Abstract: A friction stir welding system is provided that can afford a versatile and efficient welding operation. The friction stir welding spindle assembly can have a shoulder spindle and a pin spindle independently rotatable in relation to each other. The shoulder spindle and the pin spindle can be rotatably supported by a shoulder housing and a pin housing, respectively. The pin housing can be rotatably supported in the shoulder housing and adapted to be axially movable in relation to the same between a forward position and a retracted position. Additionally or alternative, the shoulder spindle and the pin spindle can each be provided with an adapter, which has a predetermined mounting surface for mounting a tool thereon.