Sewing Machine Operation
This domain tests machine parts, threading, tension balance and fault diagnosis on a lockstitch machine.
Main parts
Hand wheel, take-up lever, thread guides, tension discs, needle and needle clamp, presser foot, feed dog, bobbin and bobbin case, stitch-length regulator and reverse lever. The upper thread runs from spool through guides and tension discs, into the take-up lever, then down to the needle; the bobbin thread comes up through the needle plate.
Setting up
Use the right needle for the fabric (70/10 for georgette, 90/14 for denim) and match thread to fibre. Raise the presser foot when threading so the tension discs open, and wind the bobbin evenly. Test on a scrap: balanced stitches meet between the layers.
Faults and causes
- Loops under the fabric: bobbin case tension too loose, or upper thread not seated in the discs.
- Skipped stitches: blunt or wrong needle, wrong size, poor threading.
- Puckering: tension too tight, stitch too long on fine fabric, or needle too coarse.
- Needle breakage: pulling fabric, bent needle, needle hitting the plate.
Worked example
Stitches loop underneath while the top looks normal. The upper tension is pulling thread down and the bobbin thread is not controlled. Check that the bobbin is correctly wound and adjust bobbin case tension in small steps - do not tighten the upper tension first.
Traps
- Turning the hand wheel away from you.
- Threading with the presser foot down.
- Pulling fabric through instead of letting the feed dog move it.
- Oiling without cleaning lint first.