Working principle of sewing machine

The inventor of the sewing machine was truly a genius. As we all know, the needle of a sewing machine never fully passes through the fabric while working, yet it locks the threads tightly together. Why exactly is this? What incredible design lies inside? Traditional hand-sewing requires the needle to go all the way through, pull the thread out from the other side, release, flip, and grab again—a clearly tedious and inefficient process.

It was not until 1755 that a German inventor came up with a double-pointed needle, moving the needle eye right next to the tip, eliminating the need to flip the fabric for sewing on both sides. But this created a new problem: when the needle pierced the fabric with the thread, the thread would pull right back out as the needle was withdrawn. How to solve this? There were two mainstream solutions: chain stitch and lockstitch.

· First, the chain stitch. As the needle carries the thread down through the fabric, it leaves a loop underneath. The needle then moves slightly forward and pierces down again, passing directly through that loop, linking one loop to another to form a chain-like stitch. This is the principle of the chain stitch, the very first stitch achieved by sewing machines.

Truly stable and reliable chain-stitch sewing machines were not invented until 1857, by two geniuses working independently. One design used a hook: as the needle descended, threaded through the fabric, and rose back up, it formed a loop of upper thread; the hook quickly caught the loop and widened it, and the needle then descended again to pass through the loop, repeating the cycle. The other design was even more ingenious, using a rotating hook that continuously caught the thread, formed loops, and created new ones.

However, the chain stitch has a fatal flaw: it unravels completely if pulled. Since each loop only relies on the previous one for support, the entire row of stitches can come undone all at once once the thread loosens.

· The other, far more secure stitch—the lockstitch—solves this problem. It uses two spools of thread: one upper thread passing through the needle eye, and one lower thread hidden inside a bobbin. As the needle carries the upper thread through the fabric, the bobbin thread passes through the loop formed by the upper thread, and the two threads interlock like a tight knot, securing the fabric firmly. This structure is extremely stable, resistant to loosening, and hard to unpick. Nearly all modern household sewing machines operate on this principle.

But early lockstitch machines could only sew straight lines, and the fabric had to hang vertically, making them highly inefficient. Later improvements brought the rotary hook design, making lockstitch truly practical. The rotary hook acts like a small track, catching the upper thread, looping it around the bobbin, and returning into place—each stitch perfectly interlocks the two threads. Paired with a balanced tension mechanism, the stitches are both secure and neat. To reduce friction between the thread and fabric, engineers also added grooves to the needle, enabling fast, smooth sewing.

But how to move the fabric forward evenly after each stitch? Early users pushed the fabric by hand, resulting in crooked stitches. To fix this, the feed dog was invented—a serrated metal plate that lifts the fabric upward as the needle rises, pushes it forward by one stitch length, then lowers back into place. This design is still used in nearly every sewing machine today.