The lockstitch sewing machine, invented and developed in the 18th and 19th centuries, forms a stitch with two threads: one passed through a needle and another from a bobbin. Each thread stays on the same side of the material being sewn, interlacing with the other thread at each needle hole thanks to the machine's movement. Tension of the bobbin thread is maintained with a bobbin case, a metal enclosure with a leaf spring which keeps the thread taut. The bobbin case has to be free-floating (not attached to an axle) in order to allow the top thread to pass around the bobbin completely and hook the bobbin thread.
The lockstitch sewing machine, invented and developed in the 18th and 19th centuries, forms a stitch with two threads: one passed through a needle and another from a bobbin. Each thread stays on the same side of the material being sewn, interlacing with the other thread at each needle hole thanks to the machine's movement. Tension of the bobbin thread is maintained with a bobbin case, a metal enclosure with a leaf spring which keeps the thread taut. The bobbin case has to be free-floating (not attached to an axle) in order to allow the top thread to pass around the bobbin completely and hook the bobbin thread.