Joe, my heartfelt sympathy. The night before last, I was using my Singer 401 on a quilt and the same thing happened -- about fifty times and many swear words. As you know, the 401 and 500 are pretty much the same machine. I had quilted the interior with some thread from Thread Art, knowing it isn't the best thread. It did pretty well, not much snapping. When I went to bind it, being sure this binding would be great, oh my! Nothing but trouble. I changed threads - four manufactures. I cleaned out the tension, with which I have not had other trouble. I cleaned the already clean bobbin area. I adjusted tension, adjusted tension, adjusted tension. I changed needles several times, finally installing a titanium #16. I re-threaded so many times, cleaning out every part of that thread path. I could not get decent results with any stitch. That thread - every thread including 40 weight - snapped and frayed every minute or two. I oiled the machine. I did everything short of taking apart the tensioneer, with which there had been no previous problems. I used built in stitches and cams. I finally switched machines.
So I'd like to know too, what happened?
Aurifil is the best thread, according to what I've learned in thread construction classes. I do not use it on homeless quilts, which this one was.
Cricket