A method of using a device for homogeneously welding flat-bent structures in the form of at least two assembly parts by friction stir welding, comprising the following characteristics: a) a receiving plate guided by a guiding machine and accompanied by a drive head, and a tool bell attached to it and accompanied by a welding shoe support and a spindle bearing for a welding spindle of a welding shoe, wherein b) the welding shoe has a circular basic shape, and c) the tool bell includes a strip-shaped sensor, which is designed to determine a force, a pressure, or a displacement and is mounted on the side of the tool bell opposite to the welding process flow direction, and wherein a conical profiling is provided in the wider area of the tool receiving cone to accommodate a sensor for determining an axial force, a torque, and a bending moment on the welding spindle, and wherein another profiling is provided in the front area of the tool receiving cone with sensors for measuring the axial force acting on the welding spindle and with a piezoelectric force sensor in the longitudinal axis of the friction pin rod, also for measuring the axial force, and wherein a sensor signal amplifier is provided with a rotor antenna to receive, amplify, and transmit all determined measurement values, wherein a static antenna is provided to transmit these measurement values to a machine control, and wherein an inductive power supply system is provided to power the measurement system from a movable secondary winding and a fixed primary winding, wherein a crossbar, extending over the circular basic shape of the welding shoe above the cross-section and extending perpendicularly to this basic shape, is provided with a width ranging from 1/4 to 1/5 of a diameter of the basic shape and with an arc-shaped sliding and smoothing surface of the shoe, wherein a small flat surface in the form of a notch-shaped taper, as a chip guiding step, is located on the front face of this surface in the edge area of this surface, wherein the sliding and smoothing surface of the shoe substantially corresponds to the surface curvature of the respective assembly parts, characterized in that the pressure force acting on the welding shoe is measured and controlled, wherein the displacement of the welding shoe is carried out based on the measured axial pressure on the welding shoe, and a position control of the welding spindle and a force control of the welding shoe are carried out synchronously. a) a receiving plate guided by a guiding machine and accompanied by a drive head, and a tool bell attached to it and accompanied by a welding shoe support and a spindle bearing for a welding spindle of a welding shoe, wherein b) the welding shoe has a circular basic shape, and c) the tool bell includes a strip-shaped sensor, which is designed to determine a force, a pressure, or a displacement and is mounted on the side of the tool bell opposite to the welding process flow direction, and wherein a conical profiling is provided in the wider area of the tool receiving cone to accommodate a sensor for determining an axial force, a torque, and a bending moment on the welding spindle, and wherein another profiling is provided in the front area of the tool receiving cone with sensors for measuring the axial force acting on the welding spindle and with a piezoelectric force sensor in the longitudinal axis of the friction pin rod, also for measuring the axial force, and wherein a sensor signal amplifier is provided with a rotor antenna to receive, amplify, and transmit all determined measurement values, wherein a static antenna is provided to transmit these measurement values to a machine control, and wherein an inductive power supply system is provided to power the measurement system from a movable secondary winding and a fixed primary winding, wherein a crossbar, extending over the circular basic shape of the welding shoe above the cross-section and extending perpendicularly to this basic shape, is provided with a width ranging from 1/4 to 1/5 of a diameter of the basic shape and with an arc-shaped sliding and smoothing surface of the shoe, wherein a small flat surface in the form of a notch-shaped taper, as a chip guiding step, is located on the front face of this surface in the edge area of this surface, wherein the sliding and smoothing surface of the shoe substantially corresponds to the surface curvature of the respective assembly parts,