A method of welding a window frame provides a welding system (10) having movable head fixtures (16). Certain of the head fixtures include cutting structure (37) and each head fixture includes heat supplying structure (40). A one-piece lineal (20) has first and second ends and notches therein. The lineal is loaded with respect to the head fixtures, with portions of the lineal being folded about the notches to define folded corners. Head fixtures are moved to clamp on the lineal. The cutting structure cuts the lineal generally at each notch to separate each of the folded corners from the lineal, with each corner of the lineal being defined by a pair of ends. The head fixtures are repositioned so as to provide a space between each pair of ends. A heat supplying structure is inserted into the space to heat each pair of ends and then removed. The head fixtures are moved so that each pair of ends contact and join in a thermally welded manner.