How does a sewing machine actually make a stitch?

Most people picture a sewing machine as a fast hand — a needle going through the cloth and back. It does not work like that at all, and the difference matters, because every common fault is a failure of one specific stage of a mechanism you cannot see happening. Once you know what the machine is trying to do, a skipped stitch, a nest of loops and a puckered seam stop being three mysteries and become three different points in the same sequence.

Step by step

1. The needle goes down, carrying the top thread.It pierces both layers and passes below the needle plate with the thread through its eye. Nothing has happened yet — no stitch has been made and the thread is simply looped through the fabric.
2. The needle starts to rise, and a loop forms.This is the whole trick. The thread is gripped by friction in the fabric behind the needle, but it runs freely down the long groove milled into the front of the needle blade. So as the needle lifts a couple of millimetres, the slack has nowhere to go on the grooved side and bulges out as a small loop on the other side.
3. The rotating hook catches the loop.Below the plate, a hook on the bobbin race sweeps past the needle. Its timing is set so its point arrives at that exact fraction of a revolution when the loop exists. It catches the loop and drags it open.
4. The loop is carried right round the bobbin.The hook takes the loop of top thread all the way around the outside of the bobbin case, so the loop passes completely around the bobbin and the bobbin thread coming off it. This is what interlinks the two threads.
5. The take-up lever pulls the slack back.The arm that bobs up and down at the front of the machine head. As it rises it pulls several centimetres of top thread back up, closing the loop and drawing the interlock upwards until it is buried in the middle of the fabric sandwich.
6. The feed dogs move the fabric on.While the needle is clear of the cloth, the toothed bars under the plate rise, travel backwards by one stitch length, drop and return. Then the cycle starts again.
7. Understand what a lockstitch is.Two separate threads, from two separate supplies, knotted together once per stitch inside the fabric. It is not one thread going in and out. That is why a sewing machine seam does not unravel when a stitch breaks, and why an overlocker — which uses loopers and no bobbin — behaves completely differently.

Tips

• The needle has a long groove down the front and a small scooped hollow just above the eye, called the scarf. The groove lets the thread run; the scarf lets the hook pass close enough to catch the loop. Both are directional, which is the real reason a needle fitted the wrong way round does not work.
• The machine cannot form a stitch below a certain speed of needle rise, which is why hand-wheeling very slowly through thick fabric sometimes skips where sewing at speed does not.
• Where the interlock ends up in the fabric is what the tension dials control. Everything about tension follows from that one idea.

Common mistakes

• Assuming the bobbin is the cause of a stitch fault — Almost every fault happens on the top thread's journey — through the tension discs, the take-up lever and the needle. The bobbin does very little except sit there and unwind.
• Blaming the machine when stitches skip — Skipped stitches mean the hook found no loop to catch. That is a needle problem — blunt, bent, wrong type, wrongly fitted, or too fine for the fabric — far more often than it is a timing fault.

Questions people ask