Specialised Machine Operations: Overlock, Flatlock, Feed-off-the-Arm, Bartack and Buttonhole
This domain tests the settings and limits of the specialist machines, and whether you can choose the right one for a construction detail.
Overlock (500 class)
Upper and lower loopers, two knives (movable and stationary), differential feed. Three-thread 504 neatens edges; four-thread 514 seams and neatens knits; five-thread 516 adds a 401 chain for strength. Differential feed above 1 prevents stretched, wavy edges on knits; below 1 stretches the edge. Cutting width is typically a few millimetres - too wide wastes seam allowance, too narrow risks the edge pulling out.
Flatlock / coverstitch (600 class)
602 gives two-needle bottom cover, 605 adds a top cover thread. Produces a flat, stretchable seam used for knit hems, binding and decorative joins. Looper tension controls where the bottom cover loops sit; the top cover thread fills the seam on 605.
Feed-off-the-arm
A narrow, offset bed with top and bottom feed and a puller, used for flat-fell seams such as denim inseams and side seams. It produces a strong, flat, low-bulk seam; needle spacing must match the finished seam width.
Bartack and buttonhole
- Bartack: a dense zigzag lockstitch (class 304), commonly 28 or 42 stitches, set by bar length and stitch count. Used at pocket corners, fly ends, belt loops, and buttonhole ends.
- Buttonhole: lockstitch type with purl and gimp cord for strength; straight, keyhole or eyelet shapes; cut before or after stitching depending on the machine. Length matches the button diameter plus thickness; density too high causes bulk and pucker.
Worked example
A cotton-lycra T-shirt hem curls and the coverstitch bottom loops float. Reasoning: the fabric is stretching as it feeds, so increase differential feed and reduce looper tension slightly, then stitch a scrap, stretch it and wash-test before running production.
Traps that cost marks
Using a plain overlock as the only seam on a woven garment, inverting the differential feed direction, and setting bartack length without checking stitch count.