The present invention relates to an overlock sewing machine capable of performing a back-tack stitching. The present invention also relates to a method for performing an overlock sewing in the overlocking sewing machine.
In a conventional overlocking sewing machine, needle threads are inserted into a pair of needles, and looper threads are inserted into an overlooper and an underlooper. In cooperation with the needles and loopers, the looper threads are wound over stitch formation tongues positioned adjacent to a needle hole slot of a throat plate so as to provide an overlock stitch pattern stitched with four threads and two needles on a workpiece fabric. Upon completion of the sewing operation, a thread chain provided by the needles and the workpiece is cut, and then the thread chain is stretchedly positioned to a workpiece feed-in side while extending beyond over the needle positions, and a free end of the thread chain is held in a predetermined position. While maintaining this chain position, the thread chain is subjected to stitching to the workpiece for performing the back-tack stitching.
In the conventional sewing machine, when the thread chain is shifted to the workpiece feed-in side with respect to the needles, the chain end portion positioned at the needle side is located in a vicinity of an end portion of the needle hole slot of the throat plate. Therefore, if the new stitching is started in such a state, the needle passes through the thread chain end portion located at the needle side after the new thread chain is formed on the stitch formation tongues while the workpiece is fed to the needle location. As a result, a newly formed thread chain does not undergo stitching, and accordingly, the unstitched thread chain may entangledly be protruded in a spherical shape from the workpiece edge. Alternatively, an already formed thread chain is not sufficiently stitched, so that the unstitched part may bulge from the overlocking stitching pattern. Consequently, stitching quality may be degraded.