Originally Posted by
CatD
OK, tried a different thread and it seemed to work fine. (Although I left the rayon in the bobbin. Wonder why it's fine there but breaks in the upper?)
The upper thread endures a lot more "abuse" going through it's path than the bottom. It goes through a lot of guides, gets rubbed and tugged at with the takeup lever, and goes through the fabric several times before it finally forms a stitch. The bobbin thread has it much easier - very short path, no friction except in the tension assembly, no passing through the fabric. That's why if thread is breaking, it's pretty much always the top thread.