Looper Tension and Cutting Width

Everything in serging is relative. Looper tensions relate to each other. And cutting width relates to looper tensions.

Tension is difficult for people to understand. When the thread passes through the tension dials, it is actually sandwiched between two disks, like plates. The higher the tension setting, the harder the disks squeeze together. The harder the disks squeeze together on the thread, the less thread passes through and into the stitch. The tension disks, in effect, hold the thread back.

Cutting width is easier to understand. It refers to the distance between the needles and the knife. The larger the cutting width, the more fabric is incorporated into the stitch.

Note: Unlike on a conventional sewing machine, the needles on a serger do not move side to side. Stitch width is a factor of two things...cutting width (fabric) and looper tensions (thread.)

Remember these two principles:

1. Thread - The less looper tension, the more of that looper thread will be allowed into the stitch. The tighter the looper tension, the less of that thread will make it into the stitch.
2. Fabric - The larger the cutting width, the more fabric will be incorporated into the stitch. The smaller the cutting width, the less fabric will be incorporated into the stitch.

Really understanding those two points will enable you to make any stitch adjustments on any fabric.

To make a stitch wider, you must give it more fabric (increase cutting width), and more thread (decrease looper tensions.) If you do only one, the stitch will not be balanced. If there is too much fabric relative to the amount of thread, the fabric will curl inside the too-tight stitch. If there is too much thread relative to the amount of fabric, the loose stitches will hang off the raw edge.

For example, say you are serging along and you notice that your stitch is hanging off the edge of the fabric, like this:
If you look closely at my lousy photo you'll notice that the tops of the threads extend past the edge of the muslin. You can see purple right through them.

Either there is too much thread, or there is too little fabric. (In serging, everything is relative!) So, you can fix the problem with either principle #1 or #2 from above.

Using principle #1 - There is too much thread in the stitch, b/c it's hanging off the end of the fabric. To get less thread in the stitch, you need more tension. Tighten both looper tensions and the stitch should snug up to the edge of the fabric.

Using principle #2 - There is too little fabric in the stitch. Increase the cutting width and more fabric will be incorporated into the stitch, and the threads will be "filled up" with fabric.


Note: Principle #1 also applies to balancing looper threads against each other. If it looks like there is not enough lower looper thread, and the upper looper thread is being pulled underneath:
then you must lessen the lower looper tension.

If on the other hand, the lower looper thread is being pulled to the front:
then you must decrease the upper looper tension.

Tension is very much like a constant tug-of-war match between the upper and lower looper threads. The more tension you give to one, the more strength it has, and the more it will pull the other to its own side. (Taking the metaphor even further...the more tension a thread has, the more its muscles contract, the shorter its rope gets and the more it pulls the opposite thread over to its side.) Don't let one looper thread get the better of the other with too much tension.

Of course, you can't be forever tightening one tension then the other to keep them in balance. Eventually you will have too much tension in both loopers and the fabric will curl under the stitch (then you have a rolled hem...topic for another post.) I recommend first loosening a tension to try to balance a stitch. If that is not enough you can then tighten the opposite thread a bit as well.

Once you've internalized the way that thread tensions work with each other and cutting width, you'll be able to solve most serging problems intuitively. Really!