overlock

An overlock is a stitch formation that wraps thread around the raw edge of fabric, simultaneously cutting away excess material and binding the edge in a single pass. Unlike a lockstitch or chainstitch, the overlock uses multiple threads, typically three to five, looped over a knife blade and around the fabric edge. This produces a finished, sealed edge that prevents fraying and adds durability, making it essential in apparel and textile manufacturing where both speed and quality matter.

The overlock machine, also called a serger, cuts the fabric edge with a small blade and feeds thread loops over that freshly cut edge. The most common configuration uses four threads: one needle thread on each side and two looper threads that interlock beneath the fabric. A three-thread overlock omits one looper and is lighter, faster, and common in stretch fabrics and single-layer seaming. Five-thread versions add a separate chain-stitch looper for stronger seams, typically used in heavy wovens or where seam strength is critical.

Tension adjustment on an overlock is more complex than on conventional machines because each thread must be balanced independently. If needle tension is too tight, the edge puckers; if the looper tensions are uneven, the stitch pulls to one side or gaps form. The knife blade itself requires regular maintenance: it dulls quickly on heavy fabrics, synthetic materials, or when hitting pins, and a dull blade leaves a ragged edge rather than a clean cut.

Use and limitations

Overlocks excel at finishing seams in knits and stretch fabrics because the stitch itself stretches with the garment. They are also the standard for athletic wear, lingerie, and children's clothing where soft, flexible seams are required. However, overlocks cannot start or stop stitching as cleanly as lockstitch machines, so they are poor for topstitching, buttonholes, or any seam that requires precise beginning and end points.

The name derives from the stitch literally overhanging, or overlocking, the fabric edge. In the US and UK it is often called an overlock; in other regions the term serger, from the French sergé (serge weave), is more common. Industrial overlocks run at 4000 to 6000 stitches per minute, roughly double the speed of comparable lockstitch machines, which accounts for their dominance in high-volume production.