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 diagram below shows a fairly standard lock-stitch design.
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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 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.