Sewing Tools, Equipment and Machine Basics; Maintenance and Safety
What this domain tests
Parts and function of the single-needle lockstitch machine, needle and thread selection, routine maintenance, and fault diagnosis - usually one labelled-part question plus one troubleshooting scenario.
Machine parts and their jobs
- Spool pin and bobbin winder; tension discs and tension regulator; take-up lever; needle bar and clamp; presser foot; feed dog; throat plate; bobbin case; handwheel; reverse lever; stitch-length regulator.
- Stitch Class 301 is the two-thread lockstitch: the needle carries the top thread down, the hook catches the loop, and the bobbin thread interlocks inside the fabric.
Needles, thread and maintenance
- Needle sizes: 9/65 for voile and fine fabric, 11/75 for jersey (ballpoint), 14/90 for shirting, 16/100 for denim.
- Ballpoint needles on knits avoid cutting yarns; sharp needles on jersey cause holes.
- Daily: brush lint from feed dog and bobbin area, oil per manual, check cord and belt.
- Change the needle at shift start or after roughly eight hours; a bent or blunt needle causes skipped stitches and needle holes.
- Timing and hook adjustment belong to the mechanic, not the operator.
Worked example
Skipped stitches appear on a cotton-jersey T-shirt. Reason through it: needle blunt or bent → replace; wrong point type → switch to ballpoint 11/75; needle inserted backwards or not fully up in the clamp → reseat; incorrect needle-bar height or poor thread quality → escalate. The order matters: cheapest and most common causes first, timing last.
Traps that cost marks
- Calling the feed dog the presser foot, or the throat plate the bobbin case.
- Blaming tension for skipped stitches; tension faults show as loose or looping stitches, not missing ones.
- Adjusting machine timing as an operator.
- Using a 9/65 needle on denim.