The simplest loop stitch is chain stitch. If you want to sew chain stitches, the sewing machine uses the same length of thread to loop the back of the thread. The fabric is located on a metal plate under the needle and is fixed with a presser foot. At the beginning of each stitching, the needle passes through the fabric to draw a loop. A device that makes a loop grasps the loop before the needle is pulled out, and the device moves in synchronization with the needle. Once the needle pulls out the fabric, the feed dog device (described later) will pull the fabric forward.
When the needle passes through the fabric again, the new stitch will pass directly through the middle of the previous stitch. The coil-making device will grab the wire again and make a coil around the next coil. In this way, each coil will hold the next coil in place.
The main advantage of chain stitching is that it can be stitched very fast. However, it is not particularly strong. If one end of the thread is loosened, the entire sewing may loosen. Most sewing machines use a stronger stitch called a lockstitch. You can understand the working principle of a typical lock stitching device in the animation below.
The most important components of the lock stitching device are the hook and spool assembly. The spool is a coil of thread placed under the fabric. It is located in the center of the shuttle, which rotates under the drive of a motor, synchronized with the movement of the needle.
As with chain stitching, the needle pulls out a loop through the fabric, and it rises again while the feed dog moves the fabric forward, and then nests another loop. However, this stitching mechanism does not connect the different loops together, but connects them to another piece of wire that is loosened from the spool.
When the needle wraps the thread into the loop, the rotating shuttle uses the crochet needle to grasp the loop. As the shuttle rotates, it draws a loop around the thread from the spool. This makes the stitch very strong.