Do I need an overlocker, or will a zigzag do?

The overlocker question is really a question about volume and about what you sew. It is an addition to a sewing machine, never a substitute for one, and it is a machine with a genuine learning curve of its own — four threads, differential feed, and a knife that cuts as you go. Whether that is worth it depends entirely on how much of your sewing is the kind it makes dramatically easier.

Step by step

1. Understand what it does in one pass.Two needles and two loopers sew the seam, wrap the raw edge, and a knife trims the excess off just ahead of the stitching — all at once, and considerably faster than a sewing machine. That is what a factory seam is.
2. Understand what it does for knits.The looper chain has stretch built into it, so an overlocked seam in jersey stretches with the fabric instead of snapping. Differential feed — running the front and rear feed dogs at different speeds — also stops jersey coming out wavy. Both of these are genuinely hard to reproduce on a sewing machine.
3. Understand what it cannot do.It cannot topstitch, sew a visible hem, insert a zip, make a buttonhole, sew a dart or start and stop in the middle of a piece of fabric — it can only sew from an edge, off an edge. Everything visible on a garment is still done on the sewing machine.
4. Be honest about what you sew.If you mostly sew woven garments occasionally, a zigzag finish is perfectly adequate and you will not use the overlocker enough to justify the threading. If you sew t-shirts, leggings, children's clothes, activewear or a steady stream of garments, it changes the experience completely.
5. Factor in the threading.Four cones of thread, threaded in a specific order through a specific path. Modern machines have colour-coded paths, lay-in tension and sometimes air threading, which makes it far less painful than the reputation suggests. Older or cheaper machines can be genuinely frustrating.
6. Factor in the running costs.Four cones of thread rather than one reel, replacement knives eventually, and the fact that the machine works best if you keep several colours of cone thread in stock. Cone thread is cheap per metre and the initial outlay is real.
7. Consider a coverstitch separately.The twin parallel rows on the hem of a bought t-shirt come from a coverstitch machine, not an overlocker. Some overlockers convert; the conversion is fiddly. A twin needle on an ordinary sewing machine gives a close approximation for far less.
8. Try before you buy if you can.Many sewing shops run introductory sessions, and second-hand machines are plentiful. Threading one under supervision once tells you more than any review.

Alternatives

• Zigzag on a sewing machine: Free, on a machine you own. Finishes a raw edge perfectly adequately. Slower than an overlocker and does not trim as it goes.
• Overcast stitch and foot: Many sewing machines include a dedicated overcasting stitch and a foot with a guide bar. The closest a sewing machine gets to an overlocked edge.
• Stretch stitch on a sewing machine: A narrow zigzag or a lightning-bolt stretch stitch sews knits acceptably without an overlocker. Less elastic and less neat, and entirely workable.
• Overlocker: Trims, sews and wraps in one pass, and gives a properly stretchy seam. Fast, professional, and a second machine to buy, thread and store.
• Coverstitch machine: Produces the twin-row hem seen on ready-to-wear knitwear. A specialist third machine, worth it only if you make a lot of jersey garments.

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