Most domestic sewing machines are of the "lockstitch" variety. That means an upper thread and a lower thread "lock" together. If they don't lock together in the correct place, the tension is "off" and the seam lacks proper strength.
Improper tension can cause:
- there might be a bird's nest of thread underneath the seam you are sewing (upper tension is too loose)
- the stitches may be puckered (tension is too tight)
- the understitches may be more prominent than the upper tension and it looks like they are standing to attention
- different fabrics require different tensions
- tension problems can also arise when one thread is used in the bobbin and another is used on top. This is common in quilting and in top-stitching garments.
I had lots of problems with the tension on my machine tonight. It's a new vintage machine and I love it, but I haven't worked all the kinks out just yet.
Check your needle. This is a quick fix but can be overlooked. A blunted needle, a bent needle or the wrong strength needle can affect the way the seam looks. (for more on needles, check out my post on needle here.)
There may be thread or dust in the bobbin case. Clean this with a dry Q-tip.
Remember to have the presser foot up while threading. That way your thread will lay in the tension guides.
Your needle thread may have jumped the take-up lever. This really messes up the tension.
Try removing both your bobbin and the top thread and re-inserting them both. This can sometimes resolve the problem.
Your bobbin thread may be out of its tension spring in the bobbin case.
Check that the thread is flowing off the spool and isn't catching anywhere.
Are you threading your machine properly? I recently acquired a new vintage sewing machine by the same make I have used since I began sewing. I thought I knew how to thread it - the same way the other similar machine was threaded -but I was wrong. Check your manual for a quick re-cap. Improper threading causes tension errors.
The Tension Dial
Turn your thread tension dial as needed. If you increase the number on your tension thread dial, you will be increasing the tension on your sewing machine. If you are lowering the number on your tension thread dial, you will be decreasing the tension on your machine.
Adjust the tension by turning the knob 1/4 turn at a time. Check how that affects the tension by sewing a seam and observing it. If it has not improved the situation, continue moving the dial 1/4 of the way to the next number on the dial.
I keep note of the tension dial that works for me with each fabric. It makes life easier.
While it is possible that your bobbin tension is the culprit, most problems -99% of problems - adjustments to the upper tension will fix the problem. Adjustments to your bobbin should be your last resort. It is difficult to re-align the bobbin tension once it is tampered with - so try everything else ( twice!) before fiddling with it. If you must, use a screw driver and turn the case in minimal increments until the problem is fixed.
Three Ways to Keep Dust Out of the Sewing Machine:
1) Keep your sewing machine covered when not in use.
2) Get a narrow paintbrush from a hardware store. Use it to clean the bobbin area and the feed dog area before you start to sew. (the feed dog area often catches a lot of fluff!) Or use a dry Q-tip.
3) Purchase un-waxed dental floss and keep it with your sewing supplies. Periodically, you should “floss” the upper tension. Just thread a long piece of the dental floss through the upper tension the same way you would thread your machine. Draw it all the way through, and it will collect any bits of fluff or thread that may cause uneven upper thread tension.