Stitch line forming method

A stitch line forming method comprising:

• preparing an upholstery member and a sewing machine, wherein the sewing machine has a sewing needle having a quadrangular cross section in a plane parallel to a plane of the upholstery member, the quadrangular cross section of the sewing needle has two pairs of opposite corners, the quadrangular cross section of the sewing needle defines a pair of imaginary diagonal lines inside the quadrangular cross section between respective pairs of the two pairs of opposite corners of the quadrangular cross section, an upper thread, a lower thread, and a feeding direction in which the upholstery member is fed, wherein the upholstery member has a back face and a front face opposite to the back face;
• forming the stitch line, by performing a sewing cycle, in which, while the upholstery member is fed in the feeding direction, the sewing needle is reciprocated in a thickness direction of the upholstery member, and the sewing needle is positioned with respect to the feeding direction so that at least one of the imaginary diagonal lines between respective pairs of the two pairs of opposite corners of the quadrangular cross section of the sewing needle is inclined relative to a line extending in the feeding direction, to form needle receiving holes in the upholstery member,
• wherein each one of the needle receiving holes has two pairs of opposing corners, and each one of the needle receiving holes has two diagonal lines between respective pairs of the two pairs of opposing corners, and the imaginary diagonal lines between respective pairs of the two pairs of opposite corners of the quadrangular cross section of the sewing needle correspond to the two diagonal lines between respective pairs of the two pairs of opposing corners of the needle receiving holes, at least one of the two diagonal lines of the needle receiving holes is inclined relative to the line extending in the feeding direction,
• wherein, for each one of the needle receiving holes, the upper thread passes from the front face of the upholstery member to the back face of the upholstery member, the upper thread crosses the lower thread located on the back face of the upholstery member, the upper thread passes back to the front face of the upholstery member through the each one of the needle receiving holes, and the upper thread extends to the front face of the upholstery member from one of the two pairs of opposing corners corresponding to the one of the two diagonal lines which is inclined relative to the line extending in the feeding direction, for the each one of the needle receiving holes;
• and
• applying a tension to the lower thread, the tensioned lower thread acts on the upper thread which extends from the one of the two pairs of opposing corners of the one of the two diagonal lines of each of the needle receiving holes, to reduce an inclination, relative to the feeding direction, of the one of the two diagonal lines of each of the needle receiving holes.