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Once Upon A Time I Picked Up A Parrot And The Rest Is History Shirt, hoodie, tank top
producing a lockstitch. NikolayS [CC-BY-SA-3.0], via Wikimedia Commons.The sewing machine can’t do a backstitch with a single thread as you might expect, it’s very difficult for a machine to pass the thread though the fabric in its entirety on each stitch. Instead it uses two threads to create a stitch called lockstitch, in which each thread stays on its own side of the fabric and interlocks with the other thread only through the holes made by the needle. The upper thread comes from a spool on the top of the machine while the lower one comes from a bobbin mounted underneath its operating surface. The action of making the lockstitch has been performed by a variety of mechanisms since the invention of the sewing machine, but in the majority of those you’ll encounter it is done through a rotating hook that surrounds the bobbin. This hook picks up a loop of the upper thread pushed through the fabric by the needle, and wraps it round the lower thread from the bobbin before the needle and upper mechanism pulls the upper thread tight back up through the hole. This produces reliable and consistent stitching that can be repeated at very high speed.
You can see that there’s a lot going on here: the machine has to supply the thread at the right speed and tension, hold the fabric in place, move it forwards at the correct speed, and keep the whole lot in synchronisation. Sewing machines are complex beasts.
Know Your Machine
A sewing machine has a needle mechanism over a flat surface, suspended from a horizontal arm to give enough bed width to position the work as necessary on either side of the needle. The overwhelming majority of machines have the horizontal arm coming in from the right, coming from a vertical arm that usually contains the motor and significant parts of the drive mechanism. On the extreme right hand side of the machine is a wheel on the outside that rotates as the mechanism operates, this might historically have had a hand crank or pulley for a treadle or external motor to power the sewing. It serves to allow the operator to manually advance the stitches very slowly, or to back off the mechanism. On older machines it will be a large metal pulley that harks back to the hand-cranked or treadle days, while on newer ones it will usually take the form of a plastic knob.