Serger Stitches and Thread Tension

Serger Stitches and Thread Tension
The serger or overlock machine (interchangeable terms) rapidly trims, stitches and overcasts in one step a ravel-free seam edge using a combination of two, three, four (sometimes five) threads. The machine’s combination of components consisting of spools or cones of threads whose threading paths are color-guided on the machine; two regular machine needles (although one can be used) and two loopers (an apt name as the threads are looped or simultaneously crossed over/under a seam edge to enclose it); a metal stitch finger; cutting knives (some models have swing-away features to avoid cutting) as well as stitch length, stitch width, tension knobs or dials to control tension discs; two sets of feed dogs that move fabrics under the elongated presser foot to work at the same or slightly different speeds (differential speeds can gather or lightly stretch a seam) and all allowing for a disconcerting combination of factors that play into this marvel of time-saving professional-looking seam finishing.

The possible combinations of adjustments that go into producing a balanced serged seam finish are at times challenging to say the least.

Earlier and current models of sergers require manual threading and a strict threading order – upper looper first, then lower looper, followed by right needle and lastly left needle, as well as a necessary manual (along with a healthy dose of patience), to make sense of it all. Some newer models have a wonderous self-threading air-jet system for loopers.

A four-thread serger stitch is most durable and most common for woven and knit fabrics where both needles are threaded and both loopers threaded; three-thread stitches use only one threaded needle - left or right needle with both loopers threaded and work well on knits. A two-thread stitch known as a flatlock stitch uses one needle and one looper - for very stretchy athletic wear like yoga pants or to produce a very narrow rolled hem edge.

Needle Tension - the needle thread runs along the seam stitching line (left needle on a 4-thread stitch) It will look like a straight stitch produced by a regular sewing machine. If the tension is set too loose the resulting stitch will not knit evenly on the cut edge. Too tight and the fabric will pucker at the stitched seam line. The right needle forms another straight stitch parallel to the left needle. Both needles can be used in stitch formation or just one or the other depending on the look desired.

Looper Tension - the upper looper thread lays loops of thread on top of the seam allowance and the lower looper lays its loops on the underside. Both lock with the needle thread on the seam line and knit with each other on the cut edge of the seam allowance. Forming loops beyond the cut edge indicates tension is set too loose and puckering or rolling the seam edge to the top or bottom indicates loopers set too tight.

Stitch length will impact how much thread is released for each stitch. For some machines a stitch width adjustment dial moves the cutting blade and determines the amount of fabric within the seam. Or, using just one needle also impacts stitch width. The left needle is furthest from the cutting blade.

The stitch finger on a serger is not involved in tension but helps to control the stitches as they form and flow off the back. It sits unobtrusively near the needle plate.

It is most important to stitch a sample (or many samples) on a scrap of the same fabric before committing to a specific adjustment.

For quick thread color changes, first raise the presser foot to its highest position, then pull the needle threads out of the needle plate and temporarily off to one side. Pulling the needle threads out of the needle plate prevents the needle threads from trapping the looper threads under the needle plate mechanisms when rethreading.

Next, cut the current thread from the thread spools on each one above the machine. Replace thread cones with new ones, tie on the new color threads to snipped threads and gently pull each thread separately through each corresponding thread guide until the tension discs are reached, gently lift the new thread over the tension disc (to avoid the knot passing through and catching) and return to the tension disc and continue threading.

Lastly, the needle eyes will have to be threaded however the knots will most likely be too large to pass through. Just snip the knots near to the needle eyes and thread the needles as usual.

To balance the tension after changing fabric or thread, adjust one tension knob or dial at a time and test serge on a fabric scrap. Make small adjustments. Also, loosen or tighten the looper tensions after making any stitch length or width adjustments.

Keeping a notebook of samples detailing tension settings is invaluable. The serger is a wonderful companion to, not a replacement for, the tried and true traditional sewing machine.

Serging Info as seen on

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