Process Capability is defined as a statistical measure of the inherent process variability of a given characteristic. It consists to compares the distribution of sample values (representing the process outcome) to the specification limits.

A process is said to be capable when the output always conforms to the process specifications.

Summary

  1. Why measure process capability
  2. Process capability indices
  3. Process performance indices
  4. Process capability analysis

Why Measure Process Capability?

Monitoring Process Capability allows the manufacturing process performance to be examined and adjusted to assure products meet the design or customer’s requirements.

When used efficiently this information can:

  • Reduce scrap
  • Increase product quality and so customer satisfaction
  • Reduce number of factory defects
  • Reduce or eliminate costly reworks

Process Capability & Performance indices

Process Capability indices (short term variability) are:

  • Cp: Process Capability
  • Cpk: Process Capability Index

Process Performance indices (long term variability) are:

  • Pp: Process Performance
  • Ppk: Process Performance Index

Process Capability Indices

Process capability indices Cp and Cpk evaluate the output of a process in comparison to the specification limits determined by the target value and the tolerance range. Cp indicates if the process is capable of making parts within specifications and Cpk indicates if the process is centered between the specification limits.

The Cp index is an essential indication of process capability. Cp value is calculated using the specification limits and process standard deviation.

Cp= (USL-LSL) /6σ

USL: Upper Specification Limit

LSL: Lower Specification Limit

σ: Standard Deviation

Higher Cp values indicate a more capable process. It ignores the averages, if the system being tested isn’t centered within specification limits, it may (when used alone) give misleading impressions. Most companies require that the process Cp ≥ 1.33.

The Cpk index indicate how close the readings mean is to the center of lower and upper spec limits and how widely spread the readings are. Higher is Cpk, better is the capability of the process to meet its requirements.

Cpk is calculated using the specification limits, standard deviation (or sigma), and the mean value. The value of Cpk is the minimum of two process capability indices Cpu & Cpl. Cpu is based on the upper specification limit and Cpl is based on the lower specification limit. 

Cpk= Min(Cpu, Cpl)

Cpu= (USL-µ) /3σ

Cpl= (µ-LSL) /3σ

Most companies require that the process Cpk ≥ 1.33.

USL: Upper Specification Limit

LSL: Lower Specification Limit

σ: Standard Deviation

µ: Mean

Relationship Between Cp and Cpk for centered processes
Relationship Between Cp and Cpk for centered processes
Relationship Between Cp and Cpk for non centered processes
Relationship Between Cp and Cpk for non centered processes

CPK Conversion Table

Cpk Sigma Level Yield Defect Rate % of Spec used
0.33
1
68.27%
31.73%
0.66
2
95.45%
4.55%
1
3
99.73%
0.27%
100%
1.33
4
99.99%
0.01%
75%
1.66
5
99.9999%
0.0001%
60%
2
6
99.9999998%
0.0000002%
50%

Process Performance Indices

The Cp and Cpk indices are based on short term variability, while Pp and Ppk indices are are based on total variability. In other words, Pp and Ppk is used with a complete set of data for calculation of real performance of the system, while Cp and Cpk use sample (pre-production, batch, logical subgroups) to calculate capability of the process.

The difference between the two indices is the way the process standard deviation is calculated. Pp and Ppk use standard deviation based on whole population, while Cp and Cpk use sample standard deviation.

Ppk= Min(Ppu, Ppl)

Ppu= (USL- µ) /3σ

Ppl= (µ-LSL) /3σ

Most companies require that the process Cpk ≥ 1.33.

If the Cpk and Ppk calculations are very close or equal, that means the process is statically in control. However, if the two calculations are considerably different, even though the Cpk value may be acceptable, this is an indication that the process is not in statistical control because there are more data points that have been evaluated in actual process conditions.

Process Capability Analysis

Process capability analysis is a set of calculations used to assess whether a system is statistically able to meet a set of specifications or requirements. Capability analysis is summarized in indices. They can be monitored and reported over time to show how a system is changing.

Read more

How to Improve Process Capability using CPK Studio?

CPK Studio has been designed to simplify Process Capability improvement and help you to increase product quality, reducing scrap and defects.

Read more