No, your machine is not broken – the thread is at fault!
I spent all day yesterday trying to find the culprit of these thread pulls. Surely, the bobbin thread is catching on something! I polished the needle plate, including inside the needle hole. I polished the shuttle, inside and out, the shuttle spring and the cradle. I polished the race. I changed the needle. I changed the foot and polished it underneath. I even polished the tips of the feed dogs! My machine has never been so polished!
I changed the bobbin, that is, I wound a new one using the thread from the old one. And then it dawned on me: the thread!
Look how it is twisting!
Good Lord! What’s with that great big twisted loop?
This is a vibrating shuttle machine which I am using on low tension to sew stretch cotton with a flexible seam. So when the thread is pulled up from the shuttle, the twisted loop simply slips through as if it were a knot! The machine is not bothered by such stuff, so it just continues sewing…
So what would happen in a round bobbin machine that sews with higher tension? You’d think the tension would straighten out the twists, but the result is in fact even worse: it chokes. I tried several machines with different mechanisms – the same effect. Sure, on some it happens less often than on others, but they all suffer from the bobbin suddenly being pulled in the wrong direction, there’s a big crash, and either you break a needle, or get a rat nest on the bottom, or a pull, but something awful inevitably happens. 😦
So here is the culprit:
I don’t really know what happened with this spool of Coats Duet. I use this thread quite often, and never had such problems. But I have another spool in a different colour bought at the same time, and it twists the same way. I’ve also had some spools of Güterman thread do this, but not all. I really don’t understand it… A bad batch?
From now on, I’ll be using either cotton thread, or soft polyester. Cotton thread is usually spun softer, although come to think of it, I did come across one or the other spool with a similar high twist. I found Coats Moon to be a very nice soft spun polyester thread, that is strong enough for regular sewing (much stronger than the usual overlocking thread or the cheap and fluffy polyester thread).