Cp and Cpk compare process spread with tolerance.
Cp=(USL-LSL)/6σ compares tolerance with process spread; Cpk=min((USL-μ)/3σ,(μ-LSL)/3σ) also accounts for centring. Cpk≥1.33 is usually required, ≥1.67 for critical features. Requires a stable process and normality.
A process with Cpk=1.0 produces about 2700 ppm outside limits.
Process capability compares the natural spread of a process with the tolerance, so you know whether a process can meet the drawing.
Cp and Cpk compare process spread with tolerance.
Process capability compares the natural spread of a process with the tolerance, so you know whether a process can meet the drawing.
The ratio of the tolerance width to six times the process standard deviation, which measures spread without considering centring.
A measure that also considers how well the process mean is centred within the tolerance, so it is the more practical measure.
Typical applications include deciding whether a turning process can hold a 0.02 mm tolerance, comparing two suppliers for the capability of a critical dimension and setting a target Cpk for a new production line.
Study "Six Sigma and lean" first (topic 35 of 102 in Manufacturing, Robotics & Systems). It covers the groundwork this topic builds on.
Pp and Ppk long-term. Defect rate PPM. Improvement: centring then spread reduction.
A process with Cpk=1.0 produces about 2700 ppm outside limits.
The next topic is "Plant layout and material handling", which builds directly on this one.
It is classed as Intermediate in Manufacturing, Robotics & Systems (topic 36 of 102). It is an intermediate topic. Work through the beginner topics in Manufacturing, Robotics & Systems first.
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