On my long arm the top thread shreds and breaks when the top tension is too tight, there is a bur on a thread guide or needle. If the thread "cuts" its a problem in the take up on the bobbin. Usually for me it's the top tension being too tight. I find that I sometimes need to adjust it when changing from a poly to a cotton thread. Seems to happen much more with cotton for me. I use mostly Superior--SoFine is my favorite. Like Glide too--but must use a netting with it.