Imagine the fabric is a stream of Vibratory Polishing Equipment Machine Manufacturers. The threads are like ropes across the stream. One tug of war team pulls up and another pulls down. Each team pulls across the stream. If the lower team or bobbin team pulls harder than the upper tension team, excess thread will collect under the fabric out of sight until you lift the fabric to see ugly bunches of thread. It may even look fine on top while the threads jam up underneath. If the upper team pulls harder than the lower team, excess threads pop up on top of the fabric. Usually, you will see bubbles, balls, or bunches of thread almost as soon as they stitch.

Remember, your goal is to provide the same or equal tension from the upper and lower threads.

Begin your corrective efforts by rethreading the upper thread. Be intentional and careful to insure everything is just right. Remember to lift the presser foot while threading. Watch out for anything that might snag the thread. Check to make sure not lint or gunk has messed gotten stuck between the tension discs. Watch out for rough spots. Once you reach the needle, gently draw on the thread two or three inches. You should feel very little resistance. The thread should flow smoothly. Now drop the presser foot, and test again. Is there more resistance? Good.

Take your bobbin out. Is the thread wound evenly? It should be. Place it back in its carrier. Check the tension spring for any debris or lint that may have collected under it and clean it out. Slide the thread under the tension spring. Test it to make sure there is moderate resistance on the thread. This usually does not require adjustment unless you change the size of thread significantly. Some suggest that a professional sewing machine repair person make these adjustments when needed.

Next, adjust the upper tension to equalize the lower tension. You may need to rely on some trial and error until you get it just right. Ignore the numbers on the tension dial, unless they are really low or really high. In this case you may need to disassemble the tension assembly to make the repair, or have a technician to do.

Sew a trial seam on scrap fabric. Sew a straight stich and then a medium zig zag. Examine the stitch quality. Continue making slight adjustments until your get a stitch that pleases you. Avoid sewing on finish fabric, until you have confidence that your stitch quality is what it should be. Avoid frustration.

Sometimes things just do not work perfectly. The thread, the needle, the fabric, and the machine can all contribute to small challenges. Some of these issues can be resolved by using better thread, replacing the needle, using the right needle, or using better fabric. If you have good tension with straight stitches, but one side of the zig zag is still not just right. Minimize the flaws by setting tensions to keep the flaw on the underside instead of the facing side of the fabric. Narrowing and shortening the zig zag stitch can also help.

When you understand how your sewing machine tensions work, you can adjust them with confidence. In those rare instances where your best efforts fail, you can always rely on your local sewing machine repair technician.