Breakdown Calls Jump from Four to Seventeen: Skipped Stitches and a Cheaper Oil
A factory has 45 three-year-old lockstitch machines on a single shirt model. Breakdown calls rose from 4 per month to 17 in the last month. Eight machines on one line skip stitches; operators say the skipping gets worse after the blow-off gun has been used to clean the machines at shift end. Older machines hold their stitch length setting for only a few hours. One machine has started leaving a fine oil line along the seam. The mechanic keeps no machine history cards and answers nearly every call by loosening the hook and re-timing 'by eye'. Management has just switched to a cheaper general-purpose lubricating oil because the recommended grade went up in price. There is no standby machine, the line is committed to 5,000 pieces in nine days, and the mechanic is also expected to service the cutting room's straight-knife machine.
1. List the likely causes of the skipped stitches and set out a diagnostic sequence for the eight affected machines, explaining why the order of checks matters.
Read the worked answer
Skipped stitches mean the needle thread fails to form a proper loop with the hook, or the loop is not picked up at all. Causes fall into four groups: needle, threading/tension, needle-to-hook relationship, and fabric/feeding.
Check in this order - cheapest, most reversible and most frequent first:
- Needle: correct system and size for the fabric, sharp and straight (roll it on a flat surface), correct point for woven shirting, inserted fully into the needle bar with the scarf facing the right way. A blunt or wrong-size needle is the single commonest cause.
- Threading and tension: thread path complete, take-up lever threaded, bobbin wound correctly and inserted the right way, tension discs free of lint.
- Lint and contamination: the blow-off gun is driving lint and oil mist into the tension discs, hook and race - that explains why the problem worsens after cleaning. Blow-off should be replaced by brush and vacuum cleaning, and the affected heads stripped and cleaned.
- Needle bar height and needle-to-hook clearance - check against the manual with a gauge.
- Hook timing and feed: only after all the above.
Why the order matters: hook timing is a precision setting. Once a mechanic starts re-timing 'by eye' on every call, each adjustment introduces a new fault and destroys the reference point, so the machine never returns to a known-good state. Timing should be disturbed once, correctly, with a gauge - never as routine first aid.
2. Criticise the current maintenance practice and propose a preventive maintenance system with the records you would keep, given that no standby machine is available and 5,000 pieces are due in nine days.
Read the worked answer
What is wrong now: breakdown-only (firefighting) maintenance; no history, so no way to see that 17 calls are concentrated on a few heads; timing used as a universal cure; one mechanic also responsible for the cutting room, so planned work is always displaced by emergencies; no lubrication chart, so oil is chosen on price.
Nine-day priority (protect the shipment first):
- Triage the eight skipping machines today - needle, threading, clean, then gauge-check. Machines that pass go back to the line; the two worst are swapped with heads from the least critical operation rather than left running.
- Stop the blow-off gun practice immediately and issue brushes and a small vacuum.
- Start a simple history card per machine from today, recording date, symptom, action, parts and downtime.
Preventive system after the shipment:
- Schedule by running hours, not by calendar alone: daily (clean, oil, check needle), weekly (tension, belt, lint in race), monthly (timing and clearance check with gauge, motor and clutch, lubrication of specified points).
- Lubrication chart from the machine manual: correct grade and interval, quantity per point, signed off by the operator or a designated person.
- Records that produce information: history cards, MTBF and MTTR by machine type, breakdown Pareto, spare-parts consumption, needle consumption per 1,000 pieces.
- Justification to management: 17 breakdowns a month on 45 machines is lost production plus rework; a planned service takes a predictable 30 minutes per machine per month, while a breakdown on a committed line costs hours and missed shipment.
3. Management bought a cheaper general-purpose oil and one machine is now leaving an oil line on the seam. Evaluate the oil decision and state what you would do before the next shift.
Read the worked answer
The oil is a machine component, not a consumable to be bought on price alone.
Why cheap oil fails:
- Wrong viscosity: too thick and it will not reach the race and needle bar; too thin and it migrates onto the fabric and into the thread path. Both cause wear and stitching faults.
- Poor additive package: detergent/dispersant and anti-wear additives are what keep lint and metal fines in suspension and protect the hook and bushings; a general-purpose oil can oxidise, varnish and gum the moving parts.
- Staining risk: some cheap oils yellow or stain fabric, which is a defect that only shows after pressing - an invisible cost far larger than the oil saving.
- Warranty and manual compliance: using a non-recommended grade can void the machine warranty and invalidates any claim for premature wear.
Action before the next shift:
- Drain and replace with the grade specified in the machine manual (viscosity class and type as stated), on all 45 machines, and flush the two worst heads.
- Strip and clean the head producing the oil line; check the oil reservoir level, the wick/felt and the gasket, and look for an overfilled sump.
- Check the fabric for oil marks already produced - oil stains may not appear until after pressing, so inspect the last day's output under good light.
- Do the arithmetic for management: the saving per litre multiplied by annual consumption versus one rework batch or one worn hook assembly - the calculation justifies returning to the specified oil.