side 2 a of the upper workpiece part 2 in an area which already lies near the joining contour 4 wherein the exploratory seam 5 6 is placed substantially transverse to the joining contour 4 or in the area of a corner or curve of the joining contour 4 such that it can detect the joining contour 4 before and after the corner when the exploratory seam 5 6 is being produced the exploratory seam 5 6 or at least one typical parameter of the exploratory seam 5 6 respectively can be monitored by a detector 11 shown in fig4 when the exploratory seam 5 6 switches from a surface area a 1 of the upper workpiece part 2 which does not have any contact with the lower workpiece part 3 to a surface area a 2 in which the upper workpiece part 2 contacts the joining contour 4 the at least one parameter of the exploratory seam 5 6 being monitored by the detector 11 changes abruptly the same holds for the reverse case when the exploratory seam 5 6 switches from the surface area a 2 to the surface area a 1 this means that the detector 11 can detect boundary or contact points 7 8 in which the exploratory seam 5 6 switches from the surface area a 1 or a 2 to the surface area a.