This is almost the result of some kind of interference with the needle thread. It can be caused by any uncouth edge in the area of the path where the needle thread travels along or by too tight tension and excessive needle heat.
To avoid thread breakage or fraying
- Check the parts such as the thread guides, needle, needle hole plate, presser foot, feed dog, hook and looper are free of burrs or sharp edges.
- Use lighter tension
- Check for the correct needle size: Test with the next larger size needle.