Overlock sewing machine and thread chain back-tacker therefor

A thread chain back-tacker for an overlock sewing machine for back-tacking a thread chain to a cloth, comprising:

• a presser foot for pressing a cloth to be tacked;
• a front air nozzle disposed above said presser foot, for blowing air against a surplus of a back-tacked thread chain to set a back-tacking length of the thread chain surplus;
• an adjuster for adjusting the length of the thread chain surplus after the thread chain has been back-tacked;
• a thread drawing nozzle for sucking an end of the thread chain surplus after the thread chain has been idly tacked to the cloth;
• a thread clamp cylinder having a thread clamp for urging and securing the thread chain surplus after the thread chain surplus has been sucked into said thread drawing nozzle;
• a nozzle swing cylinder operative in response to a machine stop signal to cause rocking of said thread drawing nozzle between an advanced position and a retreated position, said advanced position being nearer than said retreated position to a needle drop position of the overlock sewing machine;
• a nozzle pulling cylinder for setting a substantially middle position of said thread drawing nozzle between the advanced and retreated positions;
• a speed setting/releasing unit for maintaining the number of needle operation cycles and rotation speed of the machine during back-tacking of the thread chain, and for releasing the pressure on said thread clamp provided in said thread clamp cylinder after a predetermined number of needle operation cycles;
• and
• a venturi unit for converting the pressure in a tube coupled to said thread drawing nozzle from positive pressure to negative pressure;
• wherein said presser foot includes means for preventing the thread chain at the back-tacking position from contacting an upper looper thread when tacking the thread chain to the cloth.