Thread chain sewing method and device for use in the two-needle overlock sewing machine

A device for backtacking thread chains in a two-needle overlock sewing machine having an outer chaining-off finger and an inner chaining-off finger, and a pair of thread-carrying needles arranged to drop one on either side of the inner chaining-off finger, comprising:

• means for controllably retracting the inner chaining-off finger from an advanced position in which it is kept alongside the outer chaining-off finger, said means for retracting having means for driving the inner chaining-off finger forward and backward in a fabric feed direction, and for holding it in its advanced and retracted positions;
• a thread tension changeover mechanism for controllably tensioning each of said needle threads tightly and slightly;
• a fabric material detection means for detecting whether the fabric material is present at the needle drop points;
• a stitching operation detection means for detecting operation of the sewing machine;
• and,
• a control circuit for causing the driving means and the thread tension changeover mechanism to operate to hold the inner chaining-off finger in its advanced position and to change over the tension of each of the needle threads from a slightly tensioned state to a tightly tensioned state when fabric material is detected by the fabric material detection means, said control circuit also causing the thread tension changeover means to operate to change over the tension of each of the needle threads from the tightly tensioned state to the slightly tensioned state when the fabric material is not detected by the fabric material detection means, and said control means also causing the driving means to hold the inner chaining-off finger in its retracted position when the operation of the sewing machine is not detected by the stitching operation detection means.