Use the fishbone to check what the 5-Why did not. Go through all six categories and mark each cause confirmed, ruled out or unknown, with the evidence beside it. A mark with no evidence does not count. Test every confirmed cause against both occurrences, and say the unknowns out loud in the report.
Wednesday, after lunch. The old fishbone is taped to the wall. It has branches for People and for Method. The other four, Machine, Material, Measurement and Environment, are empty. The team filled in 2 of 6 categories and wrote the cause the first time one of them looked likely.
The 5-Why gave you a proven cause and a proven escape. The fishbone has a different job on a repeat. It checks that you did not stop looking too soon, and it makes you say what you ruled out and why. Do the four skipped categories first.
Go through every category
For each cause on the board, put one of three marks on it: confirmed, ruled out or unknown. Then write the evidence beside the mark. A mark with no evidence does not count.
- Machine. Pilot worn below its limit: confirmed, by the three pilot runs. Press 4 against press 2: ruled out, because the defect appeared on both.
- Material. Different coil lots in the two occurrences: ruled out. The two lots came from different coils and the defect was the same.
- Measurement. The plug gage reads hole size, not spacing: confirmed. It is the reason the defect escaped.
- Method. First piece is the only spacing check in a run: confirmed. It passes before the defect starts.
- People. Operator error: ruled out. Different operators, different shifts, same defect, and both first pieces were good.
- Environment. Shop temperature: unknown. No temperature record was kept.
That is 3 confirmed, 3 ruled out and 1 unknown. Say the unknown out loud in the report. Here the three pilot runs held everything else the same, so temperature is unlikely to matter, but unlikely is not ruled out.
Test it against both occurrences
A cause belongs on the confirmed list only if it explains both lots. The worn pilot does: both lots came from the end of long runs of the same die. The gage does: neither lot was caught. The first piece check does: both first pieces passed. The old cause, the operator, fails the test, because it explains neither the different people nor the end of the run.
If a cause explains only one of the two, it is probably a contributor or a coincidence. Keep it on the board, mark it, and do not build the fix around it.
What the fishbone is for
At this point the board makes the argument for you. Two confirmed causes and a confirmed method gap point at the same place: the process around the die, not the person at the press. The fishbone does not find that. The 5-Why already found it. The fishbone is what lets you show anyone reading the report that you looked everywhere else first.
Read more: how to make a fishbone diagram. Book 1 shows the full method in Find the Cause.
The mistake to avoid
Filling the branches that feel likely and leaving the rest blank. A blank branch tells the next reader that you did not look, and the next reader may be the customer. Write ruled out or unknown on every branch, with the reason.
- Count the empty branches on your last fishbone.
- Mark each confirmed, ruled out or unknown.
- Write the evidence beside every mark.
Open the printable Monday Board
Related on ShopFloorOps
The part, the die, the lots and every number in this book (sensor bracket BK-6139) are a made-up example. The tool sidebars show ShopFloorOps tools, which I make and sell. Every chapter teaches the step itself, so you can use your own forms. New to a term? See the quality glossary.