Recently, I was spending the day going to the gemba by being a guest inspector on the assembly line. It did not take very long to see what many of the problems were in the process. One particular nonconformance that caught my attention was a persistent scratch found in the same spot on nearly 50% of the metal units. Following the established methods, I oscillated the spot and pushed the unit to the next department.
The next day, I met with the regular inspector and shared what I observed. He confirmed that this scratch was pretty common. Since it was an easy 5 second fix, nobody paid it too much attention to it. (Does this sound familiar?)
I knew that I could locate the root cause of this scratch because it was somewhat consistent and easy to see. It became a new mission and I decided to use the 50/50 Approach. This technique is pretty simple to use. Starting at the front of the line, I checked for the scratch and found none. So I split the distance between the inspection station and the front of the line. Here, at the middle of the line, I found the scratch.
Then I split the distance again, between the front of the line and the middle. At this station I found the scratch again. I continued splitting the line and moving upstream or downstream depending if the scratch was present or not. After a few rounds, I narrowed the presence of the scratch to a single station.
Focusing on this station, I saw exactly what caused the scratch. Ironically, it was a quality check. A metal square was used to check for squareness and the metal to metal contact created the scratch. It is amazing to actually follow a process and see right before your eyes the cause of a defect. A simple, homemade plastic cover on the square eliminated the problem.
By using a 50/50 approach, in some cases like this one, its easy to whittle down to the root cause. Try it sometime.