Bobbins by class
Bobbins look interchangeable and are not. We cover the classes (15, 15J, L, M), metal against plastic, and why a bobbin that is a fraction of a millimetre wrong produces tension faults that get blamed on the machine.
The commonest domestic bobbin class, in metal. Present on all six markets, with a separate US reference.
Manufacturer-matched bobbins for the Bernette b-series and Chicago machines. One reference on every Amazon store we checked.
No. The class is specific to the machine, and a bobbin of a neighbouring class can fit in the case while throwing the tension off.
The one the manual prescribes. The two do not weigh the same and do not behave identically in the case.
It is almost always the upper tension or threading to redo with the foot raised, not the bobbin. The symptom appears below but comes from above.
Yes, evenly and without jolts. A badly wound bobbin unwinds unevenly and makes the tension vary.
Better avoided: the old thread and the new do not have the same winding tension, which creates steps.
With a dry brush after every project, never with compressed air, which pushes the lint further in. The machine's most useful upkeep.
It gets scratched and distorted over time, which catches the thread. A spare part people often forget to suspect.
The bobbin class is specific to the machine, and a bobbin that fits is not a compatible bobbin. When loops appear underneath, the cause is above — tension or threading — and a dry brush in the case after every project prevents most faults.