What about breakage? How can I determine what is causing the breakage? How do I stop it?
In my experience, the first thing to look at is the end of the thread. Is there fraying? Is there a clean end or perhaps a hook on the end? Was there a pop when it broke? If there has been fraying, the needle usually causes it. The needle could be either too small or worn out. Change needles. I consistently run 4.0.
If it is a clean or hooked break, the top tension is too tight. Or, if I am running a flat thread or metallic thread, there may be a problem with the way my thread spools are mounted. I recommend spooling off the side of the spool for all metallics, and it is an absolute must for flat threads like Sulky Holoshimmer and Sliver.
If the threads pull from the end of the spool, a loop will be put in the thread for each round. These loops will pile up and cause certain breakage. If they do manage to go through the tension assembly, the kinks will almost certainly not go through the needle. Either the thread or the needle will break. The fix? There are several accessory companies that make thread holders just for this, and I do own one now, but until I found that option and decided to spend the money, a piece of bent coat hanger duct-taped near the front of the machine worked pretty well. I preferred it to be slightly up from horizontal so that as the spool turned, it was less likely to fly off the end.