Defects: Why They Are the Most Expensive Waste in Your Factory

Why are defects the most expensive waste?

A defect is expensive because the loss does not stop at the part. Material, machine time, labour and power are already spent when the fault is found. Then more is spent on rework or on making the part again. And if the part reaches the customer, it costs us their trust as well.

When a part is wrong, most owners count only the scrap value. That is the smallest number. The real cost is all the work done on the part, plus the repair, plus the delay to the order.

Where do defects hide in a factory?

Defects are not only on the shop floor.

In the shop

• A bore cut oversize because the tool had worn.
• A weld with a gap, found only at final inspection.
• A casting with a hole inside, found after machining.
• Wrong wiring in a panel, found at testing.

In the office

• A purchase order with a wrong rate or material grade.
• An old drawing revision given to the shop.
• An invoice with a wrong tax code that needs a credit note.

The office defects are cheaper per piece, but they start the shop defects. A wrong drawing makes 100 wrong parts.

What does one defect really cost?

Here is an example, with numbers that are only to explain the idea. A plant machines a batch of 80 pieces. The tool wears slowly. The first piece was checked and was good. Nobody measured again until final inspection.

At the end, 12 pieces had the bore too large. It cannot be corrected, so all 12 are scrap.

Now think about a check every 20 pieces. The problem would be found at piece 20 at the latest, with at most a few pieces affected. The cost of one check is a minute. The cost of not checking is a day and Rs. 18,000.

The cost of a defect grows at every step it travels. Catch it at the machine, not at the gate.

Why do defects happen?

• No check between the first piece and final inspection.
• Worn tools changed only when they fail, not by count.
• An old drawing revision at the machine.
• Operators not told which dimensions matter most.
• A rush, so there is no time to check. (This is overload; see the 8 wastes overview.)
• The belief that "we can always rework it".

That last belief is the most costly one. If rework is easy, nobody fixes the cause.

How do we reduce defects?

1. Check at the machine. Check the first piece, then at a fixed count. Do not wait for final inspection.
2. Stop and fix. When a fault is found, stop. Find the cause before running more.
3. Change tools by count. Note how many pieces a tool makes and change it before it fails.
4. Control drawings. Only the latest released drawing at the machine. Collect and mark the old copies as obsolete.
5. Count every defect with its reason. The top reasons become your improvement projects.
6. Make it hard to go wrong. A simple stopper, fixture or go/no-go gauge can stop the mistake from happening. This is mistake-proofing, or Poka-Yoke.

This is the idea of Fix the input not the output. Inspection at the end only sorts good from bad. It does not make parts good.

What records help?

Plants that control defects usually keep a few simple records: an in-process inspection report at the machine, a rework and rejection slip with the reason for each case, a root-cause (5 Why) sheet for the repeat cases, and a monthly rejection and rework report by stage and by reason. These already exist in many plants. The point is to fill them every day and to review them every month.

How to start in your plant

1. For one week, count every rejection and rework, with the reason.
2. Choose the top reason by cost, not by count.
3. Add one check at the machine for that reason.
4. Ask "why" five times until you reach a cause you can fix.
5. Review the numbers in the monthly meeting.

Play the video: Defects in Manufacturing: The Most Expensive Waste