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Root cause analysis (Part 2)

KZK Solutions /kzoli62/
Published by Z. Kovács in Quality · 28 May 2021
Tags: QSRootcause
Most recently, in the processing of the topic, we came to the description of the FTA / Standards. Now let's move on to the Good / Bad parts.

Root cause analysis

FTA / BAD / OK PARTS

  • Allows comparison between reality versus standard: bad versus good parts
  • Real data coming directly from measurement of BAD and GOOD parts or process.
  • EVERY TIME IT IS POSSIBLE, ORGANIZE MEASUREMENT WHEN THE PROBLEM IS STILL THERE:

Example:

  1. Ask Customer to perform measurement at Customer site jointly before send bad part back.
  2. Provide special instruction to Operator to measure (ex. the air pressure) when problem occurs next time
  3. OK Parts = Good part: value of factor on the OK parts produced in the same process than the Bad Parts
  4. Bad part: value of the factor on the Bad parts

FTA / NON-DETECTION


Make the process mapping of the process:

  • Process description step by step
  • You must start from the customer and go back to the station where the defect is created.

For each step, look at all the control points:

  • Are they designed to catch the defect ?
  • Which factor made this detection to fail ?

FTA / HOW TO REPRODUCE

  • Asking Operator to create defective parts having same signature than “bad” parts
  • Observation of process to capture facts when similar problems appears
  • Exchange of components shows that problem follows one component (Example using Shanin technique: component search)
  • Parts manufactured with same characteristics than “bad” parts show similar signature (Example using Shanin technique: component search)
  • Parts manufactured with slightly amplified deviation on suspected process parameter show similar signature

FTA / OCCURRENCE


What happened at Part just before the Bad part (T-1)?

  • How was the product just before becoming bad ?
  • What can explain the phenomenon?
    • Ask Operators
    • Observe

This is the basis for the completing the FTA:

  • You need to be at the real place
  • To observe the real facts
  • And to gather real data

FTA / SOME RECOMMENDATIONS

  • Why do you say there is no standard ? Is it the first time ?
  • Why do you say no control point? How do you verify ? Do you have start up check list, please show me ?
  • Is it easy to control ? Is repeatable ? known and done by all Operators?
  • Is there any difference between good and bad (not only parts think at situation, timing, person, process, machine …. )?
  • Did you get this measurement from the period when bad parts were manufactured ?
  • Did you compare good and bad versus standard?
  • Where is the data are coming from? How it was measured?
  • Think 4 Ms: Material, Method, Machine, Men.

LOGICAL THINKING

At this stage, it is key to be able to summarize the issue by saying without blah-blah:

  • The problem is xxx.
  • The main root cause is … This has been demonstrated by … The corrective action is … The corrective action will work because……..
  • The main reason why it has not been detected is … This has been demonstrated by … The corrective action is …

Test your conclusion by explaining why no problem occurred prior to this occurrence!


SUMMARY

  • Don’t jump to conclusion without facts
  • Compare Good/ bad/ Standard
  • Verify after each why, based on fact test data
  • Involve Operators and all related persons in the analysis
  • Follow the PDCA / 8D standard formats: you don’t need anything else!

In the next post, I will write about Japanese methods closely related to error analysis.


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