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90% of the time, we adjust the top tension to achieve the perfect stitch.

We talk a lot about tension settings and tension adjustments on machines. This time, we will discuss the other 10%, the bobbin tension. Many have been told to never touch the bobbin tension. It isn't as difficult as we have been led to believe. If you are one who has been told to never touch it, by the end of this page, hopefully you will realize that is perfectly safe and easy to adjust the bobbin case. Of course you could buy a second bobbin case, one to never adjust and the other to experiment with, but why not save $30 to $40 and learn how easy and safe it really is. Over time, tensions can change with regular use. Even though you haven't physically changed the settings, they can work themselves either tighter or looser. Thread, lint, and even temperature can affect them. There are three times when adjusting the bobbin tension might be necessary. Number one and two are obvious. Number three is the "I never thought of that before" alternative. 
    1. When using a very smooth, fine bobbin thread. If the thread is very smooth and fine, the preset setting may not apply the necessary brakes to stop it when you stop sewing. In this case, the bobbin thread continues to unwind, potentially causing backlash, and upon start up again, the thread will break. Tightening the tension will fix this.  
    2. When using a very heavy bobbin thread. The preset tension might be too tight for a heavy thread, preventing the bobbin thread from unwinding freely. Loosening the bobbin tension will solve this. (Be sure to turn the tension screw in small increments whether you are tightening or loosening the screw.)
    3. Q: "There are times when the bobbin adjustment is correct but no matter what I do to the top tension, I still can't get a perfect stitch or the thread breaks. When I loosen the top tension adequately low to run a sensitive or heavier thread, I get loops on the back. When I tighten up the top tension to get rid of the looping, the thread breaks."
      A: Looping on the back means the top tension is too loose compared to the bobbin tension so the bobbin thread is pulling too much top thread underneath. By tightening the top tension, the loops will stop, but the added tension may cause breakage, especially with sensitive threads. In this case, it might be necessary to loosen both the bobbin tension AND the upper tension. By loosening both the top and bobbin tensions, both sides of the tug-of-war give in, allowing a good stitch without breaking or looping.

How to adjust Bobbin Tension by Superior Threads

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By: Bob Purcell