I am a Manufacturing Process Engineer (MPE)

Process capability refers to the ability of a production process to consistently produce products that meet quality requirements under stable and fixed production conditions. To understand process capability, there are at least two key aspects we need to consider:

A manufacturing process’s capability is influenced by many factors, starting from product design, including materials, machines, equipment, methods, operator skills, inspection equipment, inspection methods, and the skill of the inspectors. Any changes in these factors can more or less affect process capability.

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Workingbear has already spent a good amount of time explaining what Cp (Precision) and Ck (Accuracy, sometimes called Ca) mean, and what each represents. We’ve also learned that using Cp or Ck alone can give an incomplete picture when it comes to quality control. So now, Workingbear is going to introduce the combined process capability index — Cpk — which is widely used today as a standard measure in statistical process control (SPC).

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What Is Ck or Ca?

Ck, which some people now refer to as Ca (Accuracy), represents how close a process output is to the target value. Using a target-shooting example: if someone fires six bullets and all land near the bullseye, the Ck value is good (meaning low), because the average result is close to the center of the specification — a direct hit.

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Understanding Cp as a Process Capability Indicator

Cp (short for Precision) is a measure of how tightly a process is centered and how consistently it produces results. Using a target-shooting analogy, if someone fires six bullets and they all hit the same tight spot, that means the process has high Cp. The tighter the grouping, the higher the Cp value.

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Cpk is commonly used as an overall indicator of a factory’s manufacturing process capability. However, it can also be applied across different industries to measure the ability to meet a target. A popular analogy is how well you can hit the bullseye when shooting arrows at a target.

Of course, the higher the Cpk value, the better. But Cpk is actually made up of two components: Cp and Ck.

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