Control Plans and Six Sigma Projects

Quality control plans (also known as process control plans) fit into the Control Phase of DMAIC (Define -> Measure -> Analyze -> Improve -> Control), which is the Six Sigma approach to process improvement.  Control plans play the essential role of sustaining the learnings obtained in Six Sigma projects.  Without question, the vast majority of these projects will learn that one or more process parameters must be controlled for the long run with high confidence – this is where solid control plans are essential.

A True Story

A Six Sigma team focusing on a magnet bonding process (motor manufacturing) learned that oven temperature and curing time are critical factors for meeting a magnet bond-strength specification.  In addition to learning the precise time and temperature needed for optimum bond strength, the team also went the extra distance with a PFMEA, and addressed some high-risk  process steps by changing from a batch oven to a conveyor oven, guaranteeing a fixed cure time for every magnet assembly.

Six months later, the magnet adhesion problem re-surfaced in the field.  How could this happen if the team had followed the Six Sigma process?  The root cause was investigated and the team learned that a production associate had, with good intentions, increased the belt speed on the conveyor to catch up on production.  This is a classic scenario that plays out all too often in production environments.  In this case, a simple control plan requiring a once per day check on the belt speed (or even better, removing the ability to adjust the belt speed) would have prevented the problem.

Going the extra distance with control plans will help ensure that Six Sigma teams do not have to re-visit their projects.