Abstract: An automatic bolt tightening device for assembling mechanical members placed on a buck arranged above the bolt tightening device. The bolt tightening device is provided with a headstock which is translatable along three mutually perpendicular axes. A rotatable spindle, arranged on the headstock, cooperates with the terminal part of a tang, which in turn cooperates with bolts for assembling the members. The translation of the headstock and the rotation of the spindle are actuated by motors which are controlled by a processing unit. A plurality of sensors detect the positions of the buck relative to the bolt tightening device and of the spindle relative to the tang, and output error signals to the processing unit. The processing unit controls the operation of the motors, thereby translating the headstock until the spindle and tang are coaxial.