Screw tightening apparatus

A screw tightening apparatus of the type including an electric screwdriver the bit shaft of which is arranged to carry a male screw engaged therewith to be threaded into a mounting hole of a workpiece and a drive mechanism arranged to move the screwdriver toward and away from the mounting hole of the workpiece and to apply a thrust force to the bit shaft of the screwdriver,

• wherein an electric control system for the screw tightening apparatus comprises:
• first control means for controlling operation of the screwdriver and drive mechanism in such a manner that the bit shaft is driven at a low speed and applied with a small thrust force at an initial stage during which the male screw is located to be brought into engagement with the mounting hole of the workpiece;
• second control means for controlling operation of the screwdriver and drive mechanism in such a manner that the bit shaft is driven at a higher speed than that at the initial stage and applied with a larger thrust force than that at the initial stage at an interim stage during which the male screw is threaded into the mounting hole of the workpiece;
• and
• third control means for controlling operation of the screwdriver and drive mechanism in such a manner that the bit shaft is driven at a lower speed than that at the initial stage and applied with a larger thrust force than that at the interim stage at a final stage during which the male screw is tightened and seated on the workpiece.