Monitoring Reliability and Quality Parameters in Manufacturing

Adam Bahret
Adam Bahret

Cpk Ppk

Customers often ask about controlling the parameters established as critical to reliability once in manufacturing. I believe the two best tools are the Cpk and Ppk charting. Both of these methods are apart of the core tool set in the “Six Sigma” methodology.

The power of these tools is that they provide significant value over the traditional method of monitoring established parameters. This method is commonly based in setting “alarms” at the metrics limits. Cpk and Ppk provide insight on trending within those parameters. This trending and measurement of rate of approaching the proximity of the limits allows the team to address trends before bad results have occurred.

drivingWhen driving a vehicle there is a big difference in outcome between basing your steering corrections on monitoring how far you are drifting within your lane and waiting until you have crossed the median line before correcting your path of travel.

Here are some simple overviews of the tools that can be used to discuss with both technical and non-technical team members. Ensuring the complete team has a simple understanding of the concepts can be the difference in acceptance and implementation and staying stuck in the old less effective methods.

 

 

What is Cp and Pp (well talk about the “k” in a moment)?

Cp is the monitoring of process capability. Pp is the monitoring of process performance. Cp is an index which measures how close a process is running to it’s specified limits relative to the natural variability of the process. This is very powerful. Why? A team doesn’t want to waste time reacting to changes that are within that natural variability. They will end up endlessly chasing their tails with process adjustments. But if there is a change that is not due to natural variability than something is drifting and could continue until the process is out of spec. Being aware of this “unnatural” drift early is critical to making an adjustment before the end effect is experienced.

Remember the car analogy. Franticly correcting for every small bit of feedback in the steering wheel when a pebble is hit will make for an exhausting and frantic drive. But being aware that even a smooth trend of drifting towards the center line (a limit) and correcting for it early is the difference between getting to your destination and a “Dead on Arrival” (DOA) product. In this case “you”!

Pp , process performance, produces an index number that is representative of the process capability. It will let the team know how much process the capability exhibits. A good way to express it is the Pp indicates how the process has performed looking back in time. Cp indicates how the process is performing right now and likely to continue. To continue with my car analogy a Pp index factor would be the result of a study that may be used to determine an appropriate width for a highway travel lane. In a Pp analysis the process is not in a state of control. Everything has already happened. If a process has been operating in a controlled manner successfully the values for Cp and Pp will converge.

archery-targetSo what is the “k”? More commonly Cpk and Ppk are used and not just Cp and Pp. The “k” is when the effect of a non-centered distribution is accounted for. Well what does that mean? My favorite analogy for this is an archery analogy. A Cp study would yield a metric for how tight the grouping of arrows on the target is. “Grouping” would be a metric of how big a radius has to be drawn to include all the arrows that hit the target. A smaller radius is an indication of a more controlled archer. The addition of the “k”, a non-centered distribution factor, would then add the distance of the center of the grouping to the bulls eye on the target. This is obviously a second critical parameter if you were studying which archer you were going to depend on to go and hunt for the family’s dinner. “Cp” would tell you how often they shoot in a controlled and predictable manner. ‘k” would tell you how accurately they have their sights adjusted.

Definitions:
Cp= Process Capability. A simple and straightforward indicator of process capability.
Cpk= Process Capability Index. Adjustment of Cp for the effect of non-centered distribution.
Pp= Process Performance. A simple and straightforward indicator of process performance.
Ppk= Process Performance Index. Adjustment of Pp for the effect of non-centered distribution.

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