Here is a practical article from Craftzine folk on tension and your sewing machine. Maybe of use to some folk!
Why is it that sewing machine tension can be such a bear to deal with? Here are some symptoms of tension problems:
knotting or jamming
top/bottom thread visible on the other side of the seam
Many of these problems can be alleviated by giving your machine a thorough cleaning and making sure it's threaded properly, but in some cases there's a bigger problem. Before taking it to the shop, you can change out a few parts to help troubleshoot and isolate the problem.
If you have been using plastic bobbins, they can wear out. If your needle catches the bobbin it can nick it, causing the thread to catch. Swap out your plastic bobbin for a metal one to see if it fixes the problem. Plastic bobbins can be gently sanded with a nail file to get out nicks. Over time the plastic can wear down as well from all the friction of the bobbin casing and thread, putting the bobbin out of commission.
If your needle is bent, dull, or inserted improperly, this can also cause tension problems. A bent needle is more likely to nick your bobbins, so watch out! Make sure you're using the right kind of needle for your thread and fabric; the wrong one can skip stitches (like using a sharp instead of ball-point needle on knits) or break your thread (if, for instance, the eye is too small).
Use good-quality thread. I found a site that shows magnified images of many different brands, showing how tight, loose, or fray-prone different types are. A poor quality thread causes more friction on the plastic parts of your machine and can wear them out prematurely. Which brings us to the biggest and possibly most expensive problem...
Most modern machines are made with plastic tension discs. Metal parts are great for durability and industrial use, but they are meant to be used all the time or they gum up. Plastic parts means a lighter machine and fewer maintenance and lubrication concerns when it comes to less-than-everyday use. However, as mentioned, fuzzy thread tends to wear down plastic parts over time (we're talking years, here). Plastic tension discs can become grooved, meaning you're out of luck when it comes to adjusting it, since they can't get a reliable grip on the thread. The tension discs are usually pretty far inside your machine, which makes it really hard to get in there and see if that's your problem.
NM State has a good guide for regular sewing machine maintenance to help you through the processes mentioned above. If you've tried everything short of taking the machine apart yourself, take it to a professional and explain all you've done. It's possible that your tension discs are fine and your timing is just off, which is relatively easy for a technician to adjust. Depending on the repair estimate, you might declare it time to upgrade to a new machine, vowing to only use high-quality thread (a small investment to protect your new machine). Your model might have easily-replaceable tension discs, but some manufacturers don't sell the replacement part. Thanks to Debbi Schlegel for her help on this topic. If you've got advice about tension problems, leave it in the comments below!
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