Root Cause Analysis with 5 Why: A Shop-Floor Guide for MSME Plants

What is root cause analysis with 5 Why?

Root cause analysis is finding the real reason a problem happened, so you can remove it. The 5 Why method does this by asking "why?" again and again, each time with a checked fact as the answer, until you reach a cause in the system that you can fix.

Most shop-floor problems are solved on the surface. A fuse blows, so the fuse is changed. A shaft is oversize, so it is re-turned. The machine runs again and everyone feels relieved.

Then the problem comes back next month. The cause was never touched.

A simple example of the idea

A machine stops. Ask why, and keep asking:

1. Why did the machine stop? A fuse blew because of an overload.
2. Why was there an overload? The bearing was not lubricated enough.
3. Why not? The lubrication pump was not pumping enough.
4. Why not? The pump shaft was worn.
5. Why was it worn? There was no strainer, so metal chips got in.

If you change the fuse, the machine stops again. If you fit a strainer, the cause is gone.

What are the five habits of a good 5 Why?

1. Ask the full question every time. Not just "why?" but "Why did the journal come out oversize?" Repeating the subject stops the chain from drifting at the third or fourth step.
2. Answer with a fact you have checked. Go to the machine. Look, measure, ask the operator and the setter. Write "because" only for what you have seen or measured.
3. One cause per step. If there are two answers, follow both branches to the end.
4. Stop when you reach the system. A missing standard, check, gauge, tool or training is something the plant can fix. If your answer is a person, ask one more why.
5. Read it back with "therefore". Start at the root cause and read upward: "this, therefore that, therefore the problem." If one step does not follow, a why is missing.

A useful last test: put the whole chain into one sentence. If the sentence does not hang together, there is a gap.

What if the first answer is "the operator was careless"?

This is the most common wrong stop. The chain drifts from the defect to people and targets, and the action becomes a warning letter. A warning letter cannot stop the problem returning.

Ask why the operator could make the mistake. Maybe the setup sheet has no figure. Maybe the gauge is kept in another room. Maybe the new tool cuts differently. These are things you can fix. Fix the system, respect the person.

Why do we need a second path?

Every defect that reaches inspection or the customer raises two questions.

Path A: why was it made? Our process allowed a bad part. Look at the machine, method, material, setting and standard.

Path B: why was it not caught? Our checking let it escape. Look at the check plan, how often we check, the gauge, and where the check is done.

If you fix only Path A, the next different cause will also escape. If you fix only Path B, you catch bad parts but keep making them. Two paths give two root causes, and each gets its own action.

A worked example, with Path A and Path B

This is an example with made-up details. At final inspection, 12 keyways are found wider than the drawing allows.

Path A: why were they made wide?

• Why were the keyways wide? A new cutter cuts wide in its first shafts.
• Why does it cut wide? New cutters are slightly oversize, and with normal run-out they cut wider until they wear in a little.
• Why are the cutters that size? They come from a new supplier who makes them to a wider size.
• Why were they accepted? The purchase order says only "12 mm slot drill", and cutters are not checked on receipt.

Path B: why were they not caught?

• Why did 12 wide keyways reach final inspection? The check at the milling machine did not catch them.
• Why not? The first piece is checked with a vernier, which cannot reliably see a difference of a few hundredths of a millimetre.
• Why a vernier? The go/no-go gauge is kept in the quality room.
• Why there? The inspection plan does not ask for a gauge at the machine or a check after each cutter change.

Now the actions are clear and about the system. Write the cutter size on the purchase order and check it on receipt. Keep a gauge at the machine. Add a check after every cutter change. None of them is "operator warned."

If your answer is a person, you have not asked enough whys.

How to start in your plant

1. Describe the problem in a sentence: what, where, when, how many. No cause and no solution inside it.
2. Contain the problem first. Stop making bad parts and tag what you have.
3. Gather the people who know the job, and go to the machine.
4. Ask the whys with the five habits. Write each answer on a sheet where everyone can see.
5. Run both paths, A and B.
6. Choose actions for each root cause, each with an owner and a date.
7. Wait a full month with no repeat before you close it.

Containment is not the fix. It costs time every day, and it depends on people remembering. Keep it only until the root-cause action is proved.