Thread tension becomes more critical with high speed embroidery punching machines. Each machine will require slightly different tensions and there are various methods of setting it up correctly. A description of the procedure will be included in your machine manual, but here is a basic principal which will help you set tensions on any machine.
When you look at the back of the embroidery (bobbin side) on each straight vertical column you should see 1/3 top thread, 1/3 bobbin and 1/3 top thread. The one constant thread in a multi-needle machine is the bobbin thread, so getting this tension right first will save you a lot of grief. Use the following practice and you should get it right.
Place your bobbin in the case as if you were preparing to install it in the machine. Draw out a few inches of thread and hold it up, allowing the bobbin and case to hang. 2. Gently jerk the thread to allow the case to fall. The idea is for the case to gently fall approx. 6 inches then stop. If it runs too far, the tension is too loose. If it does not run at all, the tension is too tight. Adjust accordingly by tightening or loosening the screw on the case.
Now check the top thread. It should flow freely when pulled through the eye of the needle. There should be slight tension but no catching or pulling on the way through. The pre-tensioner is designed to place enough pressure on the thread to activate the sensors below it while the tensioner fine tunes the pressure to give the lustrous look we are after.
Now that you have both tensions correct, run a sample by embroidery digitizing a number of columns of satin stitch about 1.5 inches long and 1/4 inch wide. I use a lower case L. Make as many columns as you have needles on your machine and set it up so each needle stitches a column.
The goal is to get as close as possible to correct tension displaying approx 1/3 top thread 1/3 bobbin and 1/3 top thread. The other colors require the top tension to be tightened slightly to pull the bobbin thread. In extreme cases where the bobbin is too tight and upper thread too loose no bobbin thread will show at all. When top thread is too tight and bobbin too lose there will be only bobbin thread displayed on the underside and a salt and pepper look from the top.
The tension on your embroidery punching machine is one of the most important adjustments you will have to keep in check. Once you master these techniques, however, you will notice the machine tensions need adjustments almost immediately and will make necessary changes quickly.