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Every Mechanism

Home How a Sewing Machine Works

One needle, two threads, the lockstitch

How a sewing machine works

Every stitch locks two threads together inside the cloth. Here is a machine sawn open so you can watch how.

Make one stitch, slowly
1,000
stitches a minute, flat out
2
threads in every stitch
2×
the hook turns per stitch
2.3mm
the needle rises before the catch
Under the needle of a sage-green sewing machine, its bed sawn open: the rotary hook carries a loop of the red top thread round the bobbin case, with the brass gears that turn it below. Above it are the feed dogs, the needle plate, the presser foot and the needle on its bar, and a seam of red and blue stitches runs along the cut edge of the cloth.
Sawn open under the needle: the hook is carrying a loop of the red top thread round the bobbin case.

Video

How a Sewing Machine Works - The Lockstitch in Slow Motion

Watch on YouTube

3D model · sawn open · sew with it

Sew, slow it down, take it apart

The whole machine, sawn open at the needle. Press Sew, drag to turn, scroll to zoom, click any part.

Try itPress Sew, then Slow motion to follow one stitch.
Loading the 3D model…

Meet the parts

Fifteen parts, one stitch

The casting is see-through so the parts inside show. Tap any part, or pick a name.

The sewing machine on its table with its casting drawn see-through: the motor and its belt behind the pillar, the handwheel, the main shaft along the arm, the needle bar and the take-up lever in the head, the spool on top, and below the bed the lower shaft, the toothed belt down the pillar, the gears, the hook and the feed dogs.

Hover or tap the picture. Rings mark every part.

Parts diagram

From the motor to the stitch

Numbered along the power, from the motor to the hook. The second picture is sawn open under the needle.

The sewing machine’s mechanism with the casting drawn as a faint outline: the motor behind the pillar and its V-belt to the handwheel, the main shaft along the arm with the crank at its end, the needle-bar link and the needle bar in the head, the spool, the tension discs and the take-up lever, the toothed belt down the pillar and the lower shaft along the bed, and the stitch length dial.
Under the needle, sawn open from the left: the needle coming down through the presser foot and the cloth, the needle plate, the feed dogs’ teeth in their slot, the rotary hook carrying a red loop round the bobbin case, the gears that turn the hook, and the feed cams behind them.

The cloth is cut along the seam, so the stitches show inside it.

    How it works, step by step

    Twenty steps to a lockstitch

    The video’s steps. Press Show me in 3D to see each one in the model.

      A sewing machine sewing a seam.
      Step 1 of 20

      Two threads, locked together

        Demo · the heart of it

        One stitch, slowly

        Sawn open at the needle, seen from the left. Turn the handwheel and follow the red thread round the blue one.

        • Red: the top thread.
        • Blue: the bobbin thread.
        • Amber: the hook’s point.

          Demo · one shaft keeps time

          The timing

          One shaft drives every part, so each angle of the handwheel has its own pose. Move the hook’s timing to see why stitches skip.

          –needle point above the plate
          –take-up lever
          –needle has risen when the point arrives
          –the hook

          Demo · a tug of war

          Balance the tension

          The take-up lever pulls the top thread up, the bobbin holds the bottom one back. Turn the dial and watch the lock move.

          • Tension dial: higher numbers grip the top thread harder.
          • Bobbin case screw: sets the bobbin thread’s pull. It rarely needs touching.
          • Just right: the lock hides inside the cloth.
          –pull on the top thread
          –pull on the bobbin thread
          –the lock sits
          –the cloth

          Demo · moving the cloth

          Feed dogs and stitch length

          Up, back, down, forward, while the needle is out of the cloth. The stitch length dial sets how far back they go.

          –stitch length
          –stitches per centimetre
          –stitches per inch
          –the feed dogs are

          More to know

          Kinds, history and care

          Six kinds of sewing machine

          This oneMechanical householdMotor, pedal and dials; one shaft times it all.
          Hundreds of stitchesComputerisedSmall motors set each stitch for patterns and buttonholes.
          Built for speedIndustrial lockstitchOne seam all day, thousands of stitches a minute.
          Edges and stretchOverlockerNo bobbin: loopers wrap the edge as a knife trims it.
          No motor at allTreadle and hand-crankA foot or a hand turns the handwheel.
          Pictures in threadEmbroidery machineA hoop moves the cloth under the needle to a pattern.

          Two hundred years of stitches

          1. Thomas Saint patents a machine to sew leather.
          2. Thimonnier patents a chain-stitch machine in France.
          3. Elias Howe patents a lockstitch machine.
          4. Singer’s machine and Wilson’s rotating hook are patented.
          5. Wilson’s four-motion feed: up, back, down, forward.
          6. Singer’s first electric sewing machines.
          7. Singer’s Athena 2000, sold as the first electronic machine.

          When stitches go wrong

          • Tangle under the cloth? Rethread the top thread with the presser foot up.
          • Skipped stitches? Fit a new needle, the right way round.
          • Loops underneath, or bobbin thread on top? Adjust the top tension.
          • Clean out the fluff around the feed dogs and the hook.

          Things worth knowing

          The same on both sidesA good lockstitch locks inside the cloth.
          About a thousand a minuteHome machines sew about 800 to 1,100 a minute.
          Two turns for one stitchOnly the hook’s first turn carries a loop.
          A needle’s sizeA size 90 needle is 0.9 mm thick.
          Foot up to threadRaising the foot opens the tension discs.
          Pull one threadA chain stitch unravels: that is how some sacks open.
          Quick check

          Four questions