We often talk about the importance of tension adjustments when using specialty threads. Remember the illustration showing the tug-of-war between the upper and lower threads? The illustration is on the Thread Tug of War page.
Tension adjustments are usually made for the upper thread in order to allow the machine to run decorative threads. There are times, however, when it may be necessary to make adjustments to the bobbin tension. If you use a very fine and smooth thread in the bobbin, you may need to tighten the bobbin tension. If you use a very heavy thread in the bobbin, you may need to loosen the bobbin tension. The safest way to handle this is to have two bobbin cases, one for regular use which is not adjusted and one for specialty uses when can be adjusted.
Before making any adjustments, mark the normal position of the screw by making a dot with a permanent marker. Adjusting the bobbin tension is done by turning the small screw on the bobbin case. To loosen, turn to the left or counter-clockwise. To tighten, turn to the right or clockwise. Think of the adjustment screw as a clock and adjust the tension in the amount what would be a 5 or 10 minute movement on a clock.
Check the bobbin tension by using the drop test, which is the best way to know when the tension is right. Hold the end of thread and let the bobbin and bobbin case drop. It should fall at a slow-and-steady pace and eventually stop before touching the floor. If the bobbin case falls right to the floor in a fast manner, the tension is too loose. If it doesn't fall at all, the tension is too tight.
If the tension is too loose, another problem, called backlash may occur. This is when the bobbin thread continues to unwind unchecked, even after the sewing has stopped. The loose thread then gathers in the bobbin area or under the plate and tangles, resulting in breakage and bird nesting.
By adequately tightening the bobbin tension, this problem can be avoided. Adjustments are simple. Learn to make the adjustments and you will be able to use a larger variety of threads.