The 5 Why’s, this method is very simple but highly effective in finding problems
root causes, basically it consists of making a progressive questioning of what caused
the effect and then repeat the procedure backwards.
What caused the machine failure?
Answer: A loose bolt.
Why the bolt was loose?
Answer: Because the torque meter readings were wrong.
Why the torque meter gave wrong readings?
Answer: Because the torque meter calibration was overdue for more than a month.
Why the torque meter was overdue for such a long time? And so on........
You do not have to use 5 whys, it can be as short as two or as long as ten or more
as necessary until a credible root cause is found.