In my beginning quiltmaking classes, the question I get asked the most often after the quarter inch seam allowance is how to fix a sewing machine tension problem [as well as what size needle to use for piecing and what type of thread etc.].
Notice that these two topics are largely about the sewing machine; later we get into the important quilty stuff like what kind of fabrics and patterns to pick and so forth. But you can’t get to those questions until you get better control over your sewing machine and even (gasp) gain some mastery over it. Mastery will come with practice and I’ll try to give you some pointers to get you going in the right direction. Through experience you’ll determine what works for you.
The first suggestion is to use a medium weight cotton thread for piecing (that’s probably a 50 wt. thread). You can use poly if you prefer but since it stretches a bit it’s not my favorite to sew with. I think cotton is easier to set tension-wise so I would start there. If a thread says it’s ‘all purpose’ it’s poly or cotton-covered poly (probably the worst of both worlds, very linty AND stretchy). Dressmakers often prefer poly because of its strength — a crotch seam in a pair of trousers is a great example of where strength is needed because of the stress on the seams — and sewing machine technicians like it because it doesn’t throw a lot of lint into the machine that someone will have to clean out later. But for my purposes I like to have a nicely balanced stitch and find it easier to achieve with a cotton thread. Likewise I use all high-quality cottons to piece with, and I like the idea of harmony of materials. I don’t prewash my fabric so everything is going to shrink together a bit, thread and fabric, and I’m pleased with that (even if it doesn’t all shrink at the identical rate, the crinkled effect is satisfying).
Also I recommend that you use neutral colors of thread, a warm taupe brown for warmer colors and maybe a middle gray for bluer darker colors. You won’t see the thread if the tension is balanced correctly in the final side-pressed seam so the thread color doesn’t matter so much. I do suggest that the top and bottom threads be two different colors of a similar weight thread. When there are two colors the tension problems and settings are much easier to determine than if the thread colors match.
I found a page with decent graphics that show how to look at a stitch and decide which tension (top or bobbin) needs adjusting: sewing machine tension adjustment. Try this first and if you still have difficulty you can take your machine to your dealer and ask them to help you adjust the tensions properly. The thing is that it’s worth learning how to do this for your machine because as soon as you change the thread and/or the fabrics you’re sewing the tension will probably need to be adjusted. I haven’t had a lot of experience with machines that have ‘auto-tension’ settings but what experience I do have suggests that there are times when you need to override the auto-tension and make your own adjustments.
Also if you use prewound bobbins this can change your tension. I love prewounds because they pack up to three times as much thread as you can wind on your own bobbin winder — which means you can sew three times as long before running out of bobbin thread — but sometimes the cardboard sides scrape against the inside of the shuttle case and alter the tension. Even if you pop off the cardboards the tension changes. So I always check the tension on a little sample seam with the threads I have loaded onto the machine and adjust as necessary.
For piecing, I sew with a ‘sharps’ needle and recommend a size 80. Anything smaller than that and the thread hole is mighty small and would require thin thread to sew delicate fabrics together. I don’t quilt with anything smaller than a 90 and recommend titanium needles, which bend and flex less than an ordinary needle.
When you first start sewing if you tend to tense up at the machine, you may need to adjust YOUR tension. A glass of wine and massage might help. Lock everyone out of your sewing space for a little while (unless you have babies or toddlers around, perhaps obviously) and breathe. Take your time and pay attention to what is actually in front of you. The mechanics of a sewing machine are actually pretty interesting.