The correct thread tension is crucial to embroidery success. If bobbin thread has too much slack, it will begin to loop. Conversely, too much tension may cause thread breaks. Always check your bobbin thread tension first, because it could adversely affect the tension of the top thread.
When using a bobbin tension gauge, the bobbin tensions should be 18 to 22 grams, and as much as 25 grams when embroidering caps. Rayon top tensions should be at 100 to 120 grams, and Polyester top tensions at 120 to 150 grams on a top tension gauge. If you are using a specialty thread, such as a heavy metallic thread, it is best to tighten the tension to prevent thread breaks. If your thread still breaks after tensioning your bobbin and upper thread, be sure that your needles are fresh. Old needles become worn out and could also be responsible for the thread breaks.
~Kyle Heslin, Madeira USA