Old School Process Control

By: Steve Murphy

  Mr. Murphy frequently writes about "Six Sigma in the real world." Mr. Murphy can be contacted via email by clicking here: Steve Murphy To submit your post on listen to the gemba visit: Submit a Post on Listen to the Gemba. 

​  Like a lot of university students of my era when I wasn’t “working on the post at Christmas” I worked on factory production lines. One local factory used excess yeast produced by Burton’s brewing industry to produce stock cubes. No not OXO another brand using a modern technique to produce a crumbly cube. The production machines were modified sweet making machines and operating one was a black art. The correct process depended on getting the right balance of gloop and water to produce cubes of the right weight and strength to allow the wrapping machine to individually foil wrap them. 

  Too light and weak and they disintegrated in the wrapping machine, a clean was needed and time was wasted. Too heavy and they shot through the wrapper but too much gloop was used. The theory was you weighed your cubes all the time and kept adjusting the machine to keep the weight centred. Well in practice we tried to keep the cubes on the heavy side to stop the wrapper from jamming.  To stop us from gaming the system we were regularly monitored by QA.  QA came along weighed our cubes and plotted them on their chart…. Corrections were made depending on the application of Bonnie Small’s famous rules. The routine nature of the checks meant we knew when QA were coming so we knew when to tune the weight back towards the centre a bit and HOPE the wrapper worked. Also if QA decided to break their routine there was a casual network of informants…. Good souls who could be relied on to get the word to us before the inspector arrived. Everything was going dandy UNTIL…….
…….Somebody worked out how much gloop we had used and how many cubes we had made and there was a disparity…all the gloop used should have made a lot more cubes. Eventually the plant manager decided enough was enough and a sample of cubes at the end of the line was weighed. The histograms of the inline QA data and the end of line data were compared. Low and behold there were two quite separate histograms with the in-line QA data well displaced towards the centre of the specification and ours which was well on the high side. So we were well caught as the temp I was moved from moulding to packing, picking six up and putting them in a box…… cubing was not for me.
There are lessons here for any organisation looking to implement SPC. Trying to use SPC to control an incapable process will not work. It will only make things worse. Routine is the weakness of any audit or control system. Inflicting a poorly thought out methodology on the shop floor will only drive them to gaming the system.