A sewing machine does not push a thread through the fabric the way a hand needle does. It locks two separate threads together inside the fabric.
The sequence, in order. The needle carries the top thread down through the fabric. As the needle begins to rise, the thread lags behind and a small loop forms just above the eye, in the hollow ground into the needle’s back. The hook — rotating or oscillating below the plate — arrives at exactly that moment, catches the loop, and carries it right around the bobbin case so that it passes entirely around the bobbin thread. The take-up lever then rises and pulls the slack back up, drawing the two threads tight so they lock in the middle of the fabric. Meanwhile the feed dogs advance the fabric one stitch length while the needle is clear of it. Then it repeats.
Why this is worth knowing. Nearly every common fault becomes obvious once you can picture it.
Skipped stitches mean the hook arrived and the loop was not there, or was the wrong shape — a blunt needle, a bent one, the wrong type, or one not pushed fully up.
A bird’s nest underneath means the top thread was never under tension, so the take-up lever had nothing to pull back.
Timing is simply the hook arriving at the wrong instant relative to the needle.
Not every machine works this way. Overlockers and coverstitch machines have no bobbin at all — loopers carry threads around each other to build a chain.
Questions people actually ask
Why does the needle have a groove down one side?
The long groove protects the thread on the way down, and the scarf — the scooped hollow on the back — lets the hook pass close enough to catch the loop. That is why needle type is not interchangeable.
Is this the same as a hand backstitch?
No. Hand sewing passes one thread in and out. A machine lockstitch uses two threads that never leave their own side of the fabric — they only interlock within it.
Last reviewed 2026-08-11