A bobbin case, when used with the correct bobbin, will hold the bobbin snugly in place, which will allow the machine to neatly unravel it. The bobbin case also adds tension to the bottom thread, much like the tension assembly for the top thread.
Sewing machines need an upper and a lower thread to form the stitches. The lower thread is kept in a small bobbin stored underneath the presser foot. The mechanism for winding the bobbin may vary based on the model of your sewing machine.
A bobbin is the part of a sewing machine on which the lower thread is wound. The machine makes a stitch by catching the bottom thread, from the bobbin, with the top thread, from the needle.
Have you ever tried sewing without thread? It seems easy enough, but utterly useless. However, the impossible has been reached with Sonobond’s Ultrasonic SeamMaster. This “sewing machine” operates without any thread or adhesives by using ultrasonic bonding.
A good quality bobbin thread holds up and is the best for sheer fabrics even as a top thread. However, many sewers will use the same thread as both the top thread and the bobbin thread. This is the norm and that’s usually just fine.