One of the greatest annoyances for us sewists is tension. But sometimes, when it looks as though the tension is off, it may be something else. There are four things which seem to primarily affect the stitch quality: the tension, the needle, whether you have correctly threaded the machine, and the thread.
The thicker the fabric, higher the tension must be to lift the lower thread up to the middle of the layers of fabric. You will usually be alright with a 4 or 5 on medium to medium-heavy fabrics like linen and twill weaves such as drill and denim. Thick upholstery fabrics may require a higher tension setting and a longer stitch, and lighter fabrics like cotton or even sheers will require a lower tension setting.
Make sure you thread your machine correctly. All the little milestones are there for a reason and missing one could totally ruin your stitching.
If the thread is bunching up under the fabric and you machine won't sew, you may have missed the take-up lever or "goose-neck". Without that, the thread will just stay down there and not make a stitch. It's kind of hard to explain it, but if you miss it on purpose, and turn the hand wheel with the machine switched off, you may be able to see what I mean. Until I missed the take-up lever I didn't appreciate it's importance; now I do. : )
Note: If you machine keeps locking up and the thread is tangled underneath, you may have sewn over a loose thread. Turn your machine off, snip the threads and remove the ones that are stuck. Then you can switch your machine back on and sew again, keeping all loose threads out of the way. : )
If you have been spending hours (or possibly days) trying to get the tension correct then it's probably not a tension problem. Try a bigger needle. A size 14/90 is a good average size for medium to medium-heavy cottons etc. and for when you use Sew-all thread.
People will usually tell you that to sew fine fabric such as lining fabric you must use a fine needle, e.g. size 80 or below. What they don't tell you is that that is only correct if you are using a fine thread like fine silk as well, or possibly if you are sewing a loose-weave fabric. Otherwise the needle won't make a big enough hole for the lockstitch and it will look like you have too loose upper thread tension. I seldom go below a size 14/90 unless the type of needle doesn't go up that high.
If that still doesn't fix it, make sure you are using quality thread. For some reason, even polyester cheap thread doesn't give the desired results. Use Gutermann or Coats. They are kinder to your machine anyway because they are smoother and don't leave so much fluff on your machine.
"Good in, Good out"
Remember, if you want your machine to give you good results and to last a long time, you have to look after it and that includes giving it good 'food' i.e. thread. It's the same as any other appliance, or even you. You know that junk food is bad for you. Well, junk thread is bad for your machine.
This is just what I have found out recently through experience. I don't know if it's just my machine, but my Brother XR6600 seems to be rather picky and likes to have the best. : )
Other things you can do
For tricky fabrics, like sheers and leathers etc., it helps to have the right foot, but if you don't have one, you can put tissue paper, such as Burda make, on top to help the feed, or underneath to stop fabrics from getting pulled down the needle hole. Putting it underneath also helps to get embroidery stitches to look right. You can use stabilizer if you prefer.
Save Time with This Helpful Tip
When you sew a new fabric, find the correct needle and tension and write it and the fabric's name on a swatch of the fabric. As you go along you will build a little encyclopedia of fabric. Then you won't have to keep trying to find the right tension and needle for the fabric. You can save hours! You might even try a blind hem and various stitches to find the best stitch settings and the colour and type of thread and write that down. Then you know what thread to shop for when you sew something else from the same fabric.
Until next time, happy sewing!
P.S. What problems have you had with your machine, and how have you fixed them?