No matter how careful you are, defective parts can never be reduced to zero.
What matters is responding correctly and calmly when a defect occurs.
Getting the initial response wrong can let defective parts escape into the market or cause the same defect to recur.
This page explains the steps and key points for responding when defective parts occur.
In This Article
First, Stop and Separate
When you find a defect, the first priority is to keep the damage from spreading.
First, stop machining.
Continuing to machine while defects are occurring only produces more defective parts.
Until the cause is known, pause machining and assess the situation.
Next, separate good parts from defective ones.
Defects may be mixed in among parts already machined, so quarantine any suspect lot and clearly distinguish good parts from defective ones.
In unattended operation especially, you cannot tell when the defects started, so the lot produced since the last confirmed good part must be checked carefully.
Carrying out this "stop and separate" step without fail is the iron rule of the initial response for preventing defective parts from escaping.
Isolate the Cause
Once the damage is contained, the next step is identifying the cause.
In most cases, the cause of a defect lies in one of five areas: the machine, the tooling, the material, the cutting conditions, or the setup.
Narrow down the cause from the nature of the defect.
For example, if dimensions are uniformly off, suspect tool wear or offset values; if the cross-section is distorted, suspect the workholding or roundness; if the surface is rough, suspect the tooling or cutting conditions; if defects suddenly appeared at a specific point in time, suspect tool breakage or a chip jam — infer the cause from the symptom.
The basic rule of isolation is to change one thing at a time rather than several at once.
Link It to Recurrence Prevention
Once you have found and addressed the cause, do not stop there — it is important to go further and prevent recurrence.
Dig into why the defect occurred and why it was not prevented, and put countermeasures in place as part of your system.
For example, if tool wear was the cause, review your tool-life management; if dimensional drift was the cause, adopt in-machine measurement; if a setup mistake was the cause, improve work procedures and checks — building mechanisms that ensure the same defect never happens again.
A defect is also valuable information for strengthening your recurrence-prevention system.
Reaching the Root Cause with "Five Whys" Analysis
To keep a defect from recurring, it is important to reach not just the surface cause but the true root cause behind it.
A useful approach here is "five whys" analysis — repeatedly asking "why?" to dig deeper into the cause.
For example: "A dimensional defect occurred" — why? "The tool was worn" — why? "It was past its replacement time" — why? "There was no tool-life management rule." Repeating "why" in this way reveals the systemic problem behind the surface phenomenon.
In this example, the root cause is the absence of a tool-life management system, and the countermeasure is to create tool-life management rules.
Simply removing the defective parts in front of you means the same defect will happen again.
Digging into why it occurred and building countermeasures into your system is what leads to true recurrence prevention.
Frequently Asked Questions (FAQ)
Q. What should I do first when I find a defect?
A. First stop machining, then separate good parts from defective ones to keep the damage from spreading.
In unattended operation you cannot tell when the defects started, so carefully check the lot produced since the last confirmed good part.
Q. How do I identify the cause?
A. Based on the symptoms of the defect (dimensional deviation, distorted shape, rough surface, etc.), narrow the cause down to the machine, tooling, material, cutting conditions, or setup, and check each one at a time.
Q. How do I avoid repeating the same defect?
A. It is important to go beyond addressing the immediate cause and put recurrence-prevention mechanisms in place, such as tool-life management, in-machine measurement, and well-maintained work procedures.
Summary
When defective parts occur, contain the damage by stopping and separating, isolate the cause from the symptoms, address it, and then follow through by building recurrence-prevention mechanisms.
Following the steps calmly is what prevents defective parts from escaping and improves quality.
For everyday preventive measures, see "Operational Checklist for Preventing Machining Losses".