The invention improves the quality of the double-thread stitch line by preventing it from unravelling from the end and also simplifies the construction and improves the productivity of the machine by the use of a simpler mechanism for the looper which executes rotary motion. The double-thread chain-stitch is formed and tightened with one turn of the machine's main shaft. The sewing machine in addition contains autonomous disc-shaped cam thread tighteners for the top and bottom threads, a lifter for the bottom thread which can oscillate about its own axis. The looper is mounted in such a way that it can execute rotary motion in the vertical plane parallel to the line of stitch and can execute two rotations for each cycle of movement of the needle and lifter. With the invention, a double-thread chain-stitch sewing machine can be made in three variants with flat, columnar and cylindrical platforms respectively. IMAGE
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