Design of Experiments (DOE)
The process no longer generates optimal output, but it used to. Design of Experiments (DOE) is the solution. Process inputs are optimized on day one. But, remember, all mechanical processes degrade over time. If the Inputs have not changed since the process started, are they still optimized? You know the output is no longer optimal so what do you do to compensate for process degradation?
Status quo becomes entrenched when one rests on work performed in the past. The desire to change doesn’t exist. When control variables are no longer optimized the cost of operation gradually increases.
Lean Business Position
Lean Business recommends reviewing Run Charts or your other ways of measuring process performance for output degradation.
Select a method to determine optimal settings for optimal output. The first DOE methodology was simple and strait forward. One-Factor-At-A-Time. The benefit of this method is that it is straight forward. The disadvantage is that the secondary and tertiary effects are ignored. For tertiary effects, ignoring them is reasonable, but it is dangerous to ignore secondary effects. Actually a strong secondary effect can add or subtract to a primary factor and give an undetected erroneous result. This will not happen with a Full Factorial Design.
A Full Factorial Design allows measurement of all primary, secondary and tertiary impacts of Factors. The advantage is that it is the most thorough methodology. The disadvantage is that number of experimental runs increases exponentially and the impact of every tertiary effect is measured. If an experiment is expensive or time consuming then the downside of this method becomes overwhelming. The best of both worlds is the Fractional Factorial Design.
The Fractional Factorial Design does a good job of right sizing the number of experiments. Tertiary terms are seldom or never of a magnitude that will measurably impact the output. The Full Factorial Design makes the effort to determine these impacts, the Fractional Factorial Design excludes these non-impacts. The disadvantage of the Fractional Factorial Design is its complexity. A sophisticated Lean Six Sigma Black Belt overcomes this disadvantage.
Performing a DOE will bring inputs back into their optimal states, give you profound knowledge of your process, minimize production costs and set up inputs for the future.