A standard electric sewing machine is an
amazing piece of
the sewing machine, the world would be a very different place.
Like the automobile, the cotton gin and countless other
innovations from the past 300 years, the sewing machine takes
something time-consuming and laborious and makes it fast and
easy. With the invention of the mechanized sewing machine,
manufacturers could suddenly produce piles of high-quality
clothing at minimal expense. Because of this technology, the
vast majority of people in the world can now afford the sort
of sturdy, finely-stitched clothes that were a luxury only 200
In this edition of HowStuffWorks,
we'll take look at the remarkable machine that makes all of
this possible. As it turns out, the automated stitching
mechanism at the heart of a sewing machine is incredibly
simple, though the machinery that drives it is fairly
elaborate, relying on an assembly of gears, pulleys and motors
to function properly. When you get down to it, the sewing
machine is among the most elegant and ingenious tools ever
Sew What? Sewing machines are something like
cars: There are hundreds of models on the market, and they
vary considerably in price and performance. At the low-end of
the scale, there are conventional no-frills electric designs,
ideal for occasional home use; at the high-end, there are
sophisticated electronic machines that hook up to a computer.
Textile companies have many machines to choose from, including
streamlined models specifically designed to sew one particular
But just like cars, most sewing machines are built around
one basic idea. Where the heart of a car is the internal
combustion engine, the heart of a sewing machine is the
loop stitching system.
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. In the next couple of sections, we'll see
exactly how this system works.
Lock and Chain In the last section, we saw
that the heart of a sewing machine is the loop stitch. There
are actually several different types of loop stitches, and
they all work a little differently.
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. You can see how one version of this
stitch works in the diagram below.
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
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. You can see how the typical
lock-stitch mechanism works in the animation below.
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 General Assembly 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
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 diagram below shows a fairly standard
In this diagram, 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
synch 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
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
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.
Computer Crafts 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
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
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.