This means that the bottom thread is in no view on the front of the fabric, there are no floating threads on the surface and the bottom thread is about 1/3 on the back is optimal for normal embroidery.

If the thread tension is not right, problems such as thread breakage, bird's nests, and uneven embroidery can occur. These problems can greatly hinder the embroidery process and increase costs.

Make sure that the embroidery machine is threaded correctly before making any looseness adjustments. Each step is important. Sometimes incorrect threading can cause thread looseness problems.

For more information on how to properly thread, please check out these two videos.

Turn the screw counterclockwise to loosen and clockwise to tighten. When the bottom thread of the pattern is too full and the bobbin is too loose, you can adjust it by rotating the big screw on the bobbin case clockwise; when the bottom thread of the pattern is too sparse and the bobbin is too tight, you can adjust it by rotating the big screw on the bobbin case counterclockwise.

When adjusting the bobbin case, you can also adjust the corresponding knob for the needle currently being used.

• If there is bottom thread being pulled out of the fabric surface, the knob should be turned looser to the left. 
• If there is thread throwing on the surface, the knob should be adjusted to the right to tighten. 
• One and a half turns of the upper knob is one unit of adjustment, and one half turn of the lower knob is one unit of adjustment.

For those with experience, it's still mostly experience that counts when adjusting tension. If you are a novice, it is recommended that you use a tension tester to confirm that the thread tension is in the proper range. You can refer to the values in the table below.

Sometimes a small change in the spring lever may regulate the looseness or tightness on the embroidery work. If the spring lever is all the way up, the threads will be too loose for all needles to work properly. On the contrary, if the spring lever is placed to the bottom, the threads will be too tight for all needles.

The spring lever controls the looseness or tightness of all the needles of the machine, and it should be positioned directly in the center, at 90°.

It is important to notice that the reasons for embroidery problems are varied. If you find that the problem remains after adjusting the tension settings, it's time to consider other influencing factors. For example, the embroidery speed is too fast, the design of the embroidery plate is not reasonable, the machine maintenance is not good, and other factors may also cause the embroidery process to be unsmooth, which you need to eliminate one by one.