Often times it is necessary to change bobbin thread before the bobbin is empty. I didn’t give it a thought until my Wife pointed it out to me and I realized it was worthy of mention.
Problem: Often times when changing the bobbin, people will take the bobbin out of the case, whether it be a horizontal, vertical, or shuttle bobbin, and pull the thread out thru the bobbin case. Doing this pulls the thread backwards against the bobbin tension spring and frays the thread and can trap frayed thread “fuzz” under the tension spring. If this happens, the spring tension will be adversely affected and the bobbin thread will not glide smoothly thru the bobbin case. This will show up as intermittent tension problems such as uneven stitch quality. Often, the problem shows up intermittently and the machine will sew fine, then suddenly have bad or missed stitches.
When servicing a machine, I always remove the bobbin tension spring and often find a buildup of thread that resembles felt. Now I know where it comes from!
Solution: When changing the bobbin thread, always remove the bobbin from the bobbin case without pulling the thread backwards, and cut the bobbin thread. Then pull the cut thread tail out from of the bobbin case (in the direction the machine pulls it thru). This will allow the thread to glide out of the bobbin case without fraying and lessen the build up of thread under the bobbin spring.