Stitch Types, Seams and Stitch Formation Faults
Questions here test the ISO stitch classes, the seam classes, and your ability to trace a fault to its cause rather than just naming it.
Stitch classes (ISO 4915)
- 100: single-thread chain (101).
- 300: lockstitch (301 straight; 304 zigzag, used in bartacks and buttonholes).
- 400: multi-thread chain (401 two-thread chain, strong and stretchy, common on waistbands).
- 500: overedge (504 three-thread, 514 four-thread, 516 safety stitch combining 401 plus 504).
- 600: coverstitch (602 two-needle bottom cover, 605 top and bottom cover).
Seam classes (ISO 4916)
SSa superimposed (plain seam), LSc lapped, BSa bound, FS flat seam, EFa edge finishing. Seam types you must recognise: plain, French, flat-fell, lapped, welt.
Fault diagnosis: cause before cure
- Skipped stitch: blunt or bent needle, wrong point type, wrong needle bar height, hook timing, thread tension, thick or elastic fabric.
- Needle thread break: tension too tight, burr on throat plate or hook, needle too fine for the thread, heat from friction.
- Bobbin thread break: rough bobbin case, uneven winding, trapped lint.
- Puckering: tension too tight, too many stitches per inch, thread and needle mismatch, feed problem.
- Needle cut or holes along the seam: needle too coarse or wrong point for the fabric.
- Seam slippage: too narrow a seam allowance or too few stitches per inch on a slippery weave.
Worked example
A 301 seam on woven shirting shows occasional loops underneath and two skipped stitches in a 30 cm run. Reasoning: rather than tighten the top tension, check the needle first, because skipping points to needle condition or hook timing. Replace the blunt needle, then reset upper tension, then test. Tightening tension over a damaged needle hides the real fault and creates puckering.
Traps that cost marks
Calling 504 a four-thread overlock, mixing up stitch class with seam class, and assuming that tighter tension cures a skipped stitch.