roller components (11a, 11b) on the secondary carriage against the metal parts respectively associated with them, characterised in that the connecting assembly also comprises a third pivot axis that is perpendicular to the direction of travel of the main carriage during use and is located in a plane parallel to the plane of the circular guide, said third pivot axis being outside the torch (T1, T2) holder - and pushing means being provided that act on the secondary carriage to urge it to pivot about the third axis so that the axis of the torch(es) (T1, T2) can be aligned with the axis of the mating surface and the secondary carriage can then return to its original position. - a first tracer and roller assembly (10a, 11 a) that is located at the front of the secondary carriage, in the direction of travel of the main carriage during use and comprises a front tracer component in contact during use with one of the two side walls of the groove and a front roller component rolling during use on the surface of one of the two metal parts on one side of the groove - and a second tracer and roller assembly (10b, 11b) that is located at the rear of the secondary carriage and comprises a rear tracer component in contact during use with the other side wall of the groove and a rear roller component rolling on the surface of the other metal part on the other side of the groove, the torch(es) (T1, T2) then being mounted on the secondary carriage so that they are between the first and second tracer and roller assemblies (10a, 11a; 10b, 11b), the jointed connecting assembly comprising - a first pivot axis is that is perpendicular to the direction of travel of the main carriage and is located in a plane parallel to the plane of the circular guide - and a second pivot axis that is perpendicular to the direction of travel of the main carriage and perpendicular to the plane of the circular guide, the first and second pivot axes (A1, A2) being located between the first and second tracer and roller assemblies (10a, 11a; 10b, 11b), - lateral pushing means (20, 21) being provided that act on the secondary carriage to urge it to pivot about the first axis so that each tracer component (10a, 10b) stays in contact during use with the associated wall of the groove and pushing means that constantly, during use, push the roller components (11a, 11b) on the secondary carriage against the metal parts respectively associated with them, characterised in that the connecting assembly also comprises a third pivot axis that is perpendicular to the direction of travel of the main carriage during use and is located in a plane parallel to the plane of the circular guide, said third pivot axis being outside the torch (T1, T2) holder - and pushing means being provided that act on the secondary carriage to urge it to pivot about the third axis so that the axis of the torch(es) (T1, T2) can be aligned with the axis of the mating surface and the secondary carriage can then return to its original position.