How Sewing Machines Work

The loop stitch approach is very different from ordinary hand-sewing. In the simplest hand stitch, a length of thread is tied to a small eye at the end of a needle. The sewer passes the needle and the attached thread all the way through two pieces of fabric, from one side to the other and back again. In this way, the needle runs the thread in and out of the fabric pieces, binding them together.

While this is easy enough to do by hand, it is extremely difficult to pull off with a machine. The machine would have to release the needle on one side of the fabric just as it grabbed it again on the other side. Then it would have to pull the entire length of loose thread through the fabric, turn the needle around and do the whole thing in reverse. This process is way too complicated and unwieldy for a simple machine, and even by hand it only works well with short lengths of thread.

Instead, sewing machines pass the needle only part-way through the fabric. On a machine needle, the eye is right behind the sharp point, rather than at the end.

The needle is fastened to the needle bar, which is driven up and down by the motor via a series of gears and cams (more on this later).

When the point passes through the fabric, it pulls a small loop of thread from one side to the other. A mechanism underneath the fabric grabs this loop and wraps it around either another piece of thread or another loop in the same piece of thread.

There are actually several different types of loop stitches, and they all work a little differently.

Chain Stitch The simplest loop stitch is the chain stitch. To sew a chain stitch, the sewing machine loops a single length of thread back on itself.

The fabric, sitting on a metal plate underneath the needle, is held down by a presser foot. At the beginning of each stitch, the needle pulls a loop of thread through the fabric. A looper mechanism, which moves in synch with the needle, grabs the loop of thread before the needle pulls up. Once the needle has pulled out of the fabric, the feed dog mechanism (which we'll examine later) pulls the fabric forward.

When the needle pushes through the fabric again, the new loop of thread passes directly through the middle of the earlier loop. The looper grabs the thread again and loops it around the next thread loop. In this way, every loop of thread holds the next loop in place.

The main advantage of the chain stitch is that it can be sewn very quickly. It is not especially sturdy, however, since the entire seam can come undone if one end of the thread is loosened. Most sewing machines use a sturdier stitch known as the lock stitch.

The most important element of a lock-stitch mechanism is the shuttle hook and bobbin assembly. The bobbin is just a spool of thread positioned underneath the fabric. It sits in the middle of a shuttle, which is rotated by the machine's motor in synch with the motion of the needle.

Just as in a chain-stitch machine, the needle pulls a loop of thread through the fabric, rises again as the feed dogs move the fabric along, and then pushes another loop in. But instead of joining the different loops together, the stitching mechanism joins them to another length of thread that unspools from the bobbin.

When the needle pushes a loop through the thread, the rotary shuttle grips the loop with a hook. As the shuttle rotates, it pulls the loop around the thread coming from the bobbin. This makes for a very sturdy stitch.

In the next section, we'll see how the sewing machine moves all of these components in synch.

The conventional electric sewing machine is a fascinating piece of engineering. If you were to take the outer casing off, you would see a mass of gears, cams, cranks and belts, all driven by a single electric motor. The exact configuration of these elements varies a good deal from machine to machine, but they all work on a similar idea.

The electric motor is connected to a drive wheel by way of a drive belt. The drive wheel rotates the long upper drive shaft, which is connected to several different mechanical elements. The end of the shaft turns a crank, which pulls the needle bar up and down. The crank also moves the thread-tightening arm. Moving in sync with the needle bar, the tightening arm lowers to create enough slack for a loop to form underneath the fabric, then pulls up to tighten the loop after it is released from the shuttle hook.

The thread runs from a spool on the top of the machine, through the tightening arm and through a tension disc assembly. By turning the disc assembly, the sewer can tighten the thread feeding into the needle. The tension must be tighter when sewing thinner fabric and looser when sewing thicker fabric.

The first element along the shaft is a simple belt that turns a lower drive shaft. The end of the lower drive shaft is connected to a set of bevel gears that rotates the shuttle assembly. Since both are connected to the same drive shaft, the shuttle assembly and the needle assembly always move in unison.

The lower drive shaft also moves linkages that operate the feed dog mechanism. One linkage slides the feed dog forward and backward with each cycle. At the same time, another linkage moves the feed dog up and down. The two linkages are synchronized so that the feed dog presses up against the fabric, shifts it forward, and then moves down to release the fabric. The feed dog then shifts backward before pressing up against the fabric again to repeat the cycle.

The motor is controlled by a foot pedal, which lets the sewer vary the speed easily. The cool thing about this design is that everything is linked together, so when you press on the pedal, the motor speeds all of the processes up at the same rate. The process is always perfectly synchronized, no matter how fast the motor is turning.

The sewing machine shown in the diagram can only produce a straight stitch -- a simple stitch that binds fabric with a straight seam. Most modern machines are a lot more flexible; they can produce a variety of stitches and, in some cases, can make complex designs. In the next section, we'll see how modern machines pull this off.

In the last section, we looked at the machinery inside a conventional electric sewing machine. One important addition to this basic design is the ability to sew different sorts of stitches. The typical stitch options for a conventional sewing machine are variations on the zig-zag stitch. The zig-zag stitch is exactly what it sounds like: a stich with a jagged line.

This zig-zag stitch is fairly simple to achieve. All you have to do is move the needle assembly from side to side at the same time that it is moving up and down. In a conventional electric machine, the needle bar is attached to an additional linkage, which is moved by a cam on the main drive shaft. When the linkage is engaged, the rotating cam shifts the linkage from side to side. The linkage tilts the needle bar back and forth horizontally in synch with the up and down motion.

Things work a little differently in the modern machine. Today's high-end home sewing machines have built-in computers, as well as small monitor displays for easier operation. In these models, the computer directly controls several different motors, which precisely move the needle bar, the tensioning discs, the feed dog and other elements in the machine. With this fine control, it is possible to produce hundreds of different stitches. The computer drives the motors at just the right speed to move the needle bar up and down and from side to side in a particular stitch pattern. Typically, the computer programs for different stitches are stored in removable memory disks or cartridges. The sewing-machine computer may also hook up to a PC in order to download patterns directly from the Internet.

Some electronic sewing machines also have the ability to create complex embroidery patterns. These machines have a motorized work area that holds the fabric in place underneath the needle assembly. They also have a series of sensors that tell the computer how all of the machine components are positioned. By precisely moving the work area forward, backward and side to side while adjusting the needle assembly to vary the stitching style, the computer can produce an infinite number of elaborate shapes and lines. The sewer simply loads a pattern from memory or creates an original one, and the computer does almost everything else. The computer prompts the sewer to replace the thread or make any other adjustments when necessary.

Obviously, this sort of high-tech sewing machine is a lot more complex than the fully manual sewing machines of 200 years ago, but they are both built around the same simple stitching system: A needle passes a loop of thread through a piece of fabric, where it is wound around another length of thread. This ingenious method was one of those rare, inspired ideas that changed the world forever.

To learn more about sewing machines, including details of their history, check out the links on the next page.