Most supervisors know the Pareto rule: roughly 20% of causes produce about 80% of the problems. But that is only the starting point — not the solution. To eliminate scrap sustainably, you must move from prioritisation (Pareto) to diagnosis (Ishikawa) and then to verified corrective action.
🔎 Beyond 80/20: From Pareto to Fishbone
In a modern CNC environment, simply knowing which problems are “biggest” is not enough. Real improvement comes when you systematically connect data, analysis tools, and disciplined execution. Below is the sequence I use as a manager trained in systems thinking and operational excellence.
1️⃣ Measure & Prioritise (Pareto)
Goal: Focus on the vital few issues that cause most of the waste.
- Collect defect data in a structured way: part number, operation, shift, machine, tool, timestamp.
- Build a Pareto chart to identify the handful of defect types or machines that generate most scrap. This focuses effort where the impact is highest.
2️⃣ Diagnose Deeply (Ishikawa / Fishbone)
Goal: Move from visible symptoms to a structured view of potential causes.
- For the top Pareto items, run a facilitated Fishbone session with operators, setters, QC and maintenance.
- Use the classic 6 Ms: Man, Machine, Method, Material, Measurement, Mother Nature (Environment).
- Populate the diagram with observed causes, not opinions.
Example for “out-of-tolerance parts”:
- Man: inconsistent hand-offs, poor operator training on offsets.
- Machine: spindle runout, worn tool holders.
- Method: inadequate setup checklists, missing touch-off procedure.
- Material: supplier lot variation, hard spots in barstock.
- Measurement: uncalibrated gauges, reading variability.
- Environment: temperature fluctuations, vibration from adjacent cell.
3️⃣ Validate Roots (5 Whys + Data)
Goal: Separate proximate causes from true root causes.
- For each branch, use 5 Whys to drill down into why the condition exists.
- Triangulate with evidence: tool-life logs, SPC charts, spindle runout measurements, setup records.
- Discard hypotheses that are not supported by data — keep only causes you can prove.
4️⃣ Design & Implement Countermeasures
Goal: Turn insight into concrete, testable actions on the shop floor.
Typical countermeasures include:
- Standardised setup checklists and touch-off procedures.
- Mandatory independent part inspection during idle time.
- Tool management: tool-life tracking, shadow boards, predefined replacement limits.
- Supplier containment and incoming inspection for risky materials.
- Scheduled spindle checks and preventive maintenance routines.
- Calibration regime for gauges and critical measurement equipment.
- Upskilling micro-trainings for setters and operators on key dimensions, offsets and SPC basics.
Assign clear owners, timelines, and verification metrics for each countermeasure.
5️⃣ Control & Sustain
Goal: Make the new level of performance the “new normal”.
- Deploy SPC on critical dimensions with clear reaction plans.
- Embed OEE and quality KPIs on the shop-floor dashboard.
- Run daily quality huddles to review trends, incidents, and open actions.
- If the corrective action holds, scale it across similar part families; if not, iterate and refine the analysis.
6️⃣ Leadership & Culture
Goal: Make problem-solving operator-led, not purely top-down.
- Make the process operator-led: involve them directly in diagnosis and design of fixes — adoption becomes natural.
- The supervisor’s role is to facilitate, provide resources, and remove barriers, not to “own” all answers.
- Recognise and celebrate teams who reduce scrap and stabilise processes using this method.
Result orientation: This structured path — Pareto → Fishbone → 5 Whys → verified countermeasures — routinely delivers dramatic scrap reductions (typical cases: 30–70% within months), shorter setups, and measurable OEE uplift.
👉 How do you run root cause analysis in your shop?
Would you be interested in applying a combined Pareto + Fishbone + SPC approach with your team?
