When you sew you want a nice sturdy seam, one that won't pull apart or leave gaps or cause puckering. To get this it's important to have the tensions set correctly. Many seamstresses get confused about how to set their tension. It's a fairly simple process once you understand a few basic things.
BASIC LAW OF TENSION to form the stitch: The UPPER thread tension changes the look on the UNDERSIDE of a seam while the BOBBIN tension, the bottom tension effects the appearance of the TOP SIDE of the seam. That is quite opposite of what most people think. Basically what you want is for the top and bobbin tensions to equal so that the stitch forms half way between the two.
If you can see threads from the bottom side of the stitch showing on the TOP side of the seam, like the graphic below, then either the BOBBIN tension is too loose or the TOP tension is too tight.
If there are loops on the bottom side of the seam, like the illustration below, then the TOP thread tension is too loose, or the BOBBIN tension is too tight. Typically the TOP tension is too loose.
If there is puckering on the top side then most likely the TOP tension is too tight, though it may also be the the BOBBIN tension is way too loose.
So how can we tell which it is? The best way its to begin by taking a scrap piece of the fabric you will be sewing on, or at least the same type of fabric, picking the widest zigzag stitch possible with a medium to long length of stitch and sew several inches. Now examine the seam remembering which is the top and which is the bottom. If it does not look identical on both sides then one of the tension settings is wrong. (it is also possible that the needle is the wrong type but for the moment let's assume it's the tension).
If there's a problem with the stitch you can use the above rules to figure out which tension is off. However, as the top and bottom tensions work together let's begin by going through a basic check-off list.
First examine the threading of the upper thread and make sure it is correctly following the path and isn't catching on something. This is really important when you think you've looked through everything and just can't 'see' the problem. Taking the time to go through these steps can save you a lot of pulled hair!
Next pull out the bobbin and examine it in it's case. Are the threads would around it evenly or are they all jumbled and crisscrossed? Jumbled up is a BAD thing. Try a different bobbin that is wound correctly. Now examine if the thread is coming out of the proper place, through the tension slot. Pull on the thread to see if there's total resistant. If so, something isn't right. But it can also be a problem if there is no resistance so let's now do this test.
Suspend the bobbin in it's case by the thread. Let it dangle there is space, still holding it by the thread as if it was a yoyo. It should dangle there with a little slipping, the length of thread between it and your fingers getting a bit longer. If it hits the floor there's no resistance! You have NO tension. That is BAD. Now, while still dangling it gently flick your wrist like you're holding a yoyo and wanting it to drop down a little bit, which is exactly what it should do if the tension is correct. If it doesn't release any thread at all, doesn't drop down a bit then the tension is too tight. Most bobbin tensions are adjusted by turning the little screw on the casing next to where the thread comes out. (Make sure that the bobbin turns in the case the right direction too which is the same direction of the slot!)
If the bobbin drops a lot it is too loose and you will need to tighten it a bit. Remember that a gentle flick should allow more thread to come out but not reel out. There should be some resistance.
Now that we have the bobbin correctly adjusted place it back in the machine and sew another test seam and examine it. By using the rules at the beginning of this tip determine if the upper tension is just right, too loose or too tight. Adjust the upper thread tension accordingly, first raising the pressure foot then turning the dial or however your machine adjusts tension. The higher the number the tighter the tension and vice-versa.
So that you know what the upper tension should feel like pull on the thread at a point BEFORE it goes through the needle first. Pulling after it goes through the needle puts a bit more tension on the thread and I want you to feel the tension before that point. If the tension is too tight and you pull on it after it goes through the needle it may break the needle if it's a small sized needle. You should feel some resistance. You shouldn't have to tug hard on it to pull more thread through but it also shouldn't reel out without any resistance. If the thread is breaking either the tension is very high or the thread is catching somewhere. Check the threading as well as look to see if the spool is turning freely on the spindle. Sometimes the thread will catch on the spool itself. When you buy a new spool of thread remember how the end was through a tiny slot on the side of one end? If that slot is on the bottom of the spool on the spindle it can sometimes catch the thread as it turns. Simply turn the spool upside down and re-thread the machine if needed.
Now once more do a test seam and examine it. Follow the above steps until the top and bottom of the zigzag are identical - perfect!