What QuiltedSunshine said. If you have a metal bobbin case (probably not drop in) you can use either the metal bobbin that came with your machine or a prewound plastic bobbin. If your bobbin case is not metal (probably a drop in), use the plastic one that came with your machine. Using a plastic bobbin in a metal bobbin case won't damage it, but using a metal bobbin in a bobbin case designed for plastic can ruin the case.