Like cars, most sewing machines have the same basic principles. The core of automobile is internal combustion engine, and the core of sewing machine is coil sewing system.

There is a great difference between coil stitching and manual stitching. In the simplest hand stitching, the stitcher ties a thread to the small eye at the end of the needle, and then passes the needle and thread completely through two pieces of fabric, from one side to the other, and then back to the original side. In this way, the needle moves the thread in and out of the fabric and stitches them together.

Although this is very simple for hand, it is extremely difficult to pull by machine. The machine needs to release the needle on one side of the fabric and immediately grab it again on the other side. Then, it needs to pull all the loose thread out of the fabric, turn the direction of the needle, and then repeat all the steps in the opposite direction. This process is too complicated and impractical for a simple machine, and even for manual operation, it is only easy to use when using shorter lines.

On the contrary, the sewing machine only needs to pass the needle part through the fabric. On the needle, the eye is just behind the tip, not at the end of the needle.

The needle is fixed on the needle bar, which is pulled up and down by the motor through a series of gears and cams (which will be described in detail later).

When the tip of the needle passes through the fabric, it pulls out a small coil on one side and on the other. A device under the fabric grabs the coil and wraps it around another thread or another coil of the same thread. In the next two sections, we'll see exactly how the system works.

The simplest coil suture is chain suture. To make a chain stitch, the sewing machine loops the back of the thread with the same length of thread. The fabric is placed on a metal plate under the needle and secured with a presser foot. At the beginning of each stitch, the needle pulls a loop through the fabric. A coil making device grabs the coil before the needle is pulled out, and the device moves synchronously with the needle. As soon as the needle is pulled out of the fabric, the feed dog (described later) pulls the fabric forward.

When the needle passes through the fabric again, the new coil will pass directly through the middle of the previous coil. The device that makes the coil grabs the wire again and makes the coil around the next coil. In this way, each coil holds the next coil in place.

The main advantage of chain stitch is that it can be sewed very fast. However, it is not particularly strong. If one end of the thread is loosened, the whole sewing may come loose. Most sewing machines use a stronger thread called lockstitch. You can learn how a typical sewing device works in the following animation.

The most important components of the sewing device are the shuttle hook and spool assembly. A spool is a roll of thread placed under a fabric. It is located in the center of the swing shuttle, which is driven by the motor to rotate and synchronize with the movement of the needle.

Similar to chain stitch, the needle pulls out a loop through the fabric, it rises again as the feed tooth moves the fabric forward, and then another loop is inserted. However, this stitching mechanism does not connect the different coils together, but connects them with another piece of thread loosened from the spool.

When the needle sets the thread into the coil, the rotating shuttle grabs the coil with a crochet. As the shuttle rotates, it pulls the coil around the wire from the spool. This makes the stitching very strong.

This kind of rotary shuttle is also evolved from the straight shuttle.

The coil sewing principle of sewing machine has evolved from straight shuttle to rotary shuttle, and has entered a mature stage.