In QA two schools clash: "Master the process so nothing bad can happen" vs. "Inspect at the end and sort out defects". Both have merit, both have limits. Combining them wins.
What process monitoring delivers
- Tool wear (rising friction level)
- Tool break (impulse)
- Faulty joining (e.g. screw not torqued)
- Bearing wear on machines
- Lubrication failure
Real-time reaction. Tool changed before 100 parts go to scrap.
What end-of-line testing delivers
- NVH behaviour
- Internal defects
- Heat treatment
- Material properties
Hard OK/NOK per part, documented per ID.
Comparison matrix
| Criterion | Process monitoring | EOL inspection |
|---|---|---|
| Reaction time | seconds | after part finished |
| Prevents scrap | yes | no – only detects |
| OK/NOK per part | not directly | yes |
| Data integration | continuous MES/PLC | discrete per part |
| Investment | 40–200 k€ | 50–300 k€ |
| Process intrusion | tool/machine sensors | separate station |
| Detectable defects | process-related | part-related |
Synergy
For safety-critical lines we recommend both:
- Process monitoring prevents systematic faults (dull tool, untorqued screw).
- EOL catches rare single-part defects (material crack, wrong batch).
Practical example
Steering component manufacturer: every screw joint acoustically monitored. Plus EOL resonance analysis of the assembly. No faulty components reach shipping – although either method alone would have gaps.
