Rework Pareto and first-off porosity in a fabrication shop
A fabrication shop produces about 120 m of FCAW fillet weld per day on 8–12 mm mild-steel frames. Last month’s inspection data show 11.2% rework: porosity 42%, undercut 23%, spatter 15%, incomplete fusion 12%, other 8%. Two 350 A machines share a gas manifold; machine A’s wire-feed calibration is 8% low, machine B’s gas flow is 9 L/min. Three welders rotate across both machines. Porosity is 3.1 times higher on Monday morning than on other shifts, and 68% of porosity occurs on the first 300 mm of each weld. Wire spools are stored on an open rack near the wash bay. The shop wants a 30-day improvement project and has one part-time QC inspector.
1. Analyse the data and select the most likely priority problem and root causes.
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Porosity is the largest category at 42% and has the clearest pattern. The Monday-morning and first-300-mm concentration points to start-up conditions: gas line purge, regulator creep, moisture in the torch or wire, and first-off machine settings. Machine B at 9 L/min is below the flow used elsewhere; machine A’s WFS 8% low changes current and penetration and may contribute to incomplete fusion and undercut. Wire stored on an open rack near the wash bay is a moisture source. Prioritise porosity first because it is the largest category and has a plausible common cause, then verify WFS calibration for fusion and undercut.
2. Propose a 30-day PDCA plan with measurable targets and low-cost countermeasures.
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Days 1–3: measure gas flow at each nozzle, repair leaks, set 15–20 L/min, install a purge routine and a start-up scrap test; calibrate both machines and tag them. Days 4–10: move wire to sealed storage with desiccant and FIFO, inspect tips and liners, and train all three welders on CTWD, start/stop and gas preflow. Days 11–20: run a 5-weld trial per welder per machine, record parameters and defects, and compare against baseline. Days 21–30: standardise the best settings, add a daily first-off checklist and a 30-minute QC audit. Targets: rework below 6% and porosity below 2% of inspected length; only use targets the shop can measure.
3. What metrics and documentation will show whether the change is sustained?
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Track first-pass yield, defect rate by type per 100 m, porosity rate by shift/machine/welder, gas flow at nozzle, WFS calibration deviation, wire storage audit score, and rework hours. Use a Pareto and a run chart or control chart before and after; keep calibration certificates, gas-flow logs, training records, revised work instructions, and trial weld inspection results. A single month of improvement is not enough - require three consecutive months within target and an audit by someone independent of production.