How does a sewing machine work?

The main parts

Every domestic lockstitch machine has the same core parts, whether it is a vintage treadle or a modern computerised model.

Step by step: forming one stitch

All of this happens in a fraction of a second, hundreds of times a minute at normal sewing speed.

1. 1

   The needle moves down through the fabric, carrying the upper thread through its eye.

2. 2

   As the needle starts to rise, friction holds the thread so a small loop forms behind the needle.

3. 3

   The point of the hook catches that loop.

4. 4

   The hook carries the loop all the way around the bobbin case, enclosing the bobbin thread.

5. 5

   The take-up lever rises and pulls the upper thread up, tightening the loop around the bobbin thread.

6. 6

   The two threads lock together, ideally in the middle of the fabric layers.

7. 7

   The feed dogs rise, grip the fabric against the presser foot and move it back by one stitch length, then drop down ready for the next stitch.

Why the needle is shaped as it is

A machine needle has a long groove down the front to protect the thread as it passes through the fabric, and a scooped area behind the eye called the scarf. The scarf lets the hook point pass very close to the needle to catch the loop. That is why a needle inserted backwards or not fully up causes skipped stitches: the hook misses the loop.

Needle size and point type matter for the same reason. See sewing machine needle size chart and the needle size guide.

How tension makes a good stitch

The tension discs set how much the upper thread resists, and a small spring on the bobbin case sets the lower thread resistance. When they are balanced, the lock sits inside the fabric and both sides look the same. Too much upper tension pulls the bobbin thread to the top; too little leaves loops underneath.

Because the discs only grip when the presser foot is down, threading with the foot down misses them entirely, which is why that mistake causes thread nests. Our tension guide and thread bunching fix explain more.

Zigzag and other stitches

For zigzag, the needle bar swings left and right between stitches while the feed moves the fabric forward. Decorative and stretch stitches combine needle swing with forward and backward feeding. Mechanical machines do this with cams; computerised machines use stepper motors controlled by a circuit board.

Overlockers work differently: they use loopers instead of a bobbin and a knife to trim the edge; see sewing machine vs overlocker. If you are choosing a first machine, our best sewing machines for beginners guide explains which features help.

See sewing machines for beginners →

Why stitches go wrong

Once you know how a stitch forms, most problems make sense. If the hook misses the loop, you get skipped stitches, usually from a bent, blunt or wrongly inserted needle. If the upper thread misses the tension discs or take-up lever, loose loops build up underneath. If the bobbin is the wrong type or inserted the wrong way, the lower thread tension is wrong and the lock sits on one side of the fabric.

If the feed dogs are lowered, clogged with lint or the presser foot pressure is too light, the fabric does not move and stitches pile up in one place. Our fixes for skipped stitches and fabric not feeding work through these causes in order.

Frequently asked questions

Does a sewing machine needle go all the way through the fabric? No. Unlike a hand needle, it goes down and comes back up. The hook underneath carries the thread loop around the bobbin.

Why do sewing machines need a bobbin? The bobbin supplies the lower thread that interlocks with the upper thread to form a lockstitch.

What does the take-up lever do? It pulls up the slack upper thread after the hook releases the loop, tightening the stitch.

Why does threading order matter? The thread must pass through the tension discs and take-up lever in the right order, or the stitch will not lock properly.