Seam, seam forming device, and seam forming method

A seam forming device comprising:

• two needles which are juxtaposed in a substantially perpendicular direction to a sewing direction and to which needle threads for forming chain seams and lock seams are fed;
• an upper drive means for reciprocating the two needles in a vertical direction;
• a first looper for passing a loop of the needle thread of one of the needles formed on a back face side of a sewing material through a loop of the needle thread of the other needle in synchronization with vertical movements of the two needles;
• a loop spreader for retaining the loop of the needle thread of the one needle formed by the first looper until the needle thread of the one needle of the next or later stitch is positioned in the loop of the needle thread of the one needle of the earlier stitch;
• a second looper for passing the loop of the needle thread of the one needle of the next or later stitch through the loop of the one needle thread retained by the loop spreader;
• and
• a lower drive means for causing the first and second loopers and the loop spreader to operate in synchronization with the vertical movements of the two needles,
• wherein the device forms a seam formed by tightening the chain seam by the lock seam.

A seam forming method for sewing by forming chain seams and lock seams with needle threads of two needles juxtaposed in a substantially perpendicular direction to a sewing direction, the method comprising:

• a first step of moving down the two needles by a drive means to protrude the needle thread of one needle and the needle thread of the other needle to a back face side of a sewing material;
• a second step of moving up the two needles by the drive means to form a loop of the needle thread of the one needle and a loop of the needle thread of the other needle on the back face side of the sewing material and passing the loop of the needle thread of the one needle formed on the back face side of the sewing material through the loop of the needle thread of the other needle by a first looper protruding in synchronization with the upward movements of the two needles;
• a third step of retaining the loop of the needle thread of the one needle, which is retained by the first looper during the upward movements of the two needles, on a loop spreader protruding in synchronization with the upward movements of the two needles and then for returning the first looper to an original position to separate the first looper from the one needle thread;
• and
• a fourth step of spreading the loop of the one needle thread, which is retained on the loop spreader, by a second looper protruding in synchronization with the first looper, passing the one needle out of the two lowering needles through the spread loop of the one needle thread, and returning the second looper and the loop spreader to original positions to separate the second looper and the loop spreader from the one needle thread.