On an overlock or coverstitch head there is no bobbin. When an operator says the bobbin is playing up, they mean the under-thread: the lower looper, or the bottom-covering thread. Working out which thread they mean is half of the diagnosis.
On an overlock there is no bobbin
An overlock uses loopers, not a bobbin case. The lower looper carries the under-thread from a cone on the stand. A coverstitch adds a top-covering needle thread and a bottom-covering thread under the seam.
So a reported bobbin problem can mean three different things: the lower looper thread, the bottom-covering thread, or a cone that is not feeding freely. Ask which thread looks wrong and the fault usually narrows to one path within a minute.
Reading the stitch diagram to find the fault
These machines are documented with a thread diagram, and that diagram is the diagnostic tool. Every thread has a colour and a route. Match the loose or tight thread in the seam to its route, then check that route in order: cone, guide, tension dial, take-up, looper or needle.
The first place the thread deviates from the diagram is the fault. In practice it is nearly always a missed guide, or a thread that jumped out at the tension disc when the last cone was changed.
Under-thread faults on a coverstitch
On a coverstitch the bottom-covering thread runs under the seam. If it is loose, the seam shows loops underneath while the top looks acceptable, so the operator reports a machine fault when the head is sewing exactly what it was told.
The usual causes are a covering tension too light for the fabric, a thread outside its guide, or a needle thread pulling the covering thread through to the top. Check the needle threads first; they set what the covering thread has to work against.
The loop that appears at the elastic edge
Loops at the edge of an elastic attachment are the most common complaint on an underwear line. Three sources cover most of them:
- Lower looper tension too loose for the elastic in use.
- Differential feed gathering more fabric than the elastic can hold.
- Elastic stretched as it was fed, then recovering after the seam and pushing the thread out.
Test by sewing the same seam with the elastic relaxed and comparing. If the loop disappears, the fault is in the feed, not in the tension.
When the fault follows the operator
Some faults move with the person rather than the head. If the same operator’s seams fail on three different machines, the cause is in threading, handling, or a setup habit.
Typical signs: the fault appears after a thread change, after a style change, or at the start of a shift, and the seam is fine later in the run once the operator has rethreaded. Watching one rethread usually finds it.
When the fault follows the machine
The reverse pattern is just as clear. If every operator’s seams fail on one head while the same style passes elsewhere, that head has a fault.
Typical signs: the fault is present from the first seam of the shift, it survives a full rethread with new cones, and it appears at every speed. That combination points to looper timing, a worn looper, a damaged needle plate, or a tension assembly that no longer holds.
A short triage that protects the run
Work this order and stop as soon as the fault is found:
- Ask which thread is wrong and match it to the diagram.
- Look along that thread’s full path, cone to looper or needle.
- Rethread the path with the presser foot up.
- Sew a sample at production speed on the same fabric.
- If the fault persists, swap the head out and move the style to another machine.
Swapping the head protects the run while the diagnosis continues. A line stopped for an hour to find a one-minute threading error loses that hour permanently.
Where the owner-technician boundary sits
Owner and operator scope: naming which thread is at fault, following its path, rethreading, small looper tension changes, differential feed adjustments for elastic, and deciding when to swap the head rather than keep trying.
Technician work: looper timing, looper or needle plate replacement, a tension assembly that will not hold, and any fault that follows the machine through a complete rethread at production speed. Hand over a head that has taken a needle strike even if it currently sews, because a bent looper can survive a shift before it fails.
Related guides
- Underwear line maintenance for the oil and timing round.
- Underwear line setup for calibration order before blaming the machine.
- Bobbin tension for the lockstitch side of the same complaint.
- Sewing machine tension for the thread-path diagnosis.
- Underwear line optimization when the fault is in decorative work.