Just like cars, the basic principles of most sewing machines are the same. The core of the car is the internal combustion engine, and the core of the sewing machine is the stitching system.
The stitch stitching method is very different from ordinary manual sewing. In the simplest hand stitching, the stitcher ties a thread in the small eye at the end of the needle, then passes the needle and the thread completely through the two pieces of fabric, from one side to the other, and then back to the original side. In this way, the needle drives the thread in and out of the fabric, stitching them together.
Although this is very simple for manual work, it is extremely difficult to pull with a machine. The machine needs to release the needle on one side of the fabric and then immediately grab it again on the other side. Then, it needs to pull all the loose threads out of the fabric, reverse the direction of the needle, and repeat all the steps in the opposite direction. This process is too complicated for a simple machine, and impractical, and even for manual work, it is only useful when a shorter thread is used.
Instead, the sewing machine only needs to pass the needle part through the fabric. On the needle, the eye of the needle is just behind the tip, not at the end of the needle.
The needle is fixed on the needle bar, and the needle bar is pulled up and down by a motor through a series of gears and cams (described in detail later).
When the tip of the needle passes through the fabric, it pulls a small loop on one side and the other. A device under the fabric will grab this loop and wrap it around another thread or another loop of the same thread. In the next two sections, we will see how this system works.
The simplest loop stitch is chain stitch. To sew out 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 stitch, the needle passes through the fabric to draw a loop. A loop-making device 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, 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 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.
This kind of rotary shuttle also evolved from the straight shuttle.
The sewing machine's loop stitching principle has evolved from a straight shuttle to a rotary shuttle, and it has entered a mature period.
