I have a question regarding rust and its many forms. When working on old houses and their various bits of vintage hardware (most were not replaceable) I would boil the rusty metal bits in an old dedicated pot. Hinges, latches, and doorknobs/parts all went in the pan for a good long water boil. It was also great for removing old paint from the parts without dust (think lead) or much effort.
One of two things happened, the rust came off (when cool) or changed into a hard nearly black surface. Bottom line, it stopped the rust from spreading and sometimes made the piece completely useable again! For instance, some of the old cabinet latches would be unusable due to the rough rust preventing parts from sliding back and forth. After this treatment and a bit of sanding, they worked like brand new.
I don't have rusty bits of machines around. Everything I have works. Will someone try this method on some of the bobbin shuttle parts that are rusty and let us know how it turned out? 10-15 minutes of boiling will be all it takes. Think of the saved parts potential!!