When I am teaching new sewers, if the stitch looks a little off, usually the first thing they go for is the tension. Often, it isn’t in fact the tension that is set incorrectly, rather their needle is old, the type of needle being used is incorrect or often the stitch length and width is set wrong. Sometimes though, it is actually your tension that needs correcting.
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All machines have a dial, usually with numbers along it and metal discs behind it. The higher the number, the more tension pressure is placed on the threads by the discs, and therefore distributing less thread into the machine. The lower the number, the less tension pressure is placed on the threads, and more thread is being fed into the machine.
Note, this is for the spool of thread at the top of the machine, not your bobbin. The bobbin also has its own tension dial, but as it is easy to throw the bobbin tension off alignment with the slightest of adjustment, I would suggest never changing the tension on your bobbin without the assistance of a repair shop.
Typically you can set your machine to "auto" or the middle number on the tension dial that is highlighted on your machine, but when you change to thicker or thinner fabrics, the tension might need to be adjusted to accommodate for the change in weight.
And, if you are using a serger or overlock machine, you need to check it each and every time you sew, since there is no "standard" or "automatic" setting on a serger due to the loopers that wrap themselves around the raw edge of the fabric. Below are examples of the most common stitches–straight, zigzag, and serger–and what a good stitch looks like, along side those that are too loose and too tight.
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Let's see how an ideal stitch should appear and how to achieve this stitch yourself!
Above is the top layer of the straight stitch, with the ideal setting in the center, and more tension on the left, and less tension on the right. The stitch on the left is puckering the fabric because it is too tight, and the threads on the right are a bit too loopy.
The underside of the same stitch is pictured above. The middle stitch still looks ideal, with the puckered stitch on the far left. The straight stitch with too little tension is really terrible on the underside, with the threads forming giant loose loops, resulting in a very poorly sewn seam that will not hold in place.
For a zigzag stitch, the differences are a little less obvious on the top layer. Above we have the ideal setting again in the center, with the stitch on the far left with too much tension, and the stitch on the right with too little tension.
The stitch on the left has slightly pulled the fibers of the fabric together to form a small hump between the zigzag points. The stitch on the far right is too loose from point to point, so you can see a bit of shadowing under the stitch itself.
Much like the straight stitch, the real problem for the zigzag stitch shows up on the underside. The ideal stitch is centered, the stitch with too much tension is on the far left, and the stitch with too little tension is on the far right.
The upper threads on the stitch on the far right are being pulled to the underside, creating loops where there should be points. On the stitch that's too tight, it is again forming a small hump and the actual zigzag is narrower than the ideal stitch, as it's pulling too tightly from point to point.
With a serger stitch, there are four threads to consider: the two straight stitches, and the upper and lower looper stitches. Each and every time you use your serger, you should test your stitch. The stitch in the center shows an ideal balance between the straight stitches and the looper stitches. On the right, the looper threads have not been altered, but the tension on the straight stitches has been lowered, making the stitch much too loose.
Interestingly, it makes the looper threads appear too loose, when they have not been changed at all in all three of these stitches. The stitch on the far left is the tension turned up all the way, and the only real way to spot this is that the straight stitches are so tight, they appear nearly invisible on the second row. They have also pulled the looper stitches beyond the center point.
The underside of the same stitch is pictured above. The ideal setting is in the center, nicely balanced between the threads and loopers. On the right, the lack of tension has created an absolute mess, as the threads cannot pull the loopers correctly. On the far left, the balance is off, a much more subtle correction to make, much like the previous image.
Lastly, these stitch examples show the opposite of the previous ones–these have only had their upper and lower looper threads changed, and the straight stitches have remained constant from left to right, despite the change in appearance. At the center, we again have our ideal stitch, perfectly balanced. On the right, the tension has been lowered and the loopers are much too loose. On the left, the additional tension has pulled the threads to the side of the straight stitch, as it is far too tight.
The underside of the same stitch shows again too much looping on the far right, with the much too loose tension. On the left, the only real sign that this is incorrect is that the loops are not meeting at the cut edge of the fabric, and rather have been pulled to the back side. The ideal stitch has the loopers meeting and joining directly centered on the cut edge of the fabric.
To test your stitch for sewing, always cut a scrap of exactly what you will be sewing–same fabric, interfacing, and quantity of layers–and give it a pass through all the stitches you intend to sew it with. I also suggest sewing on both the crossgrain and the length of grain, as well as on a curve or bias should it apply. That way you can get a clear image of what your stitch will really look like while sewing the finished project.
Always better to be safe than sorry!