Explainer #7
How does a sewing machine make a stitch?
A sewing machine uses two threads: one from the spool on top, and one from a small bobbin under the fabric. The needle pushes the top thread down through the fabric. As it starts back up, a loop opens, and a spinning hook catches it and carries it all the way around the bobbin. When the loop is pulled tight, the top thread has wrapped around the bobbin thread, and the two lock together inside the fabric. Then the fabric moves on and it happens again.
The principle
1. The needle takes the top thread down. Unlike a hand needle, a machine needle has its eye near the point. Along one side it has a long groove, and the top thread lies in it, so the thread is not rubbed as the needle pushes it through the fabric into the space below, where the bobbin sits.
2. The needle rises a few millimetres, and a loop forms. On the other side of the needle, just above the eye, there is a small notch. As the needle starts back up, the thread in the groove slides up with it, but on the notch side the fabric holds the thread back. That thread has nowhere to go, so it bulges out beside the needle into a small loop.
3. The hook catches the loop. Under the fabric sits a small round case holding the bobbin with the bottom thread (green in the video). The bottom thread comes out of a hole near the top of the case and goes straight up to the fabric. Around the case, a hook ring turns. Right as the loop opens, the point of the hook slips into it.
4. The loop goes around the case. The case itself stays still. The hook pulls the loop of top thread down and all the way around the outside of the case: one side of the loop goes in front of the case, the other side behind it. Because the bottom thread comes out of that case, it is now in the middle of the loop, like a post standing inside a ring of rope.
5. Pulled tight, the threads lock. An arm above the needle pulls the top thread back up. The loop slides off the case and gets smaller and smaller. It cannot come free of the bottom thread, because that thread is inside it, so it closes around it like a rope pulled tight around a post and drags it up into the fabric. The two threads now lock around each other in the middle of the fabric: that is one stitch. When both threads are pulled equally hard, the lock sits in the middle and the stitch looks the same on both sides. Then the fabric moves forward by one stitch length and it all happens again, many times a second.
Common myth
The needle ties a knot, or pushes the bobbin thread up through the fabric
No knot is tied, and the bobbin thread never goes through the fabric on its own. The top thread is wrapped once around the bobbin thread by the hook, and then the top thread is pulled up and drags the bobbin thread with it. That is why the stitch is called a lockstitch: the two threads just loop through each other. Because every stitch is locked in its own hole, a lockstitch seam does not easily unravel.How this video was made: Rendered 3D cross-section of a simplified lockstitch: one needle, a rotary hook and the bobbin, seen mostly from the front, with a side close-up of how the loop forms. The threads are drawn thicker, the hook is simplified, and the first stitch is shown in slow motion.
