Loops and nests of thread on the underside of your fabric almost always come from the top thread, not the bobbin: the thread has slipped out of the tension discs, usually because the machine was threaded with the presser foot down. Re-thread from the spool with the presser foot raised, hold both thread tails for the first stitches, and sew a test seam. Puckered seams are the opposite problem, tension too tight for the fabric or a stitch too long, and they ease with a lower top tension number, a shorter stitch and a fresh, finer needle.
Which tension problem do you have?
Look at a test seam on a doubled scrap, top and underside, and match what you see.
| Symptom | Most likely cause | Fix |
|---|---|---|
| Loose loops on the underside | Top thread not seated in the tension discs, or top tension too low | Re-thread with the foot up; then raise the top tension one number |
| Dots or loops of bobbin thread on top | Top tension too high, or bobbin thread not under the bobbin case spring | Lower the top tension; re-insert the bobbin so the thread clicks under the spring |
| A nest of thread under the first stitches | Thread tails not held, thread out of the discs, lint in the bobbin area | Hold the tails; re-thread; clean the bobbin area |
| Puckered seam on fine fabric | Tension too tight, stitch too long, needle too thick or dull | Lower the tension, stitch 2 mm (12 stitches per inch), new needle 60/8 or 70/10 |
| Seam opens to show a ladder of thread | Top tension too loose | Raise the top tension |
| Top thread keeps breaking | Tension too tight, old thread, burr on the needle or plate | Loosen; new thread and needle; check the plate for a burr |
| Fine for a while, then sudden loops | Thread catching on the spool notch, lint between the discs | Turn the spool round or use a spool cap; floss the discs |
What is thread tension on a sewing machine?
A sewing machine makes a lockstitch: the top thread goes down through the fabric, the hook catches it and wraps it around the bobbin thread, and the two threads lock together. Tension decides where that lock sits. When the two threads pull equally, the knot is hidden between the layers and the seam looks the same on both sides. When one thread pulls harder, it drags the knot to its side and the other thread shows as loops or dots.
The top thread is tensioned by a pair of spring-loaded discs, set by the numbered dial or the menu on your machine. Most dials run from 0 to 9 and sit at 4 or 5 for all-purpose thread on medium cotton. The bobbin thread is tensioned by a small flat spring on the bobbin case, set by a tiny screw. It is adjusted at the factory and rarely needs touching; nearly every tension problem is solved on the top side.
Note: A higher number means more pull on the top thread, which draws the bobbin thread up through the fabric. A lower number lets the top thread be pulled down to the underside.
Why does the bobbin thread loop on the underside?
Loops on the underside look like a bobbin problem, because that is where the bobbin thread lives. They are not. The hook pulls the top thread all the way round the bobbin on every stitch; if nothing holds the top thread back, the surplus is left under the fabric as a loop. So loops underneath mean the top thread has too little tension.
The usual reason is threading with the presser foot down. Lowering the foot closes the tension discs, and a thread laid across closed discs sits on top of them instead of between them: the dial can read 9 and the thread still runs free. Raise the foot, pull the thread out, and re-thread the whole path from the spool, making sure it snaps into the take-up lever, and sew again before you touch the dial.
Why is there a thread nest under the fabric when I start sewing?
The tangle that jams the first few stitches, often called a bird's nest, is the same problem at the moment the machine starts. The top thread has no tension, or the loose tails get pulled down into the hook and wound up with the first loops. Three habits stop it:
- Start with the take-up lever at its highest point, so the top thread is not slack. Turn the handwheel toward you one full turn before you sew.
- Hold both thread tails behind the presser foot for the first three or four stitches, or start 1 cm (3/8 in) in from the edge rather than right on it.
- Keep the bobbin area clean. Lint packed around the hook gives thread something to catch on. How to clean and oil a sewing machine takes ten minutes.
If a nest appears on every start even after re-threading, check the hole in the needle plate for a burr from a broken needle and the bobbin for a nick on its rim; both snag thread.
Why does the seam pucker?
Puckers are the fabric being gathered by the thread. On fine or tightly woven fabrics such as lawn, voile, poplin and silk, the thread takes up more room than the weave can give, or it is pulled too tight to lie flat. Three settings fix most of it: lower the top tension by one number, shorten the stitch to 2 mm, and use a sharp, fine needle, a 60/8 or 70/10 microtex. A needle that is too large makes holes the fabric closes around.
Then change how you handle the fabric. Hold it taut in front of and behind the foot, without pulling it through, so the feed dogs move it without pushing a ripple ahead of the needle. On slippery fabrics a strip of tissue paper under the seam, torn away afterwards, stops the layers shifting. If the puckers appear only on one layer, the presser foot pressure is too high for the fabric; most machines let you lower it.
How do you diagnose tension problems step by step?
Change one thing at a time and test after each, on two layers of the actual fabric, with different colors of thread on top and in the bobbin so you can read the stitch.
- Re-thread both threads from scratch, presser foot up, following the diagram on the machine. Pull the thread forward through the needle when you unthread rather than backward through the discs.
- Check the bobbin. It should be wound evenly and not overfull. Drop it in so the thread comes off in the direction printed on the plate or in the manual, and pull it under the tension spring until you feel it click.
- Fit a new needle of the right type and size for the fabric, pushed all the way up in the clamp, flat side to the back.
- Clean the bobbin area and floss the tension discs: with the foot raised, run a folded piece of thin cotton or an unwaxed dental floss between the discs to lift lint out.
- Sew a test seam and read it. Loops underneath: raise the top tension one number. Bobbin thread showing on top: lower it one number. Repeat until the lock is hidden.
- Only then consider the bobbin tension. Mark the screw's position, turn it a quarter turn (right to tighten, left to loosen) and test again.
- Check the thread. Top and bobbin thread should be the same weight; a thick top thread over a fine bobbin thread will never balance.
- Check the spool. A horizontal spool needs a cap of the right size; a thread that catches in the notch of the spool gives a sudden tight stitch every few centimeters.
When is it not a tension problem at all?
A bent needle, a burr on the plate, a needle hitting the foot or the plate, or a stitch chosen with the wrong foot all produce loops and breaks that no dial will cure. If every fabric, thread and needle gives the same uneven stitch after a full re-thread and a clean, the hook timing may have moved, usually after a hard jam. Timing is a technician's job on a modern machine. For a refresher on the whole threading path, bobbin winding and a first seam, the sewing machine basics guide walks through it. And if a particular step of a project is fighting you, say a slippery viscose that puckers, the sewing guide that comes with a Sewly pattern can be asked for tips on that step.