Sewing Technology and Quality Control
Sewing Technology and Quality Control
This domain tests stitch and seam types, sewing machine settings, seam performance and inspection systems. Expect defect diagnosis and acceptance sampling.
Core rules and concepts
- Common stitch classes: 301 lockstitch, 401 two-thread chainstitch, 504 three-thread overedge and 514 safety stitch. Stitch class is not the same as seam type.
- Seam types include plain, French, flat-felled, lapped, bound and welt. Choice depends on appearance, strength, bulk and end use.
- Needle size, thread size, tension, stitch length and presser foot pressure must suit the fabric. Too small a needle cuts the yarn; too large a needle leaves holes.
- Stitch density is checked in stitches per inch or per cm. Tension is balanced when upper and lower threads meet inside the fabric.
- The 4-point fabric inspection system assigns 1–4 points to defects by length and calculates points per 100 square yards. AQL sampling accepts a lot within a defined defect limit; it does not require zero defects.
Worked example
A 120-yard roll, 60 inches wide, has 24 defect points. Area = 120 × 60 ÷ 36 = 200 square yards. Points per 100 square yards = 24 × 100 ÷ 200 = 12. If the buyer’s limit is 20 points per 100 square yards, the roll passes on this measure, but any single severe defect may still be rejected under the contract. The reasoning is that the 4-point result is a rate, not a raw count.
Traps
Do not call a 504 overedge a seam type. Do not diagnose skipped stitches as thread tension alone; check needle, timing and fabric. Do not confuse AQL with zero-defect inspection. Do not measure points per roll without normalizing for area.