If they're set for a lighter plastic bobbin, the tension will vary if you use a heavier metal bobbin. This could cause your sewing machine to break down. Metal bobbins are stronger than plastic ones of the same size and weight. This is because they can be made from thicker material or more layers. It also means that you should not try to save money by using a lower-quality metal bobbin. These will wear out faster than premium-quality bobbins and may even be toxic if they're not manufactured properly. The best thing to do is to buy a variety of sizes of metal bobbins so you have one on hand when needed. This will help prevent any sewing machine breakdowns while still saving money.