What is the sewing principle of sewing machine manufacturers?

​Just like cars, sewing machine manufacturers tell you that the basic principles of most sewing machines are the same. At the heart of a car is the internal combustion engine, and at the heart of a sewing machine is the stitching system.

The loop stitching method is very different from ordinary hand sewing.

In the simplest form of hand stitching, the seamstress attaches a thread through the eyelet at the end of the needle, then passes the needle and thread completely through the two pieces of fabric, from one side to the other and back again. This way, the needle drives the thread in and out of the fabric, stitching them together.

​Although this is very easy to do by hand, it is extremely difficult to pull with a machine.

The machine needs to release the needle on one side of the fabric and grab it again instantly on the other side. It then entails pulling all the loose threads out of the fabric, reversing the direction of the needle, and repeating all steps in the opposite direction. The process was too complicated for a simple machine to be practical, and even by hand it only worked well with shorter threads.

Instead, a sewing machine simply passes the needle partially through the fabric. On the needle, the eye of the needle is just behind the point, not at the end of the needle.

The needle is attached to the needle bar, which is pulled up and down by a motor through a series of gears and cams (more on that later).

As the tip of the needle passes through the fabric, it pulls a small loop from one side to the other. A device underneath the fabric grabs the loop and wraps it around another thread or another loop of the same thread. In the next two sections, we'll see exactly how this system works.

The simplest coil stitch is the chain stitch. To sew a chain stitch, the machine loops the same length of thread behind the thread. The fabric sits on a metal plate under the needle, held in place with a presser foot.

At the beginning of each stitch, the needle pulls a loop through the fabric. A loop-making device that catches the loop before the needle is pulled out moves synchronously with the needle. Once the needles have pulled the fabric, the feed dogs (described later) pull the fabric forward.

When the needle passes through the fabric again, the new loop will go directly through the middle of the previous loop. The device making the coil will grab the thread again and make the coil around the next coil. This way, each coil will hold the next coil in place.

The main advantage of chain stitching is that it can be sewn very quickly. However, it's not particularly strong, and if one end of the thread comes loose, the entire sewing can come loose. Most sewing machines use a stronger type of thread called lockstitch.

You can see how a typical overlock device works in the animation below.

The most important elements of a lockstitch device are the hook and spool assembly. A spool is simply a coil of thread placed under the fabric.

It is located in the center of the shuttle, which is driven by a motor to rotate in synchronization with the movement of the needles.

As with chain stitching, the needle pulls a loop through the fabric, it rises again as the feed dog moves the fabric forward, and another loop is pulled in. Instead of joining the different loops together, though, this sewing mechanism joins them with another length of thread unwound from the spool.

As the needle pulls the thread into the loop, the spinning shuttle grabs the loop with the hook. As the shuttle rotates, it pulls a coil around the thread coming from the spool. This makes the stitching very strong.

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

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

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