Last edited Tue Apr 10, 2018, 12:39 PM - Edit history (1)
You mentioned sewing for the longest time before the thread loosens on the bobbin. So it would be good to know about how full the bobbin is when the problem starts happening.
Edited to add:
It sounds to me like your bobbin is looser at times when it starts winding. So, naturally, it's very loose after you've sewn for a long time.
When you first insert the thread through a hole in the top of the bobbin leave a few inches of thread tail sticking up. Hold onto it with a little bit of tension as it begins to wind so that the thread secures itself well enough around the bobbin at the start of the process. After it's wound around enough to cover the post pretty good let go of the thread, clip it off and wind the rest. Be sure not to hold the pedal down completely. You want to let it wind at a decent clip but not at full speed.
Just to be sure you're setting it up correctly here's a great video that covers winding the bobbin in the first few minutes. The tension disc holds the thread different from any machine I've sewn with. So check out how she puts the thread around it before it goes over to the winding post.
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