Coverstitch/flatlock tension diagnosis on a sleeve hem after a thread lot change
A T-shirt line uses a 3-needle, 5-thread flatlock (coverstitch) to hem 180 gsm 100% cotton single jersey sleeves. Specification: finished hem width 20 mm plus or minus 2 mm, 12 stitches per inch, no grin. Current settings: needle threads 2/60 polyester, top cover thread 2/120, bottom looper 2/120, differential feed 1.0, presser foot pressure set high because the operator complained the fabric slipped. A new lot of 2/120 cover thread arrived last week and was loaded on all six machines without any setting change. Since then: the fabric tunnels in ridges between the needle lines, the hem measures 22-23 mm on 4 of every 10 pieces, the top cover thread floats and needle loops show on the face side near the seam end, and after 5 home washes the hem edge waves. Machine A, threaded by the trainee, looks different again: its middle needle thread lies loose on the underside while the other two are correct.
1. State the order in which the tensions on a coverstitch must be set and why that order matters. Then say which tension group you would suspect first for the floating top cover and which for the tunneling.
Read the worked answer
Setting order on a coverstitch: looper first, then needle threads, then top cover thread.
The reason is mechanical dependency. The bottom looper supplies the thread that the needles pick up; if looper tension is wrong, the needle loops are either starved or flooded, and any needle adjustment made before that is a correction of a symptom. The needle threads in turn decide where the needle loops sit relative to the fabric, and the top cover thread only interlocks with those needle loops on the underside. Set the top cover first and you will be chasing it every time a needle tension moves.
Practical rule: adjust in half-turn steps, sew a scrap of the same fabric after every change, keep a settings sheet, and never change two groups at once.
Diagnosis here
- Floating top cover and needle loops showing on the face: the top cover thread tension is too loose relative to the needle threads, so the cover thread is not being pulled down into the interlacing point. The new thread lot, with a different coefficient of friction, is the plausible trigger.
- Tunneling between the needle lines: needle thread tension too tight (and high presser foot pressure), pulling the fabric up into ridges between the needle lines.
- Machine A's loose middle needle on the underside is a threading fault, not a tension fault - the trainee has most likely missed a thread guide or the needle is threaded from the wrong side.
2. Using the mechanics of the machine, explain why the hem is measuring 22-23 mm instead of 20 mm and why the edge waves after washing, and state the feed and pressure changes you would trial.
Read the worked answer
The hem width is set by the folded depth, but it is held by the stitch. Two mechanisms are adding 2-3 mm:
- Fabric extension during sewing. With differential feed at 1.0 and high presser foot pressure, the knit is dragged and stretched as it passes under the foot. The stitch is formed on stretched fabric; the fabric then relaxes and the hem edge has to go somewhere, so it stands up or waves. That same relaxation is what makes the wave appear only after washing, when the jersey shrinks and the stitch length does not.
- Too much thread in the seam. Tight needle threads and a loose top cover mean thread is being pushed to the face rather than buried; the extra thread volume forces the fold open, so the finished width grows.
Trials, one variable at a time, 10 pieces each:
- Differential feed 1.0 to 1.15-1.3 (positive differential feeds slightly more fabric than the needle line requires, relaxing the knit).
- Presser foot pressure from high down to the minimum that still transports the fabric without slipping.
- Needle thread tension down half a turn; top cover tension up in quarter-turn steps until the cover sits on the surface without floating.
- Re-measure hem width on 10 pieces after 24 hours relaxation, not immediately off the machine, and again after one wash.
3. Describe the checks you would run to confirm the fix and to prevent the six machines drifting apart again. Include what you would do about the trainee's machine.
Read the worked answer
Confirmation on the machine
- Sew 10 sleeves; hem width 20 plus or minus 2 mm measured after 24 hours relaxation and again after one wash.
- Visual check on the face for floating cover thread and on the underside for loose needle loops; acceptance is zero.
- Seam extension test: pull a hem to 80% extension and release; it must recover without cracking, and the hem must not wave.
- Count stitches per inch (12) with a pick glass, since a changed SPI changes both appearance and thread consumption.
Preventing drift across six machines
- One master settings card per machine, laminated at the head: thread types, needle size and point, tensions in numbered clicks, differential, SPI, presser pressure.
- Physically check and record all six machines after any thread lot change; a new lot is a change, and thread friction changes with lot, finish and moisture.
- Keep a first-off approved sample (golden sleeve) at each machine; the operator compares every new shade or lot against it.
- Restrict tension changes to the mechanic or line supervisor, and log every change with a reason.
Trainee's machine Re-thread it completely against the threading diagram, in the correct order (looper, needles, top cover), and have the trainee thread it back while being observed. Then run the same 10-piece check. Record it as a training gap, not a discipline issue - a correctly threaded machine with a loose middle needle is a threading error, and the same error will repeat on the next style unless the threading sequence is taught and signed off.