Cpk
What is the difference between Cp and Cpk?
Cp measures potential capability based on spread only; Cpk measures actual capability by including both spread and centering.
Why can Cp be high but Cpk be low?
Because the process variation may be small, but the mean is shifted toward one spec limit, so the nearest side controls capability.
Why do CNC buyers ask for Cpk?
They use it to quantify whether a supplier can consistently meet tolerance in production, not just produce first-article parts that pass inspection.
Cpk, or process capability index, is a statistical measure that quantifies how well a manufacturing process produces parts within specification limits while accounting for both variation and centering. It represents the real-world capability that quality engineers and buyers use to assess whether a process can consistently hold tolerance over time.
On the shop floor, Cpk is used to verify whether a CNC machining process—such as bore diameter, slot width, or face thickness—is statistically capable of holding tolerance over time. A stable, in-control process is sampled repeatedly; the mean and standard deviation are calculated from measured parts. The Cpk value reveals how much of the tolerance band is consumed by natural variation and any offset. In a CNC cell, a low Cpk often triggers corrective actions: tool offset adjustment, fixture review, cutter wear analysis, coolant checks, or spindle/drift investigation. Even if parts pass inspection, a low Cpk indicates the process is poorly centered. Similarly, in millwork, the same logic applies to panel thickness, edge overhang, glue line consistency, and squareness. Buyers and quality engineers rely on Cpk because it reflects actual performance, not just machine potential.
Misleading Cpk from unstable process: If the machine still shows tool wear, thermal drift, chatter, or fixture movement, the Cpk number can appear meaningful while the process lacks statistical control, giving a false sense of security.
Low Cpk despite tight spread: A process can have acceptable spread but still fail because the mean is shifted toward one tolerance limit, lowering Cpk even when Cp looks strong.
Short-term study overstates capability: A capability study based on a small, favorable sample from one setup can overstate real production performance when the process later sees tool changes, operator variation, raw-stock variation, or warm-up effects.