Plot the Variable Don’t Change the Setting

Managing a process requires understanding and tracking the critical process variables that impact the product being produced.  Temperature, speed, pressure and viscosity are all variables that will nave natural variation.

It is important to track the critical variables using a control chart to understand the trends and determine if action is need, BUT it is important to plot the variables over time and NOT change the settings to impact the variables every time.


A new process owner of an extrusion press had determined that exit temperature was a critical variable to making a good product.  Based on observations she had determined that 980 degrees was the desired exit temperature, but the process had a natural variation that could drop as low as 920 degrees and rise as high as 1040 degrees while still making a good product.

The new process owner created a control chart with an upper limit of 1040 degrees and a lower limit of 920 degrees.  She asked the press operator to plot the exit temperature every 30 minutes during the shift.  Her goal was to see the trends in the natural variation and create reaction plans for the operator under certain circumstances.

The next day, when the new process owner collected the control chart she was shocked to see each exit temperature plot was exactly 980 degrees.  A straight line of dots all connected and exactly at 980 degrees.

She quickly ran out to the press operator and asked, “Why are these plots all at 980 degrees.”

The operator proudly replied, “Well, when I measured the temperature and it was off, I would either speed up or slow down the ram speed to get your temperature. I adjusted it until I got exactly 980 degrees.  It was really hard and I had to scrap a few billets, but I GOT IT every time.”

Help your organization understand the value of plotting the variation of a critical variable and not to adjust the settings every time adding more variation.


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