Something has gone wrong and there is a job waiting. Find the symptom, work down the list in order — the cheap checks are first for a reason.
Before anything else: change the needle. It is the cheapest thing in the workshop and the most frequent culprit. A needle bends long before it looks bent, and a bend of a fraction of a millimetre moves it out of the hook’s path. Machine shops report a great many machines brought in for skipped stitches that sew perfectly the moment a new needle goes in.
Then remember the order: threading, needle, tension. Roughly four times out of five the fault is one of those three and not the machine.
- Skipped stitches
- Thread breaking or shredding
- A nest of thread under the fabric at the start of a seam
- Puckered seams
- Uneven or loose stitches on one side
- Material not feeding evenly
Skipped stitches
For a stitch to form, the hook has to pass through the loop of thread that appears at the needle eye as the needle rises a few millimetres off its lowest point. Miss that loop and you get a gap. Everything below is a reason the hook and the loop failed to meet.
- Change the needle. First, always.
A needle bends long before it looks bent, and a bend of a fraction of a millimetre moves the needle out of the hook’s path. Machine shops report that a great many machines brought in for skipped stitches sew perfectly on a new needle. Ignore the hour-counts you see quoted — those come from garment production, where the machine runs all day on fine cloth. Change it when the work tells you to: skipped stitches, a change in the sound, snagged or pulled threads in the face, or straight after the needle has met a tack, a staple or the plate. And put a fresh one in before a job you cannot afford to mark. - Check the needle is in the right way round and fully home
Pushed up short, or turned a few degrees, and the clearance goes. Worth ten seconds before anything else. - Check the needle size against the thread
Too large a needle with too fine a thread makes a loop too small for the hook to catch. There is a neat test for this: thread the needle, hold the thread at about 45 degrees and let the needle hang on it. A correctly matched needle slides down the thread under its own weight. - Rethread the machine completely
With the presser foot up, so the tension discs are open. Threading with the foot down leaves the thread sitting outside the discs and no amount of adjustment will fix it. - Check the material is being held down
If the cloth lifts with the needle — flagging — the loop never forms properly. On an industrial machine, check the inner presser foot is holding firmly on the needle’s downward stroke. - Check the throat plate
The hole should be only slightly larger than the needle and thread together. An oversized or worn hole lets the material push down into it. Check it is not warped while you are there. - Only then suspect timing
If the hook passes below or too far above the needle eye it will miss the loop regardless. Timing genuinely does drift, and a hook tip that has been sharpened more than once or twice will have moved it. That is a job for someone with the tools.
Thread breaking or shredding
Usually friction somewhere in the thread path, or a mismatch between thread and needle. The thread is telling you where it is being chafed.
- Needle too fine for the thread
The commonest cause. The thread has to pass through the eye twice on every stitch — down and back — and a tight eye files it away. Go up a needle size within the range for that thread. - Burr on the needle, hook, throat plate or thread guides
Run a finger over the thread path. A burr you can barely feel will shred thread at speed. The needle plate hole and the hook tip are the usual places. - Top tension too tight
Back it off and work up again rather than down from tight. - Wrong twist
Machine thread needs a final Z twist. S twist untwists as it runs through the machine and frays for no other reason. See the thread guide. - Unbonded thread
At upholstery speeds unbonded thread of the same size will fluff and snarl. Bonded is not a luxury here. - Thread coming off the cone badly
It should feed straight up off the cone through the stand, not be dragged sideways. A cone sitting where the spool pin should be will twist the thread as it unwinds.
A nest of thread under the fabric at the start of a seam
Almost never a tension fault, though it looks like one.
- Hold both thread tails behind the foot as you start
At the first stitch there is nothing holding the threads, so the fabric gets pushed down into the needle plate hole and the thread piles up underneath. Holding both tails for the first three or four stitches fixes it outright. - Start a few millimetres in, then reverse back
Rather than starting hard on the edge, where there is least to grip. - Check the upper thread is actually in the tension discs
If it looks like a nest every single time from the first stitch, this is why. Rethread with the presser foot up.
Puckered seams
The two layers are arriving at the needle at different rates, or the thread is pulling the seam up after it is sewn.
- Top tension too tight
The most likely cause on a straight seam. The thread is drawing the seam in after it is stitched. - Drop feed on a job that needs compound feed
If only the underside is being driven, the top layer creeps and gathers. This is the classic sign of upholstery being sewn on the wrong machine — see machines. - Blunt needle
A blunt point pushes the material down and forward rather than piercing it cleanly, and that displacement shows up as pucker. - Stitch too short for the cloth
Crowding stitches into a heavy weave gathers it. Lengthen and try again.
Uneven or loose stitches on one side
Loops on the underside mean the top tension is losing; loops on top mean the bobbin is. The loose side is the side that is failing.
- Rethread the top with the presser foot up
Before touching a single dial. Thread outside the tension discs produces exactly this and is the most common cause by a distance. - Check the bobbin case for lint
Under the tension spring in the bobbin case is where lint collects, and a small amount changes the tension noticeably. Brush it out — never blow compressed air into a machine, it drives lint further in. - Check the bobbin is wound evenly
A bobbin wound loose or lumpy will not pay off at a steady rate however the tension is set. - Adjust the top tension, not the bobbin
The bobbin tension is set and rarely wants moving. Almost every tension problem is solved on top.
Material not feeding evenly
Something is holding one surface back, or the feed is not gripping it.
- Vinyl or coated cloth sticking to the foot
A steel foot drags on a vinyl face. A roller foot or a Teflon foot solves it, and it is worth owning one before you need it. - Feed dogs clogged or worn
Lint packs between the teeth and they stop gripping. Worn teeth on an old machine do the same. - Presser foot pressure wrong
Too little and the material floats; too much and it drags. Adjust in small steps and test on an offcut. - Wrong feed type for the work
Layered or slippery assemblies need compound feed. No adjustment will make a drop feed machine handle them.
When it is worth stopping
Timing, hook clearance and a bent needle bar are real faults and they do happen. But they are the last things to suspect, not the first, and they need tools and someone who knows the machine. If you have worked through the list for a symptom and nothing has changed, that is the point to stop guessing and get it looked at — rather than three hours in, having moved every adjustment on the machine and lost your starting point.
Write down what you change. The single most useful habit in machine troubleshooting. Move one thing at a time, note it, and test. Half the machines that end up genuinely out of adjustment got there during an attempt to fix something else.
Also in this section: selector · thread · needles · machines · setup · everything.