Engineers of all disciplines will be aware of the strong shift towards continuous, data driven approaches to manufacturing in recent years. While many might be familiar with Process Capability and Process Performance, many Engineers have been unable to gain experience in these areas due to the resistance of some sectors embracing this approach.
And there are the fundamental challenges of CpK and Ppk. Some old manufacturing processes simply do not adapt well to this approach. So here is a basic overview of what Cp, Cpk, Pp and Ppk are all about.
Process Capability and Process Performance
A measure of Process Capability, CpK or Process Performance, Ppk is often used as acceptance criteria for Critical-to-Quality Attributes where variable data is collected. Variable data is also known as continuous data. (In simple terms, data that is in number format, for example a component dimension, when measured with a Vernier calipers gives a value that is “variable” 2.22mm, 2.21mm, 2.31mm etc.) Collection of variable data allows trending which is beneficial in identifying potential issues before they impact the quality of the product. For example, a shift in data may indicate that a machine tool is wearing and is nearing its end of life.
Process Capability and Process Performance refers to the ability of a process to meet specific dimensional or physical requirements and how well they fall within specification limits -how consistently do measurements fall within the upper and lower specification limits.
Pp (Process Performance) and Cp (Process Capability): quantify the stability of a process, i.e., the amount of variation in the output.
Ppk and Cpk represent both the degree of variability AND the degree that the output is centred between the lower and upper specification limits.
Pp and Ppk are calculated using the standard deviation of the entire population and represent a long term performance. Pp and Ppk values are used to describe the Process expected over the long term
Cp and Cpk are calculated using a sample standard deviation and represent the potential that could be achieved if sources of variation are identified and eliminated.
Cp and Cpk are more commonly used for short term assessment.
Check out our simple and concise Cheatsheet for Process Capability and Process Performance here
Comments
Post has no comments.