Your machine will tell you your bobbin is about to run out. Stop the machine and put in a new bobbin, don't wait until it runs out. It's always a good idea to back it up a few stitches before starting the embroidery again. Look in your manual under "Embroidering" and it will give you the steps to backing up and repositioning your project. I wind an extra bobbin before I start, it's amazing how much stitching you can get out of one bobbin. Of course if you're stitching a big design you will need a few more.