SPECSPEC8D Report › D4: the root cause8D Report › D4: the root cause
D4 root cause analysis, complaint CQ-4471D4 root cause analysis, complaint CQ-4471
Erwan·August 18, 2026
Part 774-2210-B, batch L-26-0714. Written 2026-08-06.
Status in one line: occurrence cause CONFIRMED by measurement on retained samples. Escape cause CONFIRMED by document review. One secondary hypothesis remains OPEN and does not block D5.
01 ┤Hypothesis table, occurrence: why was the burr made
| ## | HypothesisHypothesis | Test that would settle itTest that would settle it | DoneDone | ResultResult | StatusStatus |
|---|---|---|---|---|---|
| O1 | Deburring brush worn beyond useful life before its scheduled change | Measure brush filament length on the removed brush against a new one | Yes, 2026-08-03 | Removed brush 14 mm against 22 mm new, 36 percent loss. Trials on retained parts with a new brush produced no burr | CONFIRMED |
| O2 | Bore diameter drift pushing material into the intersection | Review measurement log for the batch against tolerance | Yes, 2026-08-01 | 8.02 to 8.05 mm on a tolerance of 8.00 to 8.10. Centred, no drift | RULED OUT, log ML-26-0714 |
| O3 | Drill wear on the 8 mm operation | Tool life counter and drill inspection | Yes, 2026-08-01 | 1 100 parts on a 4 000 part tool life. Drill within wear limit at inspection | RULED OUT, tool log CN06-T14 |
| O4 | Material batch harder than nominal | Hardness check on retained raw material | No, retained coupon not located | Not tested | OPEN |
O4 stays OPEN. It is unlikely given that O1 is confirmed and reproduces the condition, but no test was run and it is not written off. Hardness check is scheduled 2026-08-14 on the next coil from the same supplier lot.
02 ┤Hypothesis table, escape: why did it get out
| ## | HypothesisHypothesis | Test that would settle itTest that would settle it | DoneDone | ResultResult | StatusStatus |
|---|---|---|---|---|---|
| E1 | Control plan has no burr criterion on the 8 mm bore | Read control plan CP-774-2210 rev C | Yes, 2026-07-30 | No burr characteristic listed. Bore diameter only | CONFIRMED |
| E2 | Sampling too small to catch a 20 percent condition | Sampling calculation against the observed rate | Yes, 2026-08-04 | 5 parts per box against an observed 20 percent gives roughly a 33 percent chance of catching it per box. Contributory, not primary | CONFIRMED, secondary |
| E3 | Inspector did not follow the control plan | Review inspection records and interview shift B inspector | Yes, 2026-08-04 | Records complete and consistent with the plan. The plan itself did not ask for it | RULED OUT, record IR-26-0714 |
03 ┤Five whys, confirmed occurrence branch
Why did the parts have a burr? The deburring brush was worn to 14 mm.
Why was it worn? It ran past its useful life inside its scheduled interval.
Why did the interval not catch it? The interval is time based, seven days, with no wear measurement at change.
Why is it time based? It was set when the machine ran a softer part at half the volume, and never revisited when 774-2210-B moved to CN-06 in March 2026.
Stop at four. The next why leaves what this company controls.
04 ┤Verdict
Occurrence: a brush wear interval set for a different part and volume, with no wear check to detect early wear. CONFIRMED.
Escape: the control plan carries no burr characteristic on the 8 mm bore, so no inspection could have caught it. Sampling size contributes. CONFIRMED.
Open: O4, raw material hardness. Test scheduled 2026-08-14. Does not block D5, which addresses both confirmed causes.
🗂️8D Report
by Erwan