How do I sew over thick seams without the machine jamming?

Everyone assumes a machine that stops at a jeans seam has run out of power. It usually has not. The problem is geometry: the presser foot is a flat plate on a hinge, and when its front edge is on a thick step and its back on flat fabric, it tilts. A tilted foot pushes the needle out of vertical and lifts the fabric off the feed dogs, so the machine simultaneously loses its grip and puts the needle where it can strike the plate.

What you'll need

• A heavier needle — size 90 or 100 for denim, 100 or 110 for canvas and upholstery
• Strong thread to match
• A hump jumper, or folded scraps of the same fabric
• A walking foot, on very thick or layered work
• A hammer or a tailor's clapper

Step by step

1. Flatten the crossing before you sew it.Press the seam hard with steam, then tap the crossing firmly with a hammer through a cloth, or squeeze it under a clapper. Compressing several layers of denim into something closer to two makes more difference than any machine setting.
2. Level the presser foot.A hump jumper — a small plastic or metal levelling plate — sits behind the foot on the flat side so the foot stays horizontal as it approaches the step. A folded scrap of the same fabric does exactly the same job for nothing. Slide it in behind the foot before the foot reaches the step, and again in front as the foot comes off it.
3. Fit a needle heavy enough for the total thickness.Count the layers, not the fabric. A 90 handles a few layers of denim; a jeans seam crossing on heavy denim wants a 100. Canvas, webbing and upholstery want a 110 and a machine rated for it.
4. Lengthen the stitch.3 to 3.5mm. A short stitch packs more needle penetrations into thick fabric than it can take, and the feed dogs cannot move a thick sandwich a small distance reliably.
5. Slow right down, or stop pedalling entirely.Approach the step at a crawl. At the step itself, take your foot off the pedal and turn the handwheel by hand, one stitch at a time, until the foot is fully over. This is the single most effective thing on the list.
6. Never pull or push the fabric.It feels like helping and it deflects the needle sideways into the plate or the hook. Guide it, and if it will not move, stop and level the foot rather than forcing it.
7. Increase the presser foot pressure if the machine allows.More downward force keeps the fabric on the teeth. Reduce it again afterwards for lighter work.
8. Use a walking foot for multiple thick layers.A walking foot has its own feed teeth on top, so both faces of the sandwich are driven. On quilting, bag making, oilcloth and vinyl it is the difference between possible and not.
9. Grade the seam allowances where several seams meet.Trim each layer of allowance to a different width so the bulk steps down rather than stacking. It reduces thickness permanently and makes the finished garment lie flat as well.
10. Know your machine's limit.A lightweight domestic machine has a genuine ceiling. Heavy upholstery, thick leather, multiple layers of webbing and dog collars need a heavy-duty or industrial machine, and forcing a light one through them wrecks the timing.

Tips

• A hump jumper costs very little and there is a free version in every scrap bin. Fold a piece of the same fabric to the same thickness as the step.
• If the needle is skipping stitches only at the crossings, it is deflecting rather than blunt. Level the foot and slow down before changing the needle again.
• For leather and vinyl use a leather needle, a longer stitch, clips instead of pins — pin holes are permanent — and a roller or walking foot.
• Hammering a denim seam crossing before sewing sounds crude and is standard practice in trouser alterations.

Common mistakes

• Pushing the fabric to force it over the step — The needle bends sideways into the needle plate or the hook. That breaks needles, damages the hook, and can knock the machine's timing out so it stops picking up the bobbin thread at all.
• Using a fine needle with heavy thread — The thread has to run freely in the needle's groove. Thick thread in a fine needle frays, shreds and jams, and it looks like a tension fault.
• Sewing thick work at speed — Momentum is not the problem and speed does not solve it. Deflection and grip are, and both get worse the faster you go.

If it doesn't work

Questions people ask